STUDIES IN THE PHYSIOLOGY OF THE NERVOUS SYSTEM. XXVII.: ON THE PHENOMENON OF FACILITATION. 6: THE MOTOR ACTIVATION OF PARTS OF THE CEREBRAL CORTEX OTHER THAN THOSE INCLUDED IN THE SO‐CALLED “MOTOR” AREAS IN MONKEYS [EXCITATION OF THE POST‐central GYRUS]; WITH A NOTE ON THE THEORY OF CORTICAL LOCALISATION OF FUNCTION
1. In preceding papers of this series it has been shewn that the phenomenon of “primary facilitation” of a cortical motor flexion‐point lasts for a comparatively short time when stimuli of liminal strength are used. It may, however, occur when the stimuli are of sub‐liminal value—as is well known.2. In a similar manner facilitation of the sub‐cortical projection mechanism is also demonstrated in these papers.3. It has also been shewn that after the primary facilitation of a cortical flexion‐point the excitability of other similar flexion‐points may be raised —termed here the phenomenon of “secondary facilitation.”4. The “secondary facilitation” of a less excitable cortical flexion‐point after “primary facilitation” of a more excitable flexion‐point does not occur after the freezing (and subsequent thawing) of the former point. It may, however, still be present for other less excitable (unfrozen) points which are further removed than the frozen point from the point which is directly facilitated. This shews that the phenomenon is not one due merely to “spread” of the electric current.5. The “secondary facilitation” of a less excitable point by the “primary facilitation” of a more excitable one is not obtained after a cut in the cortex (and underlying white matter) which is placed between the two points with its plane perpendicular to the line joining them. This shews that the phenomenon of “secondary facilitation” is not solely conditioned by the facilitation (during the “primary facilitation” of the more excitable point) of a lower motor mechanism common to both points. It must be conditioned through association fibres at the cortical level.6. If this is the case it is a question whether the apparent “secondary facilitation” of a point is conditioned by a direct action upon it of the point which is “primarily facilitated” or by an increase in the effect of the direct action of the first point upon the second (which is exhibiting “primary facilitation”). In either case the phenomenon is conditioned by the action of association fibres, and in either case there is probably a double action between the two points.7. In the present paper the phenomenon of “post‐central activation” is described. After the “primary facilitation” of a motor flexion‐point a neighbouring point on the post‐central cortex may respond with a similar motor reaction to electric stimulation.8. This phenomenon of “post‐central activation” is in many ways like the “secondary facilitation” of a motor point. It is not usually obtained after the freezing of the post‐central point, nor after a longitudinal lesion set between the post‐central point and the motor flexion‐point, and with its plane perpendicular to the line which joins them. It is therefore probably a physiological phenomenon and not due to “spread” of current; and if so it is probably conditioned by association fibres between the two points, and not by the facilitation by the motor point of a lower motor mechanism common to them both.9. But just as the phenomenon of “secondary facilitation” of a less excitable motor point by the “primary facilitation” of a more excitable motor point may probably in part be conditioned by the facilitation of a lower motor mechanism common to them both, so in the case of the post‐central point there is some evidence which seems to point to a similar occurrence in certain circumstances. If this is the case it seems to shew that the post‐central mechanism may act on lower mechanisms by means of its own projection fibres, and not only through the “motor” mechanism in the pre‐central convolution.10. Nevertheless it is shewn that the phenomena both of the “secondary facilitation” of a cortical motor point and the “activation” of a post‐central point are conditioned at the cortical level. This for the first time demonstrates a phenomenon of “facilitation” in the cortical mechanism itself, after exclusion of the possibility of its location in a common lower mechanism through which the “motor” centres in the cortex act.11. In the conclusions of this paper these different phenomena are considered in their bearing upon the problem of cortical localisation of function.