"In subjects who reported seeing a second flash, the ERP measurements showed a boost of activity within the visual cortex of the brain immediately after hearing the second sound," said Mishra, adding that the second sound amplified the brain activity stimulated by the first sound. Perception of the second illusory flash was also marked by a rapid enhancement of processing in the auditory cortex of the brain. By observing the auditory boost, the researchers could predict when subjects would report seeing the visual illusion of a second flash.
"Our results provide evidence that perception of the illusory second flash is based on a very rapid and dynamic interplay between the auditory and visual cortices of the brain ?on a time scale less than one tenth the blink of an eye." Mishra said. Interestingly, the pattern was very different between individuals who did or didn't see the second flash, indicating that the brain's wiring and the strength of integration between the different sensory cortices may differ between individuals, or even vary over time. "It suggests that there are consistent differences in the neural connectivity that are possibly shaped during one's development and through experience," she said.
Next, the researchers plan to look at whether or not attention affects these illusory sensations. These studies could shed light on how people deprived of one sensation often compensate by developing another ?for instance, blind people with a more acute sense of hearing.