Binaural cross-correlation predicts the responses of neurons in the owl's auditory space map under conditions simulating summing localization.

Summing localization describes the perceptions of human listeners to two identical sounds from different locations presented with delays of 0-1 msec. Usually a single source is perceived to be located between the two actual source locations, biased toward the earlier source. We studied neuronal responses within the space map of the barn owl to sounds presented with this same paradigm. The owl's primary cue for localization along the azimuth, interaural time difference (ITD), is based on a cross-correlation-like treatment of the signals arriving at each ear. The output of this cross-correlation is displayed as neural activity across the auditory space map in the external nucleus of the owl's inferior colliculus. Because the ear input signals reflect the physical summing of the signals generated by each speaker, we first recorded the sounds at each ear and computed their cross-correlations at various interstimulus delays. The resulting binaural cross-correlation surface strongly resembles the pattern of activity across the space map inferred from recordings of single space-specific neurons. Four peaks are observed in the cross-correlation surface for any nonzero delay. One peak occurs at the correlation delay equal to the ITD of each speaker. Two additional peaks reflect "phantom sources" occurring at correlation delays that match the signal of the left speaker in one ear with the signal of the right speaker in the other ear. At zero delay, the two phantom peaks coincide. The surface features are complicated further by the interactions of the various correlation peaks.