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1Kresge Hearing Research Institute, University of Michigan, Ann Arbor, Michigan; and 2Human Cognitive Neurophysiology Laboratory, Veterans Affairs Northern California Health Care System, Martinez, California
Submitted 28 January 2005; accepted in final form 21 April 2005
We compared the spatial sensitivity of neural responses in three areas of cat auditory cortex: primary auditory cortex (A1), the posterior auditory field (PAF), and the dorsal zone (DZ). Stimuli were 80-ms pure tones or broadband noise bursts varying in free-field azimuth (in the horizontal plane) or elevation (in the vertical median plane), presented at levels 20–40 dB above units' thresholds. We recorded extracellular spike activity simultaneously from 16 to 32 sites in one or two areas of 
-chloralose-anesthetized cats. We examined the dependence of spike counts and response latencies on stimulus location as well as the information transmission by neural spike patterns. Compared with units in A1, DZ units exhibited more complex frequency tuning, longer-latency responses, increased prevalence and degree of nonmonotonic rate-level functions, and weaker responses to noise than to tonal stimulation. DZ responses also showed sharper tuning for stimulus azimuth, stronger azimuthal modulation of first-spike latency, and enhanced spatial information transmission by spike patterns, compared with A1. Each of these findings was similar to differences observed between PAF and A1. Compared with PAF, DZ responses were of shorter overall latency, and more DZ units preferred stimulation from ipsilateral azimuths, but the majority of analyses suggest strong similarity between PAF and DZ responses. These results suggest that DZ and A1 are physiologically distinct cortical fields and that fields like PAF and DZ might constitute a "belt" region of auditory cortex exhibiting enhanced spatial sensitivity and temporal coding of stimulus features.
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