How Do Visual Illusions Appear?

Have you been trying for over an hour to figure out what that Picasso picture depicts?

You should try it in complete silence...Because a new research reveals that what we hear can impair what we see.

These findings could explain us how illusions are induced in the visual cortex, due to rapid integration of auditory and visual perceptions.

When the 34 volunteers were shown a sole light flash interposed between two brief sounds, many of them reported seeing two distinct light flashes.

Focusing on the timing and location of the brain processes that triggers this illusory effect can explain how information from different senses are integrated in the brain. "This type of perceptual illusion has been described before," said first author Jyoti Mishra, graduate student in the laboratory of Dr. Steven A. Hillyard, UCSD professor of neurosciences.

"The surprising finding we made is that the illusion depends on a rapidly timed sequence of interactions between the auditory and visual cortical areas. This is part of a set of new findings by scientists in the field that show how integration of multiple sensations can happen much more rapidly than we thought before," said Mishra.

"We show physiological evidence that visual and auditory stimulation might not be processed separately, then merged together, as previously assumed, but that an almost-simultaneous integration of the sensations may actually take place in the brain."

The team assessed event-related potentials (ERPs), brain responses directly linked to the perceptual experiences provoked by sensory stimuli, employing electrophysiological or EEG records of the brain's electrical activity. "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. "The second sound amplified the brain activity stimulated by the first sound."

Perception of the second illusory flash was connected to a quick rise of processing in the auditory cortex. "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.

But the pattern varied between individuals who did or didn't see the second flash, revealing that the brain's wiring and the integration level between sensory cortices is different between individuals or even with the age. "It suggests that there are consistent differences in the neural connectivity that are possibly shaped during one's development and through experience," she said.

This research could explain how persons lacking one sensation often overdevelop another, like the case of the blind people with an extremely acute hearing perception.