Working Memory and Repetition

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Researchers have found two complementary neural mechanisms responsible for tracking items in working memory.  The first system, associated with the prefrontal cortex, is active during the maintenance and identification of a target. This area shows item specific increased activation during target maintenance.  The second system, associated with the ventral temporal and parietal cortices, is active when viewing repeated stimuli.   This system decreases in activation when responding to trial specific repetition.  Thus, two different systems are modulated during working memory performance one for target maintenance, often reported to be impaired in schizophrenia, and one for stimulus repetition.

Our task used face stimuli that had been presented previously to each subject in a familiarity task and were used multiple times, i.e., a  target on one trial may have been a distracter in a previous trial.  Target decisions could not be made by familiarity alone. In addition to holding items on line this task involved constantly updating target and distracter information creating a high demand on temporal order and contextual processing.

We predicted reduced repetition effects for targets and intact repetition effects for distracters in people with schizophrenia, consistent with a  dorsolateral prefrontal cortex dysfunction.

Each memory trial contained 1 sample face to be remembered, followed by decision faces presented one at a time.  For each working memory series the target and one distracter could be repeated up to 4 times.  The remaining faces of the series were non-repeated distracters i.e., they were not repeated in that series and were therefore novel in that trial but were used in other trials. 

Although reaction time was slower in the people with schizophrenia, schizophrenia accuracy was extremely high and did not differ significantly from healthy controls suggesting intact working memory performance on this task.   The people with schizophrenia also showed normal repetition effects to target stimuli.  Dysfunctional working memory in schizophrenia is well documented, yet the reaction time of the people with schizophrenia decreased with target repetition similar to healthy controls suggesting normal target maintenance and enhancement across repetitions.  

We are currently collected functional imaging data on people with schizophrenia and healthy comparison subjects to see if the people with schizophrenia a) show similar activation of the prefrontal cortex during target maintenance and identification and b) show decreased activity in the ventral temporal and parietal cortices when viewing repeated stimuli as healthy comparison subjects.