CODING OF SOUND LOCATION IN PRIMATE INFERIOR COLLICULUS.  J.M.Groh*;
A.M.Underhill Dept. of Psychological and Brain Sciences, Dartmouth
College, Hanover, NH, USA

The inferior colliculus (IC) has been implicated in the processing of
sound location in a variety of species such as cats, bats, barn owls,
and rodents, but little is known about how sound location is encoded
in the primate IC. Accordingly, we measured the sensitivity of IC
neurons to the location of free field acoustic stimuli in two rhesus
monkeys. The monkeys were awake and performed an eye fixation task,
but were not asked to respond to the auditory stimuli. Stimuli
consisted of broad-band noise bursts and were delivered through 15
speakers ranging in location from 90 left to 90 right along the
horizontal meridian.

About 60% of IC neurons (43 out of 73) were sensitive to sound
location (ANOVA). Generally, neurons were more responsive to sounds
located in the contralateral hemifield. However, the neurons were also
quite responsive to sounds in the ipsilateral hemifield as well. The
spatial tuning of these neurons tended to be monotonic over a broad
range of spatial locations. We did not observe clearly circumscribed
receptive fields, and non-monotonic functions such as gaussians did
not fit the data particularly well. These results suggest that the
primate IC codes the spatial location of sounds using neurons with
very broad spatial tuning with a contralateral preference.  Supported
by: ONR, Whitehall, McKnight, Sloan, John Merck Scholars, and NIH
17778-19 grants to JMG.