Memory and schizophrenia: differential link of processing speed and selective attention with two levels of encoding

The purpose of this study was to investigate how underlying cognitive deficits such as a defect in processing speed or in selective attention contributed to different types of memory impairment observed in schizophrenia (superficial vs deep encoding). 49 schizophrenic patients and 40 normal controls were administered a verbal memory task. Superficial encoding was assessed by the ability to recall items in their serial order. Deep encoding was assessed by the ability to organise words into semantic categories. Two measures of processing speed (Digit Symbol Substitution Test and Stroop colour time) and one measure of selective attention (Stroop test) were used. Regression analyses were carried out. In the patient group, processing speed contributed to both superficial and deep encoding, and to a global verbal memory score. Selective attention only contributed to the superficial encoding processes. Thus, slowing of processing speed in schizophrenia seems to be more crucial for memory performance, since it affects memory in a pervasive way.