R. Ratnam et As. Feng, Detection of auditory, signals by frog inferior collicular neurons in the presence of spatially separated noise, J NEUROPHYS, 80(6), 1998, pp. 2848-2859
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.