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Acquisition, maintenance, and generalization of a categorization strategy by children with autism. Journal of Autism and Developmental. Disorders, 24(1), 39–51 ...
Research in Autism Spectrum Disorders 6 (2012) 733–740

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Research in Autism Spectrum Disorders Journal homepage: http://ees.elsevier.com/RASD/default.asp

Generalization, overselectivity, and discrimination in the autism phenotype: A review S.M. Brown *, J.M. Bebko York University, 4700 Keele St., Toronto, Ontario, Canada M3J 1P3

A R T I C L E I N F O

A B S T R A C T

Article history: Received 17 October 2011 Received in revised form 20 October 2011 Accepted 21 October 2011

Beginning with Kanner’s (1943) seminal article on autism, through the current DSM-IV-R criteria for the disorder, children have been described as having difficulty with seeing overall gestalts, due to excess attention to the constituent part. In current terms, children with autism have been found to process objects at the local level differently, which in some cases leads to their missing more global information or understanding. These local processing biases have been proposed to lead to overselectivity, enhanced discrimination, poor generalization, and poor categorization. There has been extensive research on these separate topics over the past 40 years. The current article provides a concise review and synthesis of key research findings from these areas. Problems with previous methodology and areas in need of further research are discussed. ß 2011 Elsevier Ltd. All rights reserved.

Keywords: Overselectivity Discrimination Categorization Generalization Autism Local processing

Contents 1. 2.

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Theoretical concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Weak central coherence theory . . . . . . . . . . . . . . . . . . . 2.1. Overselectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Overselectivy unique to autism? . . . . . . . . . . 2.2.1. Discrimination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Categorization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Generalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. 2.6. Overselectivity and discrimination in face recognition Previous problems with methodology. . . . . . . . . . . . . . 2.7. 2.8. Future directions and conclusions . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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* Corresponding author at: Clinical-Developmental Psychology, York University, 4700 Keele St., Toronto, Ontario, Canada M3J 1P3. Tel.: +1 416 650 8495; fax: +1 416 650 8495. E-mail address: [email protected] (S.M. Brown). 1750-9467/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.rasd.2011.10.012

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1. Background In 1943 Kanner presented a description of 11 children with a ‘‘combination of extreme autism, obsessiveness, stereotyp[ies], and echolalia’’ (229). Kanner‘s description of the children is thought to be one of the first comprehensive reports of children that would most likely be diagnosed with what is now referred to as autism. Indeed, the characteristics that Kanner reported as common to all of the children are similar to the hallmark characteristics of autism in the current Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR; American Psychiatric Association [APA], 2000). The DSMIV-TR characterizes autism as a qualitative impairment in social skills and communication skills, with particular stereotyped patterns of behaviour and interests. One of Kanner‘s observations, that the children have an ‘‘inability to experience wholes without full attention to the constituent parts’’ (220), is also echoed in the current DSM as ‘‘a persistent preoccupation with parts of objects.’’ (APA, 2000, p. 71). 2. Theoretical concepts 2.1. Weak central coherence theory This focus on the parts or local aspects of the environment is something that can be both a deficit and an asset for children with autism (Happe´ & Frith, 2006). Frith (1989) originally proposed that individuals with autism differ from their neurotypical counterparts in the central coherence of their cognitive system. Neurotypical individuals have a tendency to view their environment in a coherent way that integrates stimuli. It is this central coherence that is the propensity for individuals to view the environment and aspects of the environment as wholes, rather than parts, such as seeing a car rather than each individual shape and piece of the car, like the hood and tires. However, for individuals with autism, this tendency towards coherence is weak; instead, the hood and tires may be processed separately. The ability to form coherent wholes of information involves taking into account context and how objects relate to each other. Frith (1989) posited that in individuals with weak central coherence, there is a deficit in global meaning and form; they see each object as separate, unrelated parts. Since Frith‘s original hypothesis, the theory of WCC has been extended and revised as further research has accumulated. Happe´ (1999) proposed that WCC is more of a cognitive style, ranging from weak to strong in a normal population bellshaped curve. This WCC cognitive style has been found to be present in relatives of children with autism and to be related to the presence of autism-like behaviours, such as resistance to change, and specialized interests (Briskman, Happe´, & Frith, 2001; Happe´, Briskman, & Frith, 2001). The WCC theory was also extended to lower level perception. Children with autism are less susceptible to visual illusions in lower level visual tasks, such as the Ebbinghaus large and small circle illusions (Happe´, 1996). Thus, the children have a weak tendency towards coherence in both higher and lower level perceptual systems. Further, children with autism have been found to perform well on tasks that require more global processing and integration when their attention was directed towards the global information (Plaisted, Swettenham, & Rees, 1999; Plaisted, Saksida, Alcantara, & Weisblatt, 2003). Thus, it appears that individuals with autism have a local processing bias rather than the global processing deficit that Frith originally proposed. Therefore, the current version of the WCC proposes that individuals with autism display a preference for local processing in both lower and higher level perceptual tasks, but this is not at the expense of being able to process information globally when attention is directed to the global information. In addition, WCC is part of a continuum of cognitive styles. WCC does not explain all aspects of autism, but can be used in conjunction with executive functioning and Theory of Mind to explain many of the characteristics of autism (Happe´ & Frith, 2006). Other researchers have offered a contrary view to the WCC account of autism. For example, Plaisted (2001) proposed a perceptual hypothesis explaining that, at the perceptual level, children with autism have enhanced discrimination abilities, which leads them to perceive stimuli as highly dissimilar and for ‘‘small and seemingly irrelevant stimuli to be perceived as important and salient features’’ (p.163). This enhanced perceptual discrimination is thought to lead to poor categorization and poor generalization. Although different theories, both Plaisted’s and Frith’s views agree that children with autism differ from typically developing peers in their processing of stimuli. Furthermore, both of the authors’ theories indicate that children with autism process objects at the local level differently, which in some cases leads to their missing more global information or understanding. Therefore, children with autism may be preferentially attending to specific aspects of stimuli, which can lead to overselectivity, enhanced discrimination, poor generalization, and poor categorization. Research on each of these components will be reviewed. There has been extensive research on these topics over the past 40 years; therefore this review is not exhaustive, but rather highlights studies to address the key findings. 2.2. Overselectivity Overselectivity refers to attending ‘‘to only a part of a relevant cue, or even a minor, often irrelevant feature of the environment, without learning about the other relevant portions of the environment’’ (Lovaas, Koegel, & Schreibman, 1979, p. 1237). The term overselectivity was used by Lovaas, Schreibman, Koegel, and Rehm (1971) to define a pattern of responding displayed by children with autism and children with a developmental delay (DD). They taught children with

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autism, children with a DD, and typically developing (TD) children to respond to a compound stimulus composed of a visual, auditory, and tactile cue. When the cues were presented separately, the TD children responded to all three cues equally, whereas the DD children tended to respond to two of the cues, and the children with autism only responded to one of the cues. It appeared that both the children with autism and the DD children were overselecting and only associating responding with one or two of the cues while ignoring the others. Lovaas et al. (1979) reasoned that this overselective responding could be the result of difficulty responding to complex stimuli, or that the children were being ‘‘super efficient’’ in their responses by attending to only one cue in order to recognize when a response was required. However, the super-efficiency hypothesis is unlikely, given that in overselectivity tasks similar to Lovaas et al. (1971), children with autism have been found to continue to only respond to one of the cues for hundreds of trials even when this cue was no longer reinforced (Koegel & Schreibman, 1977). Therefore, the overselective response was not efficient, but was at times detrimental to receiving a desired reinforcement. Overselective responding by children with autism has been found not only across modalities, as in Lovaas et al. (1971) study, but also within visually complex stimuli (Fein, Tinder, & Waterhouse, 1979; Koegel & Wilhelm, 1973) and auditory complex stimuli (Reynolds, Newsom, & Lovaas, 1974). Further, within and between each modality there does not seem to be a constant local aspect of the stimuli towards which the majority of children with autism preferentially attend, making it difficult to predict which aspect of stimuli would be associated with a particular trained response. However, Kolko et al. (1980) did find evidence that children with autism tended to be overselective towards a particular type or aspect of a stimulus if they had previously displayed a preference for it, such as in a comparison of most-preferred reinforcers. 2.2.1. Overselectivy unique to autism? It has been questioned if overselectivity may not be specific to autism. Although Lovaas et al. (1971) found that children with autism were more overselective than the children with a DD, the sample size was very small and the age ranges of the participants were extremely uneven. More importantly, the two groups of children were not matched on developmental level or verbal mental age. There have also been some equivocal findings that individuals with autism may be more or less overselective than DD peers matched on mental age or nonverbal mental age, but these studies tend to have some methodological concerns. For example, Frankel, Simmons, Fichter, and Freeman (1984) found that not only were children with autism more overselective than the children with DDs, the amount of overselectivity was unrelated to IQ. However, only three children in the DD group actually completed the study; therefore their results may not be an accurate reflection of the DD population. When children with autism have been matched with controls on nonverbal mental age, no difference in the amount of overselectivity displayed by each group has been found (Schover & Newsom, 1976; Wilhelm & Lovaas, 1976). However, Matthews, Shute, and Rees (2001) reported that adults with autism displayed less overselectivity than mental aged-matched DD peers. Therefore, it may be that for individuals with developmental disabilities there is a developmental component to the relationship between overselectivity and mental age, but for individuals with autism the processing differences persists into adulthood regardless of developmental level. If overselectivity in individuals with autism is a function of their developmental level, rather than something unique to autism, it may be that other unique characteristics of autism, such as social difficulties, may influence the ability to learn not to overselect. Zeaman and House (1963) indicated that even though individuals with a DD initially had difficulty responding to all cues, when the individuals’ attention was directed towards the relevant cue, they quickly learned to respond to all cues and stopped displaying overselective behaviour. However, individuals with autism have difficulty with joint attention, which would interfere with their attention being directed towards all relevant aspects of a stimulus. This difficulty with joint attention would explain why overselectivity in autism has been related to adaptive and social measures (Kolko, Anderson, & Campbell, 1980). The poor adaptive scores would be found in both developmentally delayed individuals and individuals with autism; however, the social difficulties would be particular to autism. 2.3. Discrimination Although individuals with autism seem to have impaired performance on tasks that require attending to all aspects of complex stimuli, this focus on the local aspects of a stimuli may cause or be caused by enhanced discrimination skills in autism. Discrimination is the ability to perceived and respond to differences in stimuli (Fellows, 1968). There have been equivocal findings with discrimination research and autism. However, many studies have involved complex stimuli or an emphasis on language, and thus may not have been accurately assessing discrimination, but more likely assessing overselectivity and communication skills. For example, Hermelin and O‘Connor (1965) had individuals with autism and peers, matched on mental age, complete three different discrimination tasks: Kinesthetic and visuospatial discrimination, visual discrimination, and orientation discrimination. They found differing results for each task, with no consistent findings for the autism group, but concluded that the autism group had weaker discrimination skills than the developmentally delayed group. However, due to the complex nature of the stimuli and the verbal skills likely needed to understand the instructions, the results may not have been an accurate reflection of discrimination skills, but rather a reflection of those who were able to understand the task and attend to the correct local aspect of the stimuli. Similarly, Fein et al. (1979) used stimuli that consisted of line drawings of snowmen that varied in their characteristics, such as missing a head in one picture, or having the orientation of the arms change in another picture. The complexity of the stimuli and the

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idiosyncratic responding to different components of the stimuli by different participants suggests that, instead of measuring discrimination abilities, the researchers were measuring overselectivity biases. Conversely, when tasks involved little to no language ability, and the stimuli were simple, to control for overselectivity, researchers found that, compared to TD individuals, individuals with autism had enhanced discrimination skills for auditory (Mottron, Peretz, & Me´nard, 2000; O’Riordan & Passetti, 2006) and visual stimuli (Litrownik, McInnis, Wetzel-Pritchard, & Filipelli, 1978). The enhanced discrimination skills of individuals with autism found with simple visual stimuli, are evident in their ability to perform better than typically developing individuals on visual search tasks, which require individuals to find a target stimulus hidden among a display of other stimuli. In feature search tasks, the target stimulus is defined by one feature that is different from the distractor stimuli (such as searching for a green X among red Ss and green Ts). For both autism and typically developing individuals, the number of distractors does not affect the time it takes to find the target. However, in conjunctive search tasks, the target shares a feature with each of the distractors (such as a green X among green Ts and red Xs) and therefore as the amount of distractors increases, it takes longer to find the target (O‘Riordan, Plaisted, Driver, & Baron-Cohen, 2001). In conjunctive tasks, individuals with autism consistently found the target faster than the typically developing group and displayed less of an increase in reaction time as the display size became larger, and as the distractors became more similar to the target (such as searching for a red F among pink Fs and red Es) (O‘Riordan & Plaisted, 2001; O‘Riordan et al., 2001; O‘Riordan, 2004; Plaistad, O‘Riordan, & Baron-Cohen, 1998). Although testing methods using visual and auditory stimuli indicate increased discrimination skills, researchers have found conflicting results for colour discrimination. High functioning children with autism actually displayed less ability to discriminate between hue, saturation, and brightness than their nonverbal cognitive ability-matched and age-matched peers (Franklin, Sowden, Burley, Notman, & Alder, 2008). Moreover, in Matthews et al. (2001) study examining overselectivity, they found that when the background colour was varied, no effect on children with autism‘s responses was found. Researches have also been unable to find an enhanced discrimination for tactile stimuli. However, there have been fewer studies examining both of these topics. O’Riordan and Passetti (2006) attempted to examine tactile discrimination judgments in individuals with autism using different grades of sand paper and by pressing fibres with different diameters against the arms of the participants. There were no differences between autism and typically developing groups in either condition. However, there was no standardized pressure for the presentation of the fibres, but merely the researchers’ perception of pressure. In addition, there may not have been a small enough difference between the sandpapers for the children with autism to demonstrate their enhanced discrimination ability compared to typically developing children. 2.4. Categorization Plaisted (2001) argued that the enhanced discrimination abilities of children with autism would interfere with their ability to categorize. ‘‘Categorization is the process by which different entities are treated as equivalent’’ (Soulie`res, Mottron, Saumier, & Larochelle, 2007, p.482). In order to treat entities as equivalent, individuals must find similarities between the entities, but enhanced discrimination skills may make finding similarities difficult. Soulie`res et al. (2007) created two tasks: a discrimination and a categorization task, with 10 ovals that ranged from fat to thin along a continuum. Using a same-different task, the children with autism and the TD children displayed the same level of ability to discriminate between ovals on a continuum from wide to thin. However, the TD children were better able to discriminate between shapes in the middle of the continuum than they were at either end of the continuum. The children with autism did not display any differences in their ability to discriminate between the shapes anywhere along the continuum. The children were also asked to classify the shapes into two categories: thin and wide. Even though the children with autism were able to discriminate between the shapes, they demonstrated an inability to categorize them. Soulie`res and colleagues argued that the ability to classify the shapes may have enhanced the TD children‘s ability to discriminate in the middle of the continuum, where their categories of fat and thin ovals approach each other. Plaisted‘s hypothesis could be used to explain the Soulie`res and colleagues results, as the children with autism may have experienced each of the ovals as different enough from each other that they found insufficient similarities to categorize them. Problems with categorization have also been found in studies examining rule-based categorization. Klinger and Dawson (2001) found that when children with autism were given the rules of how to classify novel visual stimuli, they had no difficulty making categories. However, once the specific rules were not provided, they had difficulty categorizing similar novel stimuli and were unable to categorize the prototypes of each group. Klinger and Dawson asserted that based on their findings, children with autism rely on rules for learning and have difficulty forming prototypes to categorize new information. This difficulty with forming prototypes may be due to local processing biases. Thus, children with autism may see every stimulus or object as novel unless it is exactly the same as a previously viewed stimulus or object. The formation of prototypes may be hindered further by overselectivity and discrimination in children with autism who are also developmentally delayed. That is, they are not only paying attention to the small, possibly non-relevant aspects of objects in their environment, but they may also be discriminating between the smallest changes of these local aspects of objects. Bock (1994) suggested that when individuals with autism are taught categorization strategies, rather than categorization of just specific tasks, they will spontaneously generalize the strategies to other situations. Bock based this on findings on the categorizations strategies of 4 adolescents (12- to 16-years old) with autism, among whom 3 applied these strategies to novel categorization tasks 2 months later. The one child in Bock‘s study who did not categorize was reported to be

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‘‘untestable’’ for IQ and receptive vocabulary. As Bock did not indicate why this child was untestable nor their severity of autism, it is possible that this child had the lowest IQ and severe autism, whereas the other children may have had higher IQs and/or mild or moderate autism. If so, it may be that individuals with autism can be taught categorization strategies, but that individuals who are lower functioning may not generalize the strategies to other tasks. In a recent study on categorization skills, Goldstein and Bebko (2005) found similar results to Klinger and Dawson (2001), in that when moderate to high functioning children with autism (Mean IQ = 81) were asked to categorize arrays of pictures, they did not spontaneously use systematic groupings. But when shown a means to categorize them, they learned it quickly and maintained it. However, in contrast to the Bock (1994) findings, when then asked to generalize the categorization strategy, and spontaneously generate a new classification scheme for the stimuli, they had great difficulty. The equivocal results in studies of categorization suggest the need for additional research to clarify the limits of the varied findings, and their applicability to individuals at different levels of functioning. 2.5. Generalization Plaisted (2001) hypothesized that in addition to poor categorization, enhanced discrimination would also lead to less generalization of learning across situations. A behavioural definition of generalization is the ‘‘occurrence of relevant behaviour under different non-training conditions’’ (Stokes & Baer, 1977, p. 350). Generalization refers to applying learning in one situation to different situations. Generalization can be both near and far: if there is minimal change in the conditions that elicit a specific response it is referred to as near generalization; however, if the learning or response is used in conditions that differ substantially, then far generalization has occurred (Borokowski & Cavanaugh, 1979 as cited in Blackman & Lin, 1984). If children with autism see small, possibly irrelevant aspects of their environment as highly different, then it is unlikely that they will even display near generalization. Transfer of learning may occur, which refers to repeating a strategy or behaviour learned in one task to an identical task, where the materials may be different (Blackman & Lin, 1984); however, this may also be difficult if the children associate the response with only the original materials instead of broader environmental conditions or similar materials. Even though higher functioning children with autism may not display overselectivity, they have been found to display enhanced discrimination in some tasks, and therefore may still have problems with generalization. Plaisted, O‘Riordan, and Baron-Cohen (1998b) compared generalization skills of high functioning adults with autism to neurotypical adults in a lower level perceptual task. Participants were required to discriminate between spatial locations of beach-ball like figures. The adults with high functioning autism performed better on the novel presentations of the figures and the pre-exposure condition, whereas the neurotypical adults performed better on the exposure trials. The exposure trials had three of the figures in the same location as the pre-exposure trials; thus, the adults only had to attend to the other four figures in the display, thereby facilitating their responses on the exposure trials. The neurotypical adults generalized what they saw in the pre-exposure trials to the exposure trails; however, the high functioning autism group responded to every presentation as novel. In addition to difficulties with generalization in experimental conditions with small local perceptual elements, generalization involving larger and more global elements of children‘s environments is also found to be poor. For example, in order to learn and understand spoken language, individuals must attend to multiple cues (e.g. volume, pitch, intonations, facial expressions), which would be difficult if children are overselecting aspects of spoken language (Lovaas et al., 1979). Furthermore, enhanced discrimination skills may interfere with the development of spoken language, as children with autism have been found to continue to be able to differentiate among a wide range of morphemes, including ones not belonging to their native language (Happe´ & Frith, 2006). In order to learn to speak and understand their native spoken language, children stop differentiating among all known morphemes and continue to only differentiate among the morphemes of their native language, grouping these sounds to make words by categorizing sounds together and generalizing their meaning from one specific context to others. 2.6. Overselectivity and discrimination in face recognition Enhanced discrimination and difficulties with overselectivity, categorization, and generalization have important implications for understanding and processing the environment. Scherf, Behrmann, Minshew, and Luna (2008) compared high functioning children and adults with autism to typically developing children and adults on their ability to discriminate between faces, ‘‘greebles’’ (computer generated objects that are designed to act as controls for faces, as they have a similar number of parts and configuration), and everyday objects. The high functioning autism group had more difficulty recognizing and discriminating between individuals, the sex of human faces, and greebles. However, they did not have difficulty with everyday objects. These results could be explained by the overselectivity and discrimination studies. It is possible that the faces and the greebles were more complex, and overselectivity biases would have made it difficult for the autism group to focus on differentiating between the relevant stimuli. Conversely, the everyday objects were much simpler and less complex, which would not have been as likely to elicit overselectivity in the autism group. Local processing biases have also been found with high functioning individuals with autism when they are making specific types of judgements about faces. Identifying gender has been found to involve more global, configural processing. As predicted by both Plaisted‘s hypothesis and WCC theory, in this situation where attention to the global aspects is a task

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demand, high functioning individuals with autism can attend to the global aspects when making judgements about the gender of a face. However, identifying emotion appears to not demand the use of either local or global processing exclusively, and therefore, individuals with autism will revert to local processing. Thus, high functioning individuals with autism can use global processing when required for judging the gender of human faces, but display a preference for local processing when asked to identify emotions (Deruelle, Rondan, Salle-Collemiche, Bastard-Rosset, & Da Fonse´ca, 2008; Robel et al., 2004). 2.7. Previous problems with methodology There have been some methodological weaknesses in previous research examining discrimination and generalization. Many studies have failed to adequately control for overselectivity. For example, Fein et al. (1979) study was designed to test discrimination and generalization, but due to the complex nature of the stimuli, instead seemed to be examining overselectivity. Generalization, discrimination, and categorization skills are intertwined, and it is difficult to measure them separately, since enhanced discrimination appears to lead to poor categorization and both poor categorization and enhanced discrimination seem to contribute to poor generalization. In order to test discrimination alone, same-different tasks may be the most appropriate to use, as they rule out categorization difficulties, since the tasks only require a direct comparison between the two stimuli at the same time. However, testing generalization is a complicated endeavour. Many previous studies have trained children to respond to one stimulus, referred to as the S+, and then trained no response to another stimulus, often the opposite of the S+, which is referred to as the S . After the children learned to respond to the S+ and not respond to the S , an aspect of the stimulus is changed in order to identify the limits of how different the stimulus can be from the S+ before individuals stop responding. There are two major methodological problems with this procedure: response inhibition, and shifts away from the negative stimuli, S . Training a response to one stimulus and no response to a different stimulus could be confounded by problems with inhibition. Testing children by asking for a response or no response could actually be testing the children‘s ability to inhibit responses, rather than their generalization abilities. This problem of response inhibition has been rectified in some studies by teaching one response for the S+ and a different response to the S . However, when this S+ and S procedure has been used with gradients (stimuli that vary along a continuum) it has been found that neurotypical individuals will respond away from the S and even greater responses occur on the other side of the continuum past S+. This shift away from S was demonstrated in Derenne and Breitstein (2006) study using a gradient shift of faces, where the ratios between the tip of the nose and the chin and between the hairline and the chin ranged from small to large. Half of the participants were taught with the S as the small ratio end on the continuum and the S+ as the middle of the continuum, and the other half were taught with the S as the large ratio end of the continuum and again with the middle as the S+. Individuals who were taught that the small ratios were the S , responded to the middle (the S+) and far towards the large ratios as if they were still the S+. The opposite was found for the individuals taught that the large ratios were S-; these individuals tended to respond to the middle of the continuum and even beyond to the small ratios. This shift away from the S has only been tested in TD groups and has yet to be looked at with autism groups. As children with autism can categorize readily when given rules, it may be useful to provide explicit rules in this type of task by training the middle of a gradient as one type of response (S+), and the opposite ends of the continuum as different responses (S ). Another option to control for the shift away from S would be to teach children with autism to respond to the middle of the continuum as one response, one end of the gradient as a different response, and the opposite end of the gradient as a third response. However, teaching with such a structure would be more complex, and would require more extended practice that a two-response paradigm. 2.8. Future directions and conclusions Research examining discrimination and generalization has led to a greater understanding of how individuals with autism process their environment and optimize their learning. It is apparent that the interaction between overselectivity and enhanced discrimination could have particularly detrimental impacts on categorization skills and generalization of learning across environments and stimuli. However, there are a number of other, more specific variables that can also impact outcomes. Colour discrimination and generalization seem to be particular areas of difficulty to research and teach, as variations in hue, lightness, and saturation may all affect outcome. Easily labelled stimuli have been found to make responding in children above the age of 7- to 8-years old much more selective and less generalized (Svinicki, Meier, & Svinicki, 1976). Long intertrial intervals, as well as the intrusion of self-stimulatory behaviours during testing have been found to influence the accuracy of performance on discrimination tasks (Koegel & Covert, 1972; Koegel, Dunlap, & Dyer, 1980). Therefore, for optimum outcome, it would be beneficial to eliminate or prevent these extraneous variables and use short intertrial intervals. Although the areas of overselectivity, discrimination and generalization research have had long histories, recent models of autism, such as the WCC model, have stimulated new interest in these classic questions. Methodological and conceptual weaknesses point to the need for additional research with children with autism to further examine perceptual processing with more controlled tasks.

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