Abstract
The visually evoked cortical responses we recorded in the light-adapted state consisted of primary complexes and spike-like waves. The properties of the latter waves, which we have called the light-adapted response (LAR), were studied at several levels of light adaptation.
1) The amplitude and latency of the LAR varied systematically depending upon the ratio between the intensities of photic stimulus and adapting light.
2) The LARs were classified into two groups: early and late L-waves. The early L-waves (L1-L3), which had a variable crest time, were produced predominantly at the higher level of light adaptation, whereas the late L-waves (L4-L6) with fixed crest time were produced at the lower level of adaptation.
3) When the intensity of the adapting light was increased nearly to that of the test flash, the critical LARs were recorded accompanying the suppressed primary complexes. The time interval from the primary complex to the critical LAR remained constant, regardless of stimulus intensity. At the same ratio of adapting light to stimulus, a similarity in wave-form and amplitude of the critical LAR was found. Within the intermediate range of stimulus light, the threshold (log I) of the cortical LAR increased proportionally with the increase in level of light adaptation (log L) after Weber's law. The critical contrast index calculated for this log I-log L curve was about-1.5.
4) In human subjects, brightness discrimination curves were also determined by the same photic apparatus used for the animal experiments. The results indicated that the brightness discrimination curves (log T-log B curves) which were obtained with a test field of 20° in visual angle showed a linear relationship following Weber's law. This log T-log B curve coincided with the log I-log L curve obtained in animals. The contrast index calculated for this log T-log B curve was about-1.5 to-1.8.
5) This agreement between electrophysiological and psychophysical findings suggests a close relationship between subjective liminal brightness discrimination and the critical LAR of the cortical response.
