ARTIFICIAL SYNTHESIS OF EVOKED RESPONSES TO LIGHT FLASH

A stimulus of finite duration must be considered to consist of no less than two “events.” The term “event” will be defined in this context as any alteration of input conditions, One such change occurs when the stimulus begins and another change occurs when it ends. For a rectangular stimulus wave form there are only two events. More complex stimuli may have other events interpolated between these primary ones. The concept of “duration” is derived from the fact that a stimulus has a beginning and an end. If these two events cannot be separately identified, the stimulus will not provoke a sensation having any subjective duration. It can only be experienced psychologically as “instantaneous”“now.” Such brief stimuli, as shown by the experiments of Fessard and Durrup (1931) and Charpentier (1887) are, indeed experienced as without duration. Previous studies from this laboratory (Efron 1963, a, b, c, d ) have been concerned with psychophysical measurements of temporal perception, particularly the judgments of simultaneity and of order. It is the purpose of the present paper to report some recent experiments on sensory evoked potentials which may be correlated with temporal perception. To illustrate the importance of temporal sequence in perception I would like to briefly show a film demonstrating the Pulfrich effect. You will see that the phenomenon cannot be accounted for unless we use the concept “now.” ( A film was projected to demonstrate the Pulfrich illusion. See Efron ( 1963 c ) for details). The explanation of the Pulfrich effect is explicitly based upon the idea that covering one eye with a filter will delay the conscious awareness of perceptions initiated by stimuli to that retina. I t is clear then, that this phenomenon is due to the time taken for transmission of a sensory message-and that this time is dependent on the intensity of the stimulus. Recent psychophysical studies in our laboratory (Efron 1963 b ) have shown that the transmission, for visual stimuli, is delayed about 10 msec. for each log unit reduction of intensity. The question immediately was raised as to whether the electrical correlate of this phenomenon could be seen in the evoked potential to a light flashand whether the change in latency of the first deflection showed the same 10 msec./log unit slope. Previous studies by Cobb and Dawson (1960) had indicated that this was the case, and we found the same thing. A precise demonstration of this relation has already been reported at this conference by Diamond