Figure 5.

Sniff frequency affects MT cell response amplitudes and modulation. A, Responses histogram from two MT cells recorded during playback of sniff bouts at increasing frequencies from 1 to 5 Hz. Top trace, Tracheal pressure during playback. Bottom plots, Spike histograms generated from repeated playback trials (N = 24 and 23 trials). Odorant (isoamyl acetate) was presented for the duration of the trace. In the first MT cell, responses to individual sniffs at lower frequencies were consistent throughout a bout, but as sniff frequency increased, bursts attenuated in amplitude, and responses showed less modulation by sniffing. In the second (bottom) cell, responses showed little or no attenuation in amplitude and maintained strong modulation at high frequencies. B, Spike histogram from the second cell in A during playback of the natural sniffing sequence. This cell responds faithfully to each sniff during both low-frequency sniffing and the high-frequency sniff bout. C, D, PSTHs of two different MT cells derived from sniffs at frequencies of 1–5 Hz. Plots show the mean ± SEM of instantaneous firing rate at each frequency. The gray line is the preodor firing rate. Only responses to the third and subsequent sniffs of each bout are included in this analysis. For cell 1 (C), response amplitudes and rise times were graded by sniff frequency. For cell 2 (D), there was little attenuation with frequency, but onset latencies were longer at frequencies above 1 Hz. E, F, Effect of sniff frequency on peak firing rate (E) and number of spikes per sniff (F) across all recorded MT cells (N = 37). Plots show the mean ± SEM values at each frequency. Red plots show responses to the third and subsequent sniffs; black plots show responses to the first sniff in each bout. Measurements were normalized to the value at 1 Hz sniffing. On average, higher sniff frequencies resulted in lower peak firing rates and fewer spikes per sniff with no effect on first-sniff responses. G, SMI decreases with increased sniff frequency. See Materials and Methods for calculation of the SMI. SMI values could range from 0 to 1, with 0 indicating no modulation in firing rate by sniffing; the chance SMI (see Materials and Methods) is shown in gray. Across all cells, SMI values (red) decreased only slightly (but significantly) with sniff frequency; in contrast, SMI values for ORN inputs (green) decreased sharply as sniff frequency increased. H, Distribution of SMI values for the 37 MT cells at 5 Hz sniffing. Blue plot shows cumulative probability distribution; gray dotted line indicates chance SMI for 5 Hz sniffing. Most cells showed very little loss of modulation (SMI values near 1) and thus maintained patterning. The cells with the highest SMI values at 5 Hz were generally those with shorter durations and faster rise times (see Results).