T&K-S are critical of assumptions of “Residual Normality,” that is, that “in the face of a selective developmental deficit . . . the rest of the system can develop normally” (sect. 3.1) and that Residual Normality is an assumption regarding “how development takes place.” A closer look at children with SLI shows that they are not selected directly from an underlying genotype. Instead, selectiv- ity of language impairment occurs despite appropriate marking of developmental milestones, that is, “in the context of normal de- velopment” (Bishop 1997b, p. 21). Residual Normality is, under these circumstances, a rational application of a species-specific developmental pattern, not a misguided assumption from cogni- tive models. The problem of tying a cognitive framework to de- velopmental disorders is really the problem of providing explana- tory power at multiple levels of analysis, not the use of cognitive models per se. Ultimately, the truth behind the process of devel- opment lies with exceptional children whose abilities defy expla- nation by simplistic models and challenge cognitive psychology to provide answers. Raising the bar for connectionist modeling of cognitive developmental disorders Morten H. Christiansen, a Christopher M. Conway, a and Michelle R. Ellefson b a Department of Psychology, Cornell University, Ithaca, NY 14853; b Department of Psychology, University of Warwick, Coventry CV4 7AL, United Kingdom. mhc27@cornell.edu cmc82@cornell.edu M.Ellefson@warwick.ac.uk http://cnl.psych.cornell.edu/people/chris.html http://www.psych.cornell.edu/people/Faculty/mhc27.html http://www.warwick.ac.uk/fac/sci/Psychology/staff/research.html#ME Abstract: Cognitive developmental disorders cannot be properly under- stood without due attention to the developmental process, and we com- mend the authors’ simulations in this regard. We note the contribution of these simulations to the nascent field of connectionist modeling of devel- opmental disorders and outline a set of criteria for assessing individual models in the hope of furthering future modeling efforts. Thomas & Karmiloff-Smith (T&K-S) make an important theoret- ical contribution to our understanding of cognitive developmen- tal disorders. We find their arguments regarding the problems as- sociated with the assumption of Residual Normality very com- pelling. In particular, we agree that the developmental process must be taken into account when considering the possible causes of cognitive developmental disorders. Connectionist modeling plays a crucial role in the arguments put forward by T&K-S. As highlighted in the target article, con- nectionists have recently begun to model various developmental disorders, including dyslexia (Brown 1997; Brown & Loosemore 1995; Harm & Seidenberg 1999), autism (Cohen 1998; O’Laugh- lin & Thagard 2000), selective language impairment (Hoeffner & McClelland 1993), Williams syndrome (WS) (Thomas & Karmi- loff-Smith, in press), mental retardation (Bray et al. 1997), and schizophrenia (Hoffman & McGlashan 1997). However, in a re- cent review of connectionist modeling of cognitive developmen- tal disorders, we found that most models suffer from a variety of shortcomings (Conway et al., in preparation). In this regard, we believe that the simulation approach taken by T&K-S provides a positive step forward. Using their simulations as a starting point, we discuss how connectionist modeling of cognitive developmen- tal disorders may be improved further. When modeling cognitive developmental disorders, one of the critical questions concerns how a particular deficit should be im- plemented. That is, how should the disordered startstate differ from the “normal” case? T&K-S explored three different ways of manipulating the startstate of their networks: removal of weights (lesioning), addition of noise, and changes in unit discriminability. They found that different types of startstate damage can result in very similar patterns of endstate performance. This is an impor- tant result from the viewpoint of connectionist modeling of cog- nitive developmental disorders. Many connectionists typically re- port only a single type of startstate damage (though they may have investigated others). But if a particular startstate manipulation is to have theoretical significance in terms of explaining a develop- mental deficit, it is crucial to establish that this manipulation, and no other, is the right causal factor. We therefore think that it is im- perative to follow the T&K-S example and explore several types of startstate damage. Of course, there is more to the modeling of developmental dis- orders than manipulating the startstate. It is also important that the model captures relevant aspects of behavior given reasonable input and a plausible learning task. Although existence-proof Commentary /Thomas & Karmiloff-Smith: Are developmental disorders like cases of adult brain damage? 752 BEHAVIORAL AND BRAIN SCIENCES (2002) 25:6 Figure 2 (Briscoe).