Directional tuning profiles of motor cortical cells

The directional tuning profiles of motor cortical cells are commonly descri bed by a cosine tuning function with three adjustable parameters (Georgopou los, A.P., Kalaska, J.F., Crutcher, M.D. Caminiti, R., Massey, J.T., 1982. On the relations between the direction of two-dimensional (2D) arm movement s and cell discharge in primate motor cortex. J. Neurosci. 2, 1527- 1537). In this study the variation in the shape of the directional tuning profiles among a population of cells recorded from the arm area of the motor cortex of monkeys using movements in 20 directions, every 18 degrees, was examine d systematically. This allowed the investigation of tuning functions with e xtra parameters to capture additional features of the tuning curve (i.e. tu ning breadth, symmetry, and modality) and determine an 'optimal' tuning fun ction. These functions provided better fit than the standard cosine one. Th e optimal function for the large majority of tuned cells was unimodal (84%) , and only for a few of them (16%) it was bimodal. Of the unimodal cells, 7 3% exhibited symmetric and 27% asymmetric shape. The half-width, sigma, at the midpoint of optimal tuning curves differed among cells from 30 to 90 de grees, with a median at 56 degrees. This is much narrower than in the stand ard cosine tuning function with a fixed width of sigma = 90 degrees. It was concluded that motor cortical cells are more sharply tuned than previously thought. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.