Compressed timeline of recent experience in monkey lPFC

Abstract

Cognitive theories suggest that working memory maintains not only the identity of recently-presented stimuli but also a sense of the elapsed time since the stimuli presentation. Previous studies of the neural underpinnings of working memory have focused on sustained firing, which can account for maintenance of the stimulus identity, but not for representation of the elapsed time. We analyzed single-unit recordings from the macaque lateral prefrontal cortex (lPFC) during performance of a delayed-match-to-category task. The sample stimulus triggered a consistent sequence of neurons, each neuron in the sequence firing during a circumscribed period of time. The sequences initiated by different sample stimuli were distinct but overlapping, with the degree of overlap reflecting the visual similarity of the stimuli that caused the sequences. These sequences of neurons encoded both stimulus identity and the elapsed time. The temporal code became less precise as the sample stimulus receded into the past. These findings suggest that working memory is maintained as a compressed timeline, consistent with longstanding cognitive theories of human memory.