The averaged electrical responses to diffuse and to patterned light in the human

1. 1. A marked difference was obtained in the averaged electrical responses of normal subjects when patterned light was used for stimulation instead of diffuse light. This difference is manifested mainly by the presence of a surface-positive “late wave” (180–375 msec), the amplitude of which varies with the density of contrast borders between black and white lines of the stimulus pattern. The different effect of diffuse and patterned light was also manifested with paired and with repetitive stimuli. Since the “light response”, the “cortical excitability cycle” and the “driving” vary with comparatively subtle changes of the input, these evoked phenomena cannot be considered to be only expressions of invariable cortical characteristics. 2. 2. Spontaneous or induced changes in attention or variations in light intensity affected both types of responses in the same sense and did not appear to account for the difference between them. 3. 3. Because the “late wave” depends on the peripheral input and because it is limited to the posterior head regions like the other components of light responses, it should be considered “specific” notwithstanding its long latency.