Neuronal Activity in Rat Hippocampus and Secondary Somatosensory Cortex during a Tactile Working Memory Task

A wide variety of cognitive abilities are dependent on a functional working memory (WM) system. Many attempts have been made to understand its underlying mechanism and the areas that subserve it. In the setting of tactile working memory task, two noisy vibratory stimuli separated by a delay, were applied on rats whiskers and rats had to compare the σ of the Stim1 and Stim2 to make a two-forced choice decision (Fassihi, Akrami et al. 2014). More precisely, in order to solve the task, the rats needed to perceive σ1, keep its trace in memory during delay, perceive σ2, compare σ2 to the trace of σ1 and choose an action based on this comparison. Through multi-electrode recordings, we separately explored the activity of two brain areas, SII and hippocampus, to unravel their engagement across different epochs of the parametric working memory task. In rats performing the tactile WM task, a high percentage of SII neurons in our sample showed sensory coding of the stimulus during its presentation. This activity tended to encode the comparison rule late in the presentation of the second stimulus and during the post-stimulus delay, indicating that both Stim1 and Stim2 affected the neuronal firing at this epoch. In the hippocampus of rats, place coding was prevalent among the neurons, as expected by the cognitive map theory. In contrast to SII, in the hippocampal population sensory coding was not observed. However in the hippocampus, we identified neurons with choice-correlated activity during the post-stimulus delay and therefore both the σ1 and σ2 were factors affecting neuronal response. In conclusion, sensory coding was mainly observed in SII while choice related activity was observed in both areas.