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Many investigators have prepared maps of motor cortex responses to stimulation at different sites, but hitherto none has attempted to demonstrate the alterations that occur in cortical maps when the peripheral conditions (such as the initial position of the responding limb) are altered. That this position may alter the response to stimulation of a particular cortical site has been demonstrated by Gelihorn (1948Gelihorn ( , 1949 and Gelihorn & Johnson (1950a); the influence of proprioceptive conditions on a map of cortical sites has not been shown, however. Since proprioceptive reinforcement will make subthreshold cortical stimuli effective for a particular muscle group and reduce or abolish the reactivity of the antagonistic group it seemed possible that variations in the degree of proprioceptive activity would induce changes in the size of the cortical area from which a certain muscle or group of muscles can be activated at a given intensity of stimulation.In the present paper examples of such maps will be given and described, and certain features of these maps will be discussed (see also Hyde, 1950). METHODIn a series of eleven monkeys (nine macaques, two Cynemolgi) a number of cortical sites were stimulated and electromyographic responses recorded in six muscles at a time, with the contralateral leg fixated in the two conditions of (a) knee flexed and ankle dorsi-flexed as compared with (b) knee extended and ankle plantarflexed, the hip being fixated in a neutral position and toes being free throughout. The methods of stimulation (repetitive condenser discharges from a Goodwin stimulator) and recording were the same as in earlier publications from this laboratory.In order to be sure that the cortex was relatively stable throughout the mapping, a number of different procedures were tried, of which the following was most satisfactory and was used in most if not all experiments of the present paper: four or five sites were stimulated with the leg in flexion, the same sites were then stimulated with the leg extended, and as a check the first of the sites would again be stimulated with the leg in flexion. If the first and last records of this (series) were quite similar, the series was considered valid. 24-2

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