ABSTRACT
These investigations were based on three aspects of the acquired dyslexia of certain aphasic patients: (1) the inability to read orthographically regular nonwords, (2) semantic errors made in attempting to read single words, and (3) the selective inability to read low imageability words. The first study involved an analysis of patients’ attempts to repeat and to read nonwords, and also a lexical decision task in which nonwords varied in their homophony with real words. Patients were able to make lexical decisions adequately, but unlike normal controls, they showed no effect of nonword phonology. This implies that these patients are impaired in a nonlexical route to phonology, but that lexical access is possible via a visual address. The second set of studies, with normal subjects, demonstrates in a variety of tasks that even though a word is pattern-masked such that its presence cannot be detected, its meaning affects subsequent behavior. This implies a dissociation between semantic interpretation of a word and its availability as a response. Pattern-masking interferes not with the visual encoding leading to semantic interpretation, but rather with processes, including consciousness, necessary for response production; in doing so, it may parallel certain clinical phenomena. In the third set of studies, normal subjects’ report of single words presented tachistoscopically was suggestive of the patients’ behavior in that imageability affected left but not right visual hemifield presentations. This contrasts with the results of a further experiment not involving report, which examined the degree of associative facilitation on a lexical decision task within and across hemifields. The preceding “priming” words were of high and low imageability and were pattern-masked to prevent awareness. Neither imageability nor hemifield affected latency of the size of the associative priming effect. However, contralateral priming was less effective than ipsilateral. These results suggest that (a) lexical and semantic processing of words are carried out in both cerebral hemispheres, (b) the source of word-class 210effects is in production (the realization of semantics), and (c) it is this process that is laterally asymmetric. The relationships between normal and pathological systems are discussed.
