offResponses in the Auditory Thalamus of the Guinea Pig

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Abstract
On and off auditory responses were examined in the medial geniculate body (MGB) of the guinea pig. Single- and multiunit recordings were carried out on 12 anesthetized animals, and noise-burst or pure-tone stimuli were applied to the ear contralateral to the recording hemisphere. One hundred and thirty-fiveoff or on-off neurons and 160 onneurons were studied, and the tuning curves of 21 on-off oroff neurons were examined from various nuclei of the MGB. The mean minimum threshold of the off responses (40.8 ± 20.0 dB SPL, mean ± SD; range: 0–80 dB SPL) was significantly higher than that of the on responses (28.5 ± 17.6 dB SPL, range: 0–60 dB SPL; n = 17, P < 0.001). Of 10 on-off neurons that showed identifiable tuning frequencies for both on andoff responses, 7 showed a higher off thanon best frequency (BF), 2 showed the same BF for bothon and off, and only 1 showed a slightly loweroff than on BF. Most off responses sampled from the borders of the ventral (MGv) and the rostromedial (MGrm) nuclei of the MGB showed single-peaked tuning curves, similar to those of the on responses in the MGv. The neurons located in the shell (MGs) and dorsal (MGd) nuclei of the MGB showed complicated—either multi-peaked or broad—tuning curves. Alloff responses showed long-duration-selectivity for acoustic stimuli: the mean half-maximum duration was 116.5 ± 114.8 ms ( n = 19, range: 27–411 ms). The latencies of 135off responses were studied in various divisions of the MGB. The ventral border region of MGv showed the shortest latency, followed by the dorsal border region of the MGv, the MGrm, and the caudomedial nucleus (MGcm) of the MGB. The posterior nucleus of the thalamus (Po), the MGd, and the MGs showed much longer mean latencies of >30 ms ( P < 0.05 compared with the border regions of the MGv, ANOVA), with Po showing the greatest mean latency of 60.3 ms and the greatest deviation of 25.5 ms). The latency of the offresponse (29.0 ± 14.0 ms, n = 135) was significantly greater than that of the on response (15.6 ± 9.6 ms, n = 160, P < 0.001). The present results provide valuable information about the threshold, frequency tuning characteristics, minimal response latency, and duration selectivity of off neurons in the auditory thalamus.