Fig. 206.—Diagrammatic representation of electric after-effect of stimulation. Pre-maximal stimulation produced by stoppage of light at a, gives rise to continuation of previous response followed by recovery. Stoppage of light at maximum b gives rise to recovery to equilibrium position. Stoppage of light at post-maximum c, gives rise to over-shooting below zero line.
After-effect of pre-maximum stimulation: Experiment 214.—Light is applied at arrow and stopped in different experiments at a, b, and c (Fig. 106). Continuous stimulation induces increasing galvanometric negativity; when stimulus is stopped at a before the maximum, the after-effect is a persistence of excitatory galvanometric negativity, which carries the response record higher up; after a certain interval recovery takes place and the record returns to the zero line of normal equilibrium. The after-effect of pre-maximum stimulation is thus a short-lived continuance of response followed by recovery.
After-effect at maximum: Experiment 215.—In this the photic stimulus was continued till the attainment of maximum, when light was suddenly removed at b. The after-effect was no longer a persistence of responsive movement, but disappearance of negativity and recovery to zero line of equilibrium.
Post-maximum after-effect: Experiment 216.—In this light was continued till there was a complete neutralisation, the curve of response returning to zero line; to all outer seeming the responsive indication of the tissue is the same as before excitation. But stoppage of stimulus at c causes an over-shooting at a rapid rate far below the zero line; and it is after a considerable period that the curve returns to the zero line of equilibrium.
The condition at post-maximum c is thus one of dynamic equilibrium where two opposite activities, "A" and "D," balance each other; for had the condition of the 'neutralised' tissue been exactly the same when fresh, cessation of stimulus would have kept the galvanometric spot of light at the zero position.
The electric investigation described above shows that the after-effect is modified by duration of stimulation, and that:
(1) the after-effect of pre-maximum stimulation is the continuation of response in the original direction (upward, and away from zero line), followed by recovery,
(2) the after-effect of the maximum is an electric recovery towards zero position, and
(3) the after-effect of post-maximum stimulation is an over-shooting downward below the zero line.