Previous evidence has indicated that the autistic brain uses a different balance of global and local cues for visual processing than that used by the non-autistic brain (e.g., Happé and Frith 2006
; Mottron et al. 2006
). This has, in turn led to speculation that individuals with autism would be less susceptible to visual illusions than are typically developing individuals. However, published evidence for this immunity to illusions is inconsistent, at best (Happé 1996
; Bölte et al. 2007
; but see also Ropar and Mitchell 1999
; and Hoy et al. 2004
). In the present study, we sought to determine whether a link between autism and illusion susceptibility could be more convincingly demonstrated by taking into account the extent to which individuals exhibit various autistic traits. The findings demonstrate a significant correlation between the autistic trait of systemizing and susceptibility to a subset of the tested illusions. In particular, systemizing was negatively correlated with a general illusion susceptibility factor that combined the susceptibilities to the rod-and-frame, Roelofs, Ponzo, Poggendorff and Zöllner illusions. When tested individually, the susceptibilities to four of the five illusions (with the exception of the Zöllner illusion) were also correlated with SQ scores, demonstrating that this relationship reflected a general phenomenon rather than one driven by the artifactual correlation of only a single illusion. Susceptibility to another set of illusions (induced motion and Müller-Lyer, the major components of susceptibility factor #2mod
) was not found to correlate with any of the tested autistic traits.
Baron-Cohen coined the term ‘systemizing’ to refer to the drive or tendency to analyze the variables of a system, so that one can predict and control the system’s behavior (Baron-Cohen et al. 2003
). Given that this tendency would require an attentional focus on the details of the system’s parameters, it is perhaps not surprising that systemizing would be correlated with illusion susceptibility, since a greater focus on the relevant
details of an illusory stimulus (for example, the rod of the rod-and-frame illusion), and an accompanying decrease in the processing of the illusion-causing context (i.e., the frame), would lead to a decreased susceptibility. In this way, systemizing seems very much related to the ideas of the WCC (Frith 1989
; Happé and Frith 2006
) and EPF (Mottron and Burack 2001
; Mottron et al. 2006
) theories of autism. Indeed, Baron-Cohen has theorized direct links between systemizing and WCC, noting that the “…attention to detail described by WCC may be one of the earliest manifestations of a strong drive toward systemizing, or vice versa, interest in systemizing may arise as a consequence of attention to detail,” (Baron-Cohen and Belmonte 2005
, p. 112). Happé and Frith (2006
, p. 19) propose a similar link, noting that “…some systems (e.g., bus route numbering) may be fathomable through ‘local’ coherence, provided that simple if-then rules operate without context-dependent effects. Mastery of such systems would not be counter to the coherence account.” Furthermore, Billington et al. (2008)
have recently demonstrated that SQ scores are correlated with a measure of local/global precedence using Navon figures, a finding that further suggests a relationship between systemizing and the underlying neural processes at the heart of the WCC and EPF theories.
Given the finding of a relationship between illusion susceptibility and SQ, it is somewhat surprising that there was no significant correlation between susceptibility and AQ or its subscales. In particular, the AQ subscale of ‘attention to detail’ would seem to measure at least one component of the systemizing tendency; indeed, ‘attention to detail’ was the AQ subscale that was most strongly correlated to SQ within our participants (rs = 0.322, p < 0.001; the only other subscale significantly correlated with SQ was ‘imagination’, rs = 0.119, p < 0.05). However, the relationship between ‘attention to detail’ and SQ did not also translate into a correlation between ‘attention to detail’ and susceptibility factor #1mod (rs = −0.055, p = 0.547). It is very likely that a contributing factor for this null finding is the fact that ‘attention to detail’ is assessed with only 10 of the 50 questions that comprise the AQ questionnaire, whereas systemizing is assessed with 40 questions within the SQ questionnaire. Thus, the ‘attention to detail’ subscale is undoubtedly less reliable than SQ as a measure of the systemizing trait.
The finding that illusion susceptibility is correlated with the autistic trait of systemizing echoes the results of Happé (1996)
and Bölte et al. (2007)
, who found that susceptibility in children with autism was somewhat less than that seen in a typically developing population. Given our findings, one might wonder why Ropar and Mitchell (1999
) and Hoy et al. (2004)
failed to find a difference in susceptibility in a comparison of similar groups. Our finding that the susceptibility to only a subset of illusions was associated with systemizing might lead to speculation that these other studies simply examined illusions that did not show this relationship. However, this was not the case, since the studies of Ropar and Mitchell (1999
) and Hoy et al. (2004)
did include two of the four illusions that were seen here to be correlated with SQ (including the Ponzo illusion, which had the largest effect size). In an earlier report, Brosnan et al. (2004)
speculated that the discrepancies seen in the findings of Happé (1996)
and Ropar and Mitchell (1999
) might have been due to differences in the participants’ method of responding, with Happé requiring a verbal, categorical judgment of the stimuli (e.g., “Are the two lines the same length or different lengths?”) and Ropar and Mitchell requiring participants to null the effects of the illusion by the method of adjustment. However, the current results, in which a relationship between SQ and illusion susceptibility was found using the method of adjustment, would seem to rule out differences in the method of responding as a possible explanation.
Other possible reasons for this discrepancy do exist, with the most striking difference between the studies of Ropar and Mitchell (1999
) and ours being the logic by which the paradigms were designed. In their first study of the issue, Ropar and Mitchell (1999
, experiment 1) carried out a straightforward examination of the illusion susceptibilities of three clinical populations (with diagnoses of autism, Asperger syndrome or moderate learning difficulties, and mean chronological/mental ages of 13/7, 14/14 and 10/6 years), plus three groups of typically developing controls (with mean chronological ages of 8, 11 and 17 years). Although this type of comparison is standard in the literature on developmental disorders, it is nonetheless subject to confounds brought about by the different ages of the participant groups, with illusion susceptibility known to change during development. In addition, differences in the extent to which various autistic traits and co-morbid disorders are exhibited by individuals in a clinical population could potentially obscure any real differences in the dependent variable being tested in a given study. In contrast, the present study tested a population of undergraduate college students by comparing individual differences in illusion susceptibility to a quantification of the extent to which these individuals possessed different autistic traits. As a result, this paradigm minimizes the possible confounds brought about by group differences in chronological age, mental age and co-morbidity, and has the added benefit of allowing the testing of a large number of easily-obtained participants.
A more mundane difference between the present study and those of Hoy et al. (2004)
and Ropar and Mitchell (1999
) lies in the statistical power of the analyses performed. The participants of Ropar and Mitchell were tested with far fewer trials (40, 64 and 24 trials in experiments 1 and 2 of Ropar and Mitchell 1999
, and 2001
, respectively) than the participants of the present study (200 trials for approximately half of the participants, and 80 trials for the remainder). The three experiments of Ropar and Mitchell also differed with the present study in the total number of participants, with 109 participants (13–23 participants in each of 5 experimental groups; Ropar and Mitchell 1999
, experiment 1), 99 participants (17–35 participants in each of 4 groups; Ropar and Mitchell 1999
, experiment 2) and 87 participants (11–20 participants in each of 5 groups; Ropar and Mitchell 2001
) tested. Hoy et al. (2004)
tested only 34 participants (17 participants in 2 groups). In contrast, the present study examined the relationship between illusion susceptibilities and autistic traits as continua in a more-uniform sample of 127 college students (or 321 participants for the more direct comparisons of SQ and susceptibilities to the Roelofs and rod-and-frame illusions).
It is generally assumed in the visual perception literature that visuospatial illusions are not all driven by the same underlying mechanism (Coren et al. 1976
; Coren and Porac 1987
; Prinzmetal and Beck 2001
), and indeed, some may be driven by mechanisms that impact wide ranging levels of visual processing (Dyde and Milner 2002
). Given this, it could also be assumed that the susceptibilities to different illusions would be more or less correlated to the various autistic traits. Indeed, the original work of Happé (1996)
found that although susceptibilities to a subset of illusions (Ponzo, Poggendorff, Ebbinghaus, Hering and Kanisza) were decreased in an autistic population, Müller-Lyer susceptibility was not. This pattern of findings fit well with those presented here, where only the Ponzo, Poggendorff, rod-and-frame, Roelofs, and Zöllner illusions were negatively correlated with systemizing (albeit with the Zöllner illusion only as part of susceptibility factor #1mod
). The recent work of Prinzmetal and colleagues (Prinzmetal and Beck 2001
; Shimamura and Prinzmetal 1999
) provides a strong clue as to the reason why susceptibility to these illusions might have co-varied in a way that caused them to group together within a single susceptibility factor. Prinzmetal and Beck (2001)
found that the effects of the Zöllner, Poggendorff, Ponzo and tilt-induction illusions (the latter related to the rod-and-frame effect) were magnified when observers made their judgments while the head and body were tilted 30° from vertical. In contrast, the Müller-Lyer and a size constancy illusion did not show this effect. These authors suggest that the affected illusions were all driven by context-related distortions of the observer’s perception of vertical and horizontal directions. In the present study, we found that the Roelofs effect co-varied with this group of tilt-induction illusions, suggesting that it also shares a common underlying mechanism. Previous demonstrations that the Roelofs effect involves a distortion of perceived straight-ahead (Dassonville and Bala 2004
; Dassonville et al. 2004
) suggests the need to broaden Prinzmetal and Beck’s “tilt-constancy theory of visual illusions” (Prinzmetal and Beck 2001
) to include other illusions that are manifest by context-related distortions of the observer’s spatial reference frame but are not affected by tilt constancy, per se. Further work is needed to understand why susceptibility to these illusions, but not others, is related to the systemizing trait of autism.
One obvious limitation of the present study is that it does not include the direct examination of illusion susceptibility in an autistic population, and this restricts our ability to definitively state that individuals with autism are less susceptible than the typically developing population. However, inasmuch as it is believed that systemizing is an autistic trait that forms a continuum across the general population (with autistic individuals generally falling at the upper end of this continuum, Baron-Cohen et al. 2003
), the current study clearly demonstrates a link between the systemizing trait and susceptibility to at least a subset of visuospatial illusions. These findings generally support the idea that autism entails, at least in part, an atypical balance between the use of local and global cues in visuospatial perception. However, it remains to be determined whether this imbalance is manifest as an increased focus on local elements (as suggested by the theory of EPF, Mottron et al. 2006
), a decreased use of global cues (as suggested by the theory of WCC, Happé and Frith 2006
), or some combination of the two. Previous studies specifically designed to quantify the separate local and global influences in autistic perception typically used paradigms that would have been susceptible to the various confounds of comparing participants within clinical groups that are all too heterogeneous, which might explain why the results of those studies have been so inconsistent (see of Hoy et al. 2004
). It is probable that this research question and others like it would greatly benefit from the use of a continuous-trait methodology similar to the one used in the present study, with sensory and cognitive processing abilities and tendencies directly compared to the magnitude of specific autistic traits within individuals. To fully understand autism and the related disorders, it will undoubtedly be necessary to combine the strengths of various approaches.