The early onset ramp we found in the count of cells with significantly different contra versus ipsi memory trial firing rates (orange line, Figure 3C) is paralleled in Figures 4A and 4B by an early onset in population firing rate difference for contra versus ipsi memory trials. Similarly, the late onset ramp in Figure 3C for nonmemory trials is paralleled in Figures 4C and 4D. We then turned to analyzing error trials. The activity on error trials (shaded pink for ipsi-instructed but contra motion, and blue for contra-instructed but ipsi motion; Figures 4A–4D) showed that, on average across PLX3397 price the population, cells that fire more on correctly performed
contra-instructed trials also fire more on erroneously performed ipsi-instructed trials; that is, these cells fire more INCB28060 purchase on trials where the animal orients contralateral
to the recorded side, regardless of the instruction. Similarly, ipsi preferring cells fire more on trials where the animal orients ipsilaterally, regardless of the instruction. This indicates that the firing rates of FOF cells are better correlated with the subject’s future motor response than with the instructing sensory stimulus. We quantified this observation on a cell-by-cell basis by generating a side-selectivity index (SSI) for each neuron (see Experimental Procedures for details). Positive SSIs mean that a cell fired more on contra-instructed trials. Negative SSIs mean that a cell fired more on ipsi-instructed trials. Rolziracetam If cells encode the instruction we would expect SSIcorrect ≈ SSIerror. But if cells encode the direction of the motor response, then we would expect SSIcorrect ≈ −SSIerror. We first
calculated the SSI focusing on the delay period of memory trials. We found that, over neurons, SSIcorrect correlates negatively with SSIerror (r = −0.42, p < 10−4), confirming that on memory trials, the delay period firing rates of FOF neurons encode the orienting choice of the rat, not the instruction stimulus. We then repeated this calculation for firing rates over the movement period (from Go signal to 0.5 s after the Go signal), for both memory (SSIcorrect and SSIerror correlation r = −0.59, p < 10−8) and nonmemory (r = −0.78, p < 10−17) trials. These negative correlations indicate that the FOF is again encoding the motor choice of the rat. We summarized the observations from both the delay and movement periods by calculating the SSI for the entire period, from −1.5 s before to 0.5 s after the Go cue. This again resulted in negative SSIcorrect and SSIerror correlations for both memory (r = −0.49, p < 10−5) and nonmemory (r = −0.59, p < 10−8) trials (Figure 4E). Overall, then, the firing rates of FOF neurons encode the orienting choice of the rat, not the instruction stimulus.