Spatial Segregation of Different Modes of Movement Control in the Whisker Representation of Rat Primary Motor Cortex

Abstract

What is mapped on the surface of the primary motor cortex (M1)? The classic somatotopic map holds true on the level of limb representations. However, on the small scale (at within-limb representations), neither somatotopy nor movement dynamics/kinematics seem to be organizational principles. We investigated the hypothesis that integrated into the body representation of M1 there may be separate representation of different modes of motor control, using different subcortical computations but sharing the same motor periphery. Using awake rats and long intracortical stimulation trains in M1 whisker representation (wM1) revealed that natural-like, rhythmic whisking (normally used for tactile exploration) can be evoked from a posteromedial subregion of wM1. Nonrhythmic whisker retraction, on the other hand, was evoked in an adjacent but more anterolaterally located region within wM1. Evoked whisker retraction was always accompanied by complex movements of the face, suggesting that the respective subregion is able to interact with other representations in specific behavioral contexts. Such associations were absent for evoked rhythmic whisking. The respective subregion rather seemed to activate a downstream central pattern generator, the oscillation frequency of which was dependent on the average evoked cortical activity. Nevertheless, joint stimulation of the two neighboring subregions demonstrated their potency to interact in a functionally useful way. Therefore, we suggest that the cause of cortical separation is the specific drive of subcortical structures needed to generate different types of movements rather than different behavioral contexts in which the movements are performed.

Biomechanics of the Vibrissa Motor Plant in Rat: Rhythmic Whisking Consists of Triphasic Neuromuscular Activity

“…3, 4). A myotopic map of vM1 would clarify how the control of whisking is segregated in cortex, e.g., how do "rhythmic whisking" versus "retraction-facial" areas correspond to different patterns of intrinsic and extrinsic muscle activation (Haiss and Schwarz, 2005)? follicle alone, denoted by the subscripts h and f, respectively (Fig.…”

Modeling discrete and rhythmic movements through motor primitives: a review

“…Such an hypothesis could reflect the analogy observed by Haiss and Schwarz (2005) between the representation of discrete and rhythmic movements in whisker movements in rats (see Section 6). In this model, mutual influences of movements are supposed to occur at the muscle level rather than at the spinal level, as discussed above for the Two/Two hypothesis.…”

“…Note that co-occurrence of discrete and rhythmic movements is supported by a study on whisker movements in rats by Haiss and Schwarz (2005), where it has been found that rhythmic and non rhythmic movements can be evoked through two different areas of the primary motor cortex. It has been shown in addition that simultaneous activation of both areas resulted in a shift of the offset of the whisker oscillations, that is in a combination of both movements.…”

“…In the same experiment, Haiss and Schwarz (2005) found that stimulating both "discrete" and "rhythmic" areas of the primary motor cortex resulted in a simple combination of the two behaviors: the resulting movement was the oscillation expected when only the rhythmic area is activated, but with an offset corresponding to the discrete movement resulting from the activation of the discrete area. This result is important as it shows that, even if discrete and rhythmic motor primitives result from different processes, which is not established yet, the combination of those primitives still results in a coherent, meaningful behavior.…”

“…In particular, Haiss and Schwarz (2005) have studied the electric stimulation of different types of whisker movements in the rat, namely rhythmic movement (used for tactile exploration) and whisker retraction (used to sense an object at a specific location). They found that both movements, although performed by the same set of muscles, where elicited by different (but adjacent) regions of the primary motor cortex.…”