Temporal and rate code analysis of responses to low-frequency components in the bird's own song by song system neurons

Abstract

Auditory feedback (AF) plays a critical role in vocal learning. Previous studies in songbirds suggest that low-frequency (<~1 kHz) components may be salient cues in AF. We explored this with auditory stimuli including the bird's own song (BOS) and BOS variants with increased relative power at low frequencies (LBOS). We recorded single units from BOS-selective neurons in two forebrain nuclei (HVC and Area X) in anesthetized zebra finches. Song-evoked responses were analyzed based on both rate (spike counts) and temporal coding of spike trains. The BOS and LBOS tended to evoke similar spike-count responses in substantially overlapping populations of neurons in both HVC and Area X. Analysis of spike patterns demonstrated temporal coding information that discriminated among the BOS and LBOS stimuli significantly better than spike counts in the majority of HVC (94 %) and Area X (85 %) neurons. HVC neurons contained more and a broader range of temporal coding information to discriminate among the stimuli than Area X neurons. These results are consistent with a role of spike timing in coding differences in the spectral components of BOS in HVC and Area X neurons.

Keywords: Auditory feedback; HVC; Temporal coding; Zebra finch.

Fig. 1. Examples of low-frequency BOS (LBOS) stimuli

a. The cosine curves (see Materials and Methods) used to amplify the low-frequency ranged between 200 and 1000 Hz; k(f) is the amplification factor. The maximum amplification was 320 times (A=160) or 160 times (A=80) at 600 Hz. b. Examples of the power spectrums of the BOS and LBOS (A=80, 160) are shown. The total energy (the area under the curve) is constant for all three stimuli. c. An example spectrogram and waveforms for the BOS (top), LBOS_A80 (middle), and LBOS_A160 (bottom) are shown. The same maximum value is used in the color map to graphically display the three spectrographs, emphasizing the relatively greater power at low frequencies for the LBOS and the relatively greater power at higher frequencies for the BOS.

Fig. 2. Examples of single cell responses to the BOS and LBOS₈₀ in HVC (a) and Area X (b)

Each panel shows (from the top to the bottom) the sound waveform, spectrogram, peristimulus histogram (PSTH), and raster plot of the spike trains. The HVC and Area X neurons have low and high spontaneous firing rates, respectively, and the HVC responses are more phasic than those of Area X neurons. The response profiles to the LBOS are similar to the BOS, especially for HVC neurons.

Fig. 3. Mean firing rates during presentation of the LBOS (FR_LBOS ) and BOS (FR_BOS)

For each point, the baseline firing rate of the cell was subtracted. The X-axis shows the response to the LBOS, and the Y-axis shows the response to the BOS. Red dots indicate cells with significantly higher FRs in response to the BOS than the LBOS, and black dots indicate cells with higher FR response to the LBOS than the BOS (one-sided t-test, p < 0.025). Blue circles indicate cells that did not show any significant difference in FR for the LBOS and BOS. Left: BOS vs. LBOS_A80, Right: BOS vs. LBOS_A160. a. HVC neurons (n = 52), b. Area X neurons (n = 30).

Fig. 4. d-prime values of the responses to BOS and BOS variants

a. The cumulative probability distributions of the d-prime value for the discriminability of the BOS compared to BOS variant stimuli (revBOS, Red; LBOS₈₀, Black; and LBOS₁₆₀, Blue) are shown. A positive d-prime value indicates that the BOS evoked a higher response rate than the BOS variant stimuli. Left: HVC, Right Area X. Notice the broader distribution of d-prime values for HVC compared with Area X. b. Scatter plots showing the d-prime value for each cell. Each circle corresponds to a neuron. The x-axis indicates the d-prime value for BOS compared to revBOS, while the y-axis indicates the d-prime value for BOS compared to LBOS (black, A=80, blue, A=160). Left: HVC, Right: Area X. Notice that in each nucleus, the distributions overlap.

Fig. 5. Examples of the mutual information as the function of q, the parameter for temporal precision

a. HVC, b. Area X. In each plot, the black line with circles indicates the information from the original (actual) data, and the red line with squares indicates the information from the shuffled data (see Materials and Methods). The mean and standard deviation (SD) of the shuffled data were estimated from 20 randomly shuffled data points. The error bar corresponds to ±2SD. The information from the original data reached its maximum (H_max) at q=q_max. The information from the spike count corresponds to the information at q=0 (H_count).

Fig. 6. Mutual information carried by HVC and Area X cells used to discriminate the three types of BOS

a. For each cell, the H_count (the mutual information from spike counts) and H_max (the maximum information) values were computed and plotted on the X and Y axes, respectively. Only cells with significant information (see Materials and Methods) were included (HVC, n=51, Area X, n=27). b. In the distribution of q_max , the temporal precision produces the maximum amount of information. C. Population mean (± SEM) of mutual information. Left: population means of the H_max for HVC and Area X. Right: population means of H_count for HVC and Area X. For each plot, a t-test was used to evaluate the significance of the differences in the mean values, and the p-value is shown if the difference was significant.