Temporal signals in the brain during visual perception
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The visual system is able to form relationships across a variety of timescales. These relationships could allow the temporal continuity of the retinal image and the underlying temporal structure of the world to serve as key cues in invariant object recognition (the ability of the visual system to recognize objects across a variety of angles, distances, and other conditions) as well as other visual processes at longer timescales. To utilize this temporal continuity and temporal structure the visual system needs a continuous temporal signal that spans multiple timescales and a computational mechanism for forming relationships across this temporal signal. Two studies (Chapters 2 and 3) showed evidence for a temporal signal that could be used in vision in the monkey brain. Time cells, neurons that fire at particular time intervals relative to a stimulus, could be a component of this temporal signal. Evidence of time cells was found through analysis of neural recording from monkey HPC and PFC during a memory task that requires the monkey to associate visual stimuli separated by about a second in time. After the first stimulus was presented, large numbers of units in both HPC and PFC fired in sequence. Many units fired only when a particular stimulus was presented at a particular time in the past. The temporal information of time cells might originate in another form of temporal coding: temporal context cells. Temporal context cells are neurons that quickly change in firing rate in response to a stimuli then slowly relax back to a baseline firing rate. Evidence of temporal context cells was found by analyzing the temporal responses of neural recordings from the entorhinal cortex of macaque monkeys as they viewed complex images. Many neurons in the entorhinal cortex were responsive to image onset, showing large deviations from baseline firing shortly after image onset but relaxing back to baseline at different rates. This range of relaxation rates allowed for the time since image onset to be decoded on the scale of seconds. Further, these neurons carried information about image content, suggesting that neurons in the entorhinal cortex carry information not only about when an event took place but also the identity of that event. Taken together, these findings suggest that the primate entorhinal cortex uses a spectrum of time constants to construct a temporal record of the past in support of episodic memory. A computational model was implemented that can construct and use this putative temporal record to form relationships across timescales. This model is supported by empirical results in visual experiments at timescales of saccades, seconds, and tens of seconds. At the saccadic timescale, this association across time could be relevant to forming invariant object representations.