Neurograms depicting the onset response of rat A1 neurons to 20 English consonants. Multiunit data was collected from 445 recording sites in 11 anesthetized, experimentally naive adult rats. Average poststimulus time histograms (PSTH) derived from 20 repeats are ordered by the characteristic frequency (kHz) of each recording site (y axis). Time is represented on the x axis (−5 to 40 ms). The firing rate of each site is represented in grayscale, where black indicates 400 Hz. For comparison, the mean population PSTH evoked by each sound is plotted above the corresponding neurogram. For reference, ‘gad’ evoked the highest population firing rate of 288 Hz. As in Figure 1, rows differ in the place of articulation of each consonant, and columns differ in the manner of articulation. See Supplementary Figure 2 for a direct comparison between stop-consonant spectrograms and neurograms.

Behavioral discrimination of consonant sounds. Rats successfully discriminated 9 of 11 consonant pairs evaluated. Open bars, target sound for each go/no-go task; filled bars, nontarget sound. Error bars, s.e.m. across rats. Asterisks, significant discrimination (one-tailed, paired t-test, P < 0.05).

Predictions of consonant discrimination ability based on nearest-neighbor classifier. (a) Part 1, neural discrimination of each consonant pair using the 40-ms onset response with 1-ms bins. Part 2, neural discrimination of each consonant pair using the average firing rate over the first 40 ms of the onset response. (b) Part 3, neural discrimination using the entire 700-ms stimulus duration with 1-ms bins. Part 4, neural discrimination using the average rate over the 700-ms stimulus duration. Pairs that were tested behaviorally (Fig. 4) are outlined in black.

Spectrograms of each speech sound grouped by manner and place of articulation. Words with unvoiced initial consonants are underlined. Frequency is represented on the y axis (0–35 kHz) and time on the x axis (−50 to 700 ms). Speech sounds were shifted one octave higher to accommodate the rat hearing range. See Supplementary Figure 1 for a more detailed view of the first 40 ms of each spectrogram.

Predictions of consonant discrimination ability based on onset response similarity. The euclidean distance between the neurograms evoked by each consonant pair (Fig. 2) was computed using (1) spike timing with 1-ms bins or (2) average firing rate over a 40-ms bin (see Methods). Distance measures were normalized by the most distinct pair, such that dark red squares indicate pairs that are highly distinct and blue indicates pairs that are similar. Pairs that were tested behaviorally are outlined in black. (1) refers to the upper left half of the figure and (2) refers to the lower right.

Both average A1 responses and trial-by-trial neural discrimination predicted consonant discrimination ability when temporal information was maintained. (a) The normalized euclidean distance between neurogram onset patterns (Fig. 3, part 1) correlated with behavioral performance when action potential times were binned with 1-ms precision. (b) Distance did not correlate with behavior when a single 40-ms bin was used (Fig. 3, part 2). (c,d) The average neural discrimination correlated with behavioral discrimination when 1-ms bins were used (c), but not when a 40-ms bin was used (d). Neural discrimination was performed by a nearest-neighbor classifier (see Methods) using a single sweep of neural activity. The 11 consonant pairs are printed next to each data point. Error bars, s.e.m. for behavioral and neural discrimination performance. Solid lines, best linear fits when statistically significant. Analysis using other distance measures is shown in Supplementary Table 1 online. Classifier performance on each task as a function of tone characteristic frequency is shown in Supplementary Figure 6.

Neural discrimination using the onset activity pattern from individual multiunit sites was best correlated with behavior. (a) Percent of variance across the 11 behavioral tasks that was explained using individual single units, 16 single units, individual multiunits and sets of 16 multiunits. Each population size was evaluated with or without precise temporal information using the onset response or the entire response. Filled symbols, statistically significant correlations between neural and behavioral discrimination (Pearson’s correlation coefficient, P < 0.05). Regardless of population size, neural discrimination based on mean firing rate (that is, 40-or 700-ms bin) did not correlate with behavioral performance on the 11 tasks. (b) Mean neural discrimination on the 11 consonant-discrimination tasks using individual single units, 16 single units, individual multiunits and sets of 16 multiunits. Dashed line, average behavioral discrimination; dotted lines, s.e.m. for behavioral discrimination. Error bars, s.e.m. across the 11 tasks. The complete distributions of neural discrimination by single and multiunit activity for each task are provided in Supplementary Figure 8 online.