How our brain tricks us

When walking on the street, in contact with the external world, we are constantly receiving information through our different senses (eyes, ears, touch…).

Imagine you are walking along the beach. You can feel the salty smell of the sea, the touch of the sand sneaking through your fingers while listening to the waves and birds. Your brain then, processes and combines all this information in order to create an image of the landscape you are in and ellaborate a response if needed.

Most of the time, our senses give us complementary information about an event.

For instance, during a conversation, mouth movements from our partner articulate the same words we are listening to, and so, in that case, voice and lips support each other, facilitating the understanding of the message. This effect is especially strong if our conversation is happening in a place with a lot of noise, such as a party, or if we are exposed to a non-native language.

But, what happens when two senses give contradictory information to our brain?

 In those situations, our brain gets tricked, and it suffers what is known as “ cross-modal illusions”.

One of the most famous illusions, discovered in the 70s, is the McGurk Effect. During this audiovisual illusion, one has the perception of an illusory sound, which is not the one we are listening to neither the one we are watching at in the lips of the speaker, but a new sound.

In short, what happens during the McGurk Effect (McGurk and MacDonald, 1976) is that the person in the video is saying (auditorily) /ba/, but his lips are articulating  [ga]. As visual and auditory information are incongruent, our brain perceives something in between, a new intermediate sound (i.e. /da/).

See to believe…

In the same way we sometimes listen to words that do not exist, neither what (we think) we see can always be trusted.

The Double Flash Illusion (Shams et al., 2000) is a visual illusion ocurring when a single visual flash is accompanied with two auditory “beeps”. In that situation, the single flash is perceived as two flashes. Just when we watch the video without the sound, we realize there was actually just one flash.

Feeling a fake hand as part of one‘s body

Maybe one of the most spectacular cross-modal illusions is the so- called Rubber Hand Illusion (Botvinick and Cohen, 1998).

In this illusion, we feel in our own skin how information from touch and sight can play a dirty trick to our brains.

It happens when participant's hand is obscured, while a lifelike rubber hand is visible. When the fake and real hands are simultaneous stroking with a soft brush, it leads to a subjective sense that the visible rubber hand has become a part of one's body.

When the illusion was discovered, participants in the study found themselves saying things such as: “I found myself looking at the dummy hand thinking it was actually my own”.

Crossmodal illusions can be scary sometimes.

References

Matthew Botvinick, Jonathan Cohen (1998). Rubber hands ‘feel’ touch that eyes see. Nature, 29, 756.

McGurk, H., & MacDonald, J. (1976). Hearing lips and seeing voices. Nature, 264(5588), 746-748.

Shams, L., Kamitani, Y., Shimojo, S. (2000). What you see is what you hear. Nature,408, 788.