Perception of complex sounds depends on the encoding of the dynamic and static structures within the ongoing stimulus by the auditory system. Aging has been associated with deficits in both areas, thus, the difficulty that the elderly have in speech comprehension could due to hearing loss, or to a loss of temporal sensitivity, or some combination of both. We investigated the effects of sensorineural hearing loss (SNHL) on neural correlates of temporal resolution by recording the responses of inferior colliculus neurons to a gap detection paradigm. We used C57BL/6 (C57) strain of laboratory mouse, which carries the Ahl deafness gene that initiates a progressive high frequency SNHL beginning at about 2 months of age and rapidly progresses to total deafness by 18 months. We compared gap encoding from inferior collicular neurons from young, normal-hearing C57 mice and middle-aged, hearing-impaired, C57 mice, quantifying minimal gap threshold, and recovery functions. The proportion of unit types, spontaneous rates and degree of monotonicity were comparable between young and middle-aged C57 mice. As expected, single unit thresholds were elevated by 30-40 dB in middleaged C57 mice. However, no significant differences in mean minimal gap thresholds or in the slopes of the gap recovery functions were found between the two age groups. Thus, the results suggest that moderate high frequency SNHL does not affect temporal processing as measured by the gap detection paradigm.

This research focused on the response of neurons in the inferior colliculus of the unanesthetized mustached bat, Pteronotus parnelli, to apparent auditory motion. We produced the apparent motion stimulus by broadcasting pure-tone bursts sequentially from an array of loudspeakers along horizontal, vertical, or oblique trajectories in the frontal hemifield. Motion direction had an effect on the response of 65% of the units sampled. In these cells, motion in opposite directions produced shifts in receptive field locations, differences in response magnitude, or a combination of the two effects. Receptive fields typically were shifted opposite the direction of motion (i.e., units showed a greater response to moving sounds entering the receptive field than exiting) and shifts were obtained to horizontal, vertical, and oblique motion orientations. Response latency also shifted as a function of motion direction, and stimulus locations eliciting greater spike counts also exhibited the shortest neural latency. Motion crossing the receptive field boundaries appeared to be both necessary and sufficient to produce receptive field shifts. Decreasing the silent interval between successive stimuli in the apparent motion sequence increased both the probability of obtaining a directional effect and the magnitude of receptive field shifts. We suggest that the observed directional effects might be explained by "spatial masking," where the response of auditory neurons after stimulation from particularly effective locations in space would be diminished. The shift in auditory receptive fields would be expected to shift the perceived location of a moving sound and may explain shifts in localization of moving sources observed in psychophysical studies. Shifts in perceived target location caused by auditory motion might be exploited by auditory predators such as Pteronotus in a predictive tracking strategy to capture moving insect prey.

We explored frequency and intensity encoding in the inferior colliculus of the C57 mouse model of sensorineural hearing loss. Consistent with plasticity reported in the IC of other models of hearing loss, frequency response areas (FRAs) in hearing impaired (HI) mice were broader with fewer highfrequency units than normal-hearing (NH) mice. The broad FRAs recorded from HI mice had lower cutoffs on the low frequency edge of the FRA. Characteristic frequency (CF) and sharpness of tuning (Q10) calculated from the FRA were used to divide the sample into 4 categories: low-CF sharp-FRA, low-CF broad-FRA, high-CF sharp-FRA, and high-CF broad-FRA units. Rate-intensity functions (RIFs) for CF tones and noise were used to determine the minimum and maximum response counts as well as the sound pressure levels resulting in 10%, 50%, and 90% of the maximum spike count. Tone RIFs of broad FRA units were shifted to the right of tone RIFs of sharp FRA units in both NH and HI mouse IC, regardless of the unit CF. The main effects of hearing loss were seen in the noise RIFs. The low-CF broad-FRA units in HI mice had elevated response to noise, and the high-CF sharp-FRA units in HI mice had lower maximum rates, as compared to the units recorded from NH mice. These results suggest that, as the IC responds to peripheral hearing loss with changes in the representation of frequency, an altered balance between inhibitory and excitatory inputs to the neurons recorded from the HI mice alter aspects of the units' intensity encoding. This altered balance likely occurs, at least in part, outside of the IC. Keywords plasticity; sensorineural hearing loss; C57 mouse; intensity encoding; central auditory nervous system Sensorineural hearing loss occurs in almost 10% of the US population. One of the most debilitating side effects of this condition is difficulty with word comprehension, particularly when the listening environment is noisy (Needleman and Crandell, 1995). Treatment of hearing loss includes amplification, but it cannot be assumed that amplification restores normal perception. For instance, hearing loss alters the frequency tuning curves and rate-intensity functions (RIFs) in central auditory neurons (Willott, 1984, Willott, 1986, and these deficits cannot be compensated by simple amplification alone. In addition, the sensitivity of the auditory system of hearing impaired listeners may be altered by long-term amplification. ForCorresponding Author: Joseph P. Walton, Department of Otolaryngology and Neurobiology and Anatomy, University of Rochester Medical School, 601 Elmwood Ave., Rochester, NY 14642, USA, Joseph_Walton@URMC.rochester.edu, PHONE: 585-275-1248, FAX: 585-244-4103. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that durin...

Single units in the inferior colliculus (IC) in the C57Bl/6 inbred mouse strain were tested for their temporal processing ability as measured by their minimum gap threshold (MGT), the shortest silent interval in an ongoing white-noise stimulus which a unit could encode. After ascertaining the MGT in quiet, units were re-tested in various levels of background noise. The focus of this report is on two types of tonically responding units found in the IC. Tonically inhibited (TI) units encoded gaps poorly in quiet and low levels of background noise as compared with tonically excited (TE) units. In quiet, the MGTs of TI units were about an order of magnitude longer than the MGTs typical of TE units. Paradoxically, gap encoding was improved in high levels of background noise for TI units. This result is unexpected from the traditional viewpoint that noise necessarily degrades signal processing and is inconsistent with psychophysical observations of diminished speech and gap detection processing in noisy environments. We believe the improved feature detection described here is produced by the adaptation of inhibitory input. Continuous background noise would diminish the inhibitory efficacy of the gap stimulus by increasing the latency to the onset of inhibition and decreasing its duration. This would allow more spontaneous activity to "bleed through" the silent gap, thus signaling its presence. Improved feature detection in background noise resulting from inhibitory adaptation would seem an efficient neural mechanism and one that might be generally useful in other signal detection tasks.

A computational sensorimotor model of target capture behavior by the echolocating bat, Eptesicus fuscus, was developed to understand the detection, localization, tracking, and interception of insect prey in a biological sonar system. This model incorporated acoustics, target localization processes, flight aerodynamics, and target capture planning to produce model trajectories replicating those observed in behavioral insect capture trials. Estimates of target range were based on echo delay, azimuth on the relative intensity of the echo at the two ears, and elevation on the spectral pattern of the sonar return in a match/mismatch process. Flapping flight aerodynamics was used to produce realistic model trajectories. Localization in all three spatial dimensions proved necessary to control target tracking and interception for an adequate model of insect capture behavior by echolocating bats. Target capture using maneuvering flight was generally successful when the model's path was controlled by a planning process that made use of an anticipatory internal simulation, while simple homing was successful only for targets directly ahead of the model bat.