Disrupted representation in A1 of anoxic rats. (A) Cortical maps from a control rat (Upper) and an anoxic rat (Lower). Caudal is left and rostral is right. In control animals (n = 5), progressive changes in tonotopy were observed from caudal (low frequency) to rostral (high frequency). An increased number of nonresponsive and nonselectively responding sites (open circles) and a global reduction in sites responding to middle frequencies are evident in the A1 of PA rats (n = 5). (B) Distribution of CFs in the cortex of control and PA rats. Asterisks mark significant differences between control and PA rats (P < 0.05).

Operant behavior. (A) Schematic of the auditory signal-detection task. Rats placed in an operant behavior chamber enclosed within a sound-attenuating box wait for a 9-kHz, 300-ms tone pip (1). Possible trial outcomes included: (i) a nose poke before the tone was played (2a), scored as a false-positive; (ii) failure to respond after the tone was played within a 7-s reward time-window (2b and 3a), scored as a miss; (iii) response within the reward time-window (2b and 3b), scored as a hit. (B) Plots of the performance over time. The total numbers of hits, misses, and false-positives were collected during each session. Hits and false-positives were plotted as hit rate (Upper Left), corrected hit rate (Upper Right), and false-positive rate (Lower Left) vs. time, respectively. (Lower Right) The performance index (see Methods) was obtained by the corrected hit rate × the safe rate correction factor and plotted vs. time (behavioral sessions).

ABRs in normal control and anoxic rats. (A) ABRs were obtained by averaging the electroencephalographic signals to 1,000 presentations of an acoustic click. Threshold was defined as the lowest click intensity levels that elicited an evoked response. Representative recordings from a control rat (Left) and a PA rat (Right) are shown. The recordings resulted in the typical series of five identifiable waves (I-V shown at higher magnification in B). Although ABR peaks were of lower amplitude in PA rats, in both cases all wave peaks could be consistently observed in control and PA rats at 43 and 48 dB SPL. (C) Time intervals (ms) between peaks I and III (PI-PIII), III and V (PIII-PV), and I and V (PI-PV) were estimated and plotted. IPIs were significantly prolonged in anoxic animals (marked with asterisks). Values are means ± SD.

Tone pip-evoked unit responses in the auditory cortex. (A) Left show representative examples of TCs obtained from one control (Upper) and one PA rat (Lower). Right show their PSTHs obtained in response to test tone pips. A1 neurons were identified by short-latency responses (6-7 to 20 ms) and tonotopy. (B) Bar graph showing average time (ms) to PSTH onset, peak, and offset obtained from four control (gray) and eight PA rats (black). (C) Representative example of TCs obtained from adult control (a-c) and PA (d-f) rats. A1 neurons in control rats responded continuously and reliably to test tones with a clearly delineable area or tuning curve. By contrast, discontinuous responses (lack of reliable responses to some of the test tones) and recurring blank domains within the TC increased (d) or decreased (e) sensitivity, and broad TCs (e and f) were recorded in PA rats. (D) A1 RFs were significantly degraded in PA rats as measured with the RF irregularity index. In control rats, the average RF irregularity index was 2.41 ± 0.13 and 4.20 ± 0.08 (mean ± SEM) for PA rats. An irregularity index >4, commonly observed in PA rats, was never seen in controls. (E) Response thresholds for three frequency ranges obtained from four control (gray) and eight PA rats (black). Asterisks mark significant difference bands.