CODING OF PERIPERSONAL SPACE IN INFERIOR PREMOTOR CORTEX (AREA F4)

1. We studied the functional properties of neurons in the caudal part of inferior area 6 (area F4) in awake monkeys. In agreement with previ ous reports, we found that the large majority (87%) of neurons respond ed to sensory stimuli. The responsive neurons fell into three categori es: somatosensory neurons (30%); visual neurons (14%); and bimodal, vi sual and somatosensory neurons (56%). Both somatosensory and bimodal n eurons typically responded to light touch of the skin. Their RFs were located on the face, neck, trunk, and arms. Approaching objects were t he most effective visual stimuli. Visual RFs were mostly located in th e space near the monkey (peripersonal space). Typically they extended in the space adjacent to the tactile RFs. 2. The coordinate system in which visual RFs were coded was studied in 110 neurons. In 94 neurons the RF location was independent of eye position, remaining in the same position in the peripersonal space regardless of eye deviation. The R F location with respect to the monkey was not modified by changing mon key position in the recording room. In 10 neurons the RF's location fo llowed the eye movements, remaining in the same retinal position (reti nocentric RFs). For the remaining six neurons the RF organization was not clear. We will refer to F4 neurons with RF independent of eye posi tion as somatocentered neurons. 3. In most somatocentered neurons (43 of 60 neurons) the back ground level of activity and the response to v isual stimuli were not modified by changes in eye position, whereas th ey were modulated in the remaining 17. It is important to note that ey e deviations were constantly accompanied by a synergic increase of the activity of the ipsilateral neck muscles. It is not clear, therefore, whether the modulation of neuron discharge depended on eye position o r was a consequence of changes in neck muscle activity. 4. The effect of stimulus velocity (20-80 cm/s) on neuron response intensity and RF extent in depth was studied in 34 somato-centered neurons. The results showed that in most neurons the increase of stimulus velocity produce d an expansion in depth of the RF. 5. We conclude that space is coded differently in areas that control somatic and eye movements. We sugges t that space coding in different cortical areas depends on the computa tional necessity of the effecters they control.