Thus, OMI measures the relative PD-0332991 order change in firing rate during odor application compared to baseline conditions and ranges from −1 (complete suppression of activity) to +1 (strongly driven responses). Indeed, this analysis showed that photostimulation had a suppressive action on odor responses regardless of whether the firing rate of individual odor-cell pairs was increased (p < 0.001, n = 22) or decreased (p < 0.05, n = 18) by the odor alone (Wilcoxon signed-rank test; Figure 8F). We also asked whether there was any relationship between the effects of cortical activation on spontaneous and odor-evoked responses within individual cells. To address this, we calculated a light modulation

index (LMI) ((RLED – RControl)/(RLED + RControl), where RLED = average firing rate with photostimulation,

Afatinib RControl = average rate without photostimulation) to compare the relative effects of cortical activation on both spontaneous and odor-evoked firing for each odor-cell pair (LMI ranges from −1 for complete suppression of firing by photostimulation, to +1 indicating strong enhancement of the response). This analysis revealed little correlation (r = 0.5, Spearman’s correlation coefficient) between the effects of photostimulation on spontaneous activity and responses to odors within individual cells (Figure 8G). However, across the population of M/T cells, the effect of cortical activation on odor-modulated activity was significantly greater than that on spontaneous activity (p < 0.05, Wilcoxon signed-rank

test). Thus, the effect of cortical feedback on M/T cell activity is context-dependent such that cortical activity preferentially suppresses M/T cell responses during sensory stimulation. In additional recordings, we considered whether the cortical modulation of M/T cell activity was related to features of the sensory stimulus. We investigated whether the cortical suppression of M/T cell responses depended on odor identity by examining M/T single units tested with three different odors at matched concentrations SB-3CT (50 ppm; Figure S2). Across this cell population (n = 35 single units, nine mice), cortical activation significantly suppressed odor-evoked M/T cell activity (p < 0.001, Wilcoxon signed-rank test). However, the proportion of M/T cells in which odor responses were selectively modulated (suppression of responses to only one or two of the tested odors versus suppression of responses to all three odors) was not significant (Figure S2). Thus, under our conditions, the effects of cortical feedback on M/T cell responses were not highly specific to particular odors. We next asked whether the actions of cortical feedback on odor-evoked M/T cell responses depended on odor intensity by examining responses of cells (n = 30 single units, 12 mice) to the same odor at three different concentrations.