Premotor Cortex - Initial Studies of The Premotor Cortex and Doubts About Its Existence

Initial Studies of The Premotor Cortex and Doubts About Its Existence

In the earliest work on the motor cortex, researchers recognized only one cortical field involved in motor control. Campbell in 1905 was the first to suggest that there might be two fields, a "primary" motor cortex and an "intermediate precentral" motor cortex. His reasons were largely based on cytoarchitectonics. The primary motor cortex contains cells with giant cell bodies known as "Betz cells". The Betz cells are rare or absent in the adjacent cortex.

On similar criteria Brodmann in 1909 also distinguished between his area 4 (coextensive with the primary motor cortex) and his area 6 (coextensive with the premotor cortex).

Vogt and Vogt in 1919 also suggested that motor cortex was divided into a primary motor cortex (area 4) and a higher-order motor cortex (area 6) adjacent to it. Furthermore, in their account, area 6 could be divided into 6a (the dorsal part) and 6b (the ventral part). The dorsal part could be further divided into 6a-alpha (a posterior part adjacent to the primary motor cortex) and 6a-beta (an anterior part adjacent to the prefrontal cortex). These cortical fields formed a hierarchy in which 6a-beta controlled movement at the most complex level, 6a-alpha had intermediate properties, and the primary motor cortex controlled movement at the simplest level. Vogt and Vogt are therefore the original source of the idea of a caudal (6a-alpha) and a rostral (6a-beta) premotor cortex.

Fulton in 1935 helped to solidify the distinction between a primary motor map of the body in area 4 and a higher-order premotor cortex in area 6. His main evidence came from lesion studies in monkeys. It is not clear where the term "premotor" came from or who used it first, but Fulton popularized the term.

A caveat about the premotor cortex, noted early in its study, is that the hierarchy between the premotor cortex and the primary motor cortex is not absolute. Instead both the premotor cortex and primary motor cortex project directly to the spinal cord, and each has some capability to control movement even in the absence of the other. Therefore the two cortical fields operate at least partly in parallel rather than in a strict hierarchy. This parallel relationship was noted as early as 1919 by Vogt and Vogt and also emphasized by Fulton.

Penfield in 1937 notably disagreed with the idea of a premotor cortex. He suggested that there was no functional distinction between a primary motor and a premotor area. In his view both were part of the same map. The most posterior part of the map, in area 4, emphasized the hand and fingers and the most anterior part, in area 6, emphasized the muscles of the back and neck.

Woolsey who studied the motor map in monkeys in 1956 also believed there was no distinction between primary motor and premotor cortex. He used the term M1 for the proposed single map that encompassed both the primary motor cortex and the premotor cortex. He used the term M2 for the medial motor cortex now commonly known as the supplementary motor area. (Sometimes in modern reviews M1 is incorrectly equated with the primary motor cortex.)

Given this work by Penfield on the human brain and by Woolsey on the monkey brain, by the 1960s the idea of a lateral premotor cortex as separate from the primary motor cortex had mainly disappeared from the literature. Instead M1 was considered to be a single map of the body, perhaps with complex properties, arranged along the central sulcus.