Area 4 and area 5: differences between the load direction-dependent discharge variability of cells during active postural fixation.

Previous experiments have shown that the activity of cells in both areas 4 and 5 is strongly modulated in a continuously-graded manner for movements of the arm in 8 different directions away from a central starting position. However, it is reported here that there is a clear difference between these two areas during postural fixation of the arm in one position against loads applied in 8 different directions. Area 4 cell activity shows large variations when the monkey holds its arm at the central position by exerting forces in different directions to counteract the loads. The activity of cells in area 5 is much less strongly modulated under the same isometric conditions. These observations suggest that cell activity in area 5 is less related to the level and pattern of force output than are cells in area 4. Area 5 may produce instead a signal related more to the limb position, per se, during actively maintained posture.