Trial-to-trial variability of the prefrontal neurons reveals the nature of their engagement in a motion discrimination task

Abstract

During motion discrimination tasks, many prefrontal cortex (PFC) neurons are strongly modulated by the behavioral context, suggesting their involvement in sensory discriminations. Recent studies suggest that trial-to-trial variability of spiking activity characteristic of cortical neurons could be a source of information about the state of neurons and their participation in behavioral tasks. We tested this hypothesis by examining the variability of putative pyramidal PFC neurons, a likely source of top-down influences. The variability of these neurons was calculated as a ratio of spike count variance to its mean (fano factor, FF), while monkeys compared the directions of two moving stimuli, sample and test, separated by a delay. We found that the FF tracked consecutive components of the task, dropping rapidly with the onset of stimuli being discriminated and declining more slowly before each salient event of the trial: The sample, the test, and the response. These time-dependent signals were less consistent in direction selective neurons and were largely absent during passive fixation. Furthermore, neurons with test responses that reflected the remembered sample decreased their FF well before the test, revealing the predictive nature of response variability, an effect present only during the active task. The FF was also sensitive to behavioral performance, exhibiting different temporal dynamics on error trials. These changes did not depend on firing rates and were often the only metric correlated with task demands. Our results demonstrate that trial-to-trial variability provides a sensitive measure of the engagement of putative pyramidal PFC neurons in circuits subserving discrimination tasks. direction discrimination | direction selectivity | macaque monkey | working memory N eurons in the prefrontal cortex (PFC) are thought to play a key role in cognitive control and to exert top-down influences on sensory cortical regions with which they are reciprocally connected (1, 2). Recent recordings from the PFC during motion discrimination tasks revealed direction-selective (DS) responses that depended on behavioral context, shedding light on the nature of the top-down signals PFC is likely to provide (3, 4). These and other studies (e.g., ref. 5) demonstrating the involvement of the PFC in visual discrimination tasks based their conclusions on the analysis of average firing rates over repeated trial conditions. Indeed, the average rate of action potentials has proven to be a remarkably useful measure of neural activity that not only encodes a broad range of stimulus and response dimensions but also correlates with more cognitive aspects of behavior, such as memory storage, anticipation, reward, and decision.However, there is mounting evidence that the mean firing rate is not the only measure of how neurons are affected by external events. Recent theoretical and experimental studies provide evidence that trial-by-trial variability in activity characteristic of cortical neurons (6) may also be a source of information a...

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