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1 Neurobiology, Hebrew University, Jerusalem, Israel
2 Physiology, Anatomy and Genetics, University of Oxford, Oxford, United Kingdom
3 Physiology, Anatomy and Genetics, University of Oxford, United Kingdom
* To whom correspondence should be addressed. E-mail: israel{at}md.huji.ac.il.
We used optical imaging of intrinsic signals to study the large-scale organization of ferret auditory cortex in response to complex sounds. Cortical responses were collected during continuous stimulation by sequences of sounds with varying frequency, period, or interaural level differences. We used a set of stimuli that differ in spectral structure, but have the same periodicity and therefore evoke the same pitch percept (click trains, sinusoidally amplitude modulated tones and iterated ripple noise). These stimuli failed to reveal a consistent periodotopic map across the auditory fields imaged. Rather, gradients of period sensitivity differed for the different types of periodic stimuli. Binaural interactions were studied both with single contralateral, ipsilateral and diotic broadband noise bursts, and with sequences of broadband noise bursts with varying level presented contralaterally, ipsilaterally or in opposite phase to both ears. Contralateral responses were generally largest and ipsilateral responses were smallest when using single noise bursts, but the extent of the activated area was large and comparable in all three aural configurations. Modulating the amplitude in counter phase to the two ears generally produced weaker modulation of the optical signals than the modulation produced by the monaural stimuli. These results suggest that binaural interactions seen in cortex are most likely predominantly due to subcortical processing. Thus, our optical imaging data do not support the theory that the primary or non-primary cortical fields imaged are topographically organized to form consistent maps of systematically varying sensitivity either to stimulus pitch, or to simple binaural properties of the acoustic stimuli.
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