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Assessing a new analysis of contingent color aftereffects
COGNITION Cognition 64 (1997) 207–222
Discussion
Assessing a new analysis of contingent color aftereffects Lorraine G. Allan*, Shepard Siegel Department of Psychology, McMaster University, Hamilton ON, Canada L8S 4K1
McCollough (1965) demonstrated that exposure to colored vertical and horizontal grids results in a complementary color aftereffect contingent upon bar orientation. For example, after an induction period consisting of presentations of a grid made of green and black horizontal bars alternating with another grid made of red and black vertical bars, subjects report negative color aftereffects contingent upon grid orientation; they report that achromatic horizontal grids appear pink, and achromatic vertical grids appear green. McCollough’s discovery inspired a considerable amount of research concerning contingent color aftereffects (CCAEs) (McCollough, 1965). Results of this research indicate that CCAEs are very robust, being detectable for weeks after induction (e.g., Jones and Holding, 1975). Although most CCAE research has used adult humans, the phenomenon has been observed in young children (e.g., Meyer et al., 1982), monkeys (Macquire et al., 1980), and pigeons (Roberts, 1984). A number of theoretical interpretations of CCAEs have been proposed since the publication of McCollough’s now classic paper (McCollough, 1965). Recently, Bedford (1995) presented a new account of CCAEs, which she termed a ‘perceptual learning theory". She claimed that her account explained ‘findings difficult to account for in other interpretations including which stimuli can successfully lead to contingent after-effects, the outcome of correlation manipulations, and why the effect exists at all’ (p. 253). Bedford’s account is ingenious and comprehensive, and her approach differs from other current theoretical positions: e.g., edge-detection (e.g., Broerse and O’Shea, 1995), associative (e.g., Allan and Siegel, 1986, 1993; Murch, 1976; Siegel and Allan, 1992), and error-correction (e.g., Dodwell and Humphrey, 1990, 1993). Moreover, Bedford attempts to integrate CCAEs and prism adaptation within a single theoretical framework. We present a critical appraisal of Bedford’s account of CCAEs. Although her account is admirably creative and integrative, it makes predictions that are not in * Corresponding author. Tel.: +1 905 525 9140 #23023; Fax: +1 905 529 6225; e-mail: [email protected] 0010-0277/97/$17.00 1997 Elsevier Science B.V. All rights reserved PII S00 10-0277(97)000 13-9
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accord with published data. Also, the apparent advantages of her account, over the associative account that we have favored (e.g., Siegel and Allan, 1992), derive from selective analyses of the relevant literature and from an imprecise presentation of the associative interpretation of CCAEs.
1. Bedford’s perceptual learning account of CCAEs Bedford’s perceptual learning account addresses both the circumstances that initiate perceptual learning and the content of that learning. According to the perceptual learning account, objects are constrained to behave in certain ways and perceptual learning is initiated whenever a real-world constraint about an object is violated. Perceptual learning, once initiated, involves mappings between entire stimulus dimensions or continua, rather than individual associations. According to Bedford, a CCAE will be induced when the two induction patterns are interpreted as referring to a single object and a real-world constraint about that object is violated. This is the case in the usual induction of the orientation-CCAE involving two complementarily-colored and orthogonally-oriented grids (e.g., green–horizontal and red–vertical). The two orthogonal grids can be interpreted as the same object tilted 90 deg (or the same object viewed with the head tilted 90 deg). The constraint, that an object does not change color when the object or head is tilted, is violated during induction. This constraint is preserved by an internal correction for the perceived color differences such that different orientations of the same object will no longer have different colors. If the correction is complete during induction, horizontal green bars and vertical red bars should appear the same color (gray). This internal correction results in the orientation-CCAE. When assessed with achromatic grids after induction, horizontal bars will appear pink and vertical bars will appear green. Induction of a CCAE, according to Bedford, results in new mappings between entire perceptual dimensions. The orientation-CCAE, for example, involves a new mapping between the dimensions of orientation and color. The two color–orientation pairs presented in induction (e.g., green–horizontal and red–vertical) are used to extract the underlying relation between the color and orientation dimensions. Bedford contrasts her position, that induction results in new mappings between entire perceptual dimensions, with the associative account of CCAEs (e.g., Murch, 1976; Siegel and Allan, 1992). According to the associative account, a CCAE is a manifestation of an association between the induction pattern and the induction color. The result of pairing of a pattern with color during induction is that the adaptive response of the visual system to the induction color (i.e., the complementary color) is evoked by the pattern. For example, for green–horizontal and red– vertical induction, one association is established between horizontal bars and green resulting in a pink aftereffect on an achromatic horizontal grid; another association is established between vertical bars and red resulting in a green aftereffect on an achromatic vertical grid. For the associative interpretation, it is the individual associations which are learned.
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2. Real-world constraints Dependence of the successful induction of a CCAE on the violation of constraints about real-world objects provides Bedford with a basis for predicting which stimuli are capable of inducing a CCAE and which are not: ‘The induction procedure will be successful only to the extent that the same object is involved. Otherwise, no constraint is violated and there is no discrepancy’ (p. 264). If the induction stimuli are interpreted as referring to a single object and a real world constraint is violated, a CCAE will be induced; otherwise a CCAE will not be induced. In summarizing data that support the perceptual learning account of CCAEs, Bedford emphasizes findings that are consistent with her account; that is, findings indicating that effective induction patterns are those that, when presented in different colors, violate a real-world constraint. Indeed, some effective induction patterns, such as grids that are either horizontal or vertical, or isosceles triangles that are either upright (i.e., base on bottom) or 180-deg rotated (i.e., base on top) (Siegel et al., 1992), may be interpreted as the same object in different orientations. That such patterns are effective is entirely consistent with the perceptual learning analysis of CCAEs; grids and triangles typically do not change their colors when they are rotated. CCAEs induced with patterns of different spatial frequencies (e.g., Breitmeyer and Cooper, 1972; Leppmann, 1973; Lovegrove and Over, 1972; Stromeyer, 1972) are also consistent with the perceptual learning analysis. Two vertical grids which differ in spatial frequency can be interpreted as a single object at different distances from the observer. The real-world constraint that an object does not change color with distance is violated and a CCAE is induced. The motion-CCAE (e.g., Hepler, 1968; Stromeyer and Mansfield, 1970) is also in accord with Bedford’s position. A grid moving up and down on the retina can be interpreted as the same grid viewed with different up and down head movements. The real-world constraint that an object does not change color with head motion is violated and a CCAE is induced. Although some CCAE data are consistent with Bedford’s account, there are considerable data which are not. Some of the contrary findings were available before Bedford formulated her account; others are more recent (the experiments sometimes being stimulated by the publication of her account). There are examples (1) of induction stimuli that cannot reasonably be interpreted as referring to the same object, but are effective, (2) of induction stimuli that refer to the same object and do not violate a real-world constraint, but are effective, and (3) of induction stimuli that refer to the same object and violate a real-world constraint, but are ineffective. In addition, there have been demonstrations of contingent aftereffects with stimuli that do not fit readily into Bedford’s categorization of effective induction stimuli. 2.1. Effective induction stimuli that do not refer to the same object According to Bedford, for induction stimuli to be effective the perceptual system must make the judgment that the same object is involved. Induction with stimuli that cannot reasonably be interpreted as the same object should not result in a CCAE.
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2.1.1. Lie transforms Typically, CCAEs have been induced with orthogonally-oriented and complementarily-colored grids (e.g., green–horizontal and red–vertical). As discussed by Dodwell and colleagues (e.g., Dodwell, 1992; Dodwell and Humphrey, 1990; Dodwell and O’Shea, 1987; Emerson et al., 1985; Humphrey et al., 1985, 1989), such grid stimuli are but one example of vector-fields specified in the Lie transformation group theory of visual perception (named for the Norwegian mathematician, Sophus Lie). Dodwell and colleagues suggested that CCAEs are most readily elicited by Lie derivatives. Patterns described by Lie transformations are orthogonal–if superimposed on each other, their contours will be perpendicular at every point of intersection. Thus, pairs of grids differing in orientation by 90 degrees (e.g., horizontal and vertical grids) are Lie derivatives. So is the pair constructed of concentric-circles (like a bulls-eye pattern) and radiating-lines (like the spokes of a wheel). The interpretation of a CCAE induced with concentric-circles and spokes is problematic for Bedford’s account. It is difficult to imagine that these particular Lie derivatives could refer to the same object. Bedford addressed this circle–spoke CCAE in two places in her paper. Initially, she suggested that (as predicted by her account), the circle–spoke patterns simply are ineffective: ‘The alternation of two concentric circle patterns with different spatial frequencies produces contingent after-effects, but concentric circles of one color alternated with radiating lines of another were reported not to (Fidell, 1968, reported in Skowbo et al., 1975)’ (p. 264). This cited Fidell (1968) work is an unpublished thesis. Examination of the thesis does indeed indicate that Fidell (1968) reported that the circle–spoke patterns were ineffective in her ‘first experimental series’ (the only experiments that examined these particular patterns in the thesis), but Fidell (1968) also noted that ‘the results of the first experimental series were difficult to interpret because of the small number of subjects participating in each study and the attendant variability of the data’ (p. 25). In fact, subsequent to Fidell (1968), but prior to Bedford (1995), a number of demonstrations of the effectiveness of the circle–spoke patterns in CCAE induction have been reported (e.g., Dodwell and O’Shea, 1987; Emerson et al., 1985; Humphrey et al., 1985, 1989). In apparent contradiction to her initial conclusion that these patterns are not effective, Bedford notes later in her paper that Lie transform pairs are effective, and then categorizes them as one of ‘a handful of effects that do not yet have an obvious explanation in the perceptual learning framework’ (p. 271). Although now indicating that the Lie induction aftereffects are a problem for her account, she also indicates that ‘Pavlovian models are also not particularly effective at accounting for these [Lie transform] results’ (p. 271). In fact, there is no reason, on the basis of Pavlovian models, why circle–spoke patterns should not be effective. Pavlovian interpretations of CCAEs would have difficulty explaining failures of color-paired patterns to be become elicitors of color aftereffects, not successes (see Siegel and Allan, 1992).1 2.1.2. Form-CCAE Another example of a CCAE elicited by stimuli that cannot readily be interpreted
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as referring to the same object is the form-CCAE. Foreit and Ambler (1978) alternated two outline geometric forms (square and cross) presented in complementary colors (red and green), and reported that they were unable to induce a form-CCAE. According to Bedford, the square and cross stimuli should be ineffective ‘because the retinal image of a cross and the retinal image of a square cannot refer to the same object–assuming objects maintain rigidity. Consequently pairing a red square and a green cross is not a discrepancy. Different objects can reflect different wavelengths of light... No internal malfunction will be detected, or corrected, and no illusory colors manifested’ (pp. 264–265). Bedford did note that evidence for a form-CCAE, using forms like those in Foreit and Ambler (1978), had been reported by Siegel et al. (1992). To explain these positive findings, she refers to research on object identity in apparent motion which suggests that even stimuli such as a square and a cross may be accepted as referring to the same object, though in nature they never can. Bedford then concluded that aftereffects contingent on such stimuli may be possible, but should be difficult to obtain. While the interpretation of the form-CCAE has been debated in the literature,2 there is no controversy that induction with geometric forms results in CCAEs (Allan and Siegel, 1997; Broerse and Grimbeek, 1994; Humphrey et al., 1994; Siegel et al., 1992, 1994). Moreover, contrary to Bedford’s contention, aftereffects induced with geometric forms are not difficult to obtain. For example, Broerse and Grimbeek (1994) demonstrated the presence of these aftereffects with color-naming, a relatively gross and insensitive measure. Given the many studies which have provided evidence for form-CCAEs, it is likely that the failure reported by Foreit and Ambler (1978) is anomalous. Contrary to Bedford’s prediction, CCAEs can be induced readily with form stimuli. 2.2. Effective induction stimuli that do not violate a real-world constraint Bedford claims that even when the induction stimuli can be interpreted as referring to a single object, a CCAE will be induced only if a real-world constraint is violated. Contrary to Bedford’s claim, there are examples in the literature indicating that CCAEs are induced even when a real-world constraint is not violated. 2.2.1. Induction with two grids of the same color Bedford predicts that CCAE induction with two orthogonal orientations of the same color (e.g., green–horizontal and green–vertical) would not result in a CCAE. If an object does not appear to change color as retinal orientation changes, there is no error to correct and there should be no CCAE. Bedford cites Humphrey et al. (1985) 1 More recently, Broerse and O’Shea (1995) also demonstrated that induction with spokes and concentric-circles results in CCAEs. Broerse and O’Shea (1995) (see also Broerse et al., 1994) suggested that such CCAEs are not spatiotopic (i.e., contingent on pattern) but rather are retinotopic (i.e., contingent on local orientation components of the patterns). Whether these CCAEs are spatiotopic or retinotopic is immaterial for Bedford’s account. 2 The controversy here, as with spokes and circles, is concerned with the spatiotopic vs. retinotopic nature of the CCAE.
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as providing support for her prediction. Humphrey et al. (1985) attempted to induce the orientation-CCAE with horizontal and vertical grids of the same color, and reported that CCAEs were not induced. More recently, however, Allan and Siegel (1997) demonstrated that orientation-CCAEs are induced with orthogonal grids of the same color, and that same-color CCAEs did not differ in size from CCAEs induced with complementarily-colored grids. The same-color induction and assessment procedures in Allan and Siegel (1997) differed from those used by Humphrey et al. (1985). For example, the induction phase was longer and a more sensitive assessment procedure was used. In addition, the assessment figures in Allan and Siegel (1997) consisted of only one pattern (i.e., one orientation), whereas the assessment figures used by Humphrey et al. (1985) were composites consisting of various arrangements of the induction (and also non-induction) patterns. Such composite assessment figures allow for simultaneous color contrast during assessment (see Stromeyer, 1984), and therefore are biased in favor of revealing orientationCCAEs after complementarily-colored induction relative to after same-color induction. The assessment figures in Allan and Siegel (1997) are not subject to simultaneous color contrast, and therefore are not biased in this way (see Allan and Siegel (1991) for a discussion of the role of simultaneous and successive color contrast with the method of constant stimuli). 2.2.2. Induction with black bars on colored backgrounds Bedford cites the ‘single-bar’ experiment reported by Foreit and Ambler (1978) as supporting her position that even when the induction stimuli can be interpreted as referring to a single object, a CCAE will be induced only if a real-world constraint is violated. Foreit and Ambler (1978) compared two single-bar induction conditions. In the colored-bar condition, orthogonally-oriented single bars were paired with complementary colors (e.g., a green horizontal bar and a red vertical bar) and the remainder of the screen was black. In the black-bar condition, the bars were black and the remainder of the screen was colored (e.g., a green screen for the black horizontal bar and a red screen for the black vertical bar). In the colored-bar condition, an orientation-CCAE was induced–for the above example, an achromatic vertical bar appeared green and an achromatic horizontal bar appeared pink. For the black-bar condition, an orientation-CCAE was not induced–achromatic backgrounds appeared achromatic. Bedford suggested that ‘though the background changed color, it did not clearly change orientation’ (p. 271). That is, since there was no violation of a real-world constraint, a CCAE was not induced. Foreit and Ambler (1978) noted that the ratio of black area to colored area was different in the two induction conditions; the black-to-color ratio was greater in the colored-bar condition than in the black-bar condition. They suggested that increasing the black-to-color ratio in black-bar induction might result in an orientationCCAE. Foreit and Ambler (1978) did not investigate this possibility, but Allan and Siegel (1997) did. Allan and Siegel (1997) examined the influence of the black-tocolor ratio in black-bar induction by varying both the size of the black bar and the size of the colored background. They found, as Foreit and Ambler (1978) suggested, that the black-to-color ratio was an important factor. When the black bar was small
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relative to the colored background, as in Foreit and Ambler (1978), orientationCCAEs were not induced. When the relative size of the black bar to the colored background was increased, either by increasing the size of the bar or by decreasing the size of the background, orientation-CCAEs were induced. Contrary to Bedford’s prediction, orientation-CCAEs can be induced with black bars on complementarycolored backgrounds. Allan and Siegel (1997) discussed why, on the basis of an associative account, the black-to-color ratio should be relevant in black-bar induction. It is well established that the size of the conditioned response (CR) depends on conditioned stimulus (CS) salience–the more salient the CS, the larger the CR (see Rescorla and Wagner, 1972). Siegel and Allan (1985) showed that the size of the orientation-CCAE also depends on CS salience–the more salient the orientation CS, the larger the orientation-CCAE. The Allan and Siegel (1997) results suggest that the influence of the black-to-color ratio on the size of the orientation-CCAE is attributable to the salience of the orientation CS. A larger black-to-color ratio provides a more salient black orientation CS and results in a stronger orientation-CCAE. 2.2.3. Frame lightness-CCAE Another example of a CCAE elicited by stimuli that do not violate a real-world constraint is the frame lightness-CCAE (Eissenberg et al., 1995; Siegel et al., 1992). The frame lightness-CCAE is induced by alternating two complementarily colored squares, each surrounded by a frame of a different lightness (e.g., a red square in a black frame and a green square in a white frame). After induction, an achromatic square in a black frame appears green and an achromatic square in a white frame appears pink. Although no real-world constraint is violated by presenting green and red squares in frames of different lightness, a CCAE is nevertheless induced. Although Bedford does mention that ‘two different lightnesses of a frame... produce contingent color after-effects’ (p. 263), she does not note that the frame lightnessCCAE is problematic for her account. 2.3. Ineffective induction stimuli that refer to the same object and violate a constraint In the typical induction of the orientation-CCAE, the stimuli are composed of bars that differ in luminance as well as chromaticity; that is, the colored and black bars differ in luminance, resulting in a high-contrast stimulus. A number of studies have reported that an orientation-CCAE is not induced when the induction grids are isoluminant (e.g., Allan et al., 1991, in press; Ellis, 1977; Mikaelian, 1980; see also Stromeyer, 1978). Induction with isoluminant grids, as surely as induction with high-contrast grids, violates the constraint that an object does not change color when the object or head is tilted. When discussing how her theory accounts for the orientation-CCAE, Bedford makes no mention of the failure of isoluminant grids to elicit the orientation-CCAE. As acknowledged by Allan et al. (in press), the failure of isoluminant grids to induce orientation-CCAEs is also problematic for other accounts of CCAEs, including the associative account.
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2.4. Non-visual aftereffects Bedford relies on visual contingent aftereffects when presenting data to support her position. Visual contingent aftereffects are only one example of contingent aftereffects. Contingent temporal aftereffects, such as duration contingent on pitch and duration contingent on temporal order (e.g., Allan, 1984; Walker and Irion, 1979; Walker et al., 1981) are examples of non-visual contingent aftereffects. The pitch-contingent duration aftereffect, for example, is induced by alternating two tones of different pitch which also differ in duration (e.g., a high-pitch tone of long duration and a low-pitch tone of short duration). After induction, a highpitch tone is perceived as shorter than a low-pitch tone of the same duration. Bedford’s account does not readily encompass non-visual contingent aftereffects such as the pitch-contingent duration aftereffect. Objects have a central role in her account, and auditory stimuli are not readily conceptualized as objects. In contrast (as discussed by Siegel and Allan, 1992), such non-visual contingent aftereffects are readily interpretable by the same associative analysis as that applied to visual contingent aftereffects. 2.5. Selective associations Although many CCAEs have been demonstrated and the failures are fewer than Bedford claims, it is not the case that a CCAE will be induced following pairing of any stimuli. Certain stimulus pairs are better than others at eliciting CCAEs. A number of investigators have appealed to selective associability between stimuli to explain failures of CCAE induction (e.g., Allan and Siegel, 1986; Harris, 1980; Siegel and Allan, 1985, 1992; Westbrook and Harrison, 1984). These investigators note that selective associability between stimuli is well documented in the animal conditioning literature. For example flavor stimuli, but not exteroceptive stimuli, are associated readily with gastrointestinal illness (see Domjan, 1983). Such constraints on illness-induced aversion learning likely result from the organization of the innate connections of the gustatory and visceral systems. Similarly, constraints on CCAEs may result from the organization of innate connections between luminance- and color-perception systems (see Siegel and Allan, 1992). Bedford states that an accepted procedure for demonstrating selective associability in animal learning is to show that the CS which fails to elicit conditioning with one unconditioned stimulus (US) succeeds with a different US. Without this ‘crossover,’ it is difficult to demonstrate that the selective association is not simply due to the CS itself rather than the CS–US relation. Bedford claims that investigators who have explained failures to induce CCAEs in terms of selective associability have not shown the cross-over. While the cross-over design has not been a feature of CCAE experiments, data do exist which demonstrate that the selective association is not simply due to the CS itself. The orientation-CCAE demonstrates that orientation (CS) can be successfully paired with color (US) and the spatial frequency-CCAE demonstrates that spatial frequency (CS) can be successfully paired with color (US). Allan (1996) has shown that these CSs do not elicit an aftereffect when paired with a
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different US, duration. That is, pairing orientation and duration did not result in an orientation-contingent duration aftereffect and pairing spatial frequency with duration did not result in a spatial frequency-contingent duration aftereffect These failures cannot be attributed to duration as an ineffective US since there are data which indicate that pairing the pitch of a tone (CS) with duration (US) induces a pitchcontingent duration aftereffect (Allan, 1984; Walker and Irion, 1979; Walker et al., 1981). 2.6. Summary There are data from many studies involving a variety of contingent aftereffects that are not in accord with Bedford’s account. According to Bedford, CCAEs should not be induced with Lie transforms like spokes and circles, with geometric forms like squares and crosses, with same-colored orthogonal grids, with orthogonal black bars on colored backgrounds, or with chromatic squares in frames which differ in lightness. The data indicate otherwise. Also, Bedford’s account does not differentiate between high-contrast and isoluminant grids, but the data do. Finally, Bedford’s account does not readily encompass non-visual CCAEs. For the associative account, pairings of pattern and color that result in successful induction are expected. Associative interpretations of CCAEs have difficulty explaining failures of color-paired patterns to be become elicitors of color aftereffects, not successes (see Siegel and Allan, 1992). Thus, the associative account, in common with Bedford’s perceptual learning account, would expect isoluminant grids to result in an orientation-CCAE.
3. Mappings between perceptual dimensions Bedford’s proposition that effective induction stimuli must refer to the same object implies that such induction stimuli are different values along one dimension. For example, horizontal and vertical grids differ along the orientation dimension, resulting in the orientation-CCAE, and narrow and wide bars differ in spatial frequency, resulting in the spatial frequency-CCAE. According to Bedford, ‘whenever two stimuli fall along a single dimension such that an interpretation that it is the same stimulus is possible, the stimuli can lead to a contingent after-effect’ (p. 268). This relationship between same-object referent and single-dimension provides the link to Bedford’s assertion that induction of a CCAE results in new mappings between entire perceptual dimensions. Bedford claims that the proposition that induction results in new mappings between entire dimensions provides for a better explanation of the outcome of correlational manipulations and one-pair induction than does the associative account. 3.1. Correlational manipulations The perfect correlation between pattern and color in typical CCAE induction can
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be degraded by presenting the color without the pattern, and/or by presenting the pattern without the color. To degrade the correlation between color and orientation in the induction of the orientation-CCAE, for example, chromatic gridless squares and/or achromatic grids are presented in addition to chromatic grids. For Bedford, a chromatic gridless square is a missing data point because no value on the orientation dimension is presented. A missing data point does not influence the correlation between the two dimensions, and therefore the CCAE should not be effected. In contrast, an achromatic grid is not a missing data point. Rather, orientation is paired with white, a value on the color dimension. Achromatic grids do degrade the correlation, and therefore should decrease the size of the CCAE. Bedford cites the data reported by Skowbo and Forster (1983) and Siegel and Allan (1987) as supportive of her position. These studies found that chromatic gridless squares had no effect on the size of the orientation-CCAE, whereas achromatic grids decreased the size of the orientation-CCAE. On the basis of an associative account, CCAEs, like other conditional responses, should be attenuated by decreasing the correlation between the induction pattern and the induction color. The orientation-CCAE should be deceased by presentations of an achromatic grid, since such presentations are effectively extinction trials. The data reported by Skowbo and Forster (1983) and Siegel and Allan (1987) with regard to achromatic grid presentations are consistent with the associative account as well as with Bedford’s account. On the basis of an associative account, chromatic gridless squares might also be expected to decrease the size of the CCAE. According to a prominent contemporary analysis of classical conditioning, the Rescorla–Wagner model (Rescorla and Wagner, 1972), US-alone presentations retard the formation of an CS–US association because the unpaired USs become associated with context cues, and these context–US associations compete with CS–US associations (see also Tiffany et al., 1991). Decreasing the grid–color correlation during induction by presenting chromatic gridless squares, therefore, should decrease the CCAE only if color can become associated with context cues present during induction. Siegel et al. (1992) suggested that the chromatic gridless squares were ineffective in the Skowbo and Forster (1983) and Siegel and Allan (1987) experiments because a context that could be associated with color (i.e., stimuli that result in CCAEs) was not available in those experiments. Siegel et al. (1992) used frame-lightness as their context, and showed that when an appropriate context was provided, chromatic gridless squares did decrease the size of the orientation-CCAE. Contrary to Bedford’s claim, Siegel et al. (1992) did not suggest that an appropriate context would be provided by simply conducting the experiment in an illuminated room (rather than in the dark). Rather, Siegel et al. (1992) emphasized that the context must be one that is capable of eliciting a CCAE. Moreover, Bedford’s statement that ‘We must also buy that for the McCollough effect only some stimuli can serve as background, even though no special backgrounds need be created for Pavlovian conditioning’ (p. 281) is incorrect. For both CCAE induction and Pavlovian conditioning, the context must be such that an association can be formed between the context and the US.
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Bedford does not address how her account can encompass the fact that the effectiveness of correlational manipulations depends on the availability of context cues. Chromatic gridless square presentations would be using data points both when a context is provided and when a context is not provided. The associative model, in contrast, predicts when the correlation degradation procedure will and will not decrease the CCAE. As we have discussed elsewhere (Allan and Siegel, 1993), the correlation degradation procedure should be effective when appropriate context cues are provided, but not when there are no such context cues. Rather than integrating the Siegel et al. (1992) data into her account, Bedford attempts to minimize their importance. She states, for example, that the results of the Siegel et al. (1992) correlation experiment ‘should not be a surprise to anyone. By using color stimulus trials rather than color-alone trials, the experiment essentially replicates ‘blocking’–a phenomenon already known to occur’ (p. 281). This statement indicates a misunderstanding of the Rescorla–Wagner model, and its application to correlational manipulations and the CCAE. Within the Rescorla–Wagner model, correlational manipulations are blocking of the CS–US association by the context. That is, correlation manipulations work because of blocking by the context. When no appropriate context is provided, there is no blocking and therefore the size of the CCAE is not affected (e.g., Siegel and Allan, 1987; Skowbo and Forster, 1983); when an appropriate context is provided, there is blocking and the size of the CCAE is decreased (e.g., Siegel et al., 1992). Bedford suggests that the Siegel et al. (1992) elaboration of the effects of coloralone presentations on CCAEs is too complicated: ‘we are asked to believe an increasingly convoluted account of a rather simple finding’ (p. 281). This account is the Rescorla–Wagner model, and Bedford’s characterization of the model as ‘convoluted’ is idiosyncratic. Others have extensively reviewed the influence of the model in animal learning–"the most influential theory of associative learning to emerge from the study of animal behavior in the last 25 years’ (Miller et al., 1995, p. 363). They have noted the model’s ‘elegance’ (Miller et al., 1995, p. 363), and have judged it ‘powerful’ (Walkenbach and Haddad, 1980, p. 507), ‘simple and reasonable’ (Malone, 1991, p. 302). We recently reviewed the application of the Rescorla–Wagner model to many areas, and have made the case that ‘the model provides a simple, mechanistic interpretation of seemingly complex phenomena’ (Siegel and Allan, 1996, p. 319). Bedford further asserts that the Siegel et al. (1992) discussion of the decremental effects of US-alone presentations is controversial: ‘one must adhere to a particular view of US-alone interference that not everyone studying conditioning agrees with’ (p. 281). Although it is undoubtedly true that learning theorists are not unanimous in their views of US-alone effects, and that the Rescorla–Wagner model has many shortcomings (Miller et al., 1995), the blocking by context interpretation featured in this model figures prominently in almost all current theories of associative learning (Durlach, 1989; Miller et al., 1995). 3.2. One-pair induction If induction of a CCAE results in new mappings between entire perceptual dimen-
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sions, then the two color–pattern pairs presented during typical CCAE induction is a degraded case of dimensional learning. Moreover, an induction procedure with only one color–pattern pair would, according to Bedford, be ‘a severely degraded version of connecting entire dimensions of opponent color and orientation. Consequently, ambiguity is created by using only one value from each dimension’ (p. 284). Bedford cites studies reported by Allan and Siegel (1991), Ambler and Foreit (1978), Humphrey et al. (1989) and Stromeyer III (1969) as examples of one-pair induction.3 Although only one colored pattern (e.g., a green horizontal grid) was presented during induction in these studies, the colored pattern was not presented continuously, but was alternated with a patternless stimulus. The patternless stimulus was a blank achromatic (black or white) field or a complementarily-colored patternless stimulus (e.g., a red gridless square). Eissenberg et al. (1995) have provided an analysis of the one-pair induction procedure used in these studies that reveals that, in fact, two pairs are presented and that the two pairs are not on the same dimension. As discussed by Eissenberg et al. (1995), a colored pattern is actually a compound stimulus consisting of a number of elements. A horizontal grid stimulus, for example, has a square form and is surrounded by a black frame (the area of the computer monitor around the grid). Thus, a horizontal grid stimulus is a compound consisting of at least three elements: orientation (horizontal), form (square), and frame-lightness (black). As described earlier, each of these elements is capable of eliciting a CCAE (see Siegel et al., 1992). Conceptualizing induction stimuli as compounds reveals that the one-pair induction procedure used in the studies cited by Bedford was not, in fact, one-pair induction. During induction with a green horizontal grid and a red gridless square, for example, the green–horizontal grid pairs green with a horizontal orientation, a square form, and a black frame, whereas the red–gridless square pairs red with two of these elements, the square form and the black frame. Eissenberg et al. (1995) showed that grid orientation was more salient than both square form and framelightness, and ‘overshadowed’ (Kamin, 1969) these less salient elements on the green grid presentations. However, on red gridless square presentations, the grid is absent and there would be effective pairings of the black frame and the square form with red. Eissenberg et al. (1995) assessed the aftereffect on the induced grid orientation and on the gridless square. They found that achromatic horizontal grids appeared pink and achromatic gridless squares appeared green. When assessed with the induced grid orientation, grid orientation elicited the CCAE, and the grid appeared pink. When assessed with an achromatic gridless square, frame-lightness elicited the CCAE, and the gridless square appeared green. The experiments reported by Eissenberg et al. (1995) demonstrate that the so-called one-pair induction procedure results in at least two different aftereffects–color contingent on orientation and color con3
Bedford also cites Ellis (1977) as an example of experiments in which only one color–pattern pair was presented during induction. In fact, Ellis (1977) alternated two color–orientation pairs during induction as in the typical orientation-CCAE induction procedure.
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tingent on frame-lightness. The two induction stimuli, grid and gridless square, cannot be interpreted as the same object, and the two elicitors of the CCAEs, orientation and frame lightness, do not fall on the same dimension. ‘One-pair’ induction provides one more piece of evidence against Bedford’s claim that only stimuli that can refer to the same object will result in CCAEs. Allan and Siegel (1997) presented data from an experiment in which only one grid orientation was paired with one color in induction. In that experiment a green horizontal grid was presented continuously during the 15-min induction period. Contrary to Bedford, such one-pair induction resulted in a CCAE on the induced orientation–horizontal grids appeared pink. Allan and Siegel (1997) also reported that vertical grids (which were not presented during induction) did not elicit an aftereffect. This result is expected on the basis of an associative account. When only one CS (horizontal) is paired with color (green), only that CS should elicit an orientation-CCAE (pink). 3.3. Summary Bedford claimed that conceptualizing CCAEs as new mappings between entire perceptual dimensions provides for a better explanation of the outcome of correlational manipulations and one-pair induction than does the associative account. The data, in fact, show otherwise. There are many examples of successful induction of CCAEs with stimuli that do not fall on the same dimension, including the stimuli presented during so-called ‘one-pair’ induction. Bedford’s analysis of correlational manipulations is biased, in that she ignores the fact that she cannot account for the context-dependency of correlational manipulations. Rather she attempts to minimize such effects, but in doing so reveals an idiosyncratic interpretation of the animal learning literature.
4. Conclusion Bedford’s perceptual learning analysis of CCAEs represents an original and creative approach to a phenomenon of enduring interest to students of both perception and learning. Like most significant contributions, Bedford’s account is stated with sufficient precision to generate clear predictions concerning the outcomes of various experimental manipulations. Many of these experiments have, in fact, been done (some inspired by her account). The results do not support the perceptual learning theory of CCAEs.
Acknowledgements This research was supported by grants to LGA and SS from the Natural Sciences and Engineering Research Council of Canada and the United States National Institutes of Mental Health.
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Discussion
Assessing a new analysis of contingent color aftereffects Lorraine G. Allan*, Shepard Siegel Department of Psychology, McMaster University, Hamilton ON, Canada L8S 4K1
McCollough (1965) demonstrated that exposure to colored vertical and horizontal grids results in a complementary color aftereffect contingent upon bar orientation. For example, after an induction period consisting of presentations of a grid made of green and black horizontal bars alternating with another grid made of red and black vertical bars, subjects report negative color aftereffects contingent upon grid orientation; they report that achromatic horizontal grids appear pink, and achromatic vertical grids appear green. McCollough’s discovery inspired a considerable amount of research concerning contingent color aftereffects (CCAEs) (McCollough, 1965). Results of this research indicate that CCAEs are very robust, being detectable for weeks after induction (e.g., Jones and Holding, 1975). Although most CCAE research has used adult humans, the phenomenon has been observed in young children (e.g., Meyer et al., 1982), monkeys (Macquire et al., 1980), and pigeons (Roberts, 1984). A number of theoretical interpretations of CCAEs have been proposed since the publication of McCollough’s now classic paper (McCollough, 1965). Recently, Bedford (1995) presented a new account of CCAEs, which she termed a ‘perceptual learning theory". She claimed that her account explained ‘findings difficult to account for in other interpretations including which stimuli can successfully lead to contingent after-effects, the outcome of correlation manipulations, and why the effect exists at all’ (p. 253). Bedford’s account is ingenious and comprehensive, and her approach differs from other current theoretical positions: e.g., edge-detection (e.g., Broerse and O’Shea, 1995), associative (e.g., Allan and Siegel, 1986, 1993; Murch, 1976; Siegel and Allan, 1992), and error-correction (e.g., Dodwell and Humphrey, 1990, 1993). Moreover, Bedford attempts to integrate CCAEs and prism adaptation within a single theoretical framework. We present a critical appraisal of Bedford’s account of CCAEs. Although her account is admirably creative and integrative, it makes predictions that are not in * Corresponding author. Tel.: +1 905 525 9140 #23023; Fax: +1 905 529 6225; e-mail: [email protected] 0010-0277/97/$17.00 1997 Elsevier Science B.V. All rights reserved PII S00 10-0277(97)000 13-9
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accord with published data. Also, the apparent advantages of her account, over the associative account that we have favored (e.g., Siegel and Allan, 1992), derive from selective analyses of the relevant literature and from an imprecise presentation of the associative interpretation of CCAEs.
1. Bedford’s perceptual learning account of CCAEs Bedford’s perceptual learning account addresses both the circumstances that initiate perceptual learning and the content of that learning. According to the perceptual learning account, objects are constrained to behave in certain ways and perceptual learning is initiated whenever a real-world constraint about an object is violated. Perceptual learning, once initiated, involves mappings between entire stimulus dimensions or continua, rather than individual associations. According to Bedford, a CCAE will be induced when the two induction patterns are interpreted as referring to a single object and a real-world constraint about that object is violated. This is the case in the usual induction of the orientation-CCAE involving two complementarily-colored and orthogonally-oriented grids (e.g., green–horizontal and red–vertical). The two orthogonal grids can be interpreted as the same object tilted 90 deg (or the same object viewed with the head tilted 90 deg). The constraint, that an object does not change color when the object or head is tilted, is violated during induction. This constraint is preserved by an internal correction for the perceived color differences such that different orientations of the same object will no longer have different colors. If the correction is complete during induction, horizontal green bars and vertical red bars should appear the same color (gray). This internal correction results in the orientation-CCAE. When assessed with achromatic grids after induction, horizontal bars will appear pink and vertical bars will appear green. Induction of a CCAE, according to Bedford, results in new mappings between entire perceptual dimensions. The orientation-CCAE, for example, involves a new mapping between the dimensions of orientation and color. The two color–orientation pairs presented in induction (e.g., green–horizontal and red–vertical) are used to extract the underlying relation between the color and orientation dimensions. Bedford contrasts her position, that induction results in new mappings between entire perceptual dimensions, with the associative account of CCAEs (e.g., Murch, 1976; Siegel and Allan, 1992). According to the associative account, a CCAE is a manifestation of an association between the induction pattern and the induction color. The result of pairing of a pattern with color during induction is that the adaptive response of the visual system to the induction color (i.e., the complementary color) is evoked by the pattern. For example, for green–horizontal and red– vertical induction, one association is established between horizontal bars and green resulting in a pink aftereffect on an achromatic horizontal grid; another association is established between vertical bars and red resulting in a green aftereffect on an achromatic vertical grid. For the associative interpretation, it is the individual associations which are learned.
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2. Real-world constraints Dependence of the successful induction of a CCAE on the violation of constraints about real-world objects provides Bedford with a basis for predicting which stimuli are capable of inducing a CCAE and which are not: ‘The induction procedure will be successful only to the extent that the same object is involved. Otherwise, no constraint is violated and there is no discrepancy’ (p. 264). If the induction stimuli are interpreted as referring to a single object and a real world constraint is violated, a CCAE will be induced; otherwise a CCAE will not be induced. In summarizing data that support the perceptual learning account of CCAEs, Bedford emphasizes findings that are consistent with her account; that is, findings indicating that effective induction patterns are those that, when presented in different colors, violate a real-world constraint. Indeed, some effective induction patterns, such as grids that are either horizontal or vertical, or isosceles triangles that are either upright (i.e., base on bottom) or 180-deg rotated (i.e., base on top) (Siegel et al., 1992), may be interpreted as the same object in different orientations. That such patterns are effective is entirely consistent with the perceptual learning analysis of CCAEs; grids and triangles typically do not change their colors when they are rotated. CCAEs induced with patterns of different spatial frequencies (e.g., Breitmeyer and Cooper, 1972; Leppmann, 1973; Lovegrove and Over, 1972; Stromeyer, 1972) are also consistent with the perceptual learning analysis. Two vertical grids which differ in spatial frequency can be interpreted as a single object at different distances from the observer. The real-world constraint that an object does not change color with distance is violated and a CCAE is induced. The motion-CCAE (e.g., Hepler, 1968; Stromeyer and Mansfield, 1970) is also in accord with Bedford’s position. A grid moving up and down on the retina can be interpreted as the same grid viewed with different up and down head movements. The real-world constraint that an object does not change color with head motion is violated and a CCAE is induced. Although some CCAE data are consistent with Bedford’s account, there are considerable data which are not. Some of the contrary findings were available before Bedford formulated her account; others are more recent (the experiments sometimes being stimulated by the publication of her account). There are examples (1) of induction stimuli that cannot reasonably be interpreted as referring to the same object, but are effective, (2) of induction stimuli that refer to the same object and do not violate a real-world constraint, but are effective, and (3) of induction stimuli that refer to the same object and violate a real-world constraint, but are ineffective. In addition, there have been demonstrations of contingent aftereffects with stimuli that do not fit readily into Bedford’s categorization of effective induction stimuli. 2.1. Effective induction stimuli that do not refer to the same object According to Bedford, for induction stimuli to be effective the perceptual system must make the judgment that the same object is involved. Induction with stimuli that cannot reasonably be interpreted as the same object should not result in a CCAE.
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2.1.1. Lie transforms Typically, CCAEs have been induced with orthogonally-oriented and complementarily-colored grids (e.g., green–horizontal and red–vertical). As discussed by Dodwell and colleagues (e.g., Dodwell, 1992; Dodwell and Humphrey, 1990; Dodwell and O’Shea, 1987; Emerson et al., 1985; Humphrey et al., 1985, 1989), such grid stimuli are but one example of vector-fields specified in the Lie transformation group theory of visual perception (named for the Norwegian mathematician, Sophus Lie). Dodwell and colleagues suggested that CCAEs are most readily elicited by Lie derivatives. Patterns described by Lie transformations are orthogonal–if superimposed on each other, their contours will be perpendicular at every point of intersection. Thus, pairs of grids differing in orientation by 90 degrees (e.g., horizontal and vertical grids) are Lie derivatives. So is the pair constructed of concentric-circles (like a bulls-eye pattern) and radiating-lines (like the spokes of a wheel). The interpretation of a CCAE induced with concentric-circles and spokes is problematic for Bedford’s account. It is difficult to imagine that these particular Lie derivatives could refer to the same object. Bedford addressed this circle–spoke CCAE in two places in her paper. Initially, she suggested that (as predicted by her account), the circle–spoke patterns simply are ineffective: ‘The alternation of two concentric circle patterns with different spatial frequencies produces contingent after-effects, but concentric circles of one color alternated with radiating lines of another were reported not to (Fidell, 1968, reported in Skowbo et al., 1975)’ (p. 264). This cited Fidell (1968) work is an unpublished thesis. Examination of the thesis does indeed indicate that Fidell (1968) reported that the circle–spoke patterns were ineffective in her ‘first experimental series’ (the only experiments that examined these particular patterns in the thesis), but Fidell (1968) also noted that ‘the results of the first experimental series were difficult to interpret because of the small number of subjects participating in each study and the attendant variability of the data’ (p. 25). In fact, subsequent to Fidell (1968), but prior to Bedford (1995), a number of demonstrations of the effectiveness of the circle–spoke patterns in CCAE induction have been reported (e.g., Dodwell and O’Shea, 1987; Emerson et al., 1985; Humphrey et al., 1985, 1989). In apparent contradiction to her initial conclusion that these patterns are not effective, Bedford notes later in her paper that Lie transform pairs are effective, and then categorizes them as one of ‘a handful of effects that do not yet have an obvious explanation in the perceptual learning framework’ (p. 271). Although now indicating that the Lie induction aftereffects are a problem for her account, she also indicates that ‘Pavlovian models are also not particularly effective at accounting for these [Lie transform] results’ (p. 271). In fact, there is no reason, on the basis of Pavlovian models, why circle–spoke patterns should not be effective. Pavlovian interpretations of CCAEs would have difficulty explaining failures of color-paired patterns to be become elicitors of color aftereffects, not successes (see Siegel and Allan, 1992).1 2.1.2. Form-CCAE Another example of a CCAE elicited by stimuli that cannot readily be interpreted
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as referring to the same object is the form-CCAE. Foreit and Ambler (1978) alternated two outline geometric forms (square and cross) presented in complementary colors (red and green), and reported that they were unable to induce a form-CCAE. According to Bedford, the square and cross stimuli should be ineffective ‘because the retinal image of a cross and the retinal image of a square cannot refer to the same object–assuming objects maintain rigidity. Consequently pairing a red square and a green cross is not a discrepancy. Different objects can reflect different wavelengths of light... No internal malfunction will be detected, or corrected, and no illusory colors manifested’ (pp. 264–265). Bedford did note that evidence for a form-CCAE, using forms like those in Foreit and Ambler (1978), had been reported by Siegel et al. (1992). To explain these positive findings, she refers to research on object identity in apparent motion which suggests that even stimuli such as a square and a cross may be accepted as referring to the same object, though in nature they never can. Bedford then concluded that aftereffects contingent on such stimuli may be possible, but should be difficult to obtain. While the interpretation of the form-CCAE has been debated in the literature,2 there is no controversy that induction with geometric forms results in CCAEs (Allan and Siegel, 1997; Broerse and Grimbeek, 1994; Humphrey et al., 1994; Siegel et al., 1992, 1994). Moreover, contrary to Bedford’s contention, aftereffects induced with geometric forms are not difficult to obtain. For example, Broerse and Grimbeek (1994) demonstrated the presence of these aftereffects with color-naming, a relatively gross and insensitive measure. Given the many studies which have provided evidence for form-CCAEs, it is likely that the failure reported by Foreit and Ambler (1978) is anomalous. Contrary to Bedford’s prediction, CCAEs can be induced readily with form stimuli. 2.2. Effective induction stimuli that do not violate a real-world constraint Bedford claims that even when the induction stimuli can be interpreted as referring to a single object, a CCAE will be induced only if a real-world constraint is violated. Contrary to Bedford’s claim, there are examples in the literature indicating that CCAEs are induced even when a real-world constraint is not violated. 2.2.1. Induction with two grids of the same color Bedford predicts that CCAE induction with two orthogonal orientations of the same color (e.g., green–horizontal and green–vertical) would not result in a CCAE. If an object does not appear to change color as retinal orientation changes, there is no error to correct and there should be no CCAE. Bedford cites Humphrey et al. (1985) 1 More recently, Broerse and O’Shea (1995) also demonstrated that induction with spokes and concentric-circles results in CCAEs. Broerse and O’Shea (1995) (see also Broerse et al., 1994) suggested that such CCAEs are not spatiotopic (i.e., contingent on pattern) but rather are retinotopic (i.e., contingent on local orientation components of the patterns). Whether these CCAEs are spatiotopic or retinotopic is immaterial for Bedford’s account. 2 The controversy here, as with spokes and circles, is concerned with the spatiotopic vs. retinotopic nature of the CCAE.
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as providing support for her prediction. Humphrey et al. (1985) attempted to induce the orientation-CCAE with horizontal and vertical grids of the same color, and reported that CCAEs were not induced. More recently, however, Allan and Siegel (1997) demonstrated that orientation-CCAEs are induced with orthogonal grids of the same color, and that same-color CCAEs did not differ in size from CCAEs induced with complementarily-colored grids. The same-color induction and assessment procedures in Allan and Siegel (1997) differed from those used by Humphrey et al. (1985). For example, the induction phase was longer and a more sensitive assessment procedure was used. In addition, the assessment figures in Allan and Siegel (1997) consisted of only one pattern (i.e., one orientation), whereas the assessment figures used by Humphrey et al. (1985) were composites consisting of various arrangements of the induction (and also non-induction) patterns. Such composite assessment figures allow for simultaneous color contrast during assessment (see Stromeyer, 1984), and therefore are biased in favor of revealing orientationCCAEs after complementarily-colored induction relative to after same-color induction. The assessment figures in Allan and Siegel (1997) are not subject to simultaneous color contrast, and therefore are not biased in this way (see Allan and Siegel (1991) for a discussion of the role of simultaneous and successive color contrast with the method of constant stimuli). 2.2.2. Induction with black bars on colored backgrounds Bedford cites the ‘single-bar’ experiment reported by Foreit and Ambler (1978) as supporting her position that even when the induction stimuli can be interpreted as referring to a single object, a CCAE will be induced only if a real-world constraint is violated. Foreit and Ambler (1978) compared two single-bar induction conditions. In the colored-bar condition, orthogonally-oriented single bars were paired with complementary colors (e.g., a green horizontal bar and a red vertical bar) and the remainder of the screen was black. In the black-bar condition, the bars were black and the remainder of the screen was colored (e.g., a green screen for the black horizontal bar and a red screen for the black vertical bar). In the colored-bar condition, an orientation-CCAE was induced–for the above example, an achromatic vertical bar appeared green and an achromatic horizontal bar appeared pink. For the black-bar condition, an orientation-CCAE was not induced–achromatic backgrounds appeared achromatic. Bedford suggested that ‘though the background changed color, it did not clearly change orientation’ (p. 271). That is, since there was no violation of a real-world constraint, a CCAE was not induced. Foreit and Ambler (1978) noted that the ratio of black area to colored area was different in the two induction conditions; the black-to-color ratio was greater in the colored-bar condition than in the black-bar condition. They suggested that increasing the black-to-color ratio in black-bar induction might result in an orientationCCAE. Foreit and Ambler (1978) did not investigate this possibility, but Allan and Siegel (1997) did. Allan and Siegel (1997) examined the influence of the black-tocolor ratio in black-bar induction by varying both the size of the black bar and the size of the colored background. They found, as Foreit and Ambler (1978) suggested, that the black-to-color ratio was an important factor. When the black bar was small
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relative to the colored background, as in Foreit and Ambler (1978), orientationCCAEs were not induced. When the relative size of the black bar to the colored background was increased, either by increasing the size of the bar or by decreasing the size of the background, orientation-CCAEs were induced. Contrary to Bedford’s prediction, orientation-CCAEs can be induced with black bars on complementarycolored backgrounds. Allan and Siegel (1997) discussed why, on the basis of an associative account, the black-to-color ratio should be relevant in black-bar induction. It is well established that the size of the conditioned response (CR) depends on conditioned stimulus (CS) salience–the more salient the CS, the larger the CR (see Rescorla and Wagner, 1972). Siegel and Allan (1985) showed that the size of the orientation-CCAE also depends on CS salience–the more salient the orientation CS, the larger the orientation-CCAE. The Allan and Siegel (1997) results suggest that the influence of the black-to-color ratio on the size of the orientation-CCAE is attributable to the salience of the orientation CS. A larger black-to-color ratio provides a more salient black orientation CS and results in a stronger orientation-CCAE. 2.2.3. Frame lightness-CCAE Another example of a CCAE elicited by stimuli that do not violate a real-world constraint is the frame lightness-CCAE (Eissenberg et al., 1995; Siegel et al., 1992). The frame lightness-CCAE is induced by alternating two complementarily colored squares, each surrounded by a frame of a different lightness (e.g., a red square in a black frame and a green square in a white frame). After induction, an achromatic square in a black frame appears green and an achromatic square in a white frame appears pink. Although no real-world constraint is violated by presenting green and red squares in frames of different lightness, a CCAE is nevertheless induced. Although Bedford does mention that ‘two different lightnesses of a frame... produce contingent color after-effects’ (p. 263), she does not note that the frame lightnessCCAE is problematic for her account. 2.3. Ineffective induction stimuli that refer to the same object and violate a constraint In the typical induction of the orientation-CCAE, the stimuli are composed of bars that differ in luminance as well as chromaticity; that is, the colored and black bars differ in luminance, resulting in a high-contrast stimulus. A number of studies have reported that an orientation-CCAE is not induced when the induction grids are isoluminant (e.g., Allan et al., 1991, in press; Ellis, 1977; Mikaelian, 1980; see also Stromeyer, 1978). Induction with isoluminant grids, as surely as induction with high-contrast grids, violates the constraint that an object does not change color when the object or head is tilted. When discussing how her theory accounts for the orientation-CCAE, Bedford makes no mention of the failure of isoluminant grids to elicit the orientation-CCAE. As acknowledged by Allan et al. (in press), the failure of isoluminant grids to induce orientation-CCAEs is also problematic for other accounts of CCAEs, including the associative account.
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2.4. Non-visual aftereffects Bedford relies on visual contingent aftereffects when presenting data to support her position. Visual contingent aftereffects are only one example of contingent aftereffects. Contingent temporal aftereffects, such as duration contingent on pitch and duration contingent on temporal order (e.g., Allan, 1984; Walker and Irion, 1979; Walker et al., 1981) are examples of non-visual contingent aftereffects. The pitch-contingent duration aftereffect, for example, is induced by alternating two tones of different pitch which also differ in duration (e.g., a high-pitch tone of long duration and a low-pitch tone of short duration). After induction, a highpitch tone is perceived as shorter than a low-pitch tone of the same duration. Bedford’s account does not readily encompass non-visual contingent aftereffects such as the pitch-contingent duration aftereffect. Objects have a central role in her account, and auditory stimuli are not readily conceptualized as objects. In contrast (as discussed by Siegel and Allan, 1992), such non-visual contingent aftereffects are readily interpretable by the same associative analysis as that applied to visual contingent aftereffects. 2.5. Selective associations Although many CCAEs have been demonstrated and the failures are fewer than Bedford claims, it is not the case that a CCAE will be induced following pairing of any stimuli. Certain stimulus pairs are better than others at eliciting CCAEs. A number of investigators have appealed to selective associability between stimuli to explain failures of CCAE induction (e.g., Allan and Siegel, 1986; Harris, 1980; Siegel and Allan, 1985, 1992; Westbrook and Harrison, 1984). These investigators note that selective associability between stimuli is well documented in the animal conditioning literature. For example flavor stimuli, but not exteroceptive stimuli, are associated readily with gastrointestinal illness (see Domjan, 1983). Such constraints on illness-induced aversion learning likely result from the organization of the innate connections of the gustatory and visceral systems. Similarly, constraints on CCAEs may result from the organization of innate connections between luminance- and color-perception systems (see Siegel and Allan, 1992). Bedford states that an accepted procedure for demonstrating selective associability in animal learning is to show that the CS which fails to elicit conditioning with one unconditioned stimulus (US) succeeds with a different US. Without this ‘crossover,’ it is difficult to demonstrate that the selective association is not simply due to the CS itself rather than the CS–US relation. Bedford claims that investigators who have explained failures to induce CCAEs in terms of selective associability have not shown the cross-over. While the cross-over design has not been a feature of CCAE experiments, data do exist which demonstrate that the selective association is not simply due to the CS itself. The orientation-CCAE demonstrates that orientation (CS) can be successfully paired with color (US) and the spatial frequency-CCAE demonstrates that spatial frequency (CS) can be successfully paired with color (US). Allan (1996) has shown that these CSs do not elicit an aftereffect when paired with a
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different US, duration. That is, pairing orientation and duration did not result in an orientation-contingent duration aftereffect and pairing spatial frequency with duration did not result in a spatial frequency-contingent duration aftereffect These failures cannot be attributed to duration as an ineffective US since there are data which indicate that pairing the pitch of a tone (CS) with duration (US) induces a pitchcontingent duration aftereffect (Allan, 1984; Walker and Irion, 1979; Walker et al., 1981). 2.6. Summary There are data from many studies involving a variety of contingent aftereffects that are not in accord with Bedford’s account. According to Bedford, CCAEs should not be induced with Lie transforms like spokes and circles, with geometric forms like squares and crosses, with same-colored orthogonal grids, with orthogonal black bars on colored backgrounds, or with chromatic squares in frames which differ in lightness. The data indicate otherwise. Also, Bedford’s account does not differentiate between high-contrast and isoluminant grids, but the data do. Finally, Bedford’s account does not readily encompass non-visual CCAEs. For the associative account, pairings of pattern and color that result in successful induction are expected. Associative interpretations of CCAEs have difficulty explaining failures of color-paired patterns to be become elicitors of color aftereffects, not successes (see Siegel and Allan, 1992). Thus, the associative account, in common with Bedford’s perceptual learning account, would expect isoluminant grids to result in an orientation-CCAE.
3. Mappings between perceptual dimensions Bedford’s proposition that effective induction stimuli must refer to the same object implies that such induction stimuli are different values along one dimension. For example, horizontal and vertical grids differ along the orientation dimension, resulting in the orientation-CCAE, and narrow and wide bars differ in spatial frequency, resulting in the spatial frequency-CCAE. According to Bedford, ‘whenever two stimuli fall along a single dimension such that an interpretation that it is the same stimulus is possible, the stimuli can lead to a contingent after-effect’ (p. 268). This relationship between same-object referent and single-dimension provides the link to Bedford’s assertion that induction of a CCAE results in new mappings between entire perceptual dimensions. Bedford claims that the proposition that induction results in new mappings between entire dimensions provides for a better explanation of the outcome of correlational manipulations and one-pair induction than does the associative account. 3.1. Correlational manipulations The perfect correlation between pattern and color in typical CCAE induction can
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be degraded by presenting the color without the pattern, and/or by presenting the pattern without the color. To degrade the correlation between color and orientation in the induction of the orientation-CCAE, for example, chromatic gridless squares and/or achromatic grids are presented in addition to chromatic grids. For Bedford, a chromatic gridless square is a missing data point because no value on the orientation dimension is presented. A missing data point does not influence the correlation between the two dimensions, and therefore the CCAE should not be effected. In contrast, an achromatic grid is not a missing data point. Rather, orientation is paired with white, a value on the color dimension. Achromatic grids do degrade the correlation, and therefore should decrease the size of the CCAE. Bedford cites the data reported by Skowbo and Forster (1983) and Siegel and Allan (1987) as supportive of her position. These studies found that chromatic gridless squares had no effect on the size of the orientation-CCAE, whereas achromatic grids decreased the size of the orientation-CCAE. On the basis of an associative account, CCAEs, like other conditional responses, should be attenuated by decreasing the correlation between the induction pattern and the induction color. The orientation-CCAE should be deceased by presentations of an achromatic grid, since such presentations are effectively extinction trials. The data reported by Skowbo and Forster (1983) and Siegel and Allan (1987) with regard to achromatic grid presentations are consistent with the associative account as well as with Bedford’s account. On the basis of an associative account, chromatic gridless squares might also be expected to decrease the size of the CCAE. According to a prominent contemporary analysis of classical conditioning, the Rescorla–Wagner model (Rescorla and Wagner, 1972), US-alone presentations retard the formation of an CS–US association because the unpaired USs become associated with context cues, and these context–US associations compete with CS–US associations (see also Tiffany et al., 1991). Decreasing the grid–color correlation during induction by presenting chromatic gridless squares, therefore, should decrease the CCAE only if color can become associated with context cues present during induction. Siegel et al. (1992) suggested that the chromatic gridless squares were ineffective in the Skowbo and Forster (1983) and Siegel and Allan (1987) experiments because a context that could be associated with color (i.e., stimuli that result in CCAEs) was not available in those experiments. Siegel et al. (1992) used frame-lightness as their context, and showed that when an appropriate context was provided, chromatic gridless squares did decrease the size of the orientation-CCAE. Contrary to Bedford’s claim, Siegel et al. (1992) did not suggest that an appropriate context would be provided by simply conducting the experiment in an illuminated room (rather than in the dark). Rather, Siegel et al. (1992) emphasized that the context must be one that is capable of eliciting a CCAE. Moreover, Bedford’s statement that ‘We must also buy that for the McCollough effect only some stimuli can serve as background, even though no special backgrounds need be created for Pavlovian conditioning’ (p. 281) is incorrect. For both CCAE induction and Pavlovian conditioning, the context must be such that an association can be formed between the context and the US.
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Bedford does not address how her account can encompass the fact that the effectiveness of correlational manipulations depends on the availability of context cues. Chromatic gridless square presentations would be using data points both when a context is provided and when a context is not provided. The associative model, in contrast, predicts when the correlation degradation procedure will and will not decrease the CCAE. As we have discussed elsewhere (Allan and Siegel, 1993), the correlation degradation procedure should be effective when appropriate context cues are provided, but not when there are no such context cues. Rather than integrating the Siegel et al. (1992) data into her account, Bedford attempts to minimize their importance. She states, for example, that the results of the Siegel et al. (1992) correlation experiment ‘should not be a surprise to anyone. By using color stimulus trials rather than color-alone trials, the experiment essentially replicates ‘blocking’–a phenomenon already known to occur’ (p. 281). This statement indicates a misunderstanding of the Rescorla–Wagner model, and its application to correlational manipulations and the CCAE. Within the Rescorla–Wagner model, correlational manipulations are blocking of the CS–US association by the context. That is, correlation manipulations work because of blocking by the context. When no appropriate context is provided, there is no blocking and therefore the size of the CCAE is not affected (e.g., Siegel and Allan, 1987; Skowbo and Forster, 1983); when an appropriate context is provided, there is blocking and the size of the CCAE is decreased (e.g., Siegel et al., 1992). Bedford suggests that the Siegel et al. (1992) elaboration of the effects of coloralone presentations on CCAEs is too complicated: ‘we are asked to believe an increasingly convoluted account of a rather simple finding’ (p. 281). This account is the Rescorla–Wagner model, and Bedford’s characterization of the model as ‘convoluted’ is idiosyncratic. Others have extensively reviewed the influence of the model in animal learning–"the most influential theory of associative learning to emerge from the study of animal behavior in the last 25 years’ (Miller et al., 1995, p. 363). They have noted the model’s ‘elegance’ (Miller et al., 1995, p. 363), and have judged it ‘powerful’ (Walkenbach and Haddad, 1980, p. 507), ‘simple and reasonable’ (Malone, 1991, p. 302). We recently reviewed the application of the Rescorla–Wagner model to many areas, and have made the case that ‘the model provides a simple, mechanistic interpretation of seemingly complex phenomena’ (Siegel and Allan, 1996, p. 319). Bedford further asserts that the Siegel et al. (1992) discussion of the decremental effects of US-alone presentations is controversial: ‘one must adhere to a particular view of US-alone interference that not everyone studying conditioning agrees with’ (p. 281). Although it is undoubtedly true that learning theorists are not unanimous in their views of US-alone effects, and that the Rescorla–Wagner model has many shortcomings (Miller et al., 1995), the blocking by context interpretation featured in this model figures prominently in almost all current theories of associative learning (Durlach, 1989; Miller et al., 1995). 3.2. One-pair induction If induction of a CCAE results in new mappings between entire perceptual dimen-
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sions, then the two color–pattern pairs presented during typical CCAE induction is a degraded case of dimensional learning. Moreover, an induction procedure with only one color–pattern pair would, according to Bedford, be ‘a severely degraded version of connecting entire dimensions of opponent color and orientation. Consequently, ambiguity is created by using only one value from each dimension’ (p. 284). Bedford cites studies reported by Allan and Siegel (1991), Ambler and Foreit (1978), Humphrey et al. (1989) and Stromeyer III (1969) as examples of one-pair induction.3 Although only one colored pattern (e.g., a green horizontal grid) was presented during induction in these studies, the colored pattern was not presented continuously, but was alternated with a patternless stimulus. The patternless stimulus was a blank achromatic (black or white) field or a complementarily-colored patternless stimulus (e.g., a red gridless square). Eissenberg et al. (1995) have provided an analysis of the one-pair induction procedure used in these studies that reveals that, in fact, two pairs are presented and that the two pairs are not on the same dimension. As discussed by Eissenberg et al. (1995), a colored pattern is actually a compound stimulus consisting of a number of elements. A horizontal grid stimulus, for example, has a square form and is surrounded by a black frame (the area of the computer monitor around the grid). Thus, a horizontal grid stimulus is a compound consisting of at least three elements: orientation (horizontal), form (square), and frame-lightness (black). As described earlier, each of these elements is capable of eliciting a CCAE (see Siegel et al., 1992). Conceptualizing induction stimuli as compounds reveals that the one-pair induction procedure used in the studies cited by Bedford was not, in fact, one-pair induction. During induction with a green horizontal grid and a red gridless square, for example, the green–horizontal grid pairs green with a horizontal orientation, a square form, and a black frame, whereas the red–gridless square pairs red with two of these elements, the square form and the black frame. Eissenberg et al. (1995) showed that grid orientation was more salient than both square form and framelightness, and ‘overshadowed’ (Kamin, 1969) these less salient elements on the green grid presentations. However, on red gridless square presentations, the grid is absent and there would be effective pairings of the black frame and the square form with red. Eissenberg et al. (1995) assessed the aftereffect on the induced grid orientation and on the gridless square. They found that achromatic horizontal grids appeared pink and achromatic gridless squares appeared green. When assessed with the induced grid orientation, grid orientation elicited the CCAE, and the grid appeared pink. When assessed with an achromatic gridless square, frame-lightness elicited the CCAE, and the gridless square appeared green. The experiments reported by Eissenberg et al. (1995) demonstrate that the so-called one-pair induction procedure results in at least two different aftereffects–color contingent on orientation and color con3
Bedford also cites Ellis (1977) as an example of experiments in which only one color–pattern pair was presented during induction. In fact, Ellis (1977) alternated two color–orientation pairs during induction as in the typical orientation-CCAE induction procedure.
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tingent on frame-lightness. The two induction stimuli, grid and gridless square, cannot be interpreted as the same object, and the two elicitors of the CCAEs, orientation and frame lightness, do not fall on the same dimension. ‘One-pair’ induction provides one more piece of evidence against Bedford’s claim that only stimuli that can refer to the same object will result in CCAEs. Allan and Siegel (1997) presented data from an experiment in which only one grid orientation was paired with one color in induction. In that experiment a green horizontal grid was presented continuously during the 15-min induction period. Contrary to Bedford, such one-pair induction resulted in a CCAE on the induced orientation–horizontal grids appeared pink. Allan and Siegel (1997) also reported that vertical grids (which were not presented during induction) did not elicit an aftereffect. This result is expected on the basis of an associative account. When only one CS (horizontal) is paired with color (green), only that CS should elicit an orientation-CCAE (pink). 3.3. Summary Bedford claimed that conceptualizing CCAEs as new mappings between entire perceptual dimensions provides for a better explanation of the outcome of correlational manipulations and one-pair induction than does the associative account. The data, in fact, show otherwise. There are many examples of successful induction of CCAEs with stimuli that do not fall on the same dimension, including the stimuli presented during so-called ‘one-pair’ induction. Bedford’s analysis of correlational manipulations is biased, in that she ignores the fact that she cannot account for the context-dependency of correlational manipulations. Rather she attempts to minimize such effects, but in doing so reveals an idiosyncratic interpretation of the animal learning literature.
4. Conclusion Bedford’s perceptual learning analysis of CCAEs represents an original and creative approach to a phenomenon of enduring interest to students of both perception and learning. Like most significant contributions, Bedford’s account is stated with sufficient precision to generate clear predictions concerning the outcomes of various experimental manipulations. Many of these experiments have, in fact, been done (some inspired by her account). The results do not support the perceptual learning theory of CCAEs.
Acknowledgements This research was supported by grants to LGA and SS from the Natural Sciences and Engineering Research Council of Canada and the United States National Institutes of Mental Health.
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