STUDIES OF THE PHOTICALLY STIMULATED EEG Quantification and Stability of Photic Driving Patterns

摘要

Work utilizing an electronic analyzer has presented a clear picture of the EEG response profile to photic stimulation. It was noted that the greatest response was in the frequency range 8.5 to 24 f.p.s., and hence in studies involving a normal population it would appear that stimulation outside of this band would be unnecessary. Like Mundy-Castle, we, too, have found the peak driving response to light stim¬ulation with eyes open in the alpha region. This would at first glance seem to indicate a relationship between resting and driving activity. That this is not so is indicated by the correlations between driving and eyes closed resting. Although, as in the case of the resting record, there is more activity during photic stimulation in the alpha region, one is not able to predict the amount of driving activity in the 8 to 12 region from the amount of resting activity in this band. It was apparent also from this study that driving in any one region did not necessarily indicate a marked response in other regions. One should not conclude that a subject who shows marked driving response at 8 would necessarily show a similar response at 22. To obtain a driving profile for any individual, a representative sampling throughout the frequency range 8 to 24 should be made. We found second harmonic response to be common in the range 6 to 33. Third and fourth harmonic responses were clearly elicited but were seen less frequently and subbarmonics were uncommon in this normal group. Mundy-Castle has reported that sub-harmonic responses are often seen in subjects with epilepsy or with either questionable or abnormal features in the resting EECnPrevalent theory holds that the photic driving response may be explained on the basis of resonance where, in a given individual, certain specific frequencies may tend to show recur¬rence. In light of this theory we were surprised to observe that there is no tendency for any one frequency or group of frequencies to show a response at 1:1 stimulation and a response when that frequency is the harmonic or sub-harmonic of other stimulus frequencies (i.e., although 9 c.p.s. may show a good driving response at 1:1 stimulation it does not neces¬sarily respond as a harmonic of 3 f.p.s. or as a subharmonic of 18 f.p.s.). While we have found that the shape of the driving profile was definable and consistent over a period of time, the amount of total driving and driving at most frequencies varied significantly for both group and individual from trial to trial. K'hile we are unable at this time to offer an adequate ex¬planation for the variability in amount of driving from trial to trial, it is of interest, although not significantly different from record three, that the lowest amount of activity was found in the initial record. Previous work from this laboratory suggested that tension serves to lower the driving response (18). It would seem likely that these subjects were most anxious during their first exposure to photic stimulation in a strange laboratory setting