Title: Auditory Space Processing and Multi-sensory Interactions

Gregg H Recanzone

Center for Neuroscience

University of California at Davis

Abstract:

The ability to localize sounds in space is a fundamental perception. Individual sensory receptors, the hair cells in the cochlea, cannot encode the spatial location of an acoustic stimulus. Therefore, the central nervous system must use the activity of many different hair cells in order to compute the sound source location. The cerebral cortex is known to be necessary for this perception, however, it is still unclear how this computation is performed, or in what form acoustic space is represented at the cortical level. We have investigated this issue in the alert macaque by recording the activity of single neurons to broadband noise stimuli located across 360 degrees in azimuth at four different intensity levels. The results indicate that acoustic space is serially processed through the auditory cortex. Neurons in the primary auditory cortex (A1) have broad receptive fields, whereas neurons in the caudal belt areas CM, ML, and CL have much sharper spatial tuning. Neurons located in cortical fields medial and rostral to A1 (R and MM) have very poor spatial tuning. Finally, the spike rates pooled across neurons in ML and CL contain sufficient information to account for sound localization ability across stimulus intensity, further implicating these regions in the processing of acoustic space.

Sound localization ability is also profoundly influenced by visual stimuli. The simultaneous presentation of spatially disparate auditory and visual stimuli can lead to the perception that the sound originates from the location of the visual stimulus, a phenomenon known as the ventriloquism effect. It has been hypothesized that the dominance of visual stimuli over auditory stimuli in space is the result of the higher spatial acuity of the visual system compared to the auditory system. If this hypothesis can be generalized, then perceptions where the auditory system has higher acuity than the visual system should result in the auditory stimulus dominating the percept. We have investigated this using a paradigm which measures the ability of normal human subjects to discriminate slow temporal rates of presentation of auditory, visual, and combined auditory and visual stimuli. The results show that the auditory system indeed dominates the perception of the temporal rate of visual stimuli. These results indicate that the perception of multi-modal stimuli along any given parameter is dominated by the sensory system with the highest acuity for that parameter.