In this chapter, we focus on the working memory deficit of schizophrenic patients, as assessed by delayed-response tasks. The relevance of delayed-response performance to schizophrenia has already been extensively documented by Goldman-Rakic (1987, 1991; see also Chapter 2, this volume). A typical delayed-response task involves the presentation of a stimulus, followed by a delay period and the subsequent presentation of a small set of alternative choices. Although the delayed-response task is a simple procedure, it involves a variety of cognitive functions. In order to succeed, the target stimulus or, to be more specific, the context-relevant attribute of the stimulus, must be encoded (e.g., spatial location). This information is maintained in working memory during the delay period. Then, a motor command must be successfully executed in order to elicit a response, which may involve voluntary control over the motor system. The delayed-response task is not dependent on recognition memory because at the response stage, there is no external cue in the response environment.
Deficits in delayed-response task performance remain one of the best-documented symptoms of prefrontal damage (e.g., Jacobsen, 1935; Kojima et al., 1982). Lesions in the principal sulcus impair performance on delayed-response tasks in monkeys (e.g., Blum, 1952; Goldman and Rosvold, 1970; Gross and Weiskrantz, 1964). Humans with dorsolateral prefrontal lesions show analogous deficits on tasks that require a delayed response (e.g., Oscar-Berman, 1975; Freedman and Oscar-Berman, 1986; Lewinsohn et al., 1972).