Double dissociations never license simple inferences about underlying brain organization, especially in developmental cases

Different developmental anomalies produce contrasting deficits in a single, integrated system. In a network that inflects regular and exception verbs correctly, a disproportionate deficit with exceptions occurs if connections are deleted, whereas a disproportionate deficit with regulars occurs when an auditory deficit impairs perception of the regular inflection. In general, contrasting deficits do not license the inference of underlying modularity. Thomas & Karmiloff-Smith (TK Shallice 1988). For example, a double dissociation between living things and artifacts in picture naming and property verification has been used to argue for separate modules for different semantic categories (Warrington & McCarthy 1987), and a double dissociation in reading abstract versus concrete words has been used to argue for separate modules for abstract versus concrete nouns (Warrington 1981). Closer to the example used by TK Pinker & Ullman 2002). In all these cases, connectionist/parallel-distributed processing models (Rumelhart et al. 1986) have provided alternatives to the standard interpretations of these double dissociations (Farah & McClelland 1991; Joanisse & Seidenberg 1999; Plaut 1995). In general, these models take the following form: A single integrated and interactive system is used for processing items of both categories. Because of item characteristics that covary with category membership, performance on items from one category depends more on one part of the system, whereas performance on items from the other category depends more on another part of the system. For example, Plaut (1995) suggested that concrete and abstract words may differ in the number of semantic features. He trained a network with feed-forward and recurrent connections to map both concrete and abstract words from orthography to semantics. The concrete words contained more semantic features, so they used the recurrent connections more effectively and were less dependent on the feed-forward connections. Lesioning the feed-forward connections produced a relative deficit for abstract words, whereas lesioning the recurrent connections produced a relative deficit for concrete words. Standard neuropsychological reasoning would interpret this double dissociation as evidence of separate modules for abstract and concrete words, but there is no such modularization. Similar problems arise in a developmental context. Different impairments imposed on the system at the beginning of training can differentially impact learning to perform correctly with items of different types. Again, standard neuropsychological reasoning would incorrectly imply that the underlying organization is modular. To illustrate this point, we revisited Simulation One from TK Leonard et al. 1992) that the English regular past-tense inflection may be weakly represented in the speech signal and therefore difficult to perceive for children with certain forms of language impairment (see also Tallal 1995). Specifically, the nonsyllabic forms of the English past tense (/t/ and /d/, as in “liked” and “loved”) involve very slight additions that agree in voicing with the preceding sound and can be very difficult to detect (Bird et al., in press), and the syllabic form (/^d/ as in “hated”‘) is unstressed. To simulate a deficit in perceiving these inflections, the units representing the past-tense inflection were sometimes set to 0 in the target past-tense pattern that the network is given as a model for what it should learn. This is based on the idea that children learn from what they hear and that the perceptual impairment makes the inflection sometimes imperceptible. This condition was otherwise identical to the intact condition. What we see in Table 1 is that in this new condition there is a disproportionate deficit in processing regular past tenses.2 Looking at Table 1, we clearly see a pattern of double dissociation. If we found two groups of children who exhibited the two patterns seen in the table and then employed standard neuropsyCommentary/Thomas & Karmiloff-Smith: Are developmental disorders like cases of adult brain damage? BEHAVIORAL AND BRAIN SCIENCES (2002) 25:6 763 chological reasoning, we would conclude that the normal brain contains separate systems for processing regular items and exceptions. This inference would be incorrect, however, because in this case we know that intact performance is generated by a single system that processes both regular and exceptional forms correctly. We simply have two different deficits that differentially impair learning to process the different types of items. A lesion that produces sparse connectivity reduces the ability of the network to become sensitive to particular combinations of input phonemes that must be considered simultaneously to inflect an exception correctly. An impairment that impacts perception reduces the network’s exposure to the information that indicates the correct pronunciation of the regular past tense. In summary, T&K-S have sounded an important note of caution, indicating that standard neuropsychological reasoning cannot be applied uncritically to the interpretation of patterns of deficits seen in developmental disorders. We hope we have underscored their point by noting that this caution is important in interpreting adult as well as developmental cases. Our simulation suggests that developmental “double dissociations” can be especially misleading because developmental disorders can produce contrasting differential deficits in a single integrated mechanism, but the application of standard neuropsychological reasoning would interpret this pattern as evidence for a two-part system.