Auditory Spatial Tuning Research Articles

Binaural mechanisms of spatial tuning in the cat's superior colliculus distinguished using monaural occlusion.

This study explores the mechanisms of auditory spatial tuning in the superior colliculus of the anesthetized cat by correlating spatial tuning within specific regions of space with particular types of binaural interaction. The auditory spatial tuning of units was measured using a movable, broad-band stimulus presented in a free sound field. The contribution of each ear to the response of a unit was identified by acutely plugging one or the other ear. Every unit became largely or entirely unresponsive when a foam-rubber earplug was placed in the ear contralateral to the recording site. Thus, every unit exhibited an excitatory or facilitatory influence from the contralateral ear. A plug placed in the ipsilateral ear had different effects on different units. For half of the units (16/32), an ipsilateral earplug produced increases in the sizes of the units' receptive fields and increases in the magnitudes of their responses to stimuli presented from most locations. Thus, these units exhibited inhibition from the ipsilateral ear. Another class of units (9/32) exhibited ipsilateral facilitation, in that an ipsilateral earplug caused decreases in the sizes of the units' receptive fields and prominent decreases in their response magnitudes. For the remaining units (7/32), an ipsilateral earplug resulted in decreases in the sizes of the units' receptive fields, but produced both decreases in the responses of units to stimuli presented in their best areas and increases in the responses to stimuli presented away from the best areas. Thus these units exhibited mixed facilitatory and inhibitory ipsilateral influences. The influence of an ipsilateral earplug on a unit's response tended to correlate with its spatial tuning. The region of space within which a sound source was most effective in activating a unit was its "best area". The best areas of units exhibiting ipsilateral inhibition were located furthest peripherally, those of units showing ipsilateral facilitation were located furthest frontally, and the best areas of units showing mixed ipsilateral influences were located in an intermediate area. The frequency tuning of units measured using a free-field tone source also tended to correlate with the locations of their best areas. Half of the units tested (27/54) responded to tones of the sound pressure levels (SPLs) that were used (up to 50 dB SPL).(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in external ear position modify the spatial tuning of auditory units in the cat's superior colliculus.

This study examines the influence of external ear position on the auditory spatial tuning of single units in the superior colliculus of the anesthetized cat. Unit responses to broad-band stimuli presented in a free sound field were measured with the external ears in a forward symmetrical position or with one or the other ear turned 40 degrees to the side; the ears are referred to as contra- or ipsilateral with respect to the side of the recording site. Changes in the position of either ear modified the spatial tuning of units. The region of space from which a stimulus was most effective in activating a unit is referred to as the unit's "best area". Whenever the contralateral ear was turned to the side, best areas shifted peripherally and somewhat upward, roughly in proportion to the magnitude of the change in ear position. A turn of the ipsilateral ear to the side had more variable effects, but best areas generally shifted frontally. Best areas located between approximately 10 and 40 degrees contralateral when the ears were forward were least affected by changes in ipsilateral ear position. Changes in ear position also modified the maximum response rates of many units. Units with best areas located within approximately 20 degrees of the frontal midline when the ears were forward exhibited a pronounced decrease in responsiveness when either ear was turned. Units with more peripheral best areas tended to show no change or a slight increase in responsiveness. The influence of ear position on the directionality of the external ears was determined by mapping the cochlear microphonic response to tones or one-third-octave bands of noise before and after turning the ear. When the ears were forward, maximum interaural intensity differences (IIDs) were produced by high-frequency sound sources (greater than or equal to 20 kHz) located 20-40 degrees from the frontal midline and by lower frequency sources located further peripherally. The influence of ear position on the locations from which maximum IIDs were produced was similar to the influence of ear position on unit best areas. Changes in ipsilateral ear position had different effects on high- and low-frequency IIDs that were comparable with the effects of changes in ear position on frontally and peripherally located best areas, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Experience alters the spatial tuning of auditory units in the optic tectum during a sensitive period in the barn owl

The auditory spatial tuning of bimodal (auditory-visual) units in the optic tectum of the barn owl was altered by raising animals with one ear occluded. Changes in spatial tuning were assessed by comparing the location of a unit's auditory best area with that of its visual receptive field. As shown previously, auditory best areas are aligned with visual receptive fields in the tecta of normal birds (Knudsen, E. I. (1982) J. Neurosci. 2: 1177-1194). It was demonstrated in this study that, when birds were raised with one ear occluded, best areas and visual receptive fields were aligned only as long as the earplug was in place. When the earplug was removed, best areas and visual receptive fields became misaligned, indicating that a change in auditory spatial tuning had taken place during the period of occlusion. However, in a bird that received an earplug as an adult, no such alterations in auditory spatial tuning were observed; even after 1 year of monaural occlusion, auditory best areas and visual receptive fields were misaligned so long as the earplug was in place, and were aligned when the earplug was removed. These results suggest that exposure to abnormal localization cues modifies the auditory spatial tuning of tectal units only during a restricted, sensitive period early in development. After the earplug was removed from a juvenile bird that had been raised with an occluded ear, the initial misalignment between auditory best areas and visual receptive fields decreased gradually over a period of weeks. In contrast, when earplugs were removed from two adult birds that had been raised with monaural occlusions, auditory-visual misalignments persisted for as long as measurements were made, which was up to 1 year after earplug removal. These data indicate that auditory cues become permanently associated with locations in visual space during a critical period which draws to a close at about the age when the animal reaches adulthood. Horseradish peroxidase was injected into two optic tecta (in a single animal) that contained units with permanently altered auditory spatial tuning. The positions of retrogradely labeled cells in the external nucleus of the inferior colliculus (ICX) were the same as those observed in control birds (Knudsen, E. I., and P. F. Knudsen (1983) J. Comp. Neurol. 218: 187-196). Thus, the changes in spatial tuning were not due to a shift in the topographic projection from the ICX to the optic tectum.(ABSTRACT TRUNCATED AT 400 WORDS)