Reduced spiking in entorhinal cortex during the delay period of a cued spatial response task

Abstract

Intrinsic persistent spiking mechanisms in medial entorhinal cortex (mEC) neurons may play a role in active maintenance of working memory. However, electrophysiological studies of rat mEC units have primarily focused on spatial modulation. We sought evidence of differential spike rates in the mEC in rats trained on a T-maze, cued spatial delayed response task. Animals begin at the base of the T-maze where a 1-sec white noise and visual light cue are presented on the left or right side of the maze. Rats are rewarded for responding toward the cued direction. In correct trials, we observed decreased spike rates during the delay period, the time interval between cue presentation and reward delivery. Firing-rate histograms show significant decreases during the delay period compared to 5-sec windows from both pre-cue and post-reward periods. We analyzed how running speed and trajectory specificity correlated to spike rate. Twice as many cells were responsive to cue alone compared to running speed. Trajectory specificity did not relate significantly to firing rate. Decreased spike rate may reflect active maintenance in other structures inhibiting mEC. Alternately, the reduction may reflect decreases in background activity during enhanced attention and cholinergic modulation. Lastly, animals often ran through the T-maze choice-point with varying speed. We calculated the spatial posterior probability density from spike rates during these choice-point passes. Slow passes through the choice point were characterized by greater probability of decoding to the reward locations on correct trials compared to quick passes on the maze consistent with similar "look-ahead" properties previously reported in the hippocampus and ventral striatum.