High Binaural Coherence Determines Successful Sound Localization and Increased Activity in Posterior Auditory Areas

Response preferences for "what" and "where" in human non-primary auditory cortex

Primate studies suggest the auditory cortex is organized in at least two anatomically and functionally separate pathways: a ventral pathway specializing in object recognition and a dorsal pathway specializing in object localization. The current experiment assesses the validity of this model in human listeners using fMRI to investigate the neural substrates of spatial and non-spatial temporal pattern information. Targets were differentiated from non-targets on the basis of two levels of pitch information (present vs. absent, fixed vs. varying) and two levels of spatial information (compact vs. diffuse sound source, fixed vs. varying location) in a factorial design. Analyses revealed spatially separate responses to spatial and non-spatial temporal information. The main activation associated with pitch occurred predominantly in Heschl's gyrus (HG) and planum polare, while that associated with changing sound source location occurred posterior to HG, in planum temporale (PT). Activation common to both pitch and changing spatial location was located bilaterally in anterior PT. Apart from this small region of overlap, our data support the anatomical and functional segregation of 'what' and 'where' in human non-primary auditory cortex. Our results also highlight a distinction in the sensitivity of anterior and posterior fields of PT to non-spatial information and specify the type of spatial information that is coded within early areas of the spatial processing stream.

Effects of the frequency of interaural time difference in the human brain

The two cues to the horizontal location sound sources are interaural time differences and interaural level differences. For low-frequency tones, interaural time differences provide effective and unambiguous information. For higher frequency sounds, however, interaural time differences provide ambiguous cues. In order to evaluate the effect of frequency of interaural time differences in the human auditory cortex, the auditory evoked fields to different interaural time differences of pure tone were examined. The results showed that the N1m magnitudes varied with the interaural time differences when the frequency of the pure tone was 800 Hz. The N1m magnitudes, however, did not vary with the interaural time differences when the frequency of the pure tone was 1600 Hz. These results indicate that localization performance might be reflected in N1m magnitudes.