Date Published: August 5, 2013
Publisher: Public Library of Science
Author(s): Mika Koivisto, Svetlana Kirjanen, Antti Revonsuo, Sakari Kallio, Adrian G. Dyer.
Hypnotic suggestions may change the perceived color of objects. Given that chromatic stimulus information is processed rapidly and automatically by the visual system, how can hypnotic suggestions affect perceived colors in a seemingly immediate fashion? We studied the mechanisms of such color alterations by measuring electroencephalography in two highly suggestible participants as they perceived briefly presented visual shapes under posthypnotic color alternation suggestions such as “all the squares are blue”. One participant consistently reported seeing the suggested colors. Her reports correlated with enhanced evoked upper beta-band activity (22 Hz) 70–120 ms after stimulus in response to the shapes mentioned in the suggestion. This effect was not observed in a control condition where the participants merely tried to simulate the effects of the suggestion on behavior. The second participant neither reported color alterations nor showed the evoked beta activity, although her subjective experience and event-related potentials were changed by the suggestions. The results indicate a preconscious mechanism that first compares early visual input with a memory representation of the suggestion and consequently triggers the color alteration process in response to the objects specified by the suggestion. Conscious color experience is not purely the result of bottom-up processing but it can be modulated, at least in some individuals, by top-down factors such as hypnotic suggestions.
Suggestions given with or without hypnosis may alter conscious color perception and modify neural activity in color processing areas of the brain –. Furthermore, suggestions to see specific objects in certain colors posthypnotically (i.e., after hypnosis has been cancelled) may selectively alter their perceived color . The neural mechanisms of such alterations are not known. We hypothesized that for a posthypnotic suggestion to rapidly alter the perceived color of a subset of objects selectively, some mechanism must compare the early bottom-up signal to the suggested content in order to trigger the color alteration process before the object enters consciousness. High-frequency neural oscillations provide a mechanism for rapid comparison and communication between distant brain areas. For instance, the early evoked gamma-band response is known to reflect automatic matching of bottom-up signals with memory contents about 100 ms after the stimulus-onset . Thus, object-specific posthypnotic alterations in color perception might involve an early high-frequency mechanism that compares the bottom-up input to the content of the suggestion in order to identify the objects relevant for the suggestion.
The mechanisms of hypnotic color alterations were studied by asking two very highly hypnotizable participants (TS-H and RM) to detect the colors of briefly presented shapes in the normal waking state after having been given posthypnotic suggestions in hypnosis that specific shapes will appear in altered colors. TS-H reported altered colors in the targeted shapes and her results showed that oscillatory activity in the higher beta-band correlated with the contents of the suggestion. Her evoked 22 Hz activity over the posterior cortex was enhanced in response to the suggestion-relevant shapes 70–120 ms after the stimulus-onset. When simulating the effects of suggestion, the modulation of the beta activity was not observed, although TS-H’s ERPs to the suggestion-relevant shapes showed selection negativity (SN) after 200 ms. This indicates that TS-H attended to the suggestion-relevant shapes and thus a lack of attention to the shapes during the simulation cannot explain the dissociation in the beta activity between the posthypnotic condition and the simulation.