~ ~cvic BRE 20828 A sequential representation of the occiput, arm, forearm and hand across the rostrocaudal dimension of areas 1, 2 and 5 in macaque monkeys T. P. PONS, P. E. GARRAGHTY*, C. G. CUSICK and J. H. KAAS Department of Psychology, Vanderbilt University, Nashville, TN 37240 (U. S. A. ) (Accepted January 3rd, 1985) Key words." somatosensory cortex - - somatotopic representation - - monkey cortex - - areas I, 2, 5 Receptive fields were determined for recording sites in cortical areas 1, 2 and 5 in macaque monkeys. While a mixture of cutaneous and deep receptors were represented in most regions of area 2, a specific mediolateral level of area 2 was highly responsive to cuta- neous input. Anteroposterior rows of recording sites across areas 1, 2 and 5 at this mediolateral location revealed a continuous se- quence of cutaneous receptive fields proceeding from the occiput and face in area 1 to the forearm and hand in area 5. As part of a continuing effort to determine the or- ganization of somatosensory cortex in primates, we have extended our microelectrode mapping studies of postcentral parietal cortex in macaque monkeys from architectonic areas 3b and 1 to the more caudal fields, areas 2 and 5. Previously, we presented evi- dence that areas 3b and 1 contain separate repre- sentations of the body surface that are organized in parallel and as rough mirror reversals of each other from their common bordere,L More limited record- ings from area 2 suggested that a mixture of inputs from deep and cutaneous receptors was organized in parallel and, at least in part, as a mirror reversal of the somatotopic pattern in area 1. This report docu- ments our unexpected finding of a continuous repre- sentation of the body surface from the back of the head to the hand as recording sites were sequentially moved in an anteroposterior band of cortex from caudal area 1, across area 2, and into area 5. Regions of area 2 and 5 immediately medial or lateral to this band were largely unresponsive to cutaneous stimuli in the same anesthetized preparations. These results indicate that area 2 is not uniform in responsiveness to cutaneous stimuli, and that a 'mirror reversal' rule for the organization of adjoining sensory representa- tions may not always apply. The present report is based on recordings from ap- proximately 4200 cortical sites in 7 rhesus monkeys (Macaca rnulatta) anesthetized with ketamine HC1. Our recording procedures have been described in de- tail elsewhere2. 5. Small clusters of cells were record- ed extracellularly from the middle cortical layers of areas 1, 2 and 5 using tungsten microelectrodes with low impedances of 1.0-1.5 Mff2 (at 1 kHz). The site of each penetration into cortex was marked on a pho- tograph of the exposed surface of the brain. Record- ing sites were classified as activated by cutaneous stimuli (moving hairs or lightly touching the skin), non-cutaneous stimuli (moving muscles or joints), or unresponsive to these somatic stimuli. Small electro- lytic lesions (10 ktA for 10 s) were made at selected points in cortex to aid in relating mapping data to cy- toarchitectonic fields. The representations of the occiput and neck in postcentral somatosensory cortex has long been known to be 'split' from the representation of the rest of the head10. Thus, the location of the occiput repre- sentation in area 1 was found some 9-12 mm medial to the representation of the rest of the head and face (Fig. 1). Our recordings in part of area 2 immedi- ately caudal to the representanon of the occiput re- vealed a cutaneous representation of the shoulder Correspondence: J. H. Kaas, Department of Psychology, Vanderbilt University, 134 Wesley Hall, Nashville. TN 37240. U.S.A. * Present address: Section of Neuroanatomy, Yale University School of Medicine. New Haven, CT 06510, U.S.A. 0006-8993/85/$03.30 © 1985 Elsevier Science Publishers B.V. (Biomedical Division)