Cerebral Cortex Advance Access originally published online on August 24, 2005
Cerebral Cortex 2006 16(6):791-810; doi:10.1093/cercor/bhj023
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Structure of the Excitatory Receptive Fields of Infragranular Forelimb Neurons in the Rat Primary Somatosensory Cortex Responding To Touch
1 School of Biomedical Engineering, Science and Health Systems, Drexel University, Philadelphia, PA, USA, 2 Department of Neurobiology, Drexel University College of Medicine, Philadelphia, PA, USA and 3 Fundación del Hospital Nacional de Parapléjicos para la Investigación y la Integración, SESCAM, Toledo, Spain
Address correspondence to Karen A. Moxon, PhD, Drexel University, School of Biomedical Engineering, Science and Health Systems, Philadelphia, PA 19104, USA. Email: Karen.Moxon{at}drexel.edu.
We quantitatively studied the excitatory receptive fields of 297 neurons recorded from the forelimb infragranular somatosensory cortex of the rat while touch stimuli were applied to discrete locations on the forelimbs. Receptive fields were highly heterogeneous, but they were regulated, on average, by an underlying spatio-temporal structure. We found the following. (i) Neurons responded with decreasing magnitude and increasing latency when the stimulus was moved from the primary location to secondary locations and to far ispilateral locations of their excitatory receptive fields, displaying smooth transitions from the primary location to secondary locations. (ii) Receptive field patterns revealed functional connectivity between the digits and ventral palm, which did not depend on whether the digits were stimulated dorsally or ventrally. (iii) The structure of the receptive fields (i.e. the neural responses to stimulation of secondary locations compared to the neural responses to stimulation of the primary location), reflected cortical (rather than body) distances. (iv) There was a functional separation between the forepaw and the rest of the forelimb. Namely: if the primary location was in the digits or palm, secondary locations were biased toward the digits and palm; if the primary location was in rest of the forelimb, secondary locations appeared equally distributed over forelimb, digits and palm. (v) More than 40% of neurons extended their receptive field to the ipsilateral forelimb, without any evident spatial organization. Overall, the stimuli evoked 
3 times more spikes from secondary responses than from primary responses. These results suggest that a rich repertoire of spatio-temporal responses is available for encoding tactile information. This highly distributed receptive field structure provides the electrophysiological architecture for studying organization and plasticity of cortical somatosensory processing.
Key Words: tactile • plasticity • forelimb • multi-electrode • cortical
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