If phase-locked cells are involved in generating the fast gamma oscillations associated with safety, then their activity should be inversely correlated with freezing on short-order time scales. Consistent with this idea, units that were phase locked to fast gamma showed higher firing rates during periods of decreased freezing. An example is shown in Figure 5D, depicting a unit that tended to fire more on trials when the freezing rate of the animal was lower. This relationship was significant for the sample of gamma phase-locked units (Figure 5E; p<.001, MLR), but not for non-phase-locked units (p>.05). Indeed, when we evenly divided trials into those when the animals froze the most versus those when animals froze the least, we saw a dramatic decrease in firing rate of fast gamma phase-locked units with increased freezing (Figure 5F; p<.05, sign-rank). This change was significantly different (p<.05, rank-sum) from non-phase-locked units, which were not significantly modulated by freezing (p>.05, sign-rank). Taken together, these data suggested that strongly phase-locked, doublet-firing units represent putative generators of the fast gamma oscillation.
