Verbal working memory span is decreased in patients with schizophrenia, and this might contribute to impairment in higher cognitive functions as well as to the formation of certain clinical symptoms. Processing speed has been identified as a crucial factor in cognitive efficiency in this population. We tested the hypothesis that decreased processing speed underlies the verbal working memory deficit in patients and mediates the associations between working memory span and clinical symptoms.

Forty-nine schizophrenia inpatients recruited from units for chronic and acute patients, and forty-five healthy participants, were involved in the study. Verbal working memory span was assessed by means of the letter-number span. The Digit Copy test was used to assess motor speed, and the Digit Symbol Substitution Test to assess cognitive speed.

The working memory span was significantly impaired in patients (F(1,90) = 4.6, P < 0.05). However, the group difference was eliminated when either the motor or the cognitive speed measure was controlled (F(1,89) = 0.03, P = 0.86, and F(1,89) = 0.03, P = 0.88). In the patient group, working memory span was significantly correlated with negative symptoms (r = –0.52, P < 0.0001) and thought disorganisation (r = –0.34, P < 0.025) scores. Regression analyses showed that the association with negative symptoms was no longer significant when the motor speed measure was controlled (β = –0.12, P = 0.20), while the association with thought disorganisation was no longer significant when the cognitive speed measure was controlled (β = –0.10, P = 0.26).

Decrement in motor and cognitive speed plays a significant role in both the verbal working memory impairment observed in patients and the associations between verbal working memory impairment and clinical symptoms.