Functional Modules in Primary Motor Cortex

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Classically it is held that the body's musculature is mapped in a somatotopic fashion across the surface of the motor cortex (somatotopy hypothesis). While a coarse somatotopy is an undeniable fact, finely resolved functional analysis has shown, time and again, and in different species, that the somatotopy is broken on the smaller scale. Moreover, intracortcal micro-stimulation (ICMS) covering long intervals revealed that each point on the motor cortex elicits complex, behaviorally and etiologically relevant movements. This has been interpreted to signify a representation of behavioral functions rather than elemental movements (functional modules hypothesis).Our own previous work in primary whisker motor cortex (vM1) of rodents revealed two areas sharply delineated from each other, and strong candidates for functional, modular organization. Upon ICMS, subarea RW elicits rhythmic whisking, while TZ elicits whisker retraction accompanied by whole body movements. Only TZ, not RW, is a target zone of somatosensory cortex. In this project our aim is to firstly study the causal role of each of the two candidate modules for volitional behavior, needed to decide between the somatotopy and functional module hypotheses. Secondly we aim to work out the behavioral functions and contexts served by these presumptive modules. We will first characterize the connectivity of the candidate modules, expecting to find qualitative different subsets of connections in the case of modules, or orderly shifts of a basic projection pattern across cortex surface in the case of somatotopy. In a second and third objective, integrated within one experiment, we will employ a behavioral task in head-fixed mice, designed to tap into presumed functions of the two candidate modules, and test the respective behavior after permanent lesions and acute deactivations using optogenetic silencing. The task will be comprised of two components, one involving whisking in air (thus testing the presumed function to set control parameters of a known brainstem pattern generator), and the other involving active touch, i.e adaptation of whisks against textures to maximize discrimination. Any differential involvement of the two areas in these different behaviors and sensorimotor contexts would strengthen the functional module hypothesis.