James R. Ison

The submillisecond acuity for detecting rapid spatial and temporal fluctuations in acoustic stimuli observed in humans and laboratory animals depends in part on select groups of auditory neurons that preserve synchrony from the ears to the binaural nuclei in the brainstem. These fibers have specialized synapses and axons that use a low-threshold voltage-activated outward current, I_KL, conducted through Kv1 potassium ion channels. These are in turn coupled with HCN channels that express a mixed cation inward mixed current, I_H, to support precise synchronized firing. The behavioral evidence is that their respective Kcna1 or HCN1 genes are absent in adult mice; the results are weak startle reflexes, slow responding to noise offsets, and poor sound localization. The present behavioral experiments were motivated by an in vitro study reporting increased I_KL in an auditory nucleus in Kcna2^−/− mice lacking the Kv1.2 subunit, suggesting that Kcna2^−/− mice might perform better than Kcna2^+/+ mice. Because Kcna2^−/− mice have only a 17–18-day lifespan, we compared both preweanling Kcna2^−/− vs. Kcna2^+/+ mice and Kcna1^−/− vs. Kcna1^+/+ mice at P12-P17/18; then, the remaining mice were tested at P23/P25. Both null mutant strains had a stunted physique, but the Kcna1^−/− mice had severe behavioral deficits while those in Kcna2^−/− mice were relatively few and minor. The in vitro increase of I_KL could have resulted from Kv1.1 subunits substituting for Kv1.2 units and the loss of the inhibitory “managerial” effect of Kv1.2 on Kv1.1. However, any increased neuronal synchronicity that accompanies increased I_KL may not have been enough to affect behavior. All mice performed unusually well on the early spatial tests, but then, they fell towards adult levels. This unexpected effect may reflect a shift from summated independent monaural pathways to integrated binaural processing, as has been suggested for similar observations for human infants.

It has been proposed that the high temporal and spatial acuities of human listeners and animals tested in the hearing laboratory depend in part on the short time constants of auditory neurons that are able to preserve or sharpen the information conveyed in the timing of firing of auditory nerve fibers. We tested this hypothesis in a series of in vivo experiments, based on previous in vitro experiments showing that neuronal time constants are raised in brainstem slices when HCN1 channels are blocked or in slices obtained from Hcn1 ^−/− null mutant mice. We compared Hcn1 ^−/− and Hcn1 ^+/+ mice on auditory brainstem responses (ABRs) and behavioral measures. Those measures included temporal integration for acoustic startle responses (ASRs), ASR depression by noise offset, and ASR inhibition by gaps in noise and by shifts of a noise source along the azimuth as measures of temporal and spatial acuity. Hcn1 ^−/− mice had less sensitive ABR thresholds at 32 and 48 kHz. Their wavelet P1b was delayed, and wave 2 was absent in the 16 kHz/90 SPL waveform, indicating that groups of neurons early in the auditory pathways were delayed and fired asynchronously. Baseline ASR levels were lower in Hcn1 ^−/− mice, temporal integration was delayed, time constants for ASR depression by noise offset were higher, and their sensitivity to brief gaps and spatial acuity was diminished. HCN1 channels are also present in vestibular, cutaneous, digestive, and cardiac neurons that variously may contribute to the deficits in spatial acuity and possibly in ASR levels.

The strength of the acoustic startle reflex (ASR) as a function of age was studied in adult C57BL/6J and CBA/CaJ mice, because altered ASR levels are a potential behavioral consequence of the neural reorganization that accompanies the early-onset hearing loss of the C57BL, in contrast to the normal-hearing CBA. For C57BL mice at 14–36 weeks of age, compared with 7-week-old mice, high-frequency thresholds measured with the auditory brainstem response (ABR) were less sensitive by about 25–30 dB while the hearing loss at low frequencies was 10–15 dB, but by 60 weeks losses of 45–50 dB were present across the entire spectrum. Their ASR amplitudes for 16 kHz tone pips were highest at 7 weeks and then declined with age, but, for 4 kHz tones the ASR increased in strength at 18 weeks and beyond to levels above that of the younger mice. This hyperreactivity persisted even in 60-week-old mice. The ASR for 16 kHz stimuli was positively correlated with hearing sensitivity, but the ASR for 4 kHz stimuli was positively correlated with hearing loss for mice that were 18–36 weeks of age. Furthermore, ASR amplitudes for 4 kHz stimuli were positively correlated with the 16 kHz ASR in young C57BL mice but negatively correlated in older mice. There were no similar ASR or ABR changes in adult CBA mice through 19 weeks of age. Correlations between ASR and ABR scores were always weakly positive, and correlations between 4 kHz and 16 kHz ASR amplitudes were always strongly positive. The ASR data in older C57BL mice with hearing loss are consistent with reports describing their increased neural representation of low-frequency sounds and reinforce the value of this strain for studying the functional consequences that accompany age-related cochlear degeneration.

Temporal acuity for acoustic transients in rats with bilateral auditory cortex lesions (n = 6) was compared with that of sham-surgery control rats (n = 4), using a standard gap-detection method. A comparison of sensitivity to quiet gaps in noise and dark gaps in light tested for a cross-modal effect of the lesion. The groups were compared also in their sensitivity to noise offset, to noise increments, and to noise pulses presented in quiet. Stimulus detection was assessed with the startle reflex modification procedure, which uses changes in reflex expression caused by stimuli presented immediately before reflex elicitation as the objective evidence for their detection. There were no group differences in sensitivity to noise offset, noise pulses, or dark gaps in light. In contrast, the lesion reduced sensitivity to noise increments and eliminated gap detection. These deficits were maintained for 1 month and only partially recovered 2 months after surgery. The data indicate that the auditory cortex is critically important for temporal acuity in hearing, and suggest that its contribution to gap detection is to enhance the salience of noise increments at the end of the gap.

The persistence of afferent activity at stimulus offset was examined in mice as a function of age (experiment 1) and noise level (experiment 2), using a procedure devised by von Bekesy to study the physiological decay of afferent activity. Noise offset was presented from 1 to 10 ms (the temporal gap) prior to an acoustic startle stimulus, with an abrupt edge or a linear ramp having the duration of the gap. Noise offset inhibited the startle reflex, this effect declining with age. For young (2–3 months old) and “young-old” mice (18–19 months old), the abrupt offset was always more inhibitory than the ramp, even for the one millisecond gap. In “old-old” mice (24–26 months old) abrupt and ramped offsets were not different until the gap exceeded 4 ms. The behavioral difference between abrupt and ramped decay times did not depend on noise level in young mice (4–5 months old), though overall inhibition increased with level. These data indicate that temporal acuity as measured by this gap detection method is limited in very old mice by their reduced ability to follow the abrupt change in noise level at the beginning of the gap.

Previous studies of behavior and IC single units in the mouse support theoretical expectations that gaps with ramped trailing markers have reduced detectability compared to equivalent gaps with ramped leading markers. In experiment 1, detection probability and response speeds of humans listening for gaps in broadband noise were investigated by independently varying either leading marker fall-time (FT) or trailing marker rise-time (RT). Gaps with silent duration of 1, 4, or 12 ms were presented 2 s into a 3-s noise burst, with either abrupt marker onsets and offsets or linearly ramped RT/FT of 2, 4, or 8 ms durations. Addition of a nonzero RT or FT to the gap silent period increased detectability and also increased reaction speed on trials with “Yes” response, but there was no difference in detectability or response speeds between RT and FT conditions. Experiment 2 extended this finding to gaps having two, one, or no abrupt marker edges. These findings suggest that human listeners do not make use of abrupt onset or offset information to enhance gap detection, but seem to rely on the effective sound level reduction associated with the gap for detection.

Young rats were presented with light flash prepulses varying in duration from 1 to 128 ms, with light offset or light onset fixed at 70 ms prior to an acoustic startle stimulus (Experiment 1A), and, with single or paired 1-ms flashes, the 2nd (or only) flash given 100 to 500 ms before the startle, and 1 ms to 400 ms interflash intervals (Experiment 1B). Older rats (10 and 20 months old) received the same single and double flashes but with the maximum interflash interval extended to 1,500 ms (Experiment 2). Reflex inhibition increased with increased duration from 1 to 8 ms and decreased as light onset progressively exceeded 100 ms. Inhibition for both single and double flashes also declined for onset lead times beyond 100 ms, then increased for a double flash once the interflash interval exceeded 100 ms in young and middle-aged rats and 1,500 ms in the oldest rats. Peak inhibition was much reduced in the oldest rats at short lead times but was greater than that of younger rats at long lead times. These data suggest that aged rats process visual stimuli more slowly than younger rats and show poorer temporal acuity coupled with greater visual persistence.