Fig. 7.

Relation between the best direction of a unit and its location in BI. a, Best directions as found from the threshold line fit plotted against location in the muscle for each individual subject. The two preferred directions of a bidirectional unit are marked with a star for the best direction and an open circle for the second-best direction. Regressing only the best direction (for unimodally and bimodally tuned units) on location in the muscle yielded a significant correlation in subject B only. b, Pooling data from all subjects by normalizing recording location yielded a significant correlation between best direction and location (p < 0.005). This relation suggests that units located laterally and medially are best activated for upward and up–forward forces, respectively. (For simplicity, only the best direction is shown for each unit in the first plot. In the second plot, the second-best directions are included.)c, Map of biceps with recording sites in all subjects. Sites of electrode insertion for a particular subject are marked with a corresponding letter.