Weekend bioRxiv Preprint Review: A robust role for motor cortex

Humans, and probably primates in general, use their primary motor cortex (Brodmann area 4) when executing voluntary, specifically directed motor activity, such as reaching for an object. In the human brain, Brodmann area 4 is organized spatially in the brain by the area of the body activated, as a distorted spatial map of the body (a homonculous).

It has been less clear whether non-primate animals such as the rodents use this part of their brain in a similar way. We know that fish such as the zebrafish, which lack a neocortex, probably use their pallium, which in humans is part of the basal ganglia region which modulates but does not determine voluntary motion, for such directed movements. Exactly how in the zebrafish such a basal ganglia analogue activation leads to motion does not seem to be fully understood.

The preprint below theorizes that in mammals such as rats, there is a third option between the primate's use of primary motor cortex and the absence of use in the fish: the rat may use its non-primary motor cortex regions and basal ganglia modulations for familiar, well-learned tasks, and then may activate its primary motor cortex for tasks that are unlearned, when it must move differently in the presence of novelty.

ABSTRACT

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A robust role for motor cortex

Authors: Goncalo Lopes, Joana Nogueira, Joseph J. Paton, Adam R. Kampff

doi: http://dx.doi.org/10.1101/058917

Abstract

The role of motor cortex in the direct control of movement remains unclear, particularly in non-primate mammals. More than a century of research using stimulation, anatomical and electrophysiological studies has implicated neural activity in this region with all kinds of movement. However, following the removal of motor cortex, or even the entire cortex, rats retain the ability to execute a surprisingly large range of adaptive behaviours, including previously learned skilled movements. In this work we revisit these two conflicting views of motor cortical control by asking what the primordial role of motor cortex is in non-primate mammals, and how it can be effectively assayed. In order to motivate the discussion we present a new assay of behaviour in the rat, challenging animals to produce robust responses to unexpected and unpredictable situations while navigating a dynamic obstacle course. Surprisingly, we found that rats with motor cortical lesions show clear impairments in dealing with an unexpected collapse of the obstacles, while showing virtually no impairment with repeated trials in many other motor and cognitive metrics of performance. We propose a new role for motor cortex: extending the robustness of sub-cortical movement systems, specifically to unexpected situations demanding rapid motor responses adapted to environmental context. The implications of this idea for current and future research are discussed.