Dissociation between hand motion and population vectors from neural activity in motor cortex

Preferred Direction of a cell. The activity of cells from monkeys a and b (39% of total sample) were recorded for 8 targets that were not symmetrically-distributed in space The standard approach (trigonometric method) to define the directionality of a cell 1 could not be used with these cells so we used techniques to describe planar objects (plate method) to identify the directionality of these cells. For each block of trials, vectors were constructed for each movement oriented with movement direction and scaled by cell discharge in polar space. The tips of adjacent vectors were connected by a curved line with a distance coordinate that varied linearly in magnitude in polar space between the two vector tips. Each pair of adjacent vectors and adjoining curved line was then treated as a three-sided planar object and we computed its relative area and centroid. Similar measures were made for each pair of adjacent vectors creating a total of eight objects. The total area and its centroid was then found across all eight objects. The position of the centroid relative to the origin served as a measure of the cell=s directionality for each block of trials. In order to test the validity of this plate method, the activity of 38 neurons was recorded for movements to 16 targets symmetrically distributed in space (every 22.5E). The PD of each cell computed using the trigonometric method from neural activity recorded for movements to all 16 targets was used as a baseline measure of the cell=s directionality. Table S1 shows the absolute mean error in the PDs for the different methods (trigonometric and plate) and the different target groups (16, 8 symmetric and 8 non-symmetric targets). The absolute mean error in the PD of cells using the 8 non-symmetric was 10.2E and only 2 neurons showed errors greater than the bin-size used to group the PD of cells in Fig. 2 (22.5E). Therefore, directional tuning of 95% of the cells sampled was within one bin-width of its actual tuning. Such small differences cannot account for the non-uniform distribution of preferred directions displayed in Figure 2. Relationship between cell modulation and PD. Figure S1a illustrates a polar plot of the mean cell modulation relative to movement direction. Cell modulation was defined as the highest mean discharge rate prior to and during movement (reaction time plus movement time) measured for any movement direction minus the lowest mean discharge …