Monkey prefrontal neurons during Sternberg task performance: full contents of working memory or most recent item?

To explore the brain mechanisms underlying multi-item working memory, we monitored the activity of neurons in the dorsolateral prefrontal cortex while macaque monkeys performed spatial and chromatic versions of a Sternberg working-memory task. Each trial required holding three sequentially presented samples in working memory so as to identify a subsequent probe matching one of them. The monkeys were able to recall all three samples at levels well above chance, exhibiting modest load and recency effects. Prefrontal neurons signaled the identity of each sample during the delay period immediately following its presentation. However, as each new sample was presented, the representation of antecedent samples became weak and shifted to an anomalous code. A linear classifier operating on the basis of population activity during the final delay period was able to perform at approximately the level of the monkeys on trials requiring recall of the third sample but showed a falloff in performance on trials requiring recall of the first or second sample much steeper than observed in the monkeys. We conclude that delay-period activity in the prefrontal cortex robustly represented only the most recent item. The monkeys apparently based performance of this classic working-memory task on some storage mechanism in addition to the prefrontal delay-period firing rate. Possibilities include delay-period activity in areas outside the prefrontal cortex and changes within the prefrontal cortex not manifest at the level of the firing rate.NEW & NOTEWORTHY It has long been thought that items held in working memory are encoded by delay-period activity in the dorsolateral prefrontal cortex. Here we describe evidence contrary to that view. In monkeys performing a serial multi-item working memory task, dorsolateral prefrontal neurons encode almost exclusively the identity of the sample presented most recently. Information about earlier samples must be encoded outside the prefrontal cortex or represented within the prefrontal cortex in a cryptic code.

Impairments in frontal cortical gamma synchrony and cognitive control in schizophrenia

A critical component of cognitive impairments in schizophrenia can be characterized as a disturbance in cognitive control, or the ability to guide and adjust cognitive processes and behavior flexibly in accordance with one's intentions and goals. Cognitive control impairments in schizophrenia are consistently linked to specific disturbances in prefrontal cortical functioning, but the underlying neurophysiologic mechanisms are not yet well characterized. Synchronous gamma-band oscillations have been associated with a wide range of perceptual and cognitive processes, raising the possibility that they may also help entrain prefrontal cortical circuits in the service of cognitive control processes. In the present study, we measured induced gamma-band activity during a task that reliably engages cognitive control processes in association with prefrontal cortical activations in imaging studies. We found that higher cognitive control demands were associated with increases in induced gamma-band activity in the prefrontal areas of healthy subjects but that control-related modulation of prefrontal gamma-band activity was absent in schizophrenia subjects. Disturbances in gamma-band activity in patients correlated with illness symptoms, and gamma-band activity correlated positively with performance in control subjects but not in schizophrenia patients. Our findings may provide a link between previously reported postmortem abnormalities in thalamofrontocortical circuitry and alterations in prefrontal activity observed in functional neuroimaging studies. They also suggest that deficits in frontal cortical gamma-band synchrony may contribute to the cognitive control impairments in schizophrenia.