The performance of the two monkeys on the direction accuracy task is plotted in . The average psychometric functions show that the performance of both monkeys decreased as the difference in direction between S1 and S2 decreased. However, Monkey 1 was less accurate, with direction difference thresholds 38% higher than those for Monkey 2 (Monkey 1, 50.6° ±1.9, Monkey 2, 35.5° ±1.1). This difference in performance between the two animals will be considered below in the context of the comparison effect analysis.
Of the 182 PFC neurons recorded while animals performed the direction discrimination task, 159 neurons showed task-related activity (91 cells from Monkey 1 and 68 cells from Monkey 2) and were included in the analysis. Recording sites for these neurons are provided in which shows that the majority of sites were concentrated in the pre-arcuate region (area 8ad & 8av) and around the posterior portion of the principal sulcus (9/46d and 9/46v), with the larger number of sites in its ventral region (area 9/46v)(Petrides, 2005
). The encounter rate of NS (22%, n=35) and BS (78%, n=124) neurons was uniformly distributed across recording locations.
BS putative pyramidal neurons are more active during the delay
In the analysis of delay activity we focused on the behavior of NS and BS neurons during the period in the trial where the monkeys both prepare for the upcoming comparison stimulus and maintain information about the direction of the preceding stimulus. The activity of two example neurons (one of each type) on trials where S1 and S2 are moving in the preferred (blue) and anti-preferred (red) directions is shown in . Following strong DS responses to motion during S1, both neurons showed an increase in activity with time in delay, but this increase was substantially more pronounced for the BS neuron (). Additionally, both neurons showed periods in the delay with significant differences in activity associated with the preferred and anti-preferred directions (solid colored bars along x-axis, Wilcoxon sign-rank test p<0.05). The NS cell () showed a single ~450ms period of DS dominated by the anti-preferred direction (red bar), i.e. opposite in sign relative to S1 selectivity. The BS neuron () also showed a transient period of DS, but it was dominated by the direction identified as preferred during S1 (blue bar) disappearing towards the end of delay. The activity patterns of these two types of cells are largely representative of the rest of the data presented here. Specifically, both cell types were equally likely to show DS responses to motion during S1 and the S2 and showed largely transient DS delay activity. However, as we will show below, BS neurons displayed more pronounced time-dependent delay modulation and were more likely to exhibit direction selectivity in their delay activity.
Activity of two example neurons during the direction comparison task
Periods of significant delay activity for all neurons were identified by comparing firing rates recorded during the delay to baseline activity recorded during fixation, 500ms prior to the onset of S1. Given that activity in PFC neurons often changes prior to salient events (Hussar and Pasternak, 2010
), the use of an earlier period during the fixation reduced the possibility of baseline activity being contaminated by these anticipatory rate changes. We performed a running significance test (p<0.05, Wilcoxon Signed Rank test) in 100ms non-overlapping windows stepped across the delay to identify neurons with significant baseline-deviated activity. The results of this analysis, shown in , illustrate a pronounced difference between the two cell groups, particularly later in the delay. Immediately after S1 offset, the two groups of neurons were equally likely to continue to fire above baseline levels (chi-squared test, p>0.05), likely due to S1 responses extending into the delay. During the first 750ms of the delay, the proportion of NS neurons decreased, reaching a plateau, with approximately 20% of cells showing significant baseline-deviated activity. Conversely, towards the end of the delay an increasing number of BS neurons began diverging from baseline levels. As a result, during the last 200ms of the delay there was a significant difference in the incidence of active neurons between the two groups (chi-squared test, p=0.03). This disproportionally high number of BS cells active prior to the onset of S2 suggests these neurons may play a role in preparation for the upcoming sensory stimulus. This possibility is supported by a similarly greater incidence of BS neurons active during the last 100ms of fixation leading to the onset of S1 (NS, 3%; BS, 25%, chi-squared, p=0.001). Such activity differences between the two cell groups were not observed during the S1 (NS = 58%, BS = 60%; p = 0.67, chi-squared test) or during S2 (NS = 66%, BS = 71%; chi-squared test, p = 0.54). Thus, BS neurons may indeed serve a general function in the preparation for sensory stimuli in PFC. This idea is supported by the recent observation of reduced trial-to-trial variability (Fano Factor) in these neurons prior to stimulus onset, another likely indicator of neuronal task engagement (Hussar and Pasternak, 2010
Broad-spiking neurons show anticipatory delay modulation
While this analysis revealed a higher incidence of active BS neurons during the delay, it provided no information about the time-course of this activity. In a recent paper we analyzed delay activity of BS neurons and found that some cells increased activity and some cells decreased their activity with time in delay (Hussar and Pasternak, 2010
). Because these patterns of ramping activity largely disappeared during the passive fixation task, the nature of this activity was anticipatory, reflecting task engagement. Here, we compared the patterns of delay activity observed in BS cells to those recorded in NS neurons. To quantify the magnitude and the sign of delay modulation we calculated a Delay Modulation Index (DMI), comparing activity between the middle and end of the delay (see Methods). This analysis identified cells with 3 types of activity for each cell type: neurons with increasing activity (DMI>0), decreasing activity (DMI<0), and cells with no significant modulation (DMI ~ 0). shows the distributions of this index for all BS (top) and NS neurons (bottom). The time-courses of activity (baseline subtracted) averaged for each of these three types of delay modulation are shown in . The two groups of BS neurons with significant time-dependent modulations, neurons with upward (n=50) and downward (n=23) changes in rates (broken and dotted lines) illustrate the nature of what appears to be preparatory modulation leading to the onset of the S2. In contrast, delay activity in the majority of NS cells showed no significant modulation (77%), and only a small proportion of cells (23%) showed upwards (n=5) or downwards (n=3) changes in rates with time in delay (). This difference in the incidence of delay modulation between NS and BS neurons was significant (chi-squared test; p=0.006) and suggests an active role for BS neurons in the preparation of the coming comparison process, shared by only a few NS neurons. Supporting this hypothesis is our observation that a significantly smaller percentage of BS neurons exhibited time-dependent delay activity during the passive fixation task (direction task, n=36/57, 63%; passive fixation, 11/57, 19%, chi-squared test, p=2.0x10−6
). The already small number of NS neurons with time-dependent modulation during the direction task became even smaller during passive fixation (direction task n=4/16=25%; passive fixation 1/16=6%). Considering the small number of neurons, it is not surprising that this difference did not reach significance (chi-squared test, p=0.14).
These results illustrate a striking difference in delay activity between the two groups of cells. The more numerous BS neurons were not only more active than NS cells during the delay, they were also more likely to change their activity in preparation for the upcoming comparison phase of the task.
BS neurons are more likely to represent S1 direction during the delay
We previously reported the presence of brief periods of stimulus-selective activity in the PFC during the delay in a similar task (Zaksas and Pasternak, 2006
) but did not examine the relationship of these signals to directional preferences during S1 or how this selectivity differed between cell types. In the current study, we were particularly interested in potential differences in the nature of stimulus-related delay activity represented by NS and BS neurons and whether this activity reflected direction-selective signals displayed by these cells in response to S1. To identify direction-selective activity we used ROC analysis to compare activity associated with the preferred and anti-preferred directions presented during S1 (see Methods). The results are presented separately for individual BS (top) and NS (bottom) neurons in . The plot shows DS activity for each neuron during the S1 and the delay as color-coded AROC values, with smaller and larger values represented by warmer (red) and cooler (blue) colors, respectively. In this analysis, the “preferred direction” was chosen on the basis of each neuron’s greater activity during the earliest significant DS epoch. Once identified for a given neuron, this label was maintained for the remainder of the trial. For example, a consistent direction preference during S1 and throughout the delay would be indicated by continuous blue line. Conversely, change in preference to the opposite direction between S1 and delay would be shown by switch from blue to red. The cells were sorted by the average absolute deviation from 0.5 during the task, with neurons showing greater selectivity during the delay near the top of each plot.
Direction selective delay activity
illustrates two key features of DS in the delay: its transient nature and inconsistent direction preferences. Significant periods of DS activity were short (), rarely extending beyond 500ms, and were similar between cell types (NS=245ms ± 31.1; BS=242ms ± 19.6; p=0.73, Wilcoxon signed-rank test). Although the durations of DS periods in NS and BS neurons were similar, they were substantially less common in NS cells, particularly in late delay. During S1 and in early delay, the incidence of DS cells in both groups was similar (, chi-squared, p>0.05). However, further into the delay, such signals became less common in NS neurons and largely disappeared near the time of S2 onset. This result is consistent with limited activity displayed by these neurons in late delay (see ). In contrast, the incidence of DS periods in BS neurons remained relatively constant at ~20–30%, a significantly greater percentage than NS cells (last 200ms of delay: BS, n=34, 29%; NS, n=3, 9%; chi-squared test, p = 0.011).
The second feature of delay activity is highlighted by the mixture of deep red and blue colors at the top of . This pattern is indicative of an absence of consistent preferences for the direction presented during S1, since individual neurons sometimes showed stronger activity following the direction preferred during S1 and at other times fired more following the opposite direction. We quantified this observation by directly examining the relationship between DS recorded during the S1 and during the delay (). This analysis included only neurons with significant DS during S1 (p<0.05) and was performed separately for the three consecutive 500ms delay periods: early (500–1000ms), middle (1000–1500ms), and late (1500–2000ms). In all three graphs, open symbols indicate cells with no reliable DS activity (AROC~0.5), while filled symbols indicate cells with significant DS activity (p<0.05). The filled data points in the upper half of each graph (AROC>0.5) indicate significant DS of the same sign as those recorded during S1. Filled data points in the lower half of the plot (shaded) indicate significant DS activity opposite to that during S1 (AROC<0.5) (see example in ). The bar plots on the right are summaries of these data, showing the proportions of cells preferring the same (upper columns) or opposite (lower columns) directions as during the S1 and cells with no DS delay activity (middle columns). During the first 500ms of the delay (Early delay), nearly 50% of all neurons showed DS activity that matched S1 direction, most likely reflecting S1 responses extending into the delay. The remaining neurons either showed no DS activity or a preference for the opposite direction. In the middle of the delay (Middle delay), the proportion of cells with DS matching S1 direction drastically decreased while the proportion of cells with no DS activity increased. Less than 20% of cells of both types showed DS activity, some matching the S1 and some dominated by the anti-preferred direction. At the end of the delay (Late delay), the majority of NS and BS cells carried no significant DS signals. Neurons that did display significant delay selectivity were equally likely to show a preference for the same (AROC>0.5) or opposite (AROC<0.5) direction as the S1. We should note that this analysis underestimates the proportion of cells carrying DS signals, since it excluded neurons that did not respond during S1 or responded but showed no significant DS activity. Thus, the overall proportion of cells with DS periods in the delay was higher (~25%, see ) than that shown in .
In summary, this analysis revealed that in individual neurons the representation of direction during the delay was largely independent of DS activity recorded during S1. While we found no differences in the duration of DS delay activity between the two cell types, this activity was much more likely to be carried by BS putative pyramidal neurons.
Transient direction selective delay signals depend on task demands
To determine whether the DS delay activity is utilized during the direction task, we examined whether this activity was still present during tasks not requiring direction discriminations. We used two additional tasks, neither of which required direction judgments: the speed discrimination task and the passive fixation task. During the speed task, the monkeys compared speeds of two stimuli moving either at the same or different speeds but always in the same direction. The task structure was identical to the direction task, with the exception of a unique fixation point (, bottom diagram). As during the direction task, task difficulty was manipulated by decreasing the differences between the two stimuli and measuring accuracy thresholds. Representative psychometric functions for the two animals show that both monkeys were engaged during the speed discrimination task and this engagement was comparable to that during the direction task (see ). We had sufficient data to compare DS activity of 112 neurons (NS, n=25; BS, n=87) recorded during the direction and the speed tasks.
show the comparison of DS delay activity during in the two tasks. The plots include both cell types, although the small proportion of NS cells (pink symbols) with significant DS during the delay (see ) precluded separate statistical evaluation of their effects. In the previous study, focused on the effects of task demands on DS responses, we found that DS responses decreased during the speed task and that this reduction was greater in NS neurons (Hussar and Pasternak, 2009
). This is illustrated in (S1 plot) showing pronounced decrease in the overall DS during the speed task (all cells, p=0.002; BS, p=0.087; NS, p=0.0018, Wilcoxon sign-rank test) and that this decrease was significantly greater for NS (p=0.01, Mann-Whitney U test). The scatterplots to the right extend this analysis to delay, identifying cells with DS activity during either of the two tasks. Average AROC calculated for all neurons of both types with significant selectivity in either task, shown in , illustrate that DS during S1 and the last two thirds of the delay was significantly lower when motion direction was irrelevant to the task. The data in the scatterplots () compare these effects on a cell-by-cell basis during the three portions of the delay. In early delay (700–1000ms), the overall effect was not significant (p=0.09; Wilcoxon sign-rank test), although when only cells with DS activity during the direction task (shown by circles) were examined, the drop in DS activity was highly significant (p=0.0004; Wilcoxon sign-rank test). With time in delay, the majority of cells showed less DS activity during the speed task (middle delay, 1000–1500ms, p=0.02; late delay, 1500–2000ms, p=0.02, Wilcoxon sign-rank test), displaying sensitivity of DS activity represented largely by BS cells to task demands.
Direction selective delay activity was affected by task demands
The drop in DS delay activity was even more pronounced during passive fixation, when the animals were shown S1 and S2 separated by a delay but were not required to make a response (, middle panel). We were able to directly compare the behavior of 73 neurons (NS, n=16; BS, n=57) recorded during both the passive fixation and the direction discrimination tasks (). Average DS activity recorded during S1 and the three delay periods in the two tasks, plotted in , shows that during passive fixation this activity was significantly weaker in all task epochs (S1, p=0.009; early, p=0.041; middle, p=0.042; late, p=0.049, Wilcoxon sign-rank test). Under these conditions, in contrast to the speed task, the loss of DS activity for both cell types was similar (p=0.477, Mann Whitney U test). The difference in DS on a cell-by-cell basis recorded during the two tasks is shown in .
The weakened DS delay activity of BS neurons during tasks not requiring retention of the preceding direction provides evidence for its role in sensory maintenance. Furthermore, these results further highlight the differential contribution of NS and BS neurons to different components of the sensory comparison task. While BS neurons showed only a small loss of DS in response to S1 when discriminating speeds (Hussar and Pasternak, 2009
), during the maintenance phase of the same task, these cells showed a significant loss of DS activity. On the other hand, delay activity became much less stimulus selective during the passive fixation task that did not require the animals to retain any information about S1. These results show that stimulus selectivity during the delay strongly depends on behavioral context, suggesting that it is likely to be utilized during the direction task. We should note that because of the small number NS with direction selective delay activity, these generalizations can only be applied to BS putative pyramidal neurons.
Responses during S2 reflect remembered direction
The data presented above showed that the information about the preceding stimulus was present in the form of transient distributed signals, likely to be utilized during the direction discrimination task. Our analysis also revealed that BS, rather than NS, neurons were more likely to carry these signals, giving the putative pyramidal neurons a distinct role in the maintenance of sensory information. The difference between the cell types did not hold for activity recorded during the comparison stimulus (S2), since responses to S2 of both cell types were modulated by the S1 direction. We characterized these effects by comparing responses to identical S2 stimuli on trials when they were preceded by S1 moving in the same direction (S-trials) or by S1 moving in a different direction, offset by 90° (D-trials), as shown in . The behavior of four example cells during S2, two NS (, top row) and two BS (, bottom row), illustrate the nature of response modulation we observed during the task: some cells showed stronger responses on S-trials (S>D, left plots) and some cells showed preferences for D-trials (D>S, right plots). Since these effects were equally likely to occur in NS and BS cells (NS = 39%, BS = 41%), for further analysis the data from both groups are combined. In our analysis, for cells with excitatory responses to S2 (N=66), higher firing rates on S- or D-trials were taken as an indication of a given cell’s trial preference, while for cells with suppressive responses to S2 (N=28), lower firing rates were indicative of the cell’s trial preference. We used ROC analysis to quantify the differences in responses during the two types of trials. The results for all neurons are shown in . In this analysis, AROCs >0.5 indicate stronger responses on S-trials (S>D, cooler colors) and values <0.5 indicate stronger responses on D-trials (D>S, indicated by warmer colors). The values ~0.5 (the green shades) represent cells with no reliable difference between responses during the two trial types (S=D). For display purposes the cells were grouped and sorted by the onset time of the effect (see Methods). An important feature of these results is that each type of effect, S>D and D>S, is carried by a distinct group of neurons. To further characterize the modulation of S2 responses by the preceding stimulus, all neurons were assigned into three subgroups on the basis of their ROC values: S>D (AROC ≥ 0.65; N=20, 21%), D>S (AROC ≤ 0.35; N=26, 28%) and S=D (N=48, 51%).
Comparison effects (CE) during S2
To directly compare the two types of effects (i.e. S>D and D>S), we converted AROCs in such a way that a value > 0.5 represented a greater response during that neuron’s preferred trial-type (either S-trials or D-trials). These data, shown in , illustrate both similarities and differences between S>D and D>S effects, which we will term “comparison” effects. Both effects were of similar strength and extended past the offset of the S2. However, the effect carried by the D>S group emerged significantly earlier (D>S, 250ms 34; S>D, 390ms 49; p=0.02; Mann-Whitney U test,), reaching its peak at 340ms after S2 onset. Cells in the S>D group reached their peak on average 300ms later (). These differences in onset times between the two types of effects cannot be attributed to differences in the time-course of their stimulus responses, since the two groups had similar latencies to S2 (S>D, 100ms 36; D>S cells, 130ms 24; p=0.32; Mann-Whitney U test). While both types of signals persisted after the offset of S2, the S>D effects persisted longer (; p<0.05; Mann-Whitney U test). Because in a small subset of cells with S>D effects emerged at different times after the offset of S2 (see ), the average AROC curve for the S>D group remained elevated throughout the post-S2 period until the monkeys reported their decision.
Comparison effects (CE) recorded during and after S2
The above analysis was limited to responses recorded on trials with the largest direction difference (90°), easily discriminable by both animals. However, during each recording session the animals were presented with a range of direction differences and their performance decreased as the two directions became more similar (see psychometric functions in ). We examined whether the magnitude of comparison effects paralleled behavioral performance and decreased as the two directions became more similar. This type of effect would provide compelling evidence in support of response modulations during S2 representing the sensory comparison. We tested this hypothesis by calculating comparison effects on trials with smaller differences in directions. The AROC calculated from trials of the maximal difference in direction (90°) was then subtracted from the AROC calculated for smaller differences in direction. (). In this metric, negative values indicate a decrease in the comparison effect at smaller differences in direction between S1 and S2. We found a significant relationship between the size of the comparison effect and the difference in direction, with activity during both types of trials becoming more similar with smaller differences in direction (p < 0.001, r2 = 0.419, Pearson’s correlation). This relationship held for both S>D (p < 0.001, r2 = 0.754, Pearson’s correlation) and D>S (p < 0.001, r2 = 0.613, Pearson’s correlation) cells. This scaling of the comparison effect with direction difference between S1 and S2 was similar in the two monkeys (Monkey 1 = broken line; Monkey 2 = solid line, ANOVA, p = 0.73). However, the more accurate Monkey 2 had a significantly larger proportion of cells carrying comparison effects (; 200–500ms, chi-squared test, p = 0.03). In addition to the lower incidence of cells with comparison effects, the less accurate animal also showed somewhat lower AROC values indicative of weaker comparison effects, although this difference did not reach significance (, Monkey 1, n=6 Monkey 2, n=20; p=0.13; Mann-Whitney U test). While this correlation between the incidence of cells with comparison effects and performance is compelling, it cannot be generalized since it is based on the comparison between two monkeys. Taken together, these results suggest that the two types of trials characteristic of our task were signaled by distinct neurons and these signals become smaller when the direction differences become smaller, suggesting that the observed modulation reflected the process of sensory comparison. The behavioral relevance of these signals is underscored by the correlation between the lower performance and weaker comparison effects. As we will show below, the close relationship between the comparison effects and behavior was also revealed by the analysis of activity associated with behavioral report.
To further examine the behavioral utility of comparison effects and whether these effects occur when animals are not actively engaged in the discrimination task, we compared activity in a subset of neurons during the direction task and the passive fixation task (, middle panel). This analysis was performed on a small subset of neurons with comparison effects (n=14) with a sufficient number of trials in both tasks. Because we had relatively a small number of neurons with a sufficient number of trials recorded in both tasks and cells with S>D and D>S modulation showed similar task effects, their data were combined. shows the relative response of an example neuron on S- and D-trials during the two tasks. This neuron showed a robust comparison effect during the direction task (left plot) that decreased substantially during the passive fixation task. The other PFC neurons also exhibited strong attenuation of the average comparison effect during passive fixation, as shown by the average comparison effects and the scatterplots for the neurons studied under the two behavioral conditions (). The data show that when monkeys were not required to actively engage in direction discrimination, comparison-related activity drastically decreased (p=0.008, Wilcoxon sign-ranked test). This result demonstrates that response modulation in the PFC recorded during the comparison phase of the discrimination task occurs largely when animals are actively engaged in sensory comparisons.
Attenuation of comparison effects during passive fixation
While stimulus selective delay activity was transient and did not show a consistent representation of S1 direction, its decrease during tasks not requiring direction discrimination suggests that this activity may be utilized. Since its utilization would be most advantageous during the comparison phase of the task, we asked whether the comparison effects identified in individual neurons depended on these cells also showing DS activity during the preceding delay. While 59% (n=27) of cells with comparison effects exhibited DS activity at any time during the delay, 41% (n=19) showed no significant DS delay activity at all. This result suggests that in individual neurons comparison effects do not necessarily require their own stimulus-selective delay activity and are more likely to rely on the information distributed among many neurons.
Comparison effects correlate with behavioral choice
The presence of signals reflecting similarities and differences between S1 and S2 raises the question whether these signals are utilized in the decision process. We addressed this question by first determining whether the activity that followed the onset of S2 was predictive of the behavioral report (choice probability, CP) and then examined the relationship between CP and the comparison effects.
We began by calculating CP (Britten et al., 1996
; Zaksas and Pasternak, 2006
) for all PFC neurons with a sufficient number of trials (n=84). This analysis was applied only to activity recorded on S-trials when S1 and S2 moved in the same direction in neurons with sufficient numbers of “same” and “different” reports (see Methods). The analysis revealed that more than half of the PFC neurons showed decision-related activity and identified two distinct groups of neurons associated with the behavioral report: neurons that fired more prior to “same” reports (right button, 26%, n=22), and neurons that fired more prior to “different“ reports (left button, 31%, n=26). The remaining neurons (43%, n=36) showed no differential decision-related activity. shows the average CP for neurons with higher activity prior to “same” (blue line) and “different” reports (red line) during S2 and post-S2 periods. CPs of cells associated with “different” reports reached their maximum during the S2, while CPs of cells signaling “same” reports reached their maximum later, after the offset of S2. shows the distributions of CPs contributing to each curve during three time points following the onset of S2. These distributions show that activity signaling “different” reports (shown in red) became significant early in the response (200–400ms, CP= 0.65, p= 0.0002; 600–800ms CP=0.59, p=3.6x10−4
; 1200–1400ms, CP=0.46, p=0.47; Mann-Whitney U-test) and disappeared about 500ms before the response. The distributions of CP for cells associated with “same” reports (shown in blue) became significant later, reaching their maximum during the post-S2 period and largely persisting into the period of behavioral report (200–400ms, CP=0.56; p=0.15; 600–800ms, CP=0.69, p=1.2x10−4
; 1200–1400ms, CP=0.60, p=0.045; Mann-Whitney U-test). Overall, these data show that decision-related activity appeared shortly after the onset of S2 and had a distinct time-course for each of the two types of reports, appearing shortly after the onset of the comparison stimulus and persisting until the behavioral report was made.
Activity during S2 predicts perceptual report
The two groups of neurons with complementary decision-related activity paralleled the two groups of neurons identified as carrying comparison effects (CE) (see ). This similarity prompted us to examine whether the two types of activity co-occur in the same neurons. This analysis, summarized in , revealed that 35% (n=29) of the PFC neurons carried both choice and comparison-related activity during S2 and/or post-S2 periods (CP&CE), 22% (n=18) showed only choice-related activity (CP), and the minority of cells, 13% (n=11), exhibited comparison effects but no choice-related signals (CE). We examined the relationship between the two types of activity co-occurring in the same neurons. plots the average comparison effect (CE) (solid line) and CP (broken line) computed for the same neurons. The data for S>D cells (blue lines, n=13) and D>S cells (red lines, n=16) are plotted separately. Although both groups showed average CPs comparable in strength to their CEs, choice-related activity reached its peak consistently later than did the comparison effects. compares the times of maximal effects for these two signals on a cell-by-cell basis. CPs for S>D neurons reached their average maximal effect 190ms later than the maximal comparison effects (, p=0.012, Wilcoxon Sign-Rank task). Similarly, D>S neurons showed their peak CP 145ms later than their maximal comparison effects (, p=0.006, Wilcoxon Sign-Rank task).
Comparison effects (CE) and choice-related (CP) signals
We also examined the strength and sign of these two types of signals within individual neurons. For this analysis we included all neurons carrying comparison effects during or after S2. We found a strong correlation between the strength of CE and CP (). Neurons preferring S-trials (S>D) also tended to fire more prior to “same” reports, while cells that preferred D-trials (D>S) also showed higher rates prior to “different” reports. A positive correlation between these two effects was observed during S2 (, S>D, p=0.009, r2 = 0.61; D>S, p=0.002, r2=0.58, Pearson’s correlation) and during the early post-S2 period (; S>D, p=1.2x10−4, r2 = 0.79; D>S: p=0.03, r2=0.44, Pearson’s correlation). In contrast, ROC values of cells showing no preference for trial-type (S=D) showed no significant correlation with CP (; p=0.978; r2= 0.004). This relationship supports the hypothesis that the PFC neurons with more reliable comparison effects are more strongly related to the animal’s perceptual report. Overall, the difference in the timing between the two types of effects, and the strong correlation between them, is consistent with the possibility that the information about similarities and differences between stimuli is likely to be utilized by decision-related circuitry.