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Hemispheric asymmetry in the processing of absolute versus relative spatial frequency
BRAIN
AND
COGNITION
16, 62-73 (1991)
Hemispheric Asymmetry in the Processing of Absolute versus Relative Spatial Frequency STEPHEN
CHRISTMAN
AND
University
FREDERICK
L.
KITTERLE
of Toledo AND
JOSEPH University
HELLIGE
of Southern
California
Observers indicated whether a stimulus presented to one visual field or the other consisted of two sine-wave gratings (the baseline stimulus) or those same two gratings with the addition of a 2 cycle per degree (cpd) component. When the absolute spatial frequencies of the baseline stimulus were low (0.5 and 1.0 cpd), there was a left visual field-right hemisphere (LVF-RH) advantage in reaction time (RT) to respond to the baseline stimulus which disappeared when the 2 cpd component was added (i.e., the stimulus consisted of 0.5, 1.0, and 2.0 cpd components). When the absolute spatial frequencies of the baseline stimulus were moderate to high (4.0 and 8.0 cpd), a right visual field-left hemisphere advantage in RT to respond to the baseline stimulus approached significance and shifted to a significant LVF-RH advantage when the 2 cpd component was added (i.e., the stimulus consisted of 2.0, 4.0, and 8.0 cpd components. That is, adding the same 2 cpd component caused opposite shifts in visual laterality depending on whether 2 cpd was a relatively high or relatively low frequency compared to the baseline. 0 1991 Academic
Press. Inc.
The issue of cerebral hemispheric differences in the processing of spatial frequency components of visual stimuli in normal subjects has been the focus of much recent research (e.g., Christman, 1987; Glass, Bradshaw, This research was supported by an Academic Challenge Grant from the State of Ohio to the Department of Psychology at the University of Toledo, which provided a postdoctoral fellowship to Dr. S. Christman and a visiting professorship to Dr. J. Hellige. In addition, Dr. Hellige was also supported by an NSF grant (BNS 89-08305). The comments of two anonymous reviewers are gratefully acknowledged. Address correspondence and reprint requests to Dr. Stephen Christman, Department of Psychology, University of Toledo, Toledo, OH 43606. 62 0278.2626191 $3.00 Copyright All rights
Q 1991 by Academic Press. Inc. of reproduction in any form reserved.
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Day, & Umilta, 1985; Jonsson and Hellige, 1986; Kitterle, Christman, & Hellige, 1990; Kitterle and Kaye, 1985; Sergent, 1983,1987). These studies suggest that the right visual field-left hemisphere (RVF-LH) is more efficient at processing higher spatial frequencies, while the left visual fieldright hemisphere (LVF-RH) is more efficient with lower spatial frequencies. Tests of the spatial frequency hypothesis have typically taken one of two approaches. First, studies have investigated the types of task conditions under which hemisphere by spatial frequency interactions arise. For example, Kitterle et al. (1990) have demonstrated that such interactions arise in identification tasks, but not in tasks that require mere detection of stimuli. They discuss their results in terms of the computational requirements of detection versus identification, arguing that hemispheric differences arise only in tasks with sufficient computational complexity. Similarly, others have shown that such interactions are usually not obtained in tasks requiring lexical processing (Chiarello, Senehi, & Soulier, 1986; Kersteen-Tucker & Hardyck, 1988), presumably because the strong RVF-LH dominance for lexical processing overrides input factors. A second approach, focusing directly on input factors, has been to filter out higher spatial frequencies (via digital filtering, blurring, etc.) from visual input in order to determine whether such a manipulation impairs RVF-LH performance relative to LVF-RH. The results of such manipulations have been generally consistent with predictions of the spatial frequency hypothesis (for reviews, see Christman, 1989; Sergent & Hellige, 1986). Although some recent studies have failed to find Hemisphere x Frequency interactions in identification tasks, these studies involve methodological factors that limit their ability to directly address the spatial frequency hypothesis. For example, studies by Boles and Morelli (1988) and by Moscovitch and Radzins (1987) manipulated spatial frequency via the use of complex stimuli containing broad ranges of spatial frequencies (e.g., square-wave gratings, masks composed of dots), which makes it difficult to determine which ranges of spatial frequency were mediating task performance (cf., Hellige & Sergent, 1986). Similarly, other studies have employed detection tasks (Rebai, Mecacci, Bagot, & Bonnet, 1989) and have failed to find interactions between hemisphere and spatial frequency. However, as noted above, Kitterle et al. (1990) have demonstrated that such interactions are not obtained in detection tasks. Finally, the study by Peterzell, Harvey, and Hardyck (1989) failed to find a Hemisphere x Frequency interaction in a task involving classification of letters that had been bandpass filtered. It is not clear whether their results bear on the spatial frequency hypothesis, however, because their stimulus set consisted of only four letters, and the target letters (L and H) differed
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CHRISTMAN,
KI’ITERLE,
AND
HELLIGE
from the nontarget letters (T and F) on the basis of whether a horizontal crossbar at the top of the display was absent or present. This crossbar was clearly visible at all levels of filtering. Thus, it is not clear whether subjects were performing the task on the basis of the spatial frequency content of the stimuli per se, or whether they were simply looking for a particular feature (e.g., the crossbar), regardless of the overall spatial frequency content of the input. Tests of the spatial frequency hypothesis employing methods of lowpass filtering have inherently assumed that the hemispheres differ in their efficiency at processing absolute ranges of spatial frequency. The presence of this assumption is implicitly reflected by the fact that such studies have always quantified the effects of low-pass filtering in terms of cycles per degree (cpd) of visual angle. To date, no studies have explicitly examined whether the hemispheres may also differ in processing relative ranges of spatial frequency. Relative spatial frequency is measured in terms of the periodicity of a component relative to other image components or to the rest of the image (e.g., cycles per image width). It is an empirical question whether an image component of a given absolute spatial frequency will always be processed more efficiently by a particular hemisphere, regardless of the relative frequency of other components present in the image. The manner in which a particular component (and the perceptual information that it carries) will be processed may be determined by the context created by the distribution of other components. For example, a component of intermediate absolute spatial frequency could signal global information in a very small image and signal local information in a very large image. The possibility that relative frequency may play a role in hemispheric differences in perceptual processing is an important concern, since it has been demonstrated that both relative and absolute spatial frequency play important roles in pattern recognition (Norman & Ehrlich, 1987). Methodologies of studies testing the spatial frequency hypothesis have confounded these two dimensions, making it impossible to determine whether the hemispheres differ in the processing of absolute spatial frequency, relative spatial frequency, or both. Thus, the purpose of the present experiment is to directly address the issue of whether the hemispheres differ in their ability to process relative spatial frequency components of visual stimuli. The general task involved identification of compound grating stimuli consisting of multiple sine-wave components. The advantage of using grating stimuli to test the implications of the spatial frequency hypothesis lies in the fact that the characteristics of the visual input are clearly specified (i.e., the spatial frequencies available for task performance are known) and the spatial frequencies required for effective task performance are also specified. A further advantage in using complex patterns constructed from simple components is that with complex gratings we can investigate in greater depth how the RVF-LH
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versus LVF-RH combine and compare information from different spatial frequency channels and also how the processing of a given channel is influenced by activity in other spatial frequency channels. On each trial of the experiment, one of two possible compound gratings was presented. The two differed only in that one contained a 2.0 cpd component and the other (the “baseline” stimulus) did not. In the Low Frequency Baseline condition, the other components of the compound grating were of lower frequency (specifically, 0.5 and 1.0 cpd), making the 2 cpd component a relatively high frequency compared to the components of the baseline stimulus. In the High Frequency Baseline condition, the other components were of higher frequency (specifically, 4.0 and 8.0 cpd), making the 2 cpd component a relatively low frequency compared to the components of the baseline stimulus. In terms of human contrast sensitivity to absolute spatial frequency (under conditions of brief exposure and parafoveal presentation), a 2 cpd component is a low to intermediate frequency and, on the basis of our previous research (Kitterle et al., 1990), would be expected to be slightly better processed by the LVF-RH . The straightforward prediction from the relative frequency hypothesis is that processing of the 2.0 cpd component will produce a RVF-LH advantage when the baseline stimulus consists of components lower than 2.0 cpd (e.g., 0.5 + 1.0 cpd), whereas processing of the 2.0 cpd component will produce a LVF-RH advantage when the baseline stimulus consists of components higher than 2.0 cpd (e.g., 4.0 + 8.0 cpd). However, there is a problem in testing this prediction. The problem comes from the fact that the two hemispheres are not expected to be equally efficient at processing the various baseline stimuli. For example, our earlier research suggests that there will be a LVF-RH advantage for processing a baseline stimulus consisting of 0.5 + 1.0 cpd gratings, and any RVF-LH advantage for processing the 2.0 cpd component in the context of the baseline would have to be superimposed on this LVF-RH advantage for processing the baseline stimulus without the 2.0 cpd component. Therefore, the critical prediction is that any tendency toward a LVF-RH advantage for the baseline stimulus should be reduced (or reversed) when the 2.0 cpd component is added. By way of contrast, consider the situation when the baseline consists of 4.0 + 8.0 cpd gratings. Our earlier research suggests that, in this case, there will be either a RVF-LH advantage or no visual field difference for processing the baseline stimulus (8.0 cpd is sufficiently high to produce a RVF-LH advantage, but 4.0 cpd is sufficiently low to moderate it). Therefore any LVF-RH “advantage” for processing the relatively low 2 cpd component in this higher frequency context will be superimposed on a very different advantage for processing the baseline stimulus. In this condition, the critical prediction is that any tendency toward a RVF-LH
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KITTERLE,
AND HELLIGE
advantage in the baseline condition should be reduced (or reversed) when the 2.0 cpd components is added. That is, in both of the Baseline conditions, there is predicted to be a particular pattern of Visual Field x Stimulus Type (2.0 cpd absent versus 2.0 cpd present) interaction, with the predicted interaction pattern being different for the two Baseline conditions. As a result, there should be a significant three-way interaction of Baseline Condition, Visual Field, and Stimulus Type. METHOD The stimuli consisted of compound gratings comprising multiple, vertically oriented sine-wave components. The gratings were generated by a Picasso CRT Image Synthesizer (Innisfree) under computer control and were displayed on Tektronix 608 monitors. A refresh rate of 200 Hz was employed. A large black matte surround (30 x 36” at a viewing distance of 42 in.) was placed directly in front of the monitors. Holes in the surround were used to mask the monitor screens down to two 6.8” circular apertures. A small red LED placed between the two monitors served as the fixation point, and the inner edge of each monitor screen was 3” from fixation. The mean luminance and contrasts of the displays were calibrated by means of a Tektronix 516 digital photometer and 56.523 narrow angle luminance probe. The mean luminance of the displays was 10.3 cd/m’ and the contrast of all sine-wave components was 0.5 [contrast was defined as (Maximum Luminance minus Minimum Luminance)/(Maximum Luminance plus Minimum Luminance)]. Care was taken during the course of the experiment to ensure that both CRTs were matched in luminance and contrast and that the mean luminance of the display did not change with grating presentation. Stimuli were exposed for 150 msec and were presented at a variable foreperiod (600-900 msec) following a brief warning tone. Procedure. Subjects viewed the displays through a viewing hood with a padded chin rest under near-dark conditions. Viewing was binocular. The importance of maintaining fixation on the fixation point was stressed to subjects, who were told that the purpose of the experiment was to see how well people could identify stimuli without looking directly at them. At the beginning of the experiment, subjects were light adapted for 2 min to the mean luminance of the display. The experiment consisted of Low and High Frequency Baseline conditions. In both conditions, the stimulus sets consisted of two different compound gratings. In the Low Frequency Baseline condition, one compound grating was composed of two components: 0.5 and 1.0 cpd sine-wave gratings, and the other was composed of three components: 0.5, 1.0, and 2.0 cpd gratings. In the High Frequency Baseline condition, the two-component stimulus consisted of 4.0 and 8.0 cpd gratings and the three-component stimulus was composed of 2.0, 4.0, and 8.0 cpd components. Thus, the difference between the two stimuli in each condition was defined by the presence versus absence of the two cpd component. The phase of the two cpd component relative to the other components was varied randomly in both conditions. Phase is defined as the position of a sinusoidal grating with respect to some reference point. Thus, the position (e.g., with respect to the edge of the display) of the two cpd component (when present) varied from trial to trial, while the position of the other components was held constant. Eight different phase relations for the two cpd component were used in each condition, spanning a range from 0” to 157.5” in 22.5” increments. The rationale behind this manipulation was to ensure that subjects did not base their discrimination on local luminance or contrast variability cues created by particular combinations of phase relations among the components (e.g., see Rentschler, Christen, Christen, & Landis, 1986). At the beginning of the block of trials constituting each condition, subjects were shown examples of the two stimuli composing the stimulus set and were instructed to press a key Apparatus
and
visual
display.
HEMISPHERIC 600
ASYMMETRY
1
1 +-
1
AND SPATIAL
FREQUENCY
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LVF-RH RVF-LH
300 0.5 + 1 .o 0.5+1.0+2.0 Grating Components (cpd) FIG. 1. Reaction time as a function of stimulus type and visual field of presentation in the low frequency condition. with one hand if they saw one of the stimuli, and to press a different key with their other hand if they saw the other stimulus. Subjects were instructed to respond as quickly and as accurately as possible. Both accuracy and reaction time (RT) measures were collected. The arrangement of hand of response and the order of the two conditions was counterbalanced across subjects. Each condition consisted of a block of 128 trials, resulting from the factorial combination of two different stimuli, two visual fields, four foreperiods, and eight replications. Subjects were also given practice blocks of 20 trials before each test block to help familiarize them with the nature of the task and stimuli. Subjects. Four male and four female subjects with normal or corrected-to-normal vision served as observers. All were right-handed with no left-handed relatives in their immediate family, as assessed by a handedness questionnaire. Subjects were drawn from the Psychology Department’s Human Subject Pool, were naive about the purpose of the experiment, and received course credit for their participation.
RESULTS
Since foreperiod duration was not of interest in testing the current hypothesis, the data were collapsed over this variable. Because of the low overall error rate across conditions (average = 2.9%), analyses of variance were not performed on the accuracy data. Trends in the error data mirrored trends in the RT data discussed below and there was no indication of speed-accuracy tradeoffs. RT data for the Low Frequency Baseline condition are graphed in Fig. 1 and for the High Frequency Baseline condition in Fig. 2. Analyses of variance were performed on the RT data, with hand of response and gender as between-subject variables, and visual field, stimulus type (twoversus three-component), and Baseline condition (high versus low frequency) as within-subject variables. There were no main effects or in-
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CHRISTMAN,
KI’ITERLE,
AND HELLIGE
700 -
LVF-RH
Grating Components (cpd) FIG. 2. Reaction time as a function of stimulus type and visual field of presentation in the high frequency condition.
teractions of hand of response or gender with the other variables, so further analyses were based on data which were collapsed across these two variables. Of particular interest was the presence of a significant three-way interaction between visual field, stimulus type, and Baseline condition [F(l, 7) = 40.01, p < .OOl]. In order to examine the nature of this interaction, separate analyses were conducted for the Low and High Frequency Baseline conditions. There were significant interactions between visual field and stimulus type in both the Low Frequency Baseline condition [F(l, 7) = 8.51, p < .03] and the High Frequency Baseline condition [F(l, 7) = 8.86, p < .02]. These interactions were further analyzed by examining the nature of simple effects for each condition. In the Low Frequency Baseline condition, the interaction arose due to the presence of a significant LVF-RH advantage for the two-component stimulus (0.5 + 1.0 cpd) [F(l, 7) = 10.65, p < .02], whereas there was no difference in RT between the visual fields for the three-component stimulus (0.5 + 1.0 + 2.0 cpd) [F < 11. The Visual Field x Stimulus Type interaction in the High Frequency Baseline condition arose from the presence of a nonsignificant RVF-LH advantage for the two-component stimulus (4.0 + 8.0 cpd) [F(l, 7) = 4.14, p < .08] and a significant LVF-RH advantage for the threecomponent stimulus (2.0 + 4.0 + 8.0 cpd) [F(l, 7) = 5.86, p < .05]. In the Low Frequency Baseline condition, there were main effects of stimulus type [F(l, 7) = 123.4, p < .OOOOl] and of visual field [F(l, 7) = 5.60, p < .05]. RT was quicker for the two-component stimulus and for LVF-RH presentations (as qualified by the aforementioned interac-
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tion). In the High Frequency Baseline condition, there was a main effect of stimulus type [F(l, 7) = 68.1, p < .0003], with responses being faster to the two-component stimulus. There was no main effect of visual field [F < 11. DISCUSSION
The purpose of the present experiment was to test whether the cerebral hemispheres differ in sensitivity to relative spatial frequency. Subjects were required to discriminate between compound grating stimuli composed of two versus three sinusoidal components. In the Low Frequency Baseline condition, the two components of the baseline stimulus consisted of low spatial frequencies (0.5 and 1 cpd); in the High Frequency Baseline condition, the two components of the baseline stimulus consisted of high spatial frequencies (4 and 8 cpd). The three-component stimuli in both conditions were created by adding the same 2 cpd component to the relevant two-component baseline stimuli. The results for the two-component stimuli supported previous findings of LVF-RH superiority in processing lower absolute spatial frequencies and RVF-LH superiority for processing higher absolute spatial frequencies (although the RVF-LH advantage was only marginally significant). The results for the three-component stimuli, however, suggest that hemispheric differences also exist in the processing of relative spatial frequency. Adding an identical 2 cpd component resulted in no visual field differences when it was of high spatial frequency relative to the other components present in the input, whereas it yielded a significant LVF-RH advantage when it was of low spatial frequency relative to the other components present in the input. Thus, the current results demonstrate that a stimulus composed of high frequency components (2.0 + 4.0 + 8.0 cpd) yielded a LVF-RH advantage, whereas a stimulus composed of much lower frequency components (0.5 + 1.0 + 2.0 cpd) yielded no visual field differences. This result is not predicted by the spatial frequency hypothesis in terms of absolute frequency. However, this pattern of results can be accounted for by considering the context provided by the baseline stimuli from which the aforementioned three-component stimuli had to be discriminated. It might be argued that the shift from an RVF-LH advantage for the two-component stimulus to a LVF-RH advantage for the three-component stimulus in the High Frequency Baseline condition was due to the fact that the three-component stimulus (with the addition of the 2 cpd component) had a lower mean absolute frequency than the two-component stimulus. A problem with this argument, however, lies in the fact that in the low frequency condition, the addition of the same two cpd component resulted in a shift from a LVF-RH advantage to no hemispheric advantage. If the two cpd component is an absolutely low frequency that is processed
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AND HELLIGE
better in the LVF-RH, then adding that frequency to the two-component stimulus in the low frequency condition should not have eliminated the LVF-RH advantage obtained for the two-component stimulus. It should be noted that the RT to the three-component gratings was longer than the RT to two-component gratings in both conditions. This suggests that subjects were not basing their decisions simply on the basis of the presence versus absence of the two cpd component. If this were the case, then RT to the three-component stimuli should have been quicker, since presence judgments are typically faster than absence judgments. Rather, it appears reasonable to assume that the longer RT to three-component stimuli was caused by the greater complexity of those stimuli and that subjects were not able to process the two cpd component in isolation. This confirms the notion that the manner in which the two cpd component was processed was influenced by the context of the other frequency components. The important point to be made in regard to the relative frequency hypothesis is that, although the three-component stimuli took longer to process in both conditions, the pattern of relative hemispheric impairment differed according to the relative frequency of the other components (e.g., one hemisphere was not simply better at identifying more complex stimuli). Thus, the results of the current experiment demonstrate that the RVFLH and LVF-RH differ in efficiency at processing both absolute and relative ranges of spatial frequency components in visual input. Performance on the two-component stimuli was consistent with the absolute frequency hypothesis; addition of the same two cpd component to the two-component stimuli, however, shifted performance away from a LVFRH advantage when the 2 cpd component was of high frequency relative to the baseline stimulus, but shifted performance toward a LVF-RH advantage when the 2 cpd component was of low frequency relative to the baseline stimulus. These results are consistent with general psychophysical findings that both absolute and relative frequency play important roles in stimulus identification (Norman & Ehrlich, 1987). An alternative interpretation of the current results involves what can be considered a special case of the relative frequency hypothesis. The relatively low frequency two cpd component in the High Frequency Baseline condition was the fundamental frequency of that stimulus; conversely, the relatively high frequency two cpd component in the Low Frequency Baseline condition was a harmonic frequency of that stimulus. Thus, the results are consistent with a hypothesis arguing that the LVF-RH is more efficient at processing fundamental frequencies and the RVF-LH at processing harmonic frequencies. Since fundamental versus harmonic frequencies are by definition of relatively low versus high frequency, this interpretation is essentially a special case of the more general relative frequency hypothesis. It is not clear, however, how this interpretation
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would apply to results obtained with more complex stimuli that have a very broad spectrum of frequencies, since the fundamental frequency for such stimuli is a zero cpd DC component, and all relevant pattern information is carried by higher harmonic frequencies. The current results bear on the findings of the literature review conducted by Christman (1989). Visual half-field studies were reviewed in which perceptual parameters were varied. The results for spatial manipulations such as blurring and eccentricity were mostly consistent with predictions of the spatial frequency hypothesis. The results of size manipulations on patterns of hemispheric asymmetry, however, were not as consistent. This is expected in light of the present results, since eccentricity and blurring manipulations simultaneously affect the range of both absolute and relative frequencies contained in the input. Size manipulations, on the other hand, affect only the absolute range of frequencies in the input, leaving the distribution of relative frequencies unchanged. If the hemispheres differ in the processing of both absolute and relative frequency, then manipulations which affect both dimensions would be expected to yield stronger results. The current results also have important implications for recent theories of local/global processing. It has been demonstrated that low versus high spatial frequency channels in the human visual system mediate perception of global versus local features (Shulman, Sullivan, Gish, & Sakoda, 1986). Similarly, a number of studies have demonstrated that the LVF-RH versus RVF-LH are better at global versus local processing, respectively (Martin, 1979; Robertson & Delis, 1986; Sergent, 1982). Although studies also exist that have had difficulty replicating the presence of significant hemispheric differences in local/global processing (e.g., Alivasatos & Wilding, 1982; Boles, 1984), a recent metaanalysis by Van Kleeck (1989) suggests that there are reliable hemispheric differences in local/global processing. Robertson and Lamb (1989) have combined the local/global and spatial frequency results to suggest that different relative representations of low versus high frequency channels in the LVF-RH versus RVF-LH mediate processing of global versus local features of visual input. The current finding that the LVF-RH and RVF-LH differ in the ability to process relative as well as absolute frequency lends support to this idea, since global and local aspects of an image are, by definition, relative, not absolute, constructs. In summary, the finding that the hemispheres differ in the processing of both absolute and relative ranges of spatial frequency should help further refine and clarify the initial formulations of the spatial frequency hypothesis (e.g., Sergent, 1982, 1987). Although there may be basic differences between the hemispheres in terms of perceptual-visual resolution limitations at very low and very high absolute frequencies (in which case, effects of relative frequency would be minimal or absent), in many cases
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AND HELLIGE
the relevant visual information is carried by intermediate ranges of spatial frequency. The information concerning object identity carried by components from a specific absolute spatial frequency range will be determined by the relative distribution of other frequency components present in the image. Thus, which hemisphere will be superior in the processing of a specific frequency component would be determined by the relative (as well as the absolute) frequency of that component. This finding has implications for the general nature of object recognition in the RVF-LH versus LVF-RH, since the presence of size constancy in visual perception implies that relative spatial frequency ultimately plays a decisive role in stimulus identification. REFERENCES Alisavatos, B., & Wilding, J. (1982). Hemispheric differences in matching Stroop-like letter stimuli. Cortex, 18, 5-22. Boles, D. (1984). Global versus local processing: Is there a hemispheric dichotomy? Neuropsychologia,
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Rebai, M., Mecacci, L., Bagot, J.-D., & Bonnet, C. (1989). Influence of spatial frequency and handedness on hemispheric asymmetry in visually steady-state evoked potentials. Neuropsychologia, 27, 315-324. Rentschler, I., Christen, L., Christen, S., & Landis, T. (1986). Features versus spatial phase in a tachistoscopic laterality experiment. Perception and Psychophysics, 34, 89-95. Robertson, L., & Delis, D. (1986). “Part-whole” processing in unilateral brain-damaged patients: Dysfunction of hierarchical organization. Neuropsychologiu, 24, 363-370. Robertson, L., & Lamb, M. (1991). Neuropsychological perspectives on theories of part/whole
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AND
COGNITION
16, 62-73 (1991)
Hemispheric Asymmetry in the Processing of Absolute versus Relative Spatial Frequency STEPHEN
CHRISTMAN
AND
University
FREDERICK
L.
KITTERLE
of Toledo AND
JOSEPH University
HELLIGE
of Southern
California
Observers indicated whether a stimulus presented to one visual field or the other consisted of two sine-wave gratings (the baseline stimulus) or those same two gratings with the addition of a 2 cycle per degree (cpd) component. When the absolute spatial frequencies of the baseline stimulus were low (0.5 and 1.0 cpd), there was a left visual field-right hemisphere (LVF-RH) advantage in reaction time (RT) to respond to the baseline stimulus which disappeared when the 2 cpd component was added (i.e., the stimulus consisted of 0.5, 1.0, and 2.0 cpd components). When the absolute spatial frequencies of the baseline stimulus were moderate to high (4.0 and 8.0 cpd), a right visual field-left hemisphere advantage in RT to respond to the baseline stimulus approached significance and shifted to a significant LVF-RH advantage when the 2 cpd component was added (i.e., the stimulus consisted of 2.0, 4.0, and 8.0 cpd components. That is, adding the same 2 cpd component caused opposite shifts in visual laterality depending on whether 2 cpd was a relatively high or relatively low frequency compared to the baseline. 0 1991 Academic
Press. Inc.
The issue of cerebral hemispheric differences in the processing of spatial frequency components of visual stimuli in normal subjects has been the focus of much recent research (e.g., Christman, 1987; Glass, Bradshaw, This research was supported by an Academic Challenge Grant from the State of Ohio to the Department of Psychology at the University of Toledo, which provided a postdoctoral fellowship to Dr. S. Christman and a visiting professorship to Dr. J. Hellige. In addition, Dr. Hellige was also supported by an NSF grant (BNS 89-08305). The comments of two anonymous reviewers are gratefully acknowledged. Address correspondence and reprint requests to Dr. Stephen Christman, Department of Psychology, University of Toledo, Toledo, OH 43606. 62 0278.2626191 $3.00 Copyright All rights
Q 1991 by Academic Press. Inc. of reproduction in any form reserved.
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Day, & Umilta, 1985; Jonsson and Hellige, 1986; Kitterle, Christman, & Hellige, 1990; Kitterle and Kaye, 1985; Sergent, 1983,1987). These studies suggest that the right visual field-left hemisphere (RVF-LH) is more efficient at processing higher spatial frequencies, while the left visual fieldright hemisphere (LVF-RH) is more efficient with lower spatial frequencies. Tests of the spatial frequency hypothesis have typically taken one of two approaches. First, studies have investigated the types of task conditions under which hemisphere by spatial frequency interactions arise. For example, Kitterle et al. (1990) have demonstrated that such interactions arise in identification tasks, but not in tasks that require mere detection of stimuli. They discuss their results in terms of the computational requirements of detection versus identification, arguing that hemispheric differences arise only in tasks with sufficient computational complexity. Similarly, others have shown that such interactions are usually not obtained in tasks requiring lexical processing (Chiarello, Senehi, & Soulier, 1986; Kersteen-Tucker & Hardyck, 1988), presumably because the strong RVF-LH dominance for lexical processing overrides input factors. A second approach, focusing directly on input factors, has been to filter out higher spatial frequencies (via digital filtering, blurring, etc.) from visual input in order to determine whether such a manipulation impairs RVF-LH performance relative to LVF-RH. The results of such manipulations have been generally consistent with predictions of the spatial frequency hypothesis (for reviews, see Christman, 1989; Sergent & Hellige, 1986). Although some recent studies have failed to find Hemisphere x Frequency interactions in identification tasks, these studies involve methodological factors that limit their ability to directly address the spatial frequency hypothesis. For example, studies by Boles and Morelli (1988) and by Moscovitch and Radzins (1987) manipulated spatial frequency via the use of complex stimuli containing broad ranges of spatial frequencies (e.g., square-wave gratings, masks composed of dots), which makes it difficult to determine which ranges of spatial frequency were mediating task performance (cf., Hellige & Sergent, 1986). Similarly, other studies have employed detection tasks (Rebai, Mecacci, Bagot, & Bonnet, 1989) and have failed to find interactions between hemisphere and spatial frequency. However, as noted above, Kitterle et al. (1990) have demonstrated that such interactions are not obtained in detection tasks. Finally, the study by Peterzell, Harvey, and Hardyck (1989) failed to find a Hemisphere x Frequency interaction in a task involving classification of letters that had been bandpass filtered. It is not clear whether their results bear on the spatial frequency hypothesis, however, because their stimulus set consisted of only four letters, and the target letters (L and H) differed
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from the nontarget letters (T and F) on the basis of whether a horizontal crossbar at the top of the display was absent or present. This crossbar was clearly visible at all levels of filtering. Thus, it is not clear whether subjects were performing the task on the basis of the spatial frequency content of the stimuli per se, or whether they were simply looking for a particular feature (e.g., the crossbar), regardless of the overall spatial frequency content of the input. Tests of the spatial frequency hypothesis employing methods of lowpass filtering have inherently assumed that the hemispheres differ in their efficiency at processing absolute ranges of spatial frequency. The presence of this assumption is implicitly reflected by the fact that such studies have always quantified the effects of low-pass filtering in terms of cycles per degree (cpd) of visual angle. To date, no studies have explicitly examined whether the hemispheres may also differ in processing relative ranges of spatial frequency. Relative spatial frequency is measured in terms of the periodicity of a component relative to other image components or to the rest of the image (e.g., cycles per image width). It is an empirical question whether an image component of a given absolute spatial frequency will always be processed more efficiently by a particular hemisphere, regardless of the relative frequency of other components present in the image. The manner in which a particular component (and the perceptual information that it carries) will be processed may be determined by the context created by the distribution of other components. For example, a component of intermediate absolute spatial frequency could signal global information in a very small image and signal local information in a very large image. The possibility that relative frequency may play a role in hemispheric differences in perceptual processing is an important concern, since it has been demonstrated that both relative and absolute spatial frequency play important roles in pattern recognition (Norman & Ehrlich, 1987). Methodologies of studies testing the spatial frequency hypothesis have confounded these two dimensions, making it impossible to determine whether the hemispheres differ in the processing of absolute spatial frequency, relative spatial frequency, or both. Thus, the purpose of the present experiment is to directly address the issue of whether the hemispheres differ in their ability to process relative spatial frequency components of visual stimuli. The general task involved identification of compound grating stimuli consisting of multiple sine-wave components. The advantage of using grating stimuli to test the implications of the spatial frequency hypothesis lies in the fact that the characteristics of the visual input are clearly specified (i.e., the spatial frequencies available for task performance are known) and the spatial frequencies required for effective task performance are also specified. A further advantage in using complex patterns constructed from simple components is that with complex gratings we can investigate in greater depth how the RVF-LH
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versus LVF-RH combine and compare information from different spatial frequency channels and also how the processing of a given channel is influenced by activity in other spatial frequency channels. On each trial of the experiment, one of two possible compound gratings was presented. The two differed only in that one contained a 2.0 cpd component and the other (the “baseline” stimulus) did not. In the Low Frequency Baseline condition, the other components of the compound grating were of lower frequency (specifically, 0.5 and 1.0 cpd), making the 2 cpd component a relatively high frequency compared to the components of the baseline stimulus. In the High Frequency Baseline condition, the other components were of higher frequency (specifically, 4.0 and 8.0 cpd), making the 2 cpd component a relatively low frequency compared to the components of the baseline stimulus. In terms of human contrast sensitivity to absolute spatial frequency (under conditions of brief exposure and parafoveal presentation), a 2 cpd component is a low to intermediate frequency and, on the basis of our previous research (Kitterle et al., 1990), would be expected to be slightly better processed by the LVF-RH . The straightforward prediction from the relative frequency hypothesis is that processing of the 2.0 cpd component will produce a RVF-LH advantage when the baseline stimulus consists of components lower than 2.0 cpd (e.g., 0.5 + 1.0 cpd), whereas processing of the 2.0 cpd component will produce a LVF-RH advantage when the baseline stimulus consists of components higher than 2.0 cpd (e.g., 4.0 + 8.0 cpd). However, there is a problem in testing this prediction. The problem comes from the fact that the two hemispheres are not expected to be equally efficient at processing the various baseline stimuli. For example, our earlier research suggests that there will be a LVF-RH advantage for processing a baseline stimulus consisting of 0.5 + 1.0 cpd gratings, and any RVF-LH advantage for processing the 2.0 cpd component in the context of the baseline would have to be superimposed on this LVF-RH advantage for processing the baseline stimulus without the 2.0 cpd component. Therefore, the critical prediction is that any tendency toward a LVF-RH advantage for the baseline stimulus should be reduced (or reversed) when the 2.0 cpd component is added. By way of contrast, consider the situation when the baseline consists of 4.0 + 8.0 cpd gratings. Our earlier research suggests that, in this case, there will be either a RVF-LH advantage or no visual field difference for processing the baseline stimulus (8.0 cpd is sufficiently high to produce a RVF-LH advantage, but 4.0 cpd is sufficiently low to moderate it). Therefore any LVF-RH “advantage” for processing the relatively low 2 cpd component in this higher frequency context will be superimposed on a very different advantage for processing the baseline stimulus. In this condition, the critical prediction is that any tendency toward a RVF-LH
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advantage in the baseline condition should be reduced (or reversed) when the 2.0 cpd components is added. That is, in both of the Baseline conditions, there is predicted to be a particular pattern of Visual Field x Stimulus Type (2.0 cpd absent versus 2.0 cpd present) interaction, with the predicted interaction pattern being different for the two Baseline conditions. As a result, there should be a significant three-way interaction of Baseline Condition, Visual Field, and Stimulus Type. METHOD The stimuli consisted of compound gratings comprising multiple, vertically oriented sine-wave components. The gratings were generated by a Picasso CRT Image Synthesizer (Innisfree) under computer control and were displayed on Tektronix 608 monitors. A refresh rate of 200 Hz was employed. A large black matte surround (30 x 36” at a viewing distance of 42 in.) was placed directly in front of the monitors. Holes in the surround were used to mask the monitor screens down to two 6.8” circular apertures. A small red LED placed between the two monitors served as the fixation point, and the inner edge of each monitor screen was 3” from fixation. The mean luminance and contrasts of the displays were calibrated by means of a Tektronix 516 digital photometer and 56.523 narrow angle luminance probe. The mean luminance of the displays was 10.3 cd/m’ and the contrast of all sine-wave components was 0.5 [contrast was defined as (Maximum Luminance minus Minimum Luminance)/(Maximum Luminance plus Minimum Luminance)]. Care was taken during the course of the experiment to ensure that both CRTs were matched in luminance and contrast and that the mean luminance of the display did not change with grating presentation. Stimuli were exposed for 150 msec and were presented at a variable foreperiod (600-900 msec) following a brief warning tone. Procedure. Subjects viewed the displays through a viewing hood with a padded chin rest under near-dark conditions. Viewing was binocular. The importance of maintaining fixation on the fixation point was stressed to subjects, who were told that the purpose of the experiment was to see how well people could identify stimuli without looking directly at them. At the beginning of the experiment, subjects were light adapted for 2 min to the mean luminance of the display. The experiment consisted of Low and High Frequency Baseline conditions. In both conditions, the stimulus sets consisted of two different compound gratings. In the Low Frequency Baseline condition, one compound grating was composed of two components: 0.5 and 1.0 cpd sine-wave gratings, and the other was composed of three components: 0.5, 1.0, and 2.0 cpd gratings. In the High Frequency Baseline condition, the two-component stimulus consisted of 4.0 and 8.0 cpd gratings and the three-component stimulus was composed of 2.0, 4.0, and 8.0 cpd components. Thus, the difference between the two stimuli in each condition was defined by the presence versus absence of the two cpd component. The phase of the two cpd component relative to the other components was varied randomly in both conditions. Phase is defined as the position of a sinusoidal grating with respect to some reference point. Thus, the position (e.g., with respect to the edge of the display) of the two cpd component (when present) varied from trial to trial, while the position of the other components was held constant. Eight different phase relations for the two cpd component were used in each condition, spanning a range from 0” to 157.5” in 22.5” increments. The rationale behind this manipulation was to ensure that subjects did not base their discrimination on local luminance or contrast variability cues created by particular combinations of phase relations among the components (e.g., see Rentschler, Christen, Christen, & Landis, 1986). At the beginning of the block of trials constituting each condition, subjects were shown examples of the two stimuli composing the stimulus set and were instructed to press a key Apparatus
and
visual
display.
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LVF-RH RVF-LH
300 0.5 + 1 .o 0.5+1.0+2.0 Grating Components (cpd) FIG. 1. Reaction time as a function of stimulus type and visual field of presentation in the low frequency condition. with one hand if they saw one of the stimuli, and to press a different key with their other hand if they saw the other stimulus. Subjects were instructed to respond as quickly and as accurately as possible. Both accuracy and reaction time (RT) measures were collected. The arrangement of hand of response and the order of the two conditions was counterbalanced across subjects. Each condition consisted of a block of 128 trials, resulting from the factorial combination of two different stimuli, two visual fields, four foreperiods, and eight replications. Subjects were also given practice blocks of 20 trials before each test block to help familiarize them with the nature of the task and stimuli. Subjects. Four male and four female subjects with normal or corrected-to-normal vision served as observers. All were right-handed with no left-handed relatives in their immediate family, as assessed by a handedness questionnaire. Subjects were drawn from the Psychology Department’s Human Subject Pool, were naive about the purpose of the experiment, and received course credit for their participation.
RESULTS
Since foreperiod duration was not of interest in testing the current hypothesis, the data were collapsed over this variable. Because of the low overall error rate across conditions (average = 2.9%), analyses of variance were not performed on the accuracy data. Trends in the error data mirrored trends in the RT data discussed below and there was no indication of speed-accuracy tradeoffs. RT data for the Low Frequency Baseline condition are graphed in Fig. 1 and for the High Frequency Baseline condition in Fig. 2. Analyses of variance were performed on the RT data, with hand of response and gender as between-subject variables, and visual field, stimulus type (twoversus three-component), and Baseline condition (high versus low frequency) as within-subject variables. There were no main effects or in-
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700 -
LVF-RH
Grating Components (cpd) FIG. 2. Reaction time as a function of stimulus type and visual field of presentation in the high frequency condition.
teractions of hand of response or gender with the other variables, so further analyses were based on data which were collapsed across these two variables. Of particular interest was the presence of a significant three-way interaction between visual field, stimulus type, and Baseline condition [F(l, 7) = 40.01, p < .OOl]. In order to examine the nature of this interaction, separate analyses were conducted for the Low and High Frequency Baseline conditions. There were significant interactions between visual field and stimulus type in both the Low Frequency Baseline condition [F(l, 7) = 8.51, p < .03] and the High Frequency Baseline condition [F(l, 7) = 8.86, p < .02]. These interactions were further analyzed by examining the nature of simple effects for each condition. In the Low Frequency Baseline condition, the interaction arose due to the presence of a significant LVF-RH advantage for the two-component stimulus (0.5 + 1.0 cpd) [F(l, 7) = 10.65, p < .02], whereas there was no difference in RT between the visual fields for the three-component stimulus (0.5 + 1.0 + 2.0 cpd) [F < 11. The Visual Field x Stimulus Type interaction in the High Frequency Baseline condition arose from the presence of a nonsignificant RVF-LH advantage for the two-component stimulus (4.0 + 8.0 cpd) [F(l, 7) = 4.14, p < .08] and a significant LVF-RH advantage for the threecomponent stimulus (2.0 + 4.0 + 8.0 cpd) [F(l, 7) = 5.86, p < .05]. In the Low Frequency Baseline condition, there were main effects of stimulus type [F(l, 7) = 123.4, p < .OOOOl] and of visual field [F(l, 7) = 5.60, p < .05]. RT was quicker for the two-component stimulus and for LVF-RH presentations (as qualified by the aforementioned interac-
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tion). In the High Frequency Baseline condition, there was a main effect of stimulus type [F(l, 7) = 68.1, p < .0003], with responses being faster to the two-component stimulus. There was no main effect of visual field [F < 11. DISCUSSION
The purpose of the present experiment was to test whether the cerebral hemispheres differ in sensitivity to relative spatial frequency. Subjects were required to discriminate between compound grating stimuli composed of two versus three sinusoidal components. In the Low Frequency Baseline condition, the two components of the baseline stimulus consisted of low spatial frequencies (0.5 and 1 cpd); in the High Frequency Baseline condition, the two components of the baseline stimulus consisted of high spatial frequencies (4 and 8 cpd). The three-component stimuli in both conditions were created by adding the same 2 cpd component to the relevant two-component baseline stimuli. The results for the two-component stimuli supported previous findings of LVF-RH superiority in processing lower absolute spatial frequencies and RVF-LH superiority for processing higher absolute spatial frequencies (although the RVF-LH advantage was only marginally significant). The results for the three-component stimuli, however, suggest that hemispheric differences also exist in the processing of relative spatial frequency. Adding an identical 2 cpd component resulted in no visual field differences when it was of high spatial frequency relative to the other components present in the input, whereas it yielded a significant LVF-RH advantage when it was of low spatial frequency relative to the other components present in the input. Thus, the current results demonstrate that a stimulus composed of high frequency components (2.0 + 4.0 + 8.0 cpd) yielded a LVF-RH advantage, whereas a stimulus composed of much lower frequency components (0.5 + 1.0 + 2.0 cpd) yielded no visual field differences. This result is not predicted by the spatial frequency hypothesis in terms of absolute frequency. However, this pattern of results can be accounted for by considering the context provided by the baseline stimuli from which the aforementioned three-component stimuli had to be discriminated. It might be argued that the shift from an RVF-LH advantage for the two-component stimulus to a LVF-RH advantage for the three-component stimulus in the High Frequency Baseline condition was due to the fact that the three-component stimulus (with the addition of the 2 cpd component) had a lower mean absolute frequency than the two-component stimulus. A problem with this argument, however, lies in the fact that in the low frequency condition, the addition of the same two cpd component resulted in a shift from a LVF-RH advantage to no hemispheric advantage. If the two cpd component is an absolutely low frequency that is processed
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better in the LVF-RH, then adding that frequency to the two-component stimulus in the low frequency condition should not have eliminated the LVF-RH advantage obtained for the two-component stimulus. It should be noted that the RT to the three-component gratings was longer than the RT to two-component gratings in both conditions. This suggests that subjects were not basing their decisions simply on the basis of the presence versus absence of the two cpd component. If this were the case, then RT to the three-component stimuli should have been quicker, since presence judgments are typically faster than absence judgments. Rather, it appears reasonable to assume that the longer RT to three-component stimuli was caused by the greater complexity of those stimuli and that subjects were not able to process the two cpd component in isolation. This confirms the notion that the manner in which the two cpd component was processed was influenced by the context of the other frequency components. The important point to be made in regard to the relative frequency hypothesis is that, although the three-component stimuli took longer to process in both conditions, the pattern of relative hemispheric impairment differed according to the relative frequency of the other components (e.g., one hemisphere was not simply better at identifying more complex stimuli). Thus, the results of the current experiment demonstrate that the RVFLH and LVF-RH differ in efficiency at processing both absolute and relative ranges of spatial frequency components in visual input. Performance on the two-component stimuli was consistent with the absolute frequency hypothesis; addition of the same two cpd component to the two-component stimuli, however, shifted performance away from a LVFRH advantage when the 2 cpd component was of high frequency relative to the baseline stimulus, but shifted performance toward a LVF-RH advantage when the 2 cpd component was of low frequency relative to the baseline stimulus. These results are consistent with general psychophysical findings that both absolute and relative frequency play important roles in stimulus identification (Norman & Ehrlich, 1987). An alternative interpretation of the current results involves what can be considered a special case of the relative frequency hypothesis. The relatively low frequency two cpd component in the High Frequency Baseline condition was the fundamental frequency of that stimulus; conversely, the relatively high frequency two cpd component in the Low Frequency Baseline condition was a harmonic frequency of that stimulus. Thus, the results are consistent with a hypothesis arguing that the LVF-RH is more efficient at processing fundamental frequencies and the RVF-LH at processing harmonic frequencies. Since fundamental versus harmonic frequencies are by definition of relatively low versus high frequency, this interpretation is essentially a special case of the more general relative frequency hypothesis. It is not clear, however, how this interpretation
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would apply to results obtained with more complex stimuli that have a very broad spectrum of frequencies, since the fundamental frequency for such stimuli is a zero cpd DC component, and all relevant pattern information is carried by higher harmonic frequencies. The current results bear on the findings of the literature review conducted by Christman (1989). Visual half-field studies were reviewed in which perceptual parameters were varied. The results for spatial manipulations such as blurring and eccentricity were mostly consistent with predictions of the spatial frequency hypothesis. The results of size manipulations on patterns of hemispheric asymmetry, however, were not as consistent. This is expected in light of the present results, since eccentricity and blurring manipulations simultaneously affect the range of both absolute and relative frequencies contained in the input. Size manipulations, on the other hand, affect only the absolute range of frequencies in the input, leaving the distribution of relative frequencies unchanged. If the hemispheres differ in the processing of both absolute and relative frequency, then manipulations which affect both dimensions would be expected to yield stronger results. The current results also have important implications for recent theories of local/global processing. It has been demonstrated that low versus high spatial frequency channels in the human visual system mediate perception of global versus local features (Shulman, Sullivan, Gish, & Sakoda, 1986). Similarly, a number of studies have demonstrated that the LVF-RH versus RVF-LH are better at global versus local processing, respectively (Martin, 1979; Robertson & Delis, 1986; Sergent, 1982). Although studies also exist that have had difficulty replicating the presence of significant hemispheric differences in local/global processing (e.g., Alivasatos & Wilding, 1982; Boles, 1984), a recent metaanalysis by Van Kleeck (1989) suggests that there are reliable hemispheric differences in local/global processing. Robertson and Lamb (1989) have combined the local/global and spatial frequency results to suggest that different relative representations of low versus high frequency channels in the LVF-RH versus RVF-LH mediate processing of global versus local features of visual input. The current finding that the LVF-RH and RVF-LH differ in the ability to process relative as well as absolute frequency lends support to this idea, since global and local aspects of an image are, by definition, relative, not absolute, constructs. In summary, the finding that the hemispheres differ in the processing of both absolute and relative ranges of spatial frequency should help further refine and clarify the initial formulations of the spatial frequency hypothesis (e.g., Sergent, 1982, 1987). Although there may be basic differences between the hemispheres in terms of perceptual-visual resolution limitations at very low and very high absolute frequencies (in which case, effects of relative frequency would be minimal or absent), in many cases
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the relevant visual information is carried by intermediate ranges of spatial frequency. The information concerning object identity carried by components from a specific absolute spatial frequency range will be determined by the relative distribution of other frequency components present in the image. Thus, which hemisphere will be superior in the processing of a specific frequency component would be determined by the relative (as well as the absolute) frequency of that component. This finding has implications for the general nature of object recognition in the RVF-LH versus LVF-RH, since the presence of size constancy in visual perception implies that relative spatial frequency ultimately plays a decisive role in stimulus identification. REFERENCES Alisavatos, B., & Wilding, J. (1982). Hemispheric differences in matching Stroop-like letter stimuli. Cortex, 18, 5-22. Boles, D. (1984). Global versus local processing: Is there a hemispheric dichotomy? Neuropsychologia,
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