Detection of auditory, signals by frog inferior collicular neurons in the presence of spatially separated noise

Psychophysical studies have shown that the ability to detect auditory signa ls embedded in noise improves when signal and noise sources are widely sepa rated in space; this allows humans to analyze complex auditory scenes, as i n the cocktail-part effect. Although these studies established that improve ments in detection threshold (DT) are due to binaural hearing, few physiolo gical studies were undertaken, and very little is known about the response of single neurons to spatially separated signal and noise sources. To addre ss this issue we examined the responses of neurons in the frog inferior col liculus (IC) to a probe stimulus embedded in a spatially separated masker. Frogs perform auditory scene analysis because females select mates in dense choruses by means of auditory cues. Results of the extra cellular single-u nit recordings demonstrate that 22% of neurons (A-type) exhibited improveme nts in signal DTs when probe and masker sources were progressively separate d in azimuth. In contrast, 24% of neurons (V-type) showed the opposite patt ern, namely, signal DTs were lowest when probe and masker were colocalized tin many instances lower than the DT to probe alone) and increased when the two sound sources were separated. The remaining neurons demonstrated a mix of these two types of patterns. An intriguing finding was the strong corre lation between A-type masking release patterns and phasic neurons and a wea ker correlation between V-type patterns and tonic neurons. Although not dec isive, these results suggest that phasic units may play a role in release f rom masking observed psychophysically. Analysis of the data also revealed a strong and nonlinear interaction among probe, masker, and masker azimuth a nd that signal DTs were influenced by two factors: I) the unit's sensitivit y to probe in the presence of masker and 2) the criterion level for estimat ing DT. For some units, it was possible to examine the interaction between these two factors and gain insights into the variation of DTs with masker a zimuth. The implications of these findings are discussed in relation to sig nal detection in the auditory system.