Psychophysics and electrophysiology of flicker-induced subjective experiences

Document Type: 
ASSC Conference Item
Article Type: 
Sensory Systems
flicker, subjective experiences, colour, form, frequency, phase, EEG, alpha band
Deposited by: 
Dr Cordula Becker
Date of Issue: 
Cordula Becker, Klaus Gramann, Mark A. Elliott
Event Dates: 
23-26 Jun 2006
Event Location: 
Oxford, Great Britain
Event Title: 
10th annual meeting of the Association for the Scientific Study of Consciousness
Event Type: 
ASSC Conference
Presentation Type: 
Number of Pages: 
It is generally assumed that conscious visual experiences result from the interaction of our nervous system with light reflected from spatial structures in the environment. Indeed, most conscious experiences seem to be directly related to the characteristics of the external stimulus, so that for example spatial properties of the external stimulus are directly mapped onto spatial properties of the internal experience. Yet, circumstances exist where conscious visual states are triggered by external stimulation which is not related to the phenomenal content in any direct sense. In the following we will refer to such states as subjective experiences. If these subjective experiences can be shown to be objectively attributable to processes in the causal chain of normal visual perception, they may enhance our understanding of the human visual system. In a recent study (Becker & Elliott, Consciousness and Cognition, 2006) we showed that subjective visual experiences of color and form can reliably be induced by stimulation with intermittent light over the entire visual field. These subjective experiences specifically relate to characteristics of the intermittent stimulation, such as frequency and phase. The observed frequency specificity of subjective color may be explained by the varying response latencies of the three cone types. Furthermore, opponent colors have been reported at clearly opponent phases of the stimulation rhythm suggesting that color opponency is coded in different phases of neural processing rhythms. The perception of subjective forms may have its origin in states of oscillatory resonance in the neural tissue. What is more, forms like spirals, gratings, and radials appear to be associated with distinct colors, such as red and green, suggesting that it is primarily red/green coding pathways which enter such states of resonance. Using EEG recordings we found that the report of a subjective color is preceded by a widespread decrease of power in the lower alpha frequency band. We speculate that this decrease acts as a gating process enabling the brain to establish the observed increase in gamma band activation which ultimately leads to the formation of a coherent percept of subjective color.
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