Mechanisms underlying memory impairment in schizophrenia

Background. The purpose of this experiment was to investigate mechanisms underlying commonly-observed verbal memory impairments in schizophrenia, and especially the hypothesized encoding deficit. Methods. A verbal memory task was administered to 38 patients with schizophrenia and 38 normal controls. Three functions involved in long-term memory – encoding, early phase of storage, retrieval – were investigated. First, non-organizable lists were compared to semantically-organizable lists in a free recall task, in order to vary encoding conditions. Superficial encoding (measured by a ‘sequence’ index) and deep encoding (measured by a categorization index) were assessed. Secondly, early storage was investigated by varying the delay between learning and recall. Lastly, cues were provided for organizable lists (semantic cues) and non-organizable lists (recognition sheet), in order to vary retrieval conditions. Results. An analysis of variance revealed an interaction between type of list (organizable, non-organizable) and group, showing that patients used organization less than controls. A further analysis showed that deep encoding was impaired. Also, although the propensity to use superficial encoding was unimpaired, its efficiency was less. The analysis of variance revealed no interaction with delay or with either type of cue. A correlation was found between deep processing and memory performance in both groups. Conclusions. A major deficit in encoding appeared in the patient group, with a lesser use of deep encoding and a lesser efficiency of superficial encoding. On the other hand, the early phase of storage and the retrieval function seemed unaffected. Overall memory performance appeared to be related to the depth of encoding.