Thomas J. Imig, Ph.D. - US grants

Destruction of auditory cortex in cats and primates, including humans, results in an inability to localize a sound source and implies a representation of sound direction in this structure. The long term objective of this project is to describe physiological mechanisms of sound localization, and the organization of neural networks underlying and related to the representation of sound location and frequency in the thalamocortical auditory system of the cat. Tracing anatomical pathways in relationship to physiological maps, and quantitative measurement of spatial receptive fields of single neurons are the analytical methods. This project has two major specific aims. The first focuses on the connectional organization of the tecto-thalamo-cortical pathway. These experiments utilize microelectrode mapping to guide iontophoretic injections of HRP into thalamic subdivisions whose targets in tonotopic auditory cortex have recently been characterized in detail. Each experiment will provide information on the afferent and efferent connections of a relatively small group of thalamic neurons with the midbrain, hindbrain and the cerebral cortex. The second major aim focuses on the representation of sound direction in the ventral nucleus of the medial geniculate body and primary auditory cortex (AT). Single unit discharges will be recorded in response to noise and tone bursts of various intensities and frequencies, presented in the free field from speakers located at different positions. These observations will provide information on the directional sensitivities of auditory forebrain neurons in the face of sounds which vary in frequency and intensity. In a second series of experiments, the position of each pinna individually, and both pinnae together will be varied to study the effects of changes in pinna orientation on directional sensitivities of single units. The acoustical directional sensitivity of the pinna will also be measured by recording the amplitude of the cochlear microphonic evoked by speakers at different locations. Pinna directional sensitivity will be measured for different pinna orientations. These data will provide direct comparison of changes in 1. single unit directional sensitivity and 2. directional sensitivity of the pinna resulting from changes in pinna orientation. Next, the directional sensitivities of single units will be studied as a function of location along an isofrequency controur to investigate the topographic representation of sound direction in AT. Finally, discharge rate of single neurons will be studied as a function of rate and direction of movement of a sound source.

DESCRIPTION: (Adapted From The Applicant's Abstract.) This is a continuation of research attempting to identify neural substrates of sound localization relevant to human hearing. Neurons in the cat central auditory system will be tested to determine if they are monaural directional (MD) or binaural directional (BD), two means of deriving directional selectivity for broad band noise (BBN). Properties of MD and BD cells will be evaluated in the cat inferior colliculus (IC) to determine where and how MD sensitivity is synthesized. Superior colliculus (SC) neurons will be evaluated for MD sensitivity. To determine the role of the dorsal cochlear nucleus (DCN), neurons in auditory cortex (AI) and SC will be tested after transection of DCN output fibers. Experiments on MD sensitivity in squirrel monkey auditory cortex are also proposed. Neural sensitivity to sound movement in high frequency neurons will be studied in cat AI neurons using ear plugs to determine whether movement sensitivity derives from monaural cues, binaural cues, or both. Whether the azimuth tuning of cortical cells in cat BD cells to noise stimulation reflects a linear summation of sensitivity to binaural disparities in constituent spectral components will be tested by comparing azimuth function for noise with that obtained from the linear sum of azimuth functions for tonal stimuli with frequencies distributed throughout the band pass of the noise. In all, seven hypotheses will be specifically tested.