Functional organization of inferior area 6.

The rostral part of the agranular frontal cortex (area 6) of the monkey consists of two large sectors: a superior sector lying medial to the spur of the arcuate sulcus (superior area 6) and an inferior sector lying lateral to it (inferior area 6). Single neurons have been recorded from inferior area 6 in behaving monkeys (Macaca nemestrina). The results were: (a) Proximal movements are essentially represented caudally in the histochemically defined area F4. Neurons related to these movements respond strongly to tactile and visual stimuli. Visual receptive fields are located in the space around the animal's body (peripersonal space) and their location does not change with eye movements. The direction of movements effective in triggering the neurons is congruent with the position of their visual receptive field. (b) Distal movements are represented rostrally in the anterior part of F4 and in F5. Neurons related to these movements discharge vigorously during motor acts that have a precise aim. The neurons were subdivided into four classes: grasping-with-the-hand neurons, grasping-with-the-hand-and-mouth neurons, holding neurons, and tearing neurons. Regardless of the class they belong to a large number of neurons show specificity for different types of object prehension--discharging, for example, during precision grip but not during whole-hand prehension. It is proposed that inferior area 6 contains a vocabulary of motor acts related to hand-mouth movements. The motor acts can be retrieved by visual and somatosensory stimuli. The possibility is discussed that a series of vocabularies where movements of various complexity are stored represents the neural basis of cortical motor organization.