Discussion Post: Visual Dysfunction in Children

Perception of shapes targeting local and global processes in autism spectrum disorders

Emma J. Grinter,1 Murray T. Maybery,1 Elizabeth Pellicano,1,3 Johanna C. Badcock2 and David R. Badcock1

1School of Psychology, University of Western Australia; 2Centre for Clinical Research in Neuropsychiatry/Graylands Hospital, School of Psychiatry and Clinical Neurosciences, University of Western Australia; 3Department of

Experimental Psychology, University of Bristol, UK

Background: Several researchers have found evidence for impaired global processing in the dorsal visual stream in individuals with autism spectrum disorders (ASDs). However, support for a similar pattern of visual processing in the ventral visual stream is less consistent. Critical to resolving the inconsistency is the assessment of local and global form processing ability. Methods: Within the visual domain, radial frequency (RF) patterns – shapes formed by sinusoidally varying the radius of a circle to add ‘bumps’ of a certain number to a circle – can be used to examine local and global form perception. Typically developing children and children with an ASD discriminated between circles and RF patterns that are processed either locally (RF24) or globally (RF3). Results: Children with an ASD required greater shape deformation to identify RF3 shapes compared to typically developing children, consistent with difficulty in global processing in the ventral stream. No group difference was observed for RF24 shapes, suggesting intact local ventral-stream processing. Conclusions: These outcomes support the position that a deficit in global visual processing is present in ASDs, consistent with the notion of Weak Central Coherence. Keywords: Autism, local processing, global processing, ventral visual pathway, radial frequency patterns. Abbreviations: ASD, autism spectrum disorder; TD, typically developing; WCC, Weak Central Coherence; EPF, Enhanced Perceptual Functioning; RF, radial frequency; ADI-R, Autism Diagnostic Interview – Revised.

Over the past three decades, several research groups have proposed that the cognitive profile in autism spectrum disorders (ASDs) is characterised by diffi- culties in complex information processing (Bertone, Mottron, Jelenic, & Faubert, 2005; Frith, 1989; Minshew, Goldstein, & Siegal, 1997). In particular, Weak Central Coherence (WCC) theory suggests that individuals with an ASD demonstrate a relative fail- ure to extract overall meaning, resulting in a reduced awareness of the global aspects of stimuli in con- junction with a relatively heightened awareness of the details or parts of stimuli (Frith, 1989; Happé, 1999). Several studies have shown, however, that integration abilities might be intact in ASDs (Mot- tron, Burack, Stauder, & Robaey, 1999; Ozonoff, Strayer, McMahon, & Filloux, 1994; Plaisted, Swet- tenham, & Rees, 1999). To account for these data, others have proposed, amongst several other hypotheses, that individuals with an ASD show ‘Enhanced Perceptual Functioning’ (EPF; Mottron, Dawson, Souliéres, Hubert, & Burack, 2006) in which the salience of local features is enhanced without corresponding deficits in integrative capa- bilities. Research assessing visual capabilities is uniquely positioned to clarify which of these accounts best explains atypical processing in ASDs since processes known to engage global integration can be examined (Bell & Badcock, 2008; Loffler, 2008).

At the earliest stages of visual perception, neurons in primary visual cortex (V1) extract information about local characteristics of stimuli to provide a spatially limited signal for perception (DeValois & DeValois, 1988). Because the classical receptive fields are small, however, V1 information must be integrated to enable global perception at later stages of both the dorsal (Movshon, 1990) and ventral (Loffler, 2008) visual streams. There has been con- siderable interest in visual processing in autism in recent years, and much research has investigated local and global processing in the dorsal and ventral visual pathways (see Kaiser & Shiffrar, in press; Simmons et al., 2009, for reviews). Several researchers have found higher thresholds in children with autism when compared to typically developing (TD) children on tasks targeting global dorsal stream processing that require the identifi- cation of direction of motion or the presence of coherent motion in a field of moving dots (e.g., Milne et al., 2002; Pellicano, Gibson, Maybery, Durkin, & Badcock, 2005; Spencer et al., 2000; Spencer & O’Brien, 2006; Tsermentseli et al., 2008), in con- junction with evidence of intact local dorsal stream processing (e.g., Bertone, Mottron, Jelenic, & Fau- bert, 2003; Pellicano et al., 2005). While this has been interpreted as evidence for a disturbance in higher-level global processing in the dorsal pathway in ASDs (e.g., Bertone et al., 2003; Pellicano et al., 2005), the data for a similar pattern of visual processing in the ventral visual stream is lessConflict of interest statement: No conflicts declared. Discussion Post: Visual Dysfunction in Children

Journal of Child Psychology and Psychiatry 51:6 (2010), pp 717–724 doi:10.1111/j.1469-7610.2009.02203.x

� 2010 The Authors Journal compilation � 2010 Association for Child and Adolescent Mental Health. Published by Blackwell Publishing, 9600 Garsington Road, Oxford OX4 2DQ, UK and 350 Main Street, Malden, MA 02148, USA

 

 

consistent, with some studies reporting equivalent (e.g., Blake, Turner, Smoski, Pozdol, & Stone, 2003; Milne et al., 2006; Spencer et al., 2000) and others impaired thresholds (e.g., Spencer & O’Brien, 2006; Tsermentseli et al., 2008) on measures of ventral stream global processing. Importantly, many of these studies failed to examine both local and global processing within a specified pathway or use similar stimulus characteristics to assess the two forms of processing (but see Bertone et al., 2003, 2005), making it difficult to draw firm conclusions about local and global processing in ASD.

The present study assessed both types of visual functioning in the ventral stream using shapes in which differences between the local and global stimuli were minimised. Radial frequency (RF; Wil- kinson, Wilson, & Habak, 1998) patterns are closed- contour shapes created by deforming a circle. The deformation is produced by sinusoidally varying the radius as a function of polar angle. The number of cycles of modulation in 360� corresponds to the RF number and when the amplitude of the modulating function is set to zero, a circle is produced (Fig- ure 1a). Three cycles of appropriate amplitude create a shape that looks like a triangle with rounded cor- ners (Figure 1b), and 24 cycles result in a circle with 24 ‘bumps’ (Figure 1c). For RF patterns of high fre- quency (e.g., RF24), performance for discriminating the whole shape from a circle is better than when only part of the closed shape is deformed (Loffler, Wilson, & Wilkinson, 2003), but only by an amount that can be explained by probability summation of the detection of independent local features. For this reason, the discrimination of high RF shapes is thought to be achieved by the local orientation-tuned cells in V1 (Wilkinson et al., 2003). In contrast, there is evidence that curvature and position information is pooled along the entire circumference of the pat- tern for low radial frequencies (Bell & Badcock, 2008; Bell, Badcock, Wilson, & Wilkinson, 2007), consistent with global signal integration in shapes with up to about ten cycles of modulation (Bell & Badcock, 2009; Loffler, 2008). FMRI data is consis- tent with global pooling of orientation information to extract global shape information further along the ventral pathway in V4 for RF3 patterns (Wilkinson et al., 2000).

Here we report the first study to use these stimuli with an ASD population. We used RF3 and RF24 patterns to assess global and local ventral stream processing, respectively. Loffler et al.’s (2003) examination of RF shapes showed that RF3 shapes evoke active global pooling of local curvature esti- mates, whereas RF24 shapes involve probability summation of local curvature estimates. Consistent with the idea that there is a potential difference in how RF shapes are processed, Bell, Wilkinson, Wilson, Loffler, and Badcock (2009) showed that discrimination of low RF patterns is underpinned by multiple narrow-band contour shape channels. In selecting the RF shapes for this study, we were careful to choose clear examples for which global processing of local curvature estimates was (RF3) or was not (RF24) selectively activated. This allowed determination of separate local and global contri- butions to shape processing. Discussion Post: Visual Dysfunction in Children

The WCC account can be used to predict that, relative to a neurotypical comparison group, indi- viduals with an ASD should show elevated thresh- olds on the RF3 task (i.e., poor global processing in the ventral stream), accompanied by either equivalent or lower thresholds on the RF24 task (i.e., intact/superior local processing in the ventral stream). Alternatively, EPF theory can be used to predict that individuals with an ASD should dem- onstrate equivalent or lower thresholds on the RF24 task (i.e., enhanced local processing), but, critically, equivalent thresholds on the RF3 task (i.e., intact global processing) relative to TD individuals.

Method

Group comparisons

Brock, Jarrold, Farran, Laws, and Riby (2007; see also Jarrold & Brock, 2004) demonstrated that substantial problems can be introduced when using conventional methods to match or statistically control for psycho- metric variables, such as verbal and non-verbal ability, on which children with a developmental disorder and TD children differ systematically. The analytic approach they advocate is to regress each experimental variable onto the relevant psychometric variables for a large and diverse group of TD children, and then use the regression function to generate expected scores for the

(a) (b) (c)

Figure 1 Examples of (a) a circle (b) an RF3 stimulus and (c) an RF24 stimulus

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children in the clinical group, against which their actual scores are then compared. Thomas et al. (2009) argued that this approach allows meaningful group compari- sons to be made. Accordingly, we adopted Brock et al.’s innovative approach in our group comparisons of RF pattern performance. Discussion Post: Visual Dysfunction in Children

Participants

Children with an ASD were recruited through an autism register, speech pathologists and participation in pre- vious research projects at the University of Western Australia. The 38 8–16-year-old children (32 males) in the ASD sample had received an independent clinical diagnosis from a multidisciplinary team of either autistic disorder (N = 30), Asperger’s disorder (N = 2) or pervasive developmental disorder – not otherwise specified (N = 6), according to DSM-IV (American Psy- chiatric Association, 1994) criteria. Also, each ASD child either met full criteria for autism (N = 34) or scored above the cut-off in two of the three symptom domains on the Autism Diagnostic Interview – Revised (ADI-R; Lord, Rutter, & Le Couteur, 1994) (social interaction domain: M = 20.16, SD = 6.36; communi- cation domain: M = 15.94, SD = 5.04; repetitive behaviours domain: M = 6.51, SD = 2.65). Children were excluded from the ASD sample if they had a diagnosis of any medical condition (e.g., epilepsy) or other developmental disorder (e.g., ADHD), hearing or visual problems, or were taking medication known to impact on perception or cognition. TD children (N = 132, 8–16 years old, 70 males) recruited from a metropolitan school participated after parents com- pleted a brief screening questionnaire ensuring the children had no medical, hearing or visual problems or history of developmental difficulties. Written informed consent was obtained from the parents of all children prior to participation in accordance with the policies of the University of Western Australia’s Ethics Committee. Children wore their optical corrections for visual tasks when required but were otherwise unscreened. How- ever, since both RF3 and RF24 patterns are composed of the same spatial frequency content, any group dif- ferences in acuity would impact on both patterns equally.

The groups were well matched for chronological age, t(168) = .96, p = .34, and non-verbal ability, t(168) = .31, p = .76, as measured by the Matrix Reasoning subscale of the Wechsler Intelligence Scales for Children – Version IV (WISC-IV; Wechsler, 2003; see Table 1). The ASD group had significantly lower verbal ability, as measured by the Vocabulary subscale of the WISC-IV, than the TD group, t(168) = 5.46, p < .01, consistent with communication difficulties being a core characteristic of autism. All children were considered high-functioning and were attending mainstream schools. The TD group contained more females than the ASD group, v2(1) = 14.12, p < .01. Discussion Post: Visual Dysfunction in Children

Stimuli

RF patterns were presented on an LG L1730SF touch screen driven by a Sony Vaio VGNSZ34GP laptop computer. The 1024 · 768 pixel screen had a refresh

rate of 75Hz and a mean luminance of 30 cd/m2. RF patterns were created following Bell et al. (2007). The RF patterns were formed according to the following equation:

rðhÞ¼ rmeanð1þ A sinðxh þ uÞ ð1Þ

where r and h (in radians) are the polar coordinates, rmean is the pattern’s mean radius, and A, x and u are the amplitude of modulation of the radius, radial frequency (RF), and phase of the pattern, respectively. RF patterns were always presented with random phases, rendering it impossible for the observer to predict the exact location of the lobes from trial to trial. All RF patterns had a mean radius of 1.5� with a centre- to-centre separation of 3.75�. The luminance profile of a radial cross section approximated a fourth derivative of a Gaussian set at 99% contrast and had a peak spatial frequency of 8c/�. Radial frequencies (x) of 3 and 24 were employed (Figure 1).

Procedure

The WISC Vocabulary and Matrix Reasoning subscales were given first, followed by the two RF tasks in randomised order, and forming part of a larger test battery. Each RF task began with 35 practice trials that were administered under the same conditions as the test trials to ensure that children could meet the task demands. For the psychophysical tasks, testing was conducted in a darkened room at a viewing distance of .75 m. Viewing was binocular and auditory feedback (a computer-generated tone) was provided after each practice and test trial. Discussion Post: Visual Dysfunction in Children

Two shapes were presented simultaneously for 200 ms, one a circle and the other a RF3 or RF24 pattern depending on the task. There was no time limit for response, and a 1 s delay separated the child’s touch response and the presentation of the next stimulus. Children were told: ‘In this task you will see two shapes come up on the screen. On one side the shape will be a perfect circle. On the other, the shape will not be a perfect circle, it will ‘‘have lots of little bumps around it’’ [for RF24 patterns] or ‘‘look like a squashed egg’’ [for RF3 patterns]. What you have to do is work out which side of the screen ‘‘has the shape that looks like a squashed egg’’ or ‘‘has the bumpy shape’’ – the one that’s not a perfect circle – and then touch that side of the screen.’ To ensure they understood the instructions,

Table 1 Participant characteristics

Measures Children with ASD (N = 38)

Typically developing children (N = 132)

Age (years) Mean 11.88 12.15 SD 2.54 1.94 Range 8.17–16.92 8.83–15.83

WISC Vocabulary (scaled score) Mean 8.95 11.47 SD 2.75 2.40 Range 4–15 6–19

WISC Matrix Reasoning (scaled score) Mean 10.34 10.06 SD 2.91 2.55 Range 3-16 2-18

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children were then shown two example cards that resembled the images in Figure 1 and asked which ones they would choose.

The method of constant stimuli was used to control stimulus presentation (7 stimulus levels, 15 trials per level, taking approximately 6 minutes) and thresholds were obtained from the psychometric function by fitting the equation:

Y ¼ 7:5þ 7:5

1þexpðthreshold�xr Þ ð2Þ

where threshold yields the 75% correct level, exp is the exponential function, r is a scalar determining the slope of the psychometric function, Y is the number correct out of 15, and x is the amplitude level. At least 60% of variance had to be accounted for by the psychometric function for the threshold for that observer to be included in the analyses. Each child was given two opportunities to meet this criterion if required (RF3: 6% in ASD group, 9% in TD group; RF24: 0% in ASD group, 3% in TD group). Children who did not meet the criterion on either trial run were excluded from the results for that task (RF3: 5% in ASD group, 7% in TD group, RF24: 5% in ASD group, 3% in TD group). Discussion Post: Visual Dysfunction in Children

Results

Data for each group were screened for normality, and outliers (>3 SDs from the mean) were excluded (6% for RF3 task; 2% for RF24 task). The final sample size and the descriptive statistics for each task are presented in Table 2.

Regression analyses

Following Brock et al. (2007), gender, age, verbal ability and non-verbal ability were used as predictors to conduct simple linear regression on the dependent variables for the TD children. The resulting regres- sion equations were then used to generate predicted scores for the ASD children. Next, residuals were calculated by subtracting the expected score from the observed score for each child with an ASD. These residuals were then standardised by dividing by the standard error of the regression estimate. If ability on each of the tasks developed in line with these predictors, then the mean standardised scores for the ASD children should be zero.

The mean standardised threshold for children with an ASD for RF3 patterns was .00088 (SD = .0014), which is significantly above zero, t(31) = 3.68, p < .01, Cohen’s d = .65, consistent with them requiring a greater amplitude of distortion to dis- criminate an RF3 from a circle than TD children (Figure 2a). The mean standardised threshold for the ASD group for RF24 patterns did not differ signifi- cantly from zero, t(33) = .59, p = .56, Cohen’s d = ).06, indicating that there was no significant difference between the two groups on this task (Fig- ure 2b).1

To examine whether the higher thresholds for the ASD group on the RF3 task were the result of a developmental delay in the processing of these shapes, rate of change in RF3 thresholds as a func- tion of age was compared for the two groups. There was no significant group difference in slopes as a function of age for both the RF24, t(156) = .51, p = .61 (TD mean slope = )4.42 · 10)5, ASD mean slope = )1.05 · 10)4) and RF3, t(142) = .19, p = .85 (TD mean slope = )2.81 · 10)4, ASD mean slope = )4.07 · 10)4) tasks.

Discussion

The aim of the current study was to assess local and global ventral stream functioning in children with an ASD and TD children. Importantly, we found that, relative to TD children, children with an ASD obtained significantly higher RF3 shape discrimina- tion thresholds; they required a larger distortion of the RF3 pattern to be able to discern a difference between the RF shape and the circle, indicating a difficulty with global processing within the ventral visual stream. In contrast, there was no group dif- ference for thresholds on the RF24 task, indicating intact local processing in the ventral visual stream. Poor performance on the RF3 task cannot be attrib- uted to overall poor performance on psychophysical tasks, or to uniformly weak inputs from local pro- cesses to global processes. Rather, these data sug- gest that the early stages of visual form perception are intact for individuals with an ASD, but the mechanisms required to combine local signals into a global form percept function less effectively for this group. This interpretation is consistent with WCC theory which asserts that global processing is anomalous in this population (Frith, 1989; Happé & Booth, 2008). Discussion Post: Visual Dysfunction in Children

Table 2 RF pattern modulation (A in eqn 1) thresholds for the ASD and TD groups

Task Children with ASD Typically developing

RF3 N 32 114 Mean .0137 .0111 SD .0044 .0033 Range .0068–.0240 .0036–.0210

RF24 N 34 126 Mean .0029 .0026 SD .0009 .0006 Range .0018–.0046 .0014–.0034

1 These outcomes mirror those from more traditional

analytic methods which showed that ASD children had significantly higher thresholds on the RF3 task, F(1, 141) = 7.14, p < .01, Np

2 = .05, and did not differ from TD children on the RF24 task, F(1, 154) = .13, p = .72, Np

2 = .001, when the groups (which are matched for age and non-verbal ability) were compared using ANCOVA with gender and verbal ability as covariates.

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In addition to impaired global processing, WCC theory predicts superior, or at least intact, local processing abilities. In the current study, the ASD group demonstrated no difference in ability to dis- criminate an RF24 pattern from a circle compared to the TD group. At first glance, these results appear to conflict with outcomes from a recent study which assessed sensitivity to first- and second-order defined patterns in children with an ASD and IQ-matched TD children (Bertone et al., 2005). Bertone et al. reported that their ASD group displayed superior ability to identify orientation in first-order defined patterns, in conjunction with impaired ability on second-order defined patterns, relative to a comparison group. The first-order task used by Bertone et al. measured the minimally detectable contrast threshold required to identify orientation. Conversely, in our RF24 task, contrast is at supra-threshold levels and the threshold assesses the minimum amplitude required to perceive the curvature of the local elements that comprise the RF shape. Given that these two tasks are assessing different capabilities at the local level, patterns of performance could differ. It would, however, be informative for future research to compare the same samples of ASD and TD individuals on both tasks. Importantly, both studies report impaired process- ing on visual tasks that require further processing beyond the V1 visual area, despite second-order

processing potentially occurring earlier in the hier- archy (V2, Baker, 1999) than the global shape per- ception required for RF3 patterns (V4, Wilkinson et al., 2000). These findings are consistent with the idea that individuals with an ASD experience diffi- culty in the processing of ‘complex’ information (Bertone et al., 2005; Minshew et al., 1997) in that both second-order orientation identification and RF3 shape perception invoke more complex perceptual networks to integrate multiple stimulus elements than do tasks typically used to target the local pro- cessing capabilities of V1. Discussion Post: Visual Dysfunction in Children

Other studies assessing global processing in the ventral visual stream have not produced clear group differences when comparing individuals with an ASD to matched TD control groups. Milne et al. (2006) and Spencer et al. (2000) reported no differences in form coherence thresholds using a task that required detecting the presence of a global pattern revealed by giving small line segments an orientation appropri- ate for the global pattern. In contrast, Spencer and O’Brien (2006) and Tsermentseli, O’Brien, and Spencer (2008) reported higher thresholds for children with autism compared to TD controls (but only when children with Asperger’s disorder were excluded from analyses). The task they employed required detecting global form in Glass patterns (Glass, 1969) composed of aligned dot triplets as opposed to line segments. There are many differ- ences between these studies which might have an impact. Importantly, the response of cells in V1 are facilitated by horizontal connections when adjacent cells are firing (known as collinear facilitation; see Loffler, 2008, for a review). Li and Gilbert (2002, see also Field & Hayes, 2004) demonstrated similar processes occur when elements are combined in contour detection. Therefore, one possible factor contributing to the differences between these studies is that the solid lines in the line segment stimuli may enhance processing in these collinear facilitation networks, whereas the weaker collinear facilitation produced by having three aligned dots may be less effective in activating these networks. Consequently, it is possible that the contours in line segment stimuli are facilitated by lower-level processing, whereas Glass patterns specifically target high-level integrative processing in the ventral stream (Wilson & Wilkinson, 1998). If this is the case, then the results of these combined studies are consistent with those presently reported using RF patterns. Specifi- cally, no difference between ASD and TD groups is apparent on tasks assessing lower-level ventral stream processing, whereas tasks such as RF3 and Glass pattern tasks are sensitive to the global pro- cessing difficulties experienced by individuals with an ASD. This potential explanation for the different results for the studies requires further investigation directly comparing the different stimulus types. Discussion Post: Visual Dysfunction in Children

While there are reports of nonsignificant group dif- ferences for each level of processing in the literature,

(a)

(b)

0.016

T h

re sh

o ld

( A

) T

h re

sh o

ld (

A )

RF3

0.014

0.012

0.010

0.0035 RF24

0.0030

0.0025 TD ASD

TD ASD

Figure 2 Graphs showing mean thresholds on (a) the RF3 task and (b) the RF24 task for the typically devel- oping and ASD groups (lines show 95% confidence intervals). The 95% confidence intervals are smaller in the TD group owing to the substantially larger sample size

Shape perception in autism 721

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when significant differences have been reported, for local processing the differences have invariably been in the direction of superior performance for the ASD sample relative to neurotypical comparison groups (Mottron et al., 2006). Conversely, for global processing the differences have invariably been in the direction of impaired performance for the ASD sample relative to neurotypical comparison groups (Happé & Booth, 2008). Thus, on balance, the emerging empirical literature on visual processing in ASDs favours the WCC theory given its capacity to account for both global impairment and local superiority. While the normal performance for the ASD sample on the RF24 task in the current study is compatible with the predictions derived from EPF theory, the finding of impaired global processing for the ASD group on the RF3 task cannot be accounted for by EPF theory. Thus, the patterns of findings from the current study also favour the WCC theory. The EPF account has led to valuable research showing superiority of children with ASDs in local processing in several areas, including graphic construction, visual search and the perception of hierarchical figures (see Happé & Booth, 2008, for a summary). However, many of these tasks may be differentially influenced by local and global processing, and the trade-off between the two is often unclear. For example, local and global processing on the Navon (1977) hierarchical figures are associated with spatial frequency differences (Badcock, Whitworth, Badcock, & Lovegrove, 1990), resulting in the relative contri- bution of local and global processing to task performance being unclear (see also Dakin & Frith, 2005, for a discussion). The RF shapes used in the current study are not subject to this limitation in that they have identical spatial frequencies at the local and global levels. However, performance on low RF patterns (e.g., RF3) can still be attributed to global pooling, whereas performance on high RF patterns (e.g., RF24) can be accounted for simply with reference to probability summation of infor- mation from local elements (Bell et al., 2007). Discussion Post: Visual Dysfunction in Children

An alternative explanation for the current finding of lower sensitivity to RF3 shapes in ASDs might be that global pooling in higher cortical regions devel- ops later than local processing mechanisms. As a result, the ASD group may be developmentally delayed in their ability to process global contours compared to the TD group. If this were the case, then it would be expected that the TD group would show a greater improvement with age in RF3 thresholds than the ASD children. However, the results from the current study indicated that the ASD children developed at the same rate as the TD children on both the RF3 and RF24 tasks. Never- theless, it will be important for future research to investigate whether, in adulthood, ASD samples eventually attain thresholds for low RF tasks that are comparable to those of TD groups. Additionally,

with respect to the RF stimuli, it is becoming clear that global processing occurs in shapes up to approximately an RF of 10 whereas in shapes of higher RF, local processing appears to be the pre- dominant mechanism (Bell et al., 2009; Loffler et al., 1998). In typical observers, the transfer from global to local processing with increasing RF occurs quite rapidly. Given the evidence suggesting a bias for local processing in ASDs, it is possible that their transition point is different. It will therefore be important for future research to compare ASD and TD groups on a continuum of RF shapes with increasing RF numbers. Discussion Post: Visual Dysfunction in Children

To summarise, using a novel application of RF stimuli, we have demonstrated that global pro- cessing is impaired in the ventral visual stream in a reasonably large sample of individuals with an ASD. This adds substantially to the position that a deficit in global visual processing is present in this popu- lation. The results are consistent with more general problems in neural integration in ASDs, as evi- denced by the findings from EEG (Pei et al., In Press) and fMRI (e.g., Just, Cherkassky, Keller, & Minshew, 2004) studies. Finally, the application of regression analyses to determine the difference between predicted and observed scores in a clinical population provided an innovative means to avoid the issues usually associated with matching for age, verbal and non-verbal ability. These data provide an insight into the experience of visual perception in children with ASDs. Individuals with autism often describe exceptional visual perceptual abilities that allow them to identify changes or anomalies in their environment (e.g., Grandin, 1992; Stehli, 1991) not usually noticeable to TD individuals. According to Kanner (1943), this ‘inability to experience wholes without full attention to the constituent parts’ (p. 246) is a core component of autism, but these abilities may be maladaptive in so far as they may lead to distress at small changes in the environment (Happé & Frith, 2006). Additionally, a disturbance in the ability to combine visual information in eye gaze, facial expressions and face perception has been hypothesised to contribute to impairments in social communication seen in ASD (see Dawson, Webb, & McPartland, 2005, for a review). Thus, fundamental, early-emerging difficulties in inte- grating information to form a coherent, global per- cept, as demonstrated using the RF patterns, could account for some of the major behavioural mani- festations of ASDs. Discussion Post: Visual Dysfunction in Children

Acknowledgements

This research was supported by NH&MRC Project Grant 403942 awarded to M. T. Maybery, D. R. Badcock, J. C. Badcock and E. Pellicano.

We are grateful to Judith Cullity for her assistance in programming, Rachelle Fox, Lynsey Harborow,

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Dana Sidoruk and Kelly Scaramozzino for their assistance in data collection, and two anonymous reviewers who commented on an earlier draft of the manuscript. Finally, we are extremely indebted to all the children and families who gave their time to participate in this research. Discussion Post: Visual Dysfunction in Children

Correspondence to

Emma Grinter, School of Psychology, University of Western Australia, 35 Stirling Highway, Crawley 6009, Perth, WA, Australia; Tel: +618 64882479; Email: emmagrinter@graduate.uwa.edu.au

Key points

• The study introduces a new and simple shape stimulus, radial frequency (RF) patterns, to investigate visual functioning in children with autism spectrum disorders (ASDs)

• RF patterns can be manipulated in precise ways to probe local and global mechanisms involved in pro- cessing visual form.

• Children with ASDs require more shape modulation for RF patterns that target global form processing compared to typically developing children.

• Children with ASDs show intact ability to process RF patterns targeting local form processing. • RF patterns may provide a new, readily accepted clinical tool to examine visual function in ASDs.

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Manuscript accepted 30 October 2009

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