A cat’s cocktail party: Psychophysical, neurophysiological, and computational studies of spatial release from masking

Abstract

Masked thresholds can improve substantially when a signal is spatially separatedfrom a noise masker (Saberi et al. 1991). This phenomenon, termed “spatial releasefrom masking” (SRM), may contribute to the cocktail party effect, in which alistener can hear a talker in a noisy environment. The purpose of this study is toexplore the underlying neural mechanisms of SRM.Previous psychophysical studies (Good, Gilkey, and Ball 1997) have shown thatfor high-frequency stimuli, SRM was due primarily to energetic effects related tothe head shadow, but for low-frequency stimuli, both binaural processing(presumably ITD processing) and energetic effects contributed to SRM. Therelative contributions of these two factors were not studied for broadband stimuli.Previous physiology studies have identified possible neural substrates for boththe energetic and ITD-processing components of SRM. For the energeticcomponent, our group has shown that some inferior colliculus units, “SNR units,”have masked thresholds that are predicted by the signal-to-noise ratio (SNR) in anarrowband filter centered at the unit’s CF (Litovsky et al. 2001). For the ITDcomponent, a series of studies (e.g. Jiang, McAlpine, and Palmer 1997) shows thatITD-sensitive units can exploit the differences between the interaural phasedifference (IPD) of a tone and masker to improve the neural population maskedthresholds. These studies did not describe how the units’ masked thresholds changewhen a broadband signal and masker are placed at different azimuths.Here, we examine the contributions of energetic effects and binaural processingfor broadband and low-frequency SRM using psychophysical experiments and anidealized population of SNR units. We also show that a population of ITD-sensitiveunits in the auditory midbrain exhibits a correlate of SRM. Finally, a model of ITD-sensitive units reveals that the signal’s temporal envelope influences the single-unitmasked thresholds.To appear in the proceedings of the 2003 International Symposium on Hearing, to be held inDourdan, France, 24-29 August, 2003