Review
Imaging the premotor areas

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Abstract

Recent imaging studies of motor function provide new insights into the organization of the premotor areas of the frontal lobe. The pre-supplementary motor area and the rostral portion of the dorsal premotor cortex, the ‘pre-PMd’, are, in many respects, more like prefrontal areas than motor areas. Recent data also suggest the existence of separate functional divisions in the rostral cingulate zone.

Introduction

Large regions of the brain located on the lateral surface and on the medial wall of each hemisphere participate in the generation and control of movement. The premotor areas in the frontal lobe have the anatomical substrate to influence motor output, both through connections with the primary motor cortex (M1) and through direct projections to the spinal cord (e.g. [1]). In monkeys, the frontal lobe contains six well-defined premotor areas (Fig. 1a). The presence of analogous areas in humans has been inferred from functional imaging studies (Fig. 1b) 2., 3.. However, the definition of premotor areas in humans is still evolving. Some associations between anatomy and function proposed in the past [2] have been validated by recent imaging data, and other associations are emerging.

In this review, we present the results of salient imaging studies that have helped to increase our understanding of the underlying anatomical and functional organization of the premotor areas. Much recent effort in the field of functional imaging has been to examine brain regions involved in cognitive operations. The interpretation of activations in premotor cortex during cognitive operations depends critically on a clear definition of the location and boundaries of these cortical areas. Thus, one focus of our review synthesizes the new data that is relevant to this issue.

Section snippets

Pre-supplementary motor area and supplementary motor area

In monkeys, it is now established that area 6 on the medial wall of the brain contains two separate areas: the supplementary motor area proper (SMA) in the caudal portion of area 6, and the pre-SMA in the rostral portion (Fig. 1a; reviewed in 2., 4.). The SMA and pre-SMA are equivalent to fields F3 and F6 described by Matelli et al. [5]. In humans, the level of the anterior commissure (VCA line) [6] marks the border between the two areas. The division of medial area 6 into two distinct fields

The dorsal part of the lateral premotor cortex

In monkeys, the dorsal part of the lateral premotor cortex (PMd) has been divided into rostral (F7, PMdr) and caudal (F2, PMdc) subdivisions 5., 52., on the basis of anatomical and physiological differences (Fig. 1a). These differences are analogous to those that generate the pre-SMA/SMA split. In fact, in a number of important respects, the caudal portion of the PMd has much in common with the SMA proper. Both areas project to the primary motor cortex and directly to the spinal cord 1., 8., 52.

Conclusions

The data presented in this review help to clarify the location and boundaries of the premotor areas in the frontal lobe of humans. For many of these areas, there is a clear correspondence with a specific premotor area in the monkey. For other regions, such as the motor fields in inferior frontal cortex, the correspondence with areas in the monkey brain remains to be established. Two regions have generally been considered to be motor fields: the rostral part of area 6 on the medial wall of the

Acknowledgements

Our work is supported by the Veterans Administration Medical Research and Rehabilitation Research and Development Services (PL Strick), and United States Public Health Service grant NS24328 (PL Strick).

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • •of special interest

  • ••of outstanding interest

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