Twenty-Five Years of Multielectrode Recordings in the Somatosensory System: It is All about Dynamics
Chronic multielectrode recording methods introduced 25 years ago have opened up the opportunity to simultaneously sample the activity of neurons at multiple levels of the somatosensory system while rats engage in active tactile behaviors. This chapter focuses on results gleaned from recordings in the rat whisker system. The earliest multielectrode investigations revealed that the peak of neural activity evoked by single-whisker stimuli drifts widely over the cortical and thalamic somatotopic whisker maps. This property could provide a mechanism for recognizing spatiotemporal patterns of whisker stimulation. These studies also showed that information about tactile stimulus identity is carried by the relative latencies of evoked spikes in different simultaneously recorded neurons. Subsequent experiments also revealed highly synchronized firing in neurons from brainstem to cortex, and immediate receptive field reorganization in thalamus induced by partial deafferentation or reversible inactivation of primary somatosensory cortex (S1). Even the two hemispheres of S1, long viewed as independent modules for processing exclusively contralateral stimuli, were found to interact on millisecond timescales in anesthetized and waking rats. This later finding suggested that the brain combines bilateral whisker afferents to discriminate bilateral whisker stimuli - such as the widths of tunnel openings in the dark - an idea that was confirmed by the development of bilateral tactile discrimination tasks. Multielectrode recordings during tactile discrimination revealed qualitatively distinct response modes in S1 as compared to responses to passive whisker stimulation, including task-related firing rate modulations that begin well before whisker stimulation. These data have pushed our conception of somatosensory representation - even at the earliest thalamic and cortical processing stages - away from the static classical one-barrel/one-whisker picture, toward that of a highly plastic multilevel structure whose functional architecture quickly adjusts to meet the demands of the present situation. © 2008 Elsevier Inc. All rights reserved.
Wiest, MC; Thomson, E; Nicolelis, MAL
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