Resonance, the tendency of structures to oscillate at larger amplitudes when stimulated at specific frequencies, has been employed by biological (e.g., the cochlea) and artificial (e.g., fMRI) sensing systems to facilitate detection and discrimination. We have recently discovered that rat-vibrissae resonate when stimulated at specific frequencies, such that each vibrissa has a 'tuning curve' of frequencies that amplify the range of vibrissa motion. Further, the fundamental resonance frequency that defines these tuning curves varies as a function of vibrissa length: Because vibrissa lengths vary by position on the rat face, resonance provides a somatotopic map of frequency sensitivity. Vibrissa resonance should directly and significantly impact the neural representation of the vibrissae, with potentially important perceptual implications. Here, we propose to systematically investigate the following specific hypotheses: 1. That vibrissa resonance amplifies the sensitivity of SI cortical neurons to small-amplitude high-frequency tactile stimuli; 2. That vibrissa resonance confers band-pass frequency tuning to somatotopically associated SI neurons; and, 3. That the systematic map of frequency sensitivity on the rat face is translated into a map of high-frequency sensitivity overlaid on the somatotopic SI map. To test these hypotheses with well-controlled stimuli, we will conduct electrophysiological recording and optical imaging in the anesthetized rat (Specific Aims 1 & 2). To test these hypotheses in the awake rat, we will employ identical stimuli and chronic tetrode recording in the head-posted, non-whisking rat (Specific Aim 3). To test the hypothesis that resonance is expressed during whisking, we will train head-posted rats to whisk over varying textures and conduct high-speed videography (Specific Aim 4). These studies are significant for several reasons. The biomechanics of the vibrissae have previously received relatively little attention. Critical examination of the neural correlates of vibrissa resonance may provide evidence for a new theory of tactile signal transduction. Further, Preliminary Data strongly suggest that rat SI neurons are band-pass frequency-tuned, and that a previously undiscovered, somatotopic high frequency map exists in SI, including a network of 'isofrequency' columns. The proposed studies will provide systematic examination of this potentially key aspect of sensory processing in this important model system.
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