Figure 1.
Illustration of cat auditory cortex, recording microelectrode, and primary auditory cortex activity.
(A) Schematic of cat auditory cortex organization highlighting the location of primary auditory cortex (A1). (B) Schematic of recording microelectrode. Filled circles show location of recording channels illustrated in panels C and D. (C) Peri-stimulus time histogram (PSTH) recorded in A1 during exposure to a 500-ms long upward FM sweep signal. Recording position is illustrated in panel B, (site #1). Asterisks indicate PSTH peak locations (n = 5). (D) PSTH recorded in A1 to a 500-ms long upward FM sweep signal. Location of recording in microelectrode array is illustrated in panel B, (site #11). Asterisks indicate PSTH peak locations (n = 4). Cross-correlation analyses between the PSTHs illustrated in panels C and D resulted in a cross-correlation index at time-lag zero of 0.60 and a difference in number of response peaks of 1.
Figure 2.
Time and frequency domain illustrations of acoustic signals.
(A) Illustration of an 8 kHz pure tone. The complete tonal stimulation set of pure tones was composed of 16 intensities and 129 frequencies. (B) Illustration of a white noise burst signal. The complete set of noise burst signals was composed of 25-, 50-, 100-, 250-, and 500-ms noise bursts. (C) Illustration of the first 10-ms of an upward frequency modulated (FM) sweep (1–32 kHz). Variations in duration were equivalent to the set of noise burst signals. Note that short noise bursts, FM sweeps, and pure tones share the same duration. (D) Illustration of the cat vocalization used. The duration of the vocalization was ∼0.87 sec with a rise and fall time of ∼0.2 and ∼0.5 sec, respectively. The average fundamental frequency was 570 Hz with a lowest component of 0.5 kHz and highest component of 5.2 kHz. Signals were presented at an intensity of 65 dB SPL. Note that neuronal responses to the time-reversed product of this signal were also investigated.
Figure 3.
Mean response similarity index (cross-correlation value at time-lag zero) between neuronal recordings within primary auditory cortex columns.
A–B. Response similarity indices during pure tone exposure. 25-ms (A), and 50-ms (B). C–G. Response similarity indices during white noise burst exposure. 25-ms (C), 50-ms (D) 100-ms (E), 250-ms (F), and 500-ms (G). H–L. Response similarity indices during upward FM sweep exposure. 25-ms (H), 50-ms (I) 100-ms (J), 250-ms (K), and 500-ms (L). M–N. Response similarity indices during con-specific vocalization. Forward (M), time-reversed (N). Note the progressive increase in dissimilarity between responses as a function of acoustic signal duration. Similarity indices varied between ∼0.05 and 1, with an index value of 1 corresponding to comparisons of identical responses (diagonal) and an index value of 0.05 representing the largest discrepancies in response profiles measured. Numbers in color table axes correspond to recording site numbers, see Fig. 1 for electrode site description.
Figure 4.
Mean difference in number of PSTH peak responses between neuronal recordings within primary auditory cortex columns.
A–B. Difference in number of PSTH peak responses during pure tone exposure. 25-ms (A), and 50-ms (B). C–G. Difference in number of PSTH peak responses during white noise burst exposure. 25-ms (C), 50-ms (D) 100-ms (E), 250-ms (F), and 500-ms (G). H–L. Difference in number of PSTH peak responses during upward FM sweep exposure. 25-ms (H), 50-ms (I) 100-ms (J), 250-ms (K), and 500-ms (L). M–N. Difference in number of PSTH peak responses during con-specific vocalization. Forward (M), time-reversed (N). Note the progressive increase in dissimilarity between responses as a function of acoustic signal duration during FM sweep exposure. Peak variability indices varied between 0 and 5, with an index value of 0 corresponding to no variations in the number of peak responses (diagonal) and a value of 5 representing the largest mean difference in peak response numbers measured. Numbers in color table axes correspond to recording site numbers, see Fig. 1 for electrode site description. NP: number of peaks.
Figure 5.
Distribution of neuronal response similarity levels within primary auditory cortex columns.
(A) Distribution of PSTH peak response incidence during pure tone, white noise, FM sweep, and con-specific vocalization signals. (B) Distribution of response profile similarity (cross-correlation value at time-lag zero) during pure tone, white noise, FM sweep, and con-specific vocalization signals. Note the increase in response dissimilarities as a function of acoustic signal duration. Horizontal lines in boxplots illustrate lower quartile, median, and upper quartile values. Whisker length shows limits of data distribution.
Figure 6.
Statistical significance comparisons of neuronal response irregularity within primary auditory cortex columns as a function of acoustic stimuli.
(A) Table of statistical comparisons in PSTH peak incidence across stimulus conditions. (B) Table of statistical comparisons in response similarity (cross-correlation values at time-lag zero) across stimulus conditions. Shaded squares illustrate locations of statistically significant comparisons p<0.05, ANOVA followed by Tukey-Kramer post-hoc corrections.
Figure 7.
Representative example of tuning distribution within cat A1 columns.
Characteristic frequency (CF) values are presented in Voronoi- tessellation form; colors identify tuning properties across twelve different cortical depths (rows). S, superior; V, ventral; A, anterior.
Figure 8.
Neuronal tuning within a primary auditory cortex column.
Acoustic receptive fields of neurons within the cortical track highlighted in Fig. 7. Measures of characteristic frequency (CF), defined as the tone frequency that evoked a reliable response at the lowest acoustic intensity level, were conducted by an experienced observer blind to stimulus conditions. Note that CFs remain constant irrespective of cortical depth and as such provide evidence that the electrode trajectory was orthogonal to the cortical tissue.
Figure 9.
Neuronal activation during pure tone exposure within a primary auditory cortex column.
Representative peri-stimulus time histograms (PSTHs) acquired during exposure to 25-ms (A) and 50-ms (B) pure tones across twelve cortical depth locations in primary auditory cortex. Diagram on right-hand side illustrates microelectrode array orientation with respect to cortical depth and PSTH displays. Note that irrespective of acoustic signal duration, PSTH profiles remain similar across cortical depth with a prominent single onset response. Representative acoustic signal type and duration are presented in gray. Location of recording track and corresponding receptive fields are illustrated in Fig. 7.
Figure 10.
Neuronal activation during noise burst exposure within a primary auditory cortex column. Representative peri-stimulus time histograms (PSTHs) acquired during exposure to 25-ms (A), 250-ms (B), and 500-ms (C) white noise bursts across twelve cortical depth locations in primary auditory cortex. Diagram on right-hand side illustrates microelectrode array orientation with respect to cortical depth and PSTH displays. Note that regardless of stimulus duration, PSTHs have comparable response profiles across the cortical thickness with a conspicuous single onset response. Representative acoustic signal type and duration are presented in gray. Location of recording track and corresponding receptive fields are presented in Fig. 7.
Figure 11.
Neuronal activation during frequency modulated sweep exposure within a primary auditory cortex column.
Representative peri-stimulus time histograms (PSTHs) acquired during exposure to 25-ms (A), 250-ms (B), and 500-ms (C) upward FM sweeps across twelve cortical depth locations in primary auditory cortex. Diagram on right-hand side illustrates microelectrode array orientation with respect to cortical depth and PSTH displays. Note the effect of signal duration on response profile irregularities across the cortical depth, with short acoustic stimuli resulting in regular single onset peak responses and long signals (500-msec) provoking irregular multi-peak responses. Representative start of upward FM sweeps is presented in gray. Location of recording track and corresponding receptive fields are presented in Fig. 7.
Figure 12.
Neuronal activation during con-specific vocalization exposure within a primary auditory cortex column.
Representative peri-stimulus time histograms (PSTHs) acquired during exposure to forward (A) and time-reversed (B) con-specific vocalizations across twelve cortical depth positions in primary auditory cortex. Diagram on right-hand side illustrates microelectrode array orientation with respect to cortical depth and PSTH displays. Note that irregularities in PSTH response profiles are present in both stimulus conditions. Representative acoustic signal type and duration are presented in gray. Location of recording track and corresponding receptive fields are showed in Fig. 7.
Figure 13.
Influence of acoustic features on incidence of response irregularities within primary auditory cortex columns.
In the proposed model, acoustic signals are divided into three classes: static simple (pure tones), static complex (white noise bursts), and dynamic complex (frequency modulated sweeps and con-specific vocalizations). Note that acoustic composition and duration influence the degree of response irregularity.