Abstract

Our understanding of human visual perception generally rests on the assumption that conscious visual states represent the interaction of spatial structures in the environment and our nervous system. This assumption is questioned by circumstances where conscious visual states can be triggered by external stimulation which is not primarily spatially defined. This work discusses psychophysical experiments investigating flicker induced subjective experiences of colour and form. Using the presentation of spatially uniform flicker with a precise temporal resolution it is shown that subjective experiences do not only depend on stimulation frequency, but also on stimulation phase. In addition, the occurrence of one subjective experience appears to be associated with the occurrence of others. The phenomenology of subjective colour and form is further explored and the physiological mechanisms underlying subjective colour are investigated using electroencephalography. While these data indicate that conscious visual experience may be evoked directly by particular variations in the flow of spatially unstructured light over time it may be assumed that the systems reponsible are essentially temporal in character and capable of representing a variety of visual forms and colours, coded in different frequencies or at different phases of the same processing rhythm.