Auditory properties of space-tuned units in owl's optic tectum.

Auditory units in the optic tectum of the barn owl (Tyto alba) were studied under free-field conditions with a movable sound source. These units are selective for sound location and their spatial tuning varies systematically across the tectum, forming a map of space (8). I found that frequency tuning, response latencies, and thresholds of units changed in parallel with their spatial tuning, suggesting that as a consequence these properties also are topographically distributed in the optic tectum. Response rates were determined primarily by the location of the sound source. Regardless of sound intensity, only stimuli delivered from a restricted "best area" elicited vigorous responses. Minimum response latencies were shortest for units with frontal best areas and increased systematically for units with best areas located more peripherally. The response latencies of units with best areas centered within 25 degrees of the owl's visual axis were virtually independent of sound intensity and speaker position. The latencies of units with more peripheral best areas varied with speaker position and were shortest when the speaker was in the best area. Thresholds to noise stimuli were lowest for units with best areas directly in front of the owl and increased systematically for units with best areas located more peripherally. Thus, in the representation of frontal space, where units have the smallest receptive fields and the magnification of space is the greatest (8), units also respond to the weakest sound fields. Many units (20%) could not be driven with tonal stimuli; of those that could, most were broadly tuned for frequency. Characteristic frequencies and high-frequency cutoffs shifted lower as best areas moved peripherally. High-frequency tones, which excited units with frontal best areas, either inhibited or failed to drive units with peripheral best areas. These systematic changes in unit response properties influence how sounds from different locations are represented in the tectum and reflect integrative strategies used by the owl's auditory system in deriving a representation of auditory space.