Lateral intraparietal area activity as a temporal production signal during precise timing.

Abstract

We often perform movements without external cues telling us when to move. However, the way our brains time self-initiated movements is still unclear. For example, while temporal modulations in neuronal activity have been observed in a variety of timing tasks, it is not clear if these modulations are strictly related to the timing of movements or instead reflect timing measurements of external events such as sensory cues and rewards. To isolate the temporal production signals of movement initiation, we devised a self-timed task that requires non-human primates to saccade between two fixed targets at regular intervals in the absence of external cueing and without an immediate expectation of reward. To examine the potential neural basis of this temporally dependent behavior, we recorded from single neurons in the lateral intraparietal area (LIP), which has been implicated in the cognitive planning and execution of eye movements. In contrast to previous studies that observed a build-up of activity associated with the passage of time, we found that LIP activity decreased at a constant rate over the inter-saccadic interval. Moreover, this falling activity was found to be significantly predictive of inter-saccadic interval duration on an interval by interval basis. Interestingly, the relationship of this falling activity to the actual duration of the timed interval depended on eye movement direction: it was negatively correlated when the upcoming saccade was toward the neuron's response field, and positively correlated when the upcoming saccade was directed away from the response field. This suggests that LIP activity encodes timed movements in a push-pull manner by signaling for both saccade initiation towards one target and prolonged fixation for the other target. Thus timed movements in this task appear to reflect the competition between local populations of task relevant neurons, rather than a global timing signal. Additionally, microstimulation was delivered during separate experiments to determine if a causal relationship existed between LIP activity and motor production. Stimulation affected the animals perception of time in a manner consistent with the correlation results, suggesting that LIP activity provides a motor timing signal that is utilized in the initiation of precisely timed behaviors.