Subjective visual experiences of colour and form induced by temporally modulated light

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.