Single forebrain neurons represent interval timing and reward amount during response scheduling

Climbing activity, the gradual increase of neural discharge rate across a delay, has been suggested to play a crucial role in interval timing. However, most previous studies demonstrated climbing activity only in conjunction with tasks that involved a passive tracking of the passage of time, but that did not necessitate to actively time an event, e.g. a motor response. To demonstrate the significance of climbing activity for action timing, we trained pigeons in a self-control task requiring either immediate responding to a key after the onset of a light cue ('rapid-response' trials), or waiting for a fixed interval after cue presentation before responding to the key ('wait' trials). The cue also indicated whether a correctly timed response would be rewarded with a large or a small reward. Single-cell recordings in the Nidopallium caudolaterale, the avian prefrontal cortex, revealed that some neurons showed climbing activity between cue onset and response. Their increase in firing rate was flatter and reached the peak later in wait compared with rapid-response trials. An error analysis confirmed that, relative to correct responses, premature responses were accompanied by steeper, and tardy responses by flatter ramps. In addition, the climbing discharge pattern was modulated by the amount of the anticipated reward, suggesting that timing is an intrinsic property of neurons encoding other task-related information. These results demonstrate the behavioural and motivational significance of climbing activity in prospective information encoding. Our study supports a recent paradigm shift in our understanding of the vertebrate brain evolution, and it provides further evidence for the similarity between the mammalian cortex and the avian pallium.