Top-down Modulation of the Chromatic VEP with Hypnotic Suggestion
AuthorDuncan, Chad S.
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AbstractVisual perception is composed of both bottom-up (stimulus driven) and top-down (feedback) processes. It has been demonstrated that directed spatial attention enhances processing of attended stimuli, inhibits non-attended stimuli, and increases baseline activity in portions of cortex corresponding to attended areas of the visual scene in the absence of a stimulus. However, mechanisms of attention appear to influence processing of chromatic stimuli differently than that of achromatic stimuli. Visual evoked potentials (VEPs) recorded under such conditions agree with this pattern of activation when elicited by purely achromatic grating stimuli. However, when stimuli were chosen to preferentially activate the S-(L+M) or L-M chromatically opponent pathways, no changes in signal were detected as a function of attention, suggesting different amounts of feedback, or different mechanisms of feedback, reaching the visual cortex where the VEP is thought to originate. Hypnosis is another form of top-down manipulation that can produce significant signal change in the VEP. A set of four experiments were conducted to investigate whether or not feedback extending to lower visual areas is capable of altering processing of incoming information in the presence of hypnotic suggestion. Positive and negative suggestions were used to invoke hallucinations regarding stimulus presence and absence, as well as imagined stimulus occlusions. It was hypothesized that unlike attentional manipulations hypnotic suggestions would affect VEPs elicited by stimuli that were designed to isolate the individual opponent pathways. However, similar to results from attention VEP studies, a significant difference was obtained in the achromatic waveform but not in the chromatic waveform. These data indicate that hypnotic suggestion may feedback to lower visual areas in a manner similar to that of attentional manipulations.