First-episode schizophrenia patients neuropsychologically within the normal limits: evidence of deterioration in speed of processing.

In apparent contradiction to the notion of cognitive impairment as a core feature of schizophrenia, some studies have described a subgroup of patients neuropsychologically within normal limits. It remains to be determined whether this subgroup has intact cognitive functioning or a higher premorbid functioning that attenuates the evidence of deterioration. Out of a total of 111 patients with FES or schizophreniform disorder, 25 (23%) were classified as cognitive normal (CN) according to criteria based on performance in six basic cognitive dimensions and an overall composite score, and their cognitive profile was compared with that of 28 controls. The CN subgroup had better social premorbid adjustment and had a higher premorbid IQ than the cognitive impaired subgroup. There were no differences in the other pretreatment variables examined. The CN subgroup performed similarly to controls in the cognitive dimensions, including sustained attention, verbal memory and executive functions. These profiles remained mostly unaltered after controlling for premorbid IQ. The cognitive deterioration index, calculated by ratio of performance in general knowledge and vocabulary abilities to a measure of processing speed , showed that both patient subgroups had similar levels of deterioration and that this was significantly different to that of controls. Although FES patients performed within normal limits and better than cognitive impaired patients in a processing speed task, they did nevertheless display a pattern of deterioration in processing speed (in relation to their premorbid IQ) equivalent to that of those with marked impairments.