Physiological demonstration of multiple representation in the forelimb region of the cat motor cortex

Abstract

Earlier studies in primates have demonstrated a double representation of the distal forelimb in area 4. In this study intracortical stimulation was used to map the representation of the forelimb in area 4 of the cat. Maps of individual animals revealed two spatially separate representations for the distal forelimb in area 4. Two "digit zones," regions in which threshold stimulation evoked contractions limited to digit musculature, were seen in all animals. Although the absolute location of the two digit zones varied among animals, the zones were always separated by a field in which more proximal musculature was represented. In some experiments EMG activity was monitored from selected forelimb muscles in order to determine the muscles represented in the two zones. Activity of the same digit muscle could be evoked by stimulation in each digit zone. The analysis demonstrated that some digit muscles were represented in both the digit zones. Thus, this study demonstrates that multiple representation of the distal forelimb in area 4 is not an isolated, species-specific phenomenon, but is likely to be a generalized pattern of motor cortex organization.

Electromyographic responses evoked in muscles of the forelimb by intracortical stimulation in the cat.

“…The findings demonstrate that motoneurones supplying all the muscles studied can be discharged by intracortical stimulation and they also indicate that the double representation of muscles shown in the cat motor cortex by Pappas & Strick (1981) is present in the awake animal. However, this appears not to be confined to digit muscles, but extends also to muscles acting around the wrist, elbow and shoulder.…”

“…It seems likely, therefore, that further investigation in the absence of anaesthesia will reveal a pattern of double representation in which the areas related to different joints overlap at their edges but not at their centres. Pappas & Strick (1981) found essential similarity between their rostromedial and caudolateral low-threshold zones in that both flexors and extensors of the digits were represented in each zone, as also were the extrinsic and intrinsic muscles acting on the digits. This similarity is paralleled in our own findings.…”

“…First, it is clear that for each muscle the effective points were rather widespread. This scatter may result partly from the procedure of pooling results from different animals (see Pappas & Strick, 1981;Armstrong & Drew, 1984 a) but in any one animal the locations which produced activity in a single muscle were frequently widely separated and were sometimes on opposite boundaries of the distributions in Fig. 2.…”

“…Unfortunately our experiments permit no more exact conclusion because a relatively small number of fixed electrodes was used and it was necessary to pool results from several animals. Pappas & Strick (1981) have shown that the surface landmarks in the cat pericruciate region are sufficiently variable that pooling is likely to blur maps made to reveal functional subdivisions within the cortex. However, any blurring should have acted approximately similarly on our maps for different muscles.…”

“…In view of these findings, and because Pappas & Strick (1981) have found in anaesthetized cats that contractions of digit muscles are produced from two distinct low-threshold areas in the forelimb motor cortex, we investigated e.m.g. responses evoked in a range of forelimb muscles by intracortical microstimulation.…”