Inter- and intramodal convergence in early sensory processing
Fu, Kai-Ming Gregory
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Current hierarchical models of sensory processing suggest absolute segregation of input pathways at early processing levels. For example, visual, auditory, and somatosensory stimuli are initially processed through a series of subcortical and cortical areas prior to their convergence in higher order cortical regions. Streams of information can be separated intramodally as well. For example, there appear to be different pathways for stimulus localization and identification in the visual and auditory systems. In higher order areas, both intra and intramodal streams are integrated into a richer representation of the properties of an object. My studies, using the macaque monkey model, demonstrate that both multisensory and intramodal convergence occur in early sensory cortical areas.;First, my studies in anesthetized macaques present evidence that auditory cortical neurons, located in caudo-medial auditory belt cortex (CM), respond to cutaneous stimulation. The location of the cutaneous receptive fields, primarily on the head and neck, is consistent with the hypothesis that posterior auditory cortex is primarily involved with sound localization. CM is located at the second level of the auditory cortical processing hierarchy, one step serially removed from primary core auditory cortex (A1). Second, my studies in the awake behaving macaque show that responses in posterior auditory cortex are affected by the position of the eyes. Integration of somatosensory signals and eye position data at this early stage of auditory processing may assist sound localization, again a reputed function of posterior auditory cortex.;My third study provides the initial evidence of ipsilateral input into the somatosensory hand representation in areas 3b and 1 of primary somatosensory cortex (S1). Previous models of somatosensory processing held that the topographical representation of the hands in S1 was restricted to the contralateral side. My results, obtained from stimulating the ipsilateral median nerve in the awake macaque, demonstrate a relatively long latency response present in both post-synaptic responses and action potentials. The laminar profile of the response is bilaminar, suggesting a probable feedback input. The multi unit activity response in lamina 4 is suppressed, indicating inhibition. The presence of ipsilateral inhibitory responsiveness in S1 may aid contralateral localization.
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