Can Free-Viewing Perceptual Asymmetries be Explained by Scanning, Pre-Motor or Attentional Biases?

CAN FREE-VIEWING PERCEPTUAL ASYMMETRIES BE EXPLAINED BY SCANNING, PRE-MOTOR OR ATTENTIONAL BIASES? Michael ER Nicholls and Georgina R Roberts (Department of Psychology, University of Melbourne)

ABSTRACT Judgments of relative magnitude between the left and right sides of a stimulus are generally weighted toward the features contained on the left side. This leftward perceptual bias could be the result of, (a) left-to-right scanning biases, (b) pre-motor activation of the right hemisphere, or (c) a left hemispatial attentional bias. The relative merits of these explanations of perceptual asymmetry were investigated. In Experiment 1, English and Hebrew readers made luminance judgements for two left/right mirror-reversed luminance gradients (greyscales task). Despite different reading/scanning habits, both groups exhibited a leftward perceptual bias. English and Hebrew readers also performed a line bisection task. Scanning biases were controlled by asking participants to follow a marker as it moved left-to-right or right-to-left and then stop it as it reached the midpoint of the line. Despite controlling scanning, a leftward bias was observed in both groups. In Experiment 2, peripheral spatial cues were presented prior to the greyscales stimuli. English readers showed a reduction in the leftward bias for right-sided cues as compared to left-sided and neutral cues. Right-side cues presumably overcame a pre-existing leftward attentional bias. In both experiments, pre-motor activation was controlled using bimanual responses. Despite this control, a leftward bias was observed throughout the study. The data support the attentional bias account of leftward perceptual biases over the scanning and pre-motor activation accounts. Whether or not unilateral hemispheric activation provides an adequate account of this attentional bias is discussed. Key words: neglect, left, right, pseudoneglect

INTRODUCTION Following unilateral brain damage, patients with clinical neglect typically fail to respond to stimuli delivered to the side contralateral to the lesion. This inattention may cause the person to eat only from one side of their plate, to dress one side of their body, or to collide with objects located on the unattended side. Symptoms of neglect typically arise following lesions to the parietal regions of the right hemisphere (RH) causing inattention to the left hemispace (Heilman and Valenstein, 1979). While unilateral neglect is usually associated with RH damage, it should be noted that left hemisphere (LH) lesions can also produce neglect (Ogden, 1985), albeit often in a weaker form (Mattingley et al., 1992). There is a general consensus that the symptoms of neglect stem from a cognitive rather than a sensory impairment (Halligan and Marshall, 1994; Ishiai et al., 1987). Insights into the brain mechanisms that control attention have also come Cortex, (2002) 38, 113-136

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from the study of perceptual asymmetries that occur in non-clinical populations under free-viewing conditions. In these conditions, participants are normally required to make a judgement of relative magnitude between the left and right sides of a stimulus that is presented bilaterally to the visual fields/hemispheres. Under such conditions, normal individuals generally bias judgements of relative magnitude towards the information contained on the left side of the stimulus (e.g., Luh, 1995; Luh et al., 1991). A good example of a task that can induce free-viewing perceptual asymmetries is the line bisection task. Whereas clinical neglect patients bisect lines to the right of their midpoint (Schenkenberg et al., 1980), normal individuals tend to bisect lines to the left of centre (Jewell and McCourt, 2000). This leftward error in line bisection, also known as pseudoneglect (Bowers and Heilman, 1980), occurs across a broad range of research protocols, including paper and pencil tests (Luh, 1995), computerised tests requiring forced ‘left’ or ‘right’ judgements of pre-bisected lines (McCourt and Jewell, 1999), and tasks requiring the bisection of rods though touch alone (Sampaio and Chokron, 1992). While the leftward error for line bisection appears to be a highly reliable phenomenon (McCourt, 2001), it should be noted that a number of researchers have failed to find such an effect (e.g., ReuterLorenz and Posner, 1990; Werth and Poeppel, 1988). Chimeric facial stimuli, which are a composite of two faces joined along their midsagittal axes, have also been used to induce perceptual asymmetries under free-viewing conditions (Mattingley et al., 1994). For tasks that require participants to judge, which within a pair of left-right mirror reversed chimeric stimuli is more emotional, a bias towards choosing the stimulus with the emotive features contained on the left-hand side is typically observed (Moreno et al., 1990). This leftward bias may, however, depend upon the emotional valence of the face (Reuter-Lorenz and Davidson, 1981). Leftward biases have also been observed for a group of tasks that require simple judgments of relative magnitude within pairs of left/right mirror-reversed stimuli. An example of such a task is the ‘greyscales task’ devised by Mattingley et al. (1994). This task requires participants to make a forced two-choice discrimination of the relative brightness of two simultaneously presented horizontal bars. The bars change incrementally from white on one side to black on the other and are arranged so that they are left/right reversals of each other (see Figure 1). Nicholls et al. (1999) found that, when required to select the bar that was darker, participants chose the greyscale that was darker on the left-hand side 67% of the time. This bias occurred despite the fact that the greyscales within the pair were equiluminant. When asked to select the bar that was lighter, participants reversed their decisions and selected the bar that was lighter on the left-hand side. In addition to the response bias, leftward responses were significantly faster than rightward responses, suggesting that participants were more confident of their leftward responses. Similar leftward biases for judgments of relative magnitude have been reported for judgements of size (Nicholls et al., 1999) and numerosity (Luh, 1995; Luh et al., 1991; Nicholls et al., 1999). From the evidence reviewed above, it can be seen that when non-clinical participants are required to ‘balance’ the features of a stimulus that crosses from left to right, they tend to overestimate the features located on the left. A variety

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of mechanisms have been put forward to account this phenomenon. Manning et al. (1990) proposed that leftward perceptual biases were the result of left-to-right scanning habits, which predominate among readers of English (see Abed, 1991). Left-to-right scans could lead to an overrepresentation of the leftward extent of a stimulus compared to the right because scans always starts on the left edge of the stimulus, but can be terminated before the right edge is reached (cf. Kim et al., 1997). The scanning bias theory has been examined through the direct control of eye scanning. For example, Brodie and Pettigrew (1996) instructed participants to scan left-to-right or right-to-left during a line bisection task. They found that left-to-right scanning resulted in a significantly higher deviation to the left than did right-to-left scanning. Chokron et al. (1998) controlled scanning during a line bisection task by asking participants to follow a marker as it moved rightto-left or left-to-right. Unlike Brodie and Pettigrew (1996), Chokron et al. (1998) found that left- and right-ward scanning shifted bisections symmetrically toward the origin of the scan. A number of studies have failed to demonstrate that direct manipulations of scanning affect perceptual asymmetries. McCourt and Olafson (1997) controlled overt scanning by presenting stimuli for 150 msec and found that participants continued to bisect lines to the left of centre. To rule out the possibility that scanning may operate covertly on an iconic trace, McCourt and Jewell (1999) used a backward mask to disrupt iconic storage. Despite short exposure times and backward masking, leftward errors persisted for the task. Scanning has also been controlled by requiring participants to fixate centrally during a rod bisection task (Bradshaw et al., 1985). Once again, despite controlling scanning, leftward errors were observed. However, unlike McCourt and Olafson (1997), Bradshaw et al. (1985) cannot rule out the possibility that covert scanning biases contributed to the leftward error. The effect of scanning habits on leftward perceptual biases has also been investigated by comparing readers with different scanning directions. Chokron et al. (1997) found that readers of Hebrew (right-to-left) showed no significant bias for a line extension task, whereas readers of French (left-to-right) significantly under-constructed the left side of the line. For a line bisection task, Chokron and Imbert (1993) found that Hebrew readers bisected lines to the right of centre whereas French readers bisected lines to the left (see also Chokron and De Agostini, 1995). Sakhuja et al. (1996) and Vaid and Singh (1989) have observed similar results for the perception of chimeric faces. They found that readers of Hindi (left-to-right) attended to the leftward features of the face whereas readers of Urdu (right-to-left) attended to the rightward features of the face. An effect of reading direction between native Hebrew readers and Arabic-Hebrew bilinguals for the perception of chimeric faces has also been reported (Eviatar, 1997). An alternative account of leftward perceptual biases has emphasised the role of unilateral motor activity (Heilman and Valenstein, 1979). Brodie and Pettigrew (1996) found that dextrals bisected the line more to the left when the bisection was performed with the left hand rather than with the right hand (see also Bradshaw et al., 1986, and Sampaio and Chokron, 1992). McCourt et al. (2001) found a leftward bias for line bisection when either the left or right hands

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were used to make response. The bias was stronger, however, when the left hand was used. Halligan et al. (1991) have demonstrated that hemispace interacts with unimanual activity for a line bisection task. A leftward bias was observed for both hands when their starting position was to the left end of the line, whereas no asymmetry was apparent when the starting position was to the right end of the line. These results suggest that it is not unilateral motor activity per se that affects perception, but rather, it is the way in which attention is distributed in space. McCourt and Olafson (1997) have also found evidence against a motor explanation of perceptual asymmetry. They eliminated motor biases by requiring participants to make simple button-press responses to a series of pre-bisected lines. Hand of response was balanced across trials. Despite controlling motor activity, participants tended to judge lines transected slightly to the left as being equally bisected (see also Milner et al., 1992). Together, these results suggest that motor activation only partly accounts for leftward biases in perception Leftward perceptual asymmetries may also be the result of an attentional bias directed toward the left hemispace. Milner et al. (1992) examined the role of attentional biases by requiring participants to report lateralised cues during a line bisection task. They found that the cueing procedure caused a shift of the bisection mark towards the cued end of the line and thus concluded that attentional biases played an important role in line bisection (see also Harvey et al., 1995; Nichelli et al., 1989; Reuter-Lorenz et al., 1990). Mennemeier et al. (1997) have also reported that cues affect line bisection. In this case, however, an effect was observed for leftward cues, but not for rightward cues. Mattingley et al. (1993) have reported that lateralised cues do not affect line bisection in normals. They argued that previous experiments had introduced unwanted perceptual effects by presenting visible cues beyond the end of the line. Such a configuration may be problematic because it makes the cued end appear larger. To avoid this effect, Mattingley et al. (1993) conducted two experiments where (a) the cue did not extend beyond the horizontal endpoint of the line, and (b) where the cues were invisible. In both experiments, cueing had no effect on line bisection performance in non-clinical participants. Harvey et al. (2000) have also reported that invisible cues do not affect asymmetries in line bisection when participants draw an invisible mark at the left or right ends of the line. If the experimenter makes the invisible mark, however, the bisection mark was drawn toward the cued end. Harvey et al. (2000) suggested that the invisible cueing procedure was not effective when participants made the response because the motor component of the cueing procedure overrode the attentional effect of the cue. A more specific version of the attentional bias theory suggests that the attentional bias observed under free-viewing conditions arises from asymmetries in hemispheric activation. This theory is derived from the early work of Kinsbourne (1970) which proposed that unilateral activation of a hemisphere causes an attentional bias to the contralateral hemispace. Thus, the act or expectancy of performing spatial tasks such as face recognition, length estimation, or darkness discrimination might be expected to activate the RH more than the LH, leading to a bias of attention to the left hemispace. In support of this proposition, Vingiano (1991) demonstrated that concurrent spatial activity

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(activating the RH) caused right inattention. Schiff and Truchon (1993) investigated the effects of unilateral hand contraction on perceptual asymmetries in a chimeric face task. The expected leftward bias emerged during left-hand activity, but this bias was reduced during right-hand activity. Nicholls et al. (2001), however, found that unimanual activity has no effect on the leftward bias for the greyscales task. They required participants to either tap one finger or clench one hand during performance of the greyscales task. Because Robertson and North (1992) have demonstrated manipulations of hemispace can affect the impact of unimanual activity in neglect, lateral and midline hand placement conditions were used. None of the unimanual activation conditions had an impact on the degree or direction of the leftward bias. From the evidence reviewed above, it becomes clear that the exact causes of the leftward bias observed under free-viewing conditions remain unclear. While there appears to be some support for each explanation, each explanation also has its detractors. The present paper describes two experiments that attempt to clarify the causes of the leftward perceptual bias. Given that the leftward and rightward line bisection errors observed for non-clinical and clinical populations are thought to reflect the operation of the same set of brain mechanisms (McCourt and Jewell, 1999), the identification of the causes of leftward perceptual biases could provide important insights into clinical disorders of attention. Indeed, each of the three explanations of perceptual asymmetries in normals has a counterpart in neglect. Thus, the literature has highlighted the role of overt/covert visual orienting (Reuter-Lorenz and Posner, 1990; Weintraub and Mesulam, 1987), pre-motor activation (Rizzolatti and Berti, 1990), and activation/attention (Kinsbourne, 1987) in clinical disorders of attention. EXPERIMENT 1 The first experiment examined the effect of scanning biases in English and Hebrew reading participants. The greyscales task was used in the first part of this study because of the particularly strong leftward bias that it generates (Nicholls et al., 1999, 2001) and because measures of reaction time (RT) can be obtained. If reading direction plays a significant role in perceptual asymmetry, a bias toward selecting the stimulus with the salient feature on the left-hand side should be observed for English readers and a rightward bias for Hebrew readers. The RT data should show a similar pattern of asymmetry with faster RTs for leftward responses for English readers and faster rightward responses for Hebrew readers. If the leftward bias in responses and RT persists across both reading groups, this would suggest that reading direction does not contribute to the leftward bias observed for the greyscales task. In the second part of the study, the same groups of participants were required to complete a passive line bisection task modeled on that used by Chokron et al. (1998). They manipulated scanning direction by asking participants to follow a marker as it moved from left-to-right or right-to-left and stop it as it reached the middle of the line. A significant effect of scanning direction was found whereby bisection points were shifted towards the origin of the scan. The effect of left-

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and right-ward scanning was symmetrical and appears to have overridden any pseudoneglect that may have existed for the task. Chokron et al. (1998) concluded that their experimental technique had successfully controlled scanning biases and that pseudoneglect was dependent upon these biases. The present study made a number of changes to the design used by Chokron et al. (1998). First, participants were asked to make a bimanual response with their index fingers to stop the marker. This mode of response is preferable to the right hand response used by Chokron. Indeed, given that hand of response has been found to affect perceptual asymmetry (McCourt et al., 2001; Schiff and Truchon, 1993) it is possible that the symmetry observed by Chokron et al. (1998) was related to LH activation rather than a control of scanning direction. The second change was concerned with randomising the horizontal position of the line to be bisected. Chokron et al. always placed the line in the horizontal centre of the display screen. This procedure may have allowed participants to use landmark cues to bisect the line rather than attending to the line itself. Although McCourt et al. (2000) have reported leftward errors for a line bisection task irrespective of whether the stimuli were centred or moved about the centre of the display, centreing the stimuli does allow participants to adopt a strategy that could counteract leftward perceptual biases. To prevent this strategy being used, the current study randomly shifted the midpoint of the line slightly to the left or right of the horizontal centre of the screen. A number of predictions can be made for the second part of the study. If scanning biases are important to the leftward perceptual bias, and if the marker effectively controls scanning, no bias should be observed for either group. Alternatively, it is possible that scanning biases are important to perceptual asymmetries, but that the procedure did not effectively control these biases. In this case, a leftward bias would be expected for readers of English and a rightward bias for readers of Hebrew. Finally, it is possible that scanning biases do not contribute to the leftward perceptual bias. If this proposition is true, a leftward bias of equal magnitude should be observed for both groups. Chokron et al. (1998) found that bisections were made to the left of the centre when the marker moved from left-to-right and vice versa when the marker moved right-to-left. They attributed the effect of marker direction to changes in the direction of scanning. Thus, lines were bisected towards the side on which the marker started because the scan led to an overrepresentation of that side of the line. However, Manning et al. (1990) have demonstrated that individuals adopt different strategies when bisecting lines. Some make ‘undershoot’ errors when they bisect a line as the scan enters a central indifference zone. Others, however, make ‘overshoot’ errors as they bisect the line as the scan passes outside the central indifference zone. These differences in strategy might be expected to cancel one another, leading to no bias in either direction. The present study sought to re-examine this issue by examining rightto-left and left-to-right trials separately. The greyscales and line bisection tasks both controlled unilateral motor activity through a bimanual response procedure. If no perceptual asymmetries are found for either task, this would implicate pre-motor activity as an important component of perceptual asymmetries. If a leftward perceptual bias is observed in

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any part of the experiment, however, this would effectively rule out pre-motor activity as a possible cause of perceptual asymmetries. Performance on both tasks was compared to determine whether individual asymmetries on the greyscales and line bisection tasks were related to one another. A positive correlation between the two tests would suggest that both were tapping the same set of processes. Research investigating the correlation between different measures of perceptual asymmetry have produced mixed results, with some finding a significant positive correlation (Luh, 1995) and others finding positive correlations of marginal significance (Hoptman and Levy, 1988; Nicholls et al., 1999). Materials and Methods Participants Twenty psychology students and 20 Israeli tourists (M = 8, F = 12 in both groups) took part in this experiment. The Psychology students participated in the study as part of their course requirements. Hebrew reading participants were collected by recruiting Israeli backpackers as they visited Melbourne. A number of steps were made to ensure that the Israelis had been exposed to a minimum of English text. First, Israelis were approached only if they had been away from Israel for less than two weeks. Second, tests designed to assess how much English text they had been exposed to and a test of reading ability was administered to all participants. By selecting participants who had been exposed to a minimum of English text, we believed we would have a group of Hebrew readers that would be comparable to those that have yielded reading direction effects previously (e.g., Chokron, et al., 1997; Eviatar, 1997). The Israeli backpackers were paid for their participation. All psychology students were fluent readers of English only. The modal age of the psychology students and Israelis was 18 and 22 years, respectively. Participants’ hand preference was measured using the Edinburgh Handedness Inventory (Oldfield, 1971). On a scale ranging from –100 (left-handed) to +100 (right-handed), all participants were right-handed (mean = 87.5, SD = 16.6). Participants reported having normal or correctedto-normal vision and were naïve as to the aims and expected outcomes of the experiment. Apparatus The experiment was controlled using a PC interfaced with a digital input/output card with an on-board 1.0 msec timer (Blue Chip Technology, DCM-16). The stimuli were presented on a 280 × 210mm (480 × 360 pixels) VGA monitor, surrounded by a black screen to reduce distractions and prevent the use of external cues as landmarks. Participants responded using a four-button response panel. The buttons were arranged symmetrically about the centre of the panel, allowing participants to make a bimanual response Stimuli An example of the stimuli used in the greyscales task is shown in Figure 1. Each stimulus was defined by a thin black rectangle and displayed against a light grey background. The stimuli were viewed at a distance of 500 mm and presented at five different lengths: 110, 132, 154, 176 and 198 mm. As Figure 1 illustrates, participants were presented with two bars, each changing from black at one end to white at the other end. The bars were positioned one on top of the other and arranged so that they were left-right mirror reversals of each other, i.e. if the upper stimulus was darker on the left side, the lower stimulus was darker on the right side. The horizontal midline of each stimulus pair was aligned with the centre of the display screen, while the vertical midline of the upper and lower stimuli were placed 29 mm above and below the centre of the screen, respectively. Each bar was 50 pixels (22 mm) high and was divided into 50 increments,

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Fig. 1 – Example of the greyscales stimuli.

with the size of each increment varying according to the length of the stimulus. Each bar was made to change luminance from one end to the other by adjusting the ratio of light to dark pixels within each increment. For example, at the light end of the stimulus, the first increment contained no dark pixels, the second increment contained one dark pixel per vertical line, the third increment two dark pixels per line and so on, until the final increment contained no white pixels resulting in a comparably dark section. The vertical position of the pixels within each line was randomised to create the impression of a smooth change in brightness and make each stimulus within a pair appear slightly different. Although each stimulus was configured differently at a local level, the two bars within each pair contained the same number of light and dark pixels, making them equiluminant at a global level. For the line bisection task, a white horizontal line, 3 pixels thick, was drawn onto a grey background along the vertical midline of the screen. The length of the lines varied along the same dimensions described for the greyscales task. A vertical marker, 1 pixel wide and 18 mm high, was placed with its vertical midpoint located on either the right or left ends of the line. The marker travelled along the line at a constant speed of 22 mm/s and did not disturb the horizontal line as it moved. To prevent the use of landmark cues, the horizontal midpoint of the lines was randomly shifted between trials from 9 mm to the left and right of the centre of the screen. Procedure The testing session lasted 45 minutes and was conducted in a quiet and well-lit room, devoid of visual and auditory distractions. For the greyscales and line bisection tasks, participants were seated in front of the computer so that their mid-sagittal axes were aligned with the centre of the response panel and display screen. The greyscales and line bisection tasks were administered in separate blocks, with 50% of participants in each group completing the greyscales task first and 50% completing the line bisection task first. Twelve practice trials were given prior to the commencement of each task to familiarise participants with the demands of the task. Greyscales Task This task contained 100 trials. Each of the five different lengths was presented an equal number of times. In 50% of trials, the upper stimulus was darker on the left side and the

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lower stimulus, darker on the right side. In the remaining trials, the reverse was true. The order in which the different factors were presented was balanced and randomised for each participant. Each trial began with the presentation of a greyscales stimulus. Participants were asked to determine which bar within the stimulus pair was darker overall. To select the top bar, participants pressed the upper two buttons on the response panel simultaneously using both middle fingers. To select the bottom bar, participants pressed the lower two buttons simultaneously using both index fingers. To maintain compatibility in stimulusresponse mapping, finger of response was not manipulated. If participants failed to respond within 4000 msec following presentation of the stimulus, the trial was rejected a replaced later in the block with a trial of an identical configuration. A beeper sounded automatically on these rejected trials, warning subjects to respond more quickly. Once a response was recorded, the screen was cleared and a new trial was begun in 1500 msec. Line Bisection Task This task contained 100 trials. Each of the five stimulus lengths and two cursor starting positions (left, right) appeared an equal number of times. The order in which the different factors appeared was randomised for each participant. Each trial began with the presentation of the horizontal line to be bisected. Participants started the marker moving by pushing two buttons simultaneously with both index fingers. Participants pressed both buttons again to indicate when the marker had reached the middle of the horizontal line. If participants ‘missed’ the middle and failed to depress the buttons, the marker continued to the end of the line and then reappeared at its original starting point. The trial was then repeated. A new trial was only begun once an attempt was made to stop the marker. Following a response, the display was cleared and a new trial was begun in 1500 msec. Once participants had completed the greyscales and line bisection tasks, the National Adult Reading Test (NART) was administered. This task requires participants to read 50 English words aloud. Responses were recorded using a Dictaphone and scored as correct or incorrect later. Finally, a reading questionnaire was administered. Participants were asked if (a) they could read English fluently, (b) if they read a second language fluently, and (c) how much English they had read over the past six months.

Results Sixty-five percent of Hebrew readers indicated that they were fluent readers of English. Despite this, the NART revealed that error scores for English readers (mean = 22.5, SD = 4.8) were significantly lower than for Hebrew readers (mean = 40.9, SD = 3.7) whose scores approached the maximum of 50 [t (38) = 13.51, p < 0.001]. On average, Hebrew readers estimated that 89% of material they had read over the past six months was written in Hebrew. Greyscales Task Responses were categorised as either ‘right’ or ‘left’ according to whether participants selected the bar that was dark on the right or left sides, respectively. A measure of response bias was calculated using Equation 1. Bias =

(Nright – Nleft)

× 100

(1)

Ntotal N.B. Nright and Nleft equal the total number of response made to the left and right, respectively. Ntotal represents the total number of responses.

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Scores can range between –100 to +100 with positive scores reflecting a bias toward the right and negative scores reflecting a bias to the left. The response bias data are shown in Figure 2. The average response bias for English and Hebrew readers was –31.9 and –19.1, respectively. A pair of t-tests revealed that the biases for English and Hebrew readers were significantly different from zero [t (19) = 3.31, p < 0.005; t (19) = 2.43, p < 0.05]. The data were then analysed with a mixed between/within subjects analysis of variance (ANOVA) with length (5 levels) as a within-participants factor and group (English, Hebrew) as a between-participants factor. There was no significant effect of length [F (4, 152) = 0.3, ns] or group [F (1, 38) = 1.1, ns] and no interaction between the factors.

Fig. 2 – Mean response bias (with ± SE bars) for the greyscales task for English and Hebrew readers as a function of stimulus length. Higher negative scores indicate an increased bias towards the left.

Mean RT was calculated for left and right responses within each of the five lengths. Where no responses were made within a condition, the series mean was substituted for the missing value. Six substitutions were made out of a total of 400 cells. The RT data satisfied the assumptions of homogeneity of variance and normality. Three univariate outliers were detected via stem and leaf plots, however kurtosis values were all normal (range = – 0.66 to 1.01). A mixed model ANOVA was performed on the data using reading direction (English, Hebrew) as a between-participants factor and stimulus length (five levels) and response type (left, right) as within-participants factors. The RT data are shown in Figure 3. Increases in stimulus length were associated with progressively longer RTs [F (4, 152) = 3.2, p < 0.005]. There was some suggestion that left responses were faster than rightward. However, the effect of side failed to reach significance [F (1, 38) = 2.4, ns]. There was no significant

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effect of group [F (1, 38) = 0.1, ns] and no interaction between group and side [F (1, 38) = 0.3, ns]. No other interactions were statistically significant.

Fig. 3 – Mean reaction times (with ± SE bars) for the greyscales task for English and Hebrew readers as a function of stimulus length and response-type (right/left).

Line Bisection Task A measure of response bias was gained by calculating the mean deviation of the subjective bisection point from the objective centre of the line. Measures were made in units of one pixel and these were converted to metric measures of length (mm) by multiplying each pixel by 0.583 (screen width (280 mm) / 480 pixels). Negative scores reflect a tendency to bisect the line to the left of the true centre whereas positive scores indicate a tendency to bisect to the right of centre. The response bias data are shown in Figure 4. The average response bias for English and Hebrew readers was – 0.98 mm and – 1.16 mm, respectively. A pair of t-tests revealed that the biases for English and Hebrew readers were significantly different from zero [t (19) = 2.42, p < 0.05; t (19) = 2.16, p < 0.05]. The data were then analysed with a mixed model ANOVA with group as a between-participants factor and starting position of the marker (left, right) and stimulus length as within-participants factors. The right-start condition was associated with a significantly stronger leftward bias than the left-start condition [F (1, 38) = 13.6, p < 0.005]. There were no significant main effects for group [F (1, 38) = 0.1, ns] or stimulus length [F (4, 152) = 1.79, ns]. There was a significant interaction between the side on which the marker started and stimulus

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length [F (4, 152) = 3.48, p < 0.005]. Inspection of Figure 4 suggests that this interaction arose because of a reduction in the effect of starting position with increases in stimulus length. Post-hoc comparisons supported this proposition. The effect of starting position was significant for lengths of 110 mm [F (1, 38) = 55.1, p < 0.001], 132 mm [F (1, 38) = 36.4, p < 0.001] and 154 mm [F (1, 38) = 17.6, p < 0.001]. The effect of starting position was not significant for lengths of 176 mm [F (1, 38) = 0.6, ns] or 198 mm [F (1, 38) = 0.1, ns]. No other interactions approached statistical significance.

Fig. 4 – Mean response bias (with ± SE bars) for the line bisection task for English and Hebrew readers as a function of stimulus length and the side on which the marker started (right/left).

A Pearson product-moment correlation was performed between the response bias measures for the greyscales and line bisection tasks. This analysis revealed a positive correlation between the tasks that approached statistical significance [r (40) = 0.29, p = 0.07]. Discussion In accord with previous research (Nicholls et al., 1999, 2001), there was a strong leftward bias for the greyscales task. On average, participants selected the bar that was dark on the left-hand side 75.5% of the time. Unlike Nicholls et al. (2001), however, the bias toward the left did not increase with increases in stimulus length. While the pseudoneglect and perceptual asymmetry literature often reports increases in the leftward bias with increases in stimulus length

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(Luh, 1995; McCourt and Jewell, 1999), this effect is not always found (Mattingley et al., 1993; Nicholls et al., 1999). The leftward response bias observed in this study cannot be the result of asymmetrical pre-motor activation because this activity was controlled throughout the study using a bimanual response procedure. The data also do not support the scanning bias model. If scanning biases imposed by reading habits were an important consideration in the greyscales task, a leftward bias should have been observed for English readers and a rightward bias observed for Hebrew readers. Instead of this, a strong leftward bias was observed for both groups. Reaction times for the greyscales task became longer with increases in stimulus length. This effect has been reported previously (Nicholls et al., 1999) and most probably reflects the fact that the longer stimuli spread the relevant information over a larger area; increasing the time taken to assimilate the information. Despite some suggestion that leftward responses were faster than rightward responses, this effect failed to reach statistical significance. This nonsignificant finding stands in contrast to the faster leftward responses reported previously for the greyscales task (Nicholls et al., 1999). The discrepancy between the studies is hard to explain given the procedural similarities between the studies. It is possible, however, that the experimenter who conducted this study emphasised the importance of consistent reaction times less than previous experimenters did. There was no suggestion that side of response interacted with reading direction so that Hebrew readers were faster for rightward responses and English readers were faster for leftward responses This result, once again, militates against a scanning bias interpretation of leftward perceptual biases. It could be argued that we failed to find any differences between the English and Hebrew readers because the Israeli backpackers were not ‘pure’ readers of Hebrew. That is, the Hebrew readers we recruited had been exposed to more English text than the Hebrew readers used in previous studies. There are number of reasons, however, why we feel that this is not a significant problem. First, our Israeli participants, like those used in previous studies (e.g., Chokron et al., 1998), were in their early twenties. The large majority of these participants are taught English at an early age and can speak and read English. Second, despite considering themselves to be fluent in English, the NART revealed that the Israelis were less proficient readers of English than their Australian counterparts. In addition, our Israeli participants revealed that 89% of the material they had read over the past six months had been written in Hebrew. These data demonstrate that the Israeli participants were significantly more familiar with Hebrew than English and, in this regard, were most probably not different from previous Hebrew reading samples. In any case it seems improbable that a relatively brief exposure to English text could overcome a right-to-left reading habit that is acquired over a lifetime. The line bisection task used in the current study was modelled on the one used by Chokron et al. (1998). These authors found that leftward scans resulted in a shift of the perceived midpoint of a line to the left and vice versa for rightward scans. The effect of left and right scanning appear to counteract each other, resulting in no overall leftward bias for the task for either the French or Hebrew readers. Chokron et al. (1998) attributed this null effect to their

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experimental procedure, which effectively controlled scanning biases. Despite adopting a similar scanning control technique, the present study found a leftward bias for both reading groups. Thus, the present study suggests that leftward perceptual biases arise even when scanning is controlled. It could be argued that the experimental procedure was not effective in its control of scanning habits. If this was true, however, and if the scanning hypothesis was correct, a leftward bias should have been observed for English readers and a rightward bias for Hebrew readers. Contrary to this prediction, a leftward bias was observed for both groups. This null effect, not only contradicts the study by Chokron et al. (1998), but also challenges a large body of other well-conducted studies (Chokron and Imbert, 1993; Eviatar, 1997; Sakhuja et al., 1996; Vaid and Singh, 1989). Procedural differences between the current study and Chokron et al. (1998) may account for the discrepancies observed between the studies. The present study used a bimanual response procedure whereas Chokron et al. required a right hand response. There is some evidence (Schiff and Truchon, 1993) that right hand activity can reduce leftward perceptual biases through activation of the LH (though see Nicholls et al., 2001, for contrary evidence). Thus, it is possible that Chokron et al. (1998) did not observe a leftward perceptual bias because they used the right hand to make responses. Another procedural difference between the studies is the positioning of the lines to be bisected. The present study randomly shifted the middle of the lines around the midpoint of the screen whereas Chokron et al. always centred the lines in the middle of the screen. Chokron’s procedure may have allowed participants to adopt a strategy where they attended to the middle of the screen rather than to the line. Although McCourt et al. (2000) have demonstrated that leftward errors persist for line bisection tasks despite being centred on the screen, this procedure is not ideal and could counteract leftward perceptual biases. Response bias for the line bisection task was affected by the starting position of the marker. On average, when the marker moved right-to-left, bisections were made –1.98 mm to the left of centre. This leftward bias was reduced to – 0.16 mm when the marker moved left-to-right. The fact that performance on the task was affected by the starting position of the marker demonstrates that participants were attending to the marker. The effect of the marker, however, is in the opposite direction to that reported by Chokron et al. (1998) and a number of other researchers (e.g., Brodie and Pettigrew, 1996; see Jewell and McCourt, 2000, for a review). Chokron et al. (1998) found that lines were bisected towards the side on which the marker started. The model of perceptual asymmetry proposed by Manning et al. (1990) can explain these results. This model proposes that a ‘central zone of indifference’ surrounds the middle of a line and that this zone corresponds to the region of ‘just noticeable difference’ between the two halves of the line. One strategy discussed by Manning et al. involved making the bisection as the scan entered the indifference zone. This strategy would cause the effect observed by Chokron et al. (1998) where bisection marks fell toward the side on which the scan began. However, Manning et al. also discussed the possibility that participants bisected the line as the scan exited the central indifference zone. This strategy would account for the

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present results, where participants tended to stop the marker after it had passed the middle of the line. It is also possible that the effect of starting position observed in the present study reflects response delays. It is well known that manual responses to stimuli are not instantaneous and that minimum response times are in the order of 100-150 msec (Alegria, 1974). This delay may have also contributed to the overshoot errors observed in the current study. It is unclear why the present study found an effect of starting position opposite to that reported by Chokron et al. (1998). It is unlikely that the discrepancy stems from differences in the rate at which the marker moved: The marker moved at 22 mm/sec in the present study whereas Chokron used a speed of 20 mm/sec. It is possible, however, that Chokron’s procedure, which centred the lines in the middle of the display, allowed participants to anticipate the centre of the line more so than in the present study. This anticipation may have encouraged participants to make their bisections as the marker moved into the central zone of indifference. The present study also revealed that the effect of starting position diminished with increases in stimulus length. This interaction may reflect differences in preparedness to respond. For shorter stimuli, the marker reached the middle of line more quickly and this may have given participants less time to make their judgment and respond. In contrast, the marker took longer to reach the middle of the line for the longer stimuli. For these cases, fewer overshoot errors were made because participants had more time to make their judgements and indicate their decision. Finally, there was a positive correlation that approached statistical significance between the greyscales and line bisection tasks. This result suggests that the tasks tap some common mechanisms, but also suggests that a considerable amount of variation is unique to each test. Similar weak correlations between tests of perceptual asymmetry under free-viewing conditions have been reported by a number of researchers (Hoptman and Levy, 1988; Nicholls et al., 1999). The results of this study and others (Hoptman and Levy, 1988; Nicholls et al., 1999) suggest that perceptual asymmetries observed under free-viewing conditions are task-dependent and reflect the operation of a number of independent processes, only some of which are common between the tasks. EXPERIMENT 2 The first experiment demonstrated that scanning biases and asymmetrical premotor activation do not account for the leftward perceptual bias observed for either the greyscales or line bisection tasks. The second experiment explored the possibility that the perceptual asymmetry for the greyscales tasks is the result of an attentional bias. The contribution of attention to perceptual asymmetry has been investigated using unilateral activation paradigms such as hand clenching with mixed results (e.g., Nicholls et al., 2001; Schiff and Truchon, 1993). Cueing techniques have also been used to manipulate attention during line bisection tasks. In most studies conducted to date (e.g., Milner et al., 1992;

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Nichelli et al., 1989), attention has been manipulated by asking participants to report a letter that was presented concurrently with the stimulus to be bisected. In general, these experiments appear to demonstrate that bisection marks are shifted towards the side on which the cue appeared (Jewell and McCourt, 2000). Fischer (1994), however, suggested that the cueing effect observed by Milner et al. (1992) was related to a ‘gestalt-like’ grouping of the cue and the line (see Shuren et al., 1997). Fischer (1994) controlled mass effects by presenting cues to both ends of the line. Attention was manipulated by requiring participants to report the cues in a left-right or right-left order. The order of report had no affect on the perceptual asymmetry. Thus, Fischer concluded that the cueing effects observed in previous studies were the result of perceptual effects rather than an attentional bias (see also Mattingley et al., 1992). The present study sought to investigate the role of attentional biases in perceptual asymmetries using a cueing paradigm that has not been used before in this field. Previous studies have manipulated attention with cues that were presented concurrently with the experimental stimulus. The present study used non-informative peripheral spatial cues like those used by Posner and Cohen (1984). These types of cues are thought to result in the automatic orientation of attention to the cued location (Yantis and Jonides, 1990). Because they can occur (and disappear) before the presentation of the target stimulus, they are less likely to result in a grouping effect with the target stimulus. Cues were presented to either the left or right sides of the greyscales stimuli. To provide a measure of baseline performance, a neutral condition was added where cues were presented to both sides of the greyscales stimuli. The cues appeared for 51 msec and then disappeared 51 msec before the onset of the greyscales stimuli. To accentuate the effect of spatial cueing, the exposure duration of the greyscales stimuli was reduced to 408 msec. If the leftward perceptual bias for the greyscales stimuli is the result of a bias of attention directed to the left hemispace, then cueing to the left side should have little effect on performance relative to the baseline condition. In contrast, cueing to the right should cause a shift in attention away from the left, leading to a reduction of the leftward bias. If attentional processes do not contribute to the leftward perceptual bias, however, the cueing procedure should have no effect on performance on the greyscales task. Materials and Methods Participants Sixteen first year psychology students (M = F) participated in the study as part of their course requirements. The Edinburgh Handedness Inventory indicated that all participants were right-handed (mean = 85.0, SD = 17.4). The modal age of participants was 18 years and all reported to have normal, or corrected to normal, vision. All participants were naïve in relation to the specific aims of the study. Stimuli and Procedure The greyscales stimuli used in the experiment were the same as those described in the first study. White vertical bars, 51 mm high and 3 mm wide, gave spatial cues. The

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horizontal position of the cues varied in line with the length of the greyscales stimuli and always appeared 12 mm from the outer edge of the stimuli. The cues stretched from the lower edge of the bottom greyscale to the upper edge of the top greyscale. The cues were presented to (a) the left side (b) the right side, or (c) both sides of the greyscales stimuli. Each trial began with the presentation of the cue for a period of 51 msec against a uniform grey background. The cue was then removed and, after a further period of 51 msec, the greyscales stimuli were presented. The greyscales stimuli were presented for a period of 408 msec, after which, the display was cleared. To accentuate the cueing effect, participants were encouraged to respond quickly. Trials with RTs greater than 2000 msec were automatically rejected and replaced with trials of identical composition. A total of 240 trials were presented. Equal numbers of trials with left, right and neutral cues were given. The orientation of the greyscales stimuli (left/right) and the five stimulus lengths were tested within the cue factor. Participants were instructed to ignore the cues and attend to the greyscales stimuli. All other instructions and procedural details of this study were the same as those outlined for the greyscales task in the first experiment.

Results A measure of response bias was calculated using the procedure described in the first experiment (see Equation 1). To provide more stable data across the three cueing conditions, the data were collapsed across the stimulus length factor. To determine whether levels of bias were significantly different from zero, a series of t-tests were conducted on the data from the left-cue, neutral and right-cue conditions. Levels of bias were significantly different from zero for the left-cue [t (15) = 2.87, p < 0.05] and neutral conditions [t (15) = 2.72, p < 0.05], but not for the right-cue condition [t (15) = 1.76, ns]. The data were then analysed with an ANOVA with cue condition (left, neutral and right) as a within participants factor. The ANOVA revealed a significant effect of cueing condition [F (2, 30) = 4.05, p < 0.05]. Inspection of

Fig. 5 – Mean response bias (with ± SE bars) for the greyscales task as a function of cue-type (left/neutral/right).

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Figure 5 suggests that the effect of the cues stems from a difference between the neutral and right-cue conditions. Post-hoc tests confirmed this interpretation with a significant difference between the right-cue and neutral condition [t (15) = 2.25, p < 0.05] and right-cue and left-cue condition [t (15) = 2.64, p < 0.05], but no difference between the left-cue and neutral condition [t (15) = 0.14, ns]. Reaction time was calculated using the procedure described in the first experiment. The data were analysed with an ANOVA with response type (right, left) and cue condition (left, neutral, right) as within participants factors. Figure 6 illustrates that leftward responses were faster than rightward responses [F (1, 15) = 4.86, p < 0.05]. The effect of cue [F (2, 30) = 1.32, ns] and the interaction between response type and cue [F (2, 30) = 1.74, ns] failed to reach statistical significance.

Fig. 6 – Mean reaction times (with ± SE bars) for the greyscales task as a function of cue-type and response-type (left/right).

Discussion Like the first experiment, a strong leftward bias was observed for the greyscales task. On average, participants selected the stimulus with the salient feature on the left-hand side, 74.6% of the time. The leftward bias was significant for left-sided and neutral cues, but not for right-sided cues. While the leftward bias was not significant for right-sided cues, it should be noted that the bias was still relatively strong (69.1%). Thus, it seems parsimonious to conclude that there was a reduction in the leftward bias for right-sided cues relative to the neutral trials rather than an elimination of this bias. There are a number of reasons why it is unlikely that the cueing effect is

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related to perceptual grouping (Fischer, 1994). First, the cues were not present by the time the greyscales appeared, making it less likely that they were grouped together. Second, if a grouping effect had occurred, it would seem reasonable to expect a pattern of results opposite to those observed. Given that the cues were white against a background of grey and that the discrimination required participants to select the darker stimulus, one might expect participants to select the stimulus with the dark end away from the cue. That is, the white cue, when combined with the greyscales stimuli would make the dark end of one stimulus appear lighter. The data do not support this interpretation and show no sign of a rightward bias for left-sided cues and vice versa for right-sided cues. The response bias data are consistent with an attentional bias interpretation of perceptual asymmetry (Milner et al., 1992). Thus, left-sided cues had no effect on the bias because attentional resources were already directed to this side of space. In contrast, right-sided cues caused a shift of attentional resources away from the left hemispace towards the right. This shift in attention may have caused the leftward features of the stimuli to be less salient, resulting in more right-sided decisions. The RT data revealed that leftward responses were 102 msec faster than rightward responses. This asymmetry confirms previous reports of faster leftward responses for the greyscales task (Nicholls et al., 1999) and suggests that participants were more confident when they responded ‘left’ rather than ‘right’. There was no indication that the RT advantage for leftward responses, present for left-sided and neutral cues, was reduced or reversed for right-sided cues. Thus, the RT data do not support an attentional bias interpretation of the leftward perceptual asymmetry. The lack of a cueing effect may be related to the relatively long mean RTs for the greyscales task (sometimes in excess of 600 msec). These long RTs are no doubt related to the complicated nature of the discrimination. It is possible that the long RTs masked any exogenous cueing effects, which are usually found for RTs in the range of 300 to 400 msec (Posner and Cohen, 1984). GENERAL DISCUSSION The present study sought to identify the mechanisms that underlie leftward perceptual asymmetries. In the first experiment, the effect of scanning biases was manipulated using a moving probe. Although scanning direction affected the perceived midpoint of the line, it did not eliminate an overall leftward bias for the task. Scanning was also manipulated by testing groups with different reading habits. Reading direction did not affect the leftward bias for either the greyscales or line bisection tasks. In the first and second experiments, asymmetries in premotor activation were controlled through a bimanual response procedure. Despite this control, leftward perceptual asymmetries were observed in both studies. Thus, the present study also failed to support the predictions of a premotor activation account of asymmetry. The attentional bias model of perceptual asymmetry was supported by the second experiment. This study showed that right-sided cues significantly reduced the leftward bias whereas left-sided cues

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had no effect on the perceptual asymmetry. The leftward attentional bias observed in the current study could be generated through unilateral hemispheric activation of the type described by Kinsbourne (1970). Thus, spatially-oriented, non-verbal tasks such as they greyscales task might activate the RH more than the LH, leading to a bias of attention towards the left hemispace. This leftward bias of attention would be expected to increase the salience of the features contained on the left side of the stimuli relative to those contained on the right side. Thus, when asked to select the greyscale that appeared darker, participants tended to select the stimulus that was dark on the left-hand side. Alternatively, it is possible that the attentional bias for the greyscales task reflects a process that is independent of hemispheric activation. There are a number or reasons for entertaining this notion. First, studies that have explicitly tested the applicability of Kinsbourne’s (1970) model to free-viewing perceptual asymmetries have provided mixed results. While Vingiano (1991) did show a leftward bias in attention for a concurrent spatial task, there was no significant rightward bias for a concurrent verbal task. Research by Schiff and Truchon (1993) using chimeric facial stimuli has also produced mixed results. Contraction of the right hand reduced the leftward bias when the faces had a negative expression on the left side. When the negative expression was on the right side, however, neither contraction of the left or right hands affected the leftward bias. Finally, research conducted in our laboratory (Nicholls et al., 2001) has shown that unimanual activity of the hands (clenching and tapping) does not affect the leftward bias for the greyscales task. Another reason for suspecting that unilateral activation is not central to leftward perceptual asymmetries is related to the strength of hemispheric activation. For the greyscales stimuli used in the present study, participants selected the stimulus with the salient feature on the left-hand side 3 out of 4 times. Within the realm of perceptual asymmetry research, this is a substantial asymmetry. The perceptual asymmetry for the greyscales task stands in contrast to hemispheric asymmetries in brightness discrimination, which most probably underlie the task. Indeed, while brightness discrimination has been associated with the RH (Davidoff, 1975), asymmetries of this type are notoriously weak (Basso et al., 1977; Moscovitch, 1979). Bearing this in mind, it is difficult to envisage how task-specific activation of the RH for a weakly lateralised function such as brightness discrimination can account for the leftward bias on the greyscales task. However, this argument may not extend to other tasks that induce leftward perceptual biases under free-viewing conditions. Thus, while some research points to no lateralisation for line bisection (Andreassi and Juszczak, 1984; Greenwood et al., 1980), other research suggests RH involvement (Fink et al., 2000a; Fink et al., 2000b; Galati et al., 2000). Similarly, the RH has clearly been implicated in the perception of facial expression (e.g., Benton, 1990). As an alternative to Kinsbourne’s (1970) activation model, it is possible that attentional asymmetries reflect a general tendency to attend more to the leftward features of a stimulus than to those on the right. This asymmetry is not driven by unilateral activation, but rather, by asymmetries in the neural mechanisms

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that control attention. Heilman et al. (1987) have suggested that the RH is specialised for all spatial functions, including attention. Support for this notion comes from clinical studies showing that neglect occurs more frequently, and with a greater severity, following lesions to the RH (Mattingley et al., 1992). Tests of hemispheric function using unilateral sodium amobarbital injections confirm the importance of the RH in attentional processes. Spiers et al. (1990) demonstrated that anaesthetising the RH disrupted attention in the contralateral and ipsilateral fields whereas anaesthetising the LH did not disrupt attentional processes. Right hemisphere specialisation for attentional processes may cause more attention to be paid to the contralateral hemispace. A leftward attentional bias may account, not only for the present data, but also for a number of other findings within the non-clinical literature. For example, Jeeves and Dixon (1970) found that simple RTs were significantly faster to stimuli presented to the left visual field than to the right visual field. Similar results have been reported by a number of other researchers (Anzola et al., 1977; Bradshaw and Perriment, 1970). This left visual field advantage could reflect a left hemispatial attentional bias, which facilitates the detection of signals located in the left visual field. A leftward bias of attention may also account for the asymmetrical cueing effects observed by Heilman and van den Abell (1979). They examined simple RTs to the onset of a central target following cues to either the left or right visual fields and found that cues delivered to the left visual field (hence RH) resulted in a significantly larger cueing effect than for the right visual field. Sturm et al. (1989) have reported a similar left visual field advantage for cueing in a choice RT task. A left visual field superiority in reaction time has also been demonstrated under sustained attention conditions (Whitehead, 1991). The literature relating to a leftward bias in attention is far from conclusive, however. For example, the left visual field advantage observed for simple RT reverses to a right visual field advantage when a two-choice RT paradigm is employed (Anzola et al., 1977). A right visual field advantage has also been observed when cues of varying validity have been used to predict the location of an ensuing target (Egly and Homa, 1984). The critical difference between studies that have found a left or right visual field attentional advantage may be related to whether the target location is predictable. Umiltà and Nicoletti (1985) found a right visual field advantage only when visual field (right-left) was randomised rather than blocked. The stimuli used to generate perceptual asymmetries usually fall in both visual fields in a predictable fashion. As a result, the stimuli may lend themselves to a processing strategy where a left hemispace attentional advantage is generated. It is possible that subtle variations in processing strategy and the associated shifts in attention may account for some of the discrepancies observed in the perceptual asymmetry literature. In conclusion, the data collected in this study failed to support the scanning bias or pre-motor activation accounts of perceptual asymmetry. The data did, however, demonstrate that the leftward perceptual bias is reduced when attention is shifted away from the left hemispace. Traditionally, this attentional bias has been attributed to asymmetries in hemispheric activation. However, support for role of hemispheric activation in hemispatial attentional biases is not strong and

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(Received 21 February 2001; reviewed 24 May 2001; revised 27 July 2001; accepted 13 August 2001; Action Editor: Giuseppe Vallar)