Cobalt applied to the sensorimotor area of the cortex cerebri of the rat.

1. In anaesthetized rats, the area of the cerebral cortex where the electrical response to stimulation of the fore or hind paw has a minimum latency was found to bear a fairly constant relation to bony landmarks of the skull. Reasons are given for accepting the site of minimum latency as being the area most closely associated with the periphery.2. In animals under anaesthesia, the cortical areas responding with minimum latency to fore- and hind paw stimulation are spatially distinct.3. Cobalt powder has been applied to one of the two cortical areas where the electrical response to contralateral fore- or hind paw stimulation is a minimum. The animals have been allowed to recover and the time course of the resulting muscular jerks has been followed.4. Jerks occur most frequently and earliest in the contralateral forelimb after the application of cobalt to the forelimb area of the cortex. When the cobalt is placed on the hind limb area, these jerks appear first and occur most frequently in the hind limb. This implies that the cortical areas most closely connected with fore and hind limbs are physiologically distinct in the absence of anaesthesia.5. The limb jerks start to appear after 24 hr, occur with increasing frequency until the 10th day and thereafter decline. The decline in jerking is attributed to the formation around the cobalt of a connective tissue capsule.