UC Berkeley

CHAPTER 1: Somatosensory maps in rodents & primates—a review

This chapter reviews basic principles and recent findings in primate, human, and rodent somatosensory maps. Topographic maps of the body surface are a major feature of somatosensory cortex. In primates, parietal cortex contains four somatosensory areas, each with its own map, with the primary cutaneous map in area 3b. Rodents have at least three somatosensory areas, and the whisker map in rodent primary somatosensory cortex is a canonical system for studying cortical microcircuits, sensory coding, and map plasticity.

Maps are not isomorphic to the body surface, but magnify behaviorally important skin regions, which include the hands and face in primates, and the whiskers in rodents. Within each map, intracortical circuits process tactile information, mediate spatial integration, and support active sensation. Functional representations are more overlapping than suggested by textbook depictions of map topography. Maps may also contain fine-scale representations of touch sub-modalities, or direction of tactile motion. In addition, somatosensory maps are plastic throughout life in response to altered use or injury.

CHAPTER 2: Somatotopic precision of whisker tuning in layer 2/3 of rat barrel cortex

Although cortical maps of the sensory periphery are topographically organized at a large scale, the fine-scale precision of the map at the level of neighboring cells varies between species, sensory modalities, and cortical layers. In rodent somatosensory cortex, where each whisker is mapped to a dedicated cortical column, cells in layer 4 of each column are predominantly tuned to the appropriate whisker (columnar whisker; CW). However, in mice, cells in the upper cortical layers display locally heterogeneous "salt-and-pepper" tuning. It remains unclear whether the same heterogeneity is found in the upper layers of the rat whisker map. To address this question, we examined whisker tuning in regular-spiking (RS) and fast-spiking (FS) units recorded from layer 2/3 whisker columns in anesthetized rats. Among RS units, we observe a narrow majority (59%) best tuned to the CW; a clear majority (88%) with the CW among a statistically comparable group of "equal best" whiskers; and a small number (12%) of units best driven by one or more surround whiskers and poorly responsive to the CW. CW-tuned units display sharper tuning and faster responses to their best whisker than non-CW-tuned units, but are distributed evenly along vertical and horizontal dimensions of the column. These results demonstrate tuning in rat L2/3 that is more homogeneous than in mouse layer 2, but less so than in rat or mouse layer 4. FS units are tuned to the CW more frequently (78%) than RS units, consistent with broadly sampled input from local excitatory cells most responsive, as a group, to the columnar whisker.