Such findings led to the enhanced discrimination-reduced generalisation (EDRG) theory of autism (Plaisted et al., 1998b) which successfully explains not only the superior results demonstrated in visual search paradigms but also the superior performance commonly seen by ASC individuals on embedded figures tasks (Jolliffe & Baron Cohen, 1997; Shah & Frith, 1983) and block design tests (Rumsey & Hamberger, 1988; Shah & Frith, 1993). Proponents of this theory suggest that there is hyper-sensitivity to differences between stimuli and a reduced perception of similarities in ASC (Plaisted, 2001). Plaisted (2001) draws on elemental theories of generalisation, which suggest that stimuli that share enough common features will be responded to comparably whereas stimuli that share fewer similarities will be treated as different (Estes, 1950). Plaisted (2001) argues that in ASC, similarities between objects and situations are not perceived and processed in this typical manner and as a result, minute details of difference take precedent over the commonalities, rendering even highly similar stimuli as different.
Plaisted (2001) offers that the ASC superiority on both the Embedded Figures Task and the Block Design Task is due to the EDRG theory rather than WCC. She points out that the target image in the embedded figures task (e.g. the triangle) comprises features that are in common with the whole and other features that are unique to the target. It is offered that target detection will be enhanced if the common features are processed poorly and the unique features are processed efficiently. For individuals who are hyper-sensitive to differences, such as those with ASC, this would cause a pop out effect of the target object and detection times would be faster. Similarly, processing unique features of the required block faces in the block design test and not being distracted by blocks whose features are similar to other block faces would provide an advantage to ASC individuals in the block design task.
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Furthermore, evidence of enhanced discrimination provided by the Plaisted et al. (1998b) study support the EDRG theory’s notion of hyper-sensitivity to differences. Discrimination of very similar stimuli in typically developing individuals is usually confounded by the drive to hold the commonalities that exist between the stimuli as most salient and the differences as negligible. It has been found therefore that fine discriminations of highly similar stimuli rely on exposure to the stimuli so that the unique features of each stimuli can be learnt and made more salient for subsequent discrimination purposes (McLaren, Kaye, & Mackintosh, 1989). Exposure is negated in ASC however since there is an automatic bias to process the differences between stimuli rather than the commonalities. Both typical perceptual learning (exposure to highly similar stimuli enables later discrimination) in the non-ASC group and a bias to process differences rather than commonalities in the ASC group was demonstrated in Plaisted et al.’s
(1998b) study. It was found that non-ASC adults better discriminated between pre-exposed highly similar stimuli than they did novel highly similar stimuli, reflecting that they had learnt the differences between highly similar stimuli in the pre-exposed condition. However, ASC adults showed no benefit of pre-exposure, performing just as well with novel stimuli as with pre- exposed stimuli and outperformed the non-ASC group on the novel discrimination task, demonstrating a propensity to attend to and process the differences across conditions. In a later study, O’Riordan and Plaisted (2001) also found that HF ASC children outperformed non- ASC matched children on conjunctive search tasks where the target and distractor similarity was high. Whereas performance in the non-ASC group was hindered when target-distracter similarity was increased, this did not apply to the ASC group of children. Superior performance for visual search has recently been observed in infants as young as nine months old, who went on to receive a diagnosis of ASC (Gliga et al., 2015).
In addition to visual discrimination, hyper discrimination abilities in ASC have also been found in the auditory modality. For example, Bonnel et al. (2003) found that ASC individuals demonstrated superior performance in discrimination of frequency of pure tones in both same- different tasks and in identification tasks.
If there is, as is suggested by the EDGR theory, hyper-sensitivity to differences and attenuated perception of commonalities, then the implications are clear; the relative inability to perceive similarities would result in diminished categorisation and a lack of information available to be taken from one situation and applied to another (Plaisted et al., 1998). Categorisation has been
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defined as ‘…the process by which different entities are treated as equivalent’’ (Soulieres et al., 2007, p.482) and as such relies on the processing of commonalities between stimuli.
As a consequence of diminished categorisation, the world, and in particular, the social world, would become highly complex and difficult to interpret (Klinger & Dawson, 2001). Research has demonstrated atypical generalisation and categorical perception (Soulieres et al., 2007) in ASC individuals and has therefore contributed to our understanding of the difficulties (including FER deficits and transfer of knowledge from one context to another) experienced by this population. For example, a reduced ability to categorise in ASC was demonstrated way before the advent of the WCC or EDRG theories of autism. Hermelin and O’Connor (1967) identified that ASC children could not rely on a chunking method in a memory task. Whereas developmentally delayed non-ASC children were able to chunk words together, which belonged to the same semantic category in order to aid recall, ASC children were not. Similar results of a categorisation deficit in ASC were yielded in a study by Tager-Flusberg (1991) who demonstrated that ASC children did not appear to gain from the facilitative effect of semantically similar nouns in a memory task.
In a more recent study, Soulieres et al. (2007) found that HF ASC children differed to non-ASC matched controls in both discrimination and categorisation tasks. Children were presented with ten ovals that differed in width along a continuum (narrow ovals at the beginning of the continuum gradually increasing in width to the end of the continuum). In the first task, participants were shown two ovals side by side and had to indicate whether they were the same or different. Although there were no differences overall in the ability to discriminate between the ovals, the ASC children were able to discriminate between ovals regardless of where they sat on the continuum whilst the non-ASC children demonstrated improved discrimination of ovals in the middle of the continuum. The researchers argued that ovals from the middle of the continuum sat on the boundaries of the narrow versus fat categories, which may have led to easier discriminations in the non-ASC group. It is suggested that the non-ASC children acquired a representation for each category when they were first exposed to each (narrow and fat) which facilitated their discriminations. The finding that discrimination in the ASC group was not facilitated by proximity to category boundaries was interpreted as evidence of either; discrimination being autonomous in the ASC group with no classification of stimuli (narrow versus fat) since they were not explicitly instructed to categorise stimuli, or if categories were
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created during the same-different task, diminished top-down influences rendered them irrelevant for the current task and therefore they did not affect discrimination performance. Although there were no differences in the categorisation task between the groups, this study revealed that HF ASC children do not apply categorical knowledge to facilitate discrimination processes and therefore points to attenuated top-down processes in ASC and enhanced autonomy of low-level perceptual processes. Although categorisation is possible, it is not used to inform low-level perceptual processes. These findings are in line with the EDRG theory of autism in that the ASC children may have perceived the ovals to be unique enough to discriminate without the need for classification to aid their responses.
In a study by Klinger and Dawson (2001) it was found that although ASC children are able to categorise stimuli, this is only the case when they are provided with explicit rules to do so. Results such as these suggest that in order for ASC to learn and form prototypes, rules are needed which will enable them to focus on commonalities between stimuli. Without these rules, ASC children do not spontaneously classify novel objects by grouping them together by commonalities but rather appear to perceive each and every stimulus as unique, influenced by their hyper-perception of differences, which prevents automatic categorisation. However, more recently, Soulieres and colleagues (2010) demonstrated that ASC individuals can learn to categorise provided they are exposed to the to be learnt materials for a sufficient amount of time since they appear to adopt a guessing strategy in the first instance before employing more efficient top-down processes. Results suggested that category learning is slower since it may not be automatically engaged and furthermore, when categorisation does occur, ASC individuals appear to use fewer dimensions to categorise.
Poor categorisation and thus reduced generalisation is also argued by the EDRG theory to account for the restricted interests prevalent in ASC. ASC individuals often have circumscribed interests and their object of fascination is all encompassing with rigid boundaries. They do not have a tendency to broaden this fascination and apply it to a similar prototype. For example, should a non-ASC child become interested in cars, they are likely to transfer their interest for one car make to another, broadening their interest to encompass other exemplars of the same category. In ASC however, interests are so circumscribed that an attentiveness to one make of car would not tend to translate to an interest in any other car makes.
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Although the WCC and EDRG theories differ in a number of respects, there are many overlaps. Both theories posit that the processing of stimuli differs in ASC relative to typically developing individuals. Collectively, these theories and the numerous empirical research findings, which serve to support their hypotheses, explain a wide range of ASC phenomenon such as enhanced discrimination, reduced generalisation and attenuated categorisation. Both theories ascribe such phenomenon as arising from a tendency to process their environment at a detail-focussed level, which can mean they often fail to see the bigger picture by integrating information into a meaningful coherent global gist.
The EDRG theory offers an explanation for the lack of generalisation of acquired knowledge to real life situations (e.g. FER) whilst allowing an insight into the sensory-perceptual anomalies characteristic of autism. It provides a window with which to view the ways in which ASC individuals perceive their world. This positive view of autism focuses on the superior perceptual abilities of those on the spectrum and as mentioned earlier, it is theories such as these which may allow for innovative interventions to be developed; interventions which capitalise on such strengths, take account of the differential ways in which autistic populations perceive their world and as such be designed to suit their individual learning style.