Chen, L. & Rogers, T.
University of Wisconsin-Madison
Patients with semantic impairments frequently show deficits in lexical tasks like word-recognition and reading aloud. This association has suggested to some that lexical and semantic processing are interdependent. Very occasionally, however, patients present with serious semantic impairment and normal lexical processing, raising the possibility that lexical and semantic processes are independent. To resolve the tension between these views, we hypothesize that individuals vary in the degree to which semantic and lexical representations automatically interact, and that the extent of their interaction depends on two factors: (a) the strength of an individual?s cognitive control, that is, her ability to potentiate different pathways in posterior cortex to suit the current task, and (b) the nature of her experience with different linguistic tasks. To see how these factors account for the range of neuropsychological data, we studied the behavior of a recurrent neural network model that learned distributed orthographic and semantic representations and the mappings between them. The model incorporated localist task representations that potentiated different subsets of units in the network, making it easier for them to respond to their inputs. In some tasks (e.g. reading, writing), both semantic and orthographic units were potentiated; in others (e.g. Scrabble, thinking), either orthographic or semantic units alone were potentiated. We orthogonally varied the strength of the potentiation and the frequency of different tasks during learning, then tested the model with a word-recognition task under simulated semantic lesions. When potentiation was strong, word-recognition remained near ceiling even with severe semantic lesions-lexical and semantic representations were effectively independent. With moderate or weak potentiation, word-recognition was always impaired and performance varied with the nature of the training experience. The model thus predicts that lexical processing should depend less on semantics in individuals with strong control abilities-a prediction subsequently confirmed in a dual-task interference study.