The purpose of this study was to investigate the relationship between the visual after-image and corresponding brain waves (EEG) unde more specific stimulus conditions thar Jasper & Cruikshank (1936, 1937) and Motokawa & Mita (1941). The stimulus conditiom used in the present experiment were : 15 to 200 lux in intensity, 3 to 60 sec. in duration and 1×1 sq. cm. to 60×60 sq. cm. in size. Analysis of EEG corresponding to after-images was made in two ways : (1) in terms of the amplitude of glpha waves, and (2) by the use of a band-filtered method developed by Komizo & Yamaoka (1954).Procedure and SubjectsThe subject was asked to be seated facing a stimulus figure in the dark room. EEG's were recorded of the subject while he was closing his eyes. As soon as the light stimulus was exposed he was instructed to open his eyes and when the stimulus was extinguished after a certain duration of time, he was required to close the eyes. When an after-image was perceived, he had to press on an electric key.When the stimulus intensity was varied, a constant duration of 30 sec. and a constant size of 3×3 sq. cm. were used. The stinlulus duration was changed with the same area and intensity (3×3 sq. cm. and 50 lux). When the stimulus size was variable, a constant intensity of 50 lux and a constant duration of 30 sec. were utilized. An EEG recorder with 6 channels, manufactured by Sanei Co. was used for experimentation. Four experienced subjects were observed.Results and Discussions1. Appearance of after-images agreed with the decrease in alpha wave amplitude and its disappearance with the increase in the amplitude. This was most conspicuous at the beginning of the whole phase, but was obscured at the end where alpha waves reached a certain level of amplitude.2. After-images were seen longer and more frequently when the stimulus intensity was 200 lux. With the optimal intensity for the subject, after-images became clearer in contour and their intensity increased. The corresponding EEG showed regular fluctuations and the amplitude of alpha waves indicated regular increases and decreases.3. When the duration of a light stimulus was varied, corresponding changes were shown in after-images and EEG. The longer the stimulus duration was, longer was its after-image prolonged and the EEG more regular in fluctuations.4. When the stimulus size was of a optimal visual angle (7.2°to 13.9°) its atter-imags was seen longer and the EEG more regular in fluctuations.5. A band-filtered analysis of EEG showed that wave components of 20, 10, 5 and 2.5 c/s were involved in the EEG of after-images. Components of 10 c/s changed with components of 5 c/s while those of 20 c/s varied with those of 2.5 c/s. They behaved antagonistically.6. A slight difference was suggested between the EEG of a light stimulus and that of its after-image. In the former dominance of 20 c/s components was shown for a certain length of time, while in the latter 10 c/s and 5 c/s components were more conspicuous and 20 c/s components were inhibited, being shifted to a lower frequency of 18 c/s.
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Apr 1, 1959
Open Access