The senses of hearing and vision also cooperate at a more basic level prior to each of them even producing an estimate, suggests a new study.
Kim, Megan Peters, and Ladan Shams from the University of California Los Angeles, have shown behavioural evidence that this interplay happens in the earliest workings of perception, not just before that logical decision, making stage, but before the pre-conscious combination of behavioural evidence.
To demonstrate that one sense can affect another even before perception, the researchers showed 63 participants a bunch of dots on a screen, in two phases with a pause between them. In one phase, the dots moved around at random; in the other, some proportion moved together from right to left.
The participants had to indicate in which phase the dots moved together horizontally. In experiment 1, the subjects were divided into three groups. While they looked at the dots, one group heard sound moving in the same direction as the right-to-left dots, and stationary sound in the random phase.
A second group heard the same right-to-left sound in both phases. The third group heard the identical sound in both phases, but it moved in the opposite direction of the dots.
In the second and third conditions, because the sound was exactly the same in both phases, it added no cognitively useful information about which phase had the leftward-moving dots. In experiment 2, each participant experienced trials in all three conditions.
The researchers found that all the participants did best under the first condition when the sound moved only in the leftward-motion phase. The opposite-moving sound neither enhanced nor worsened the visual perception.
However, surprisingly the uninformative sound, the one that travelled leftward both with the leftward-moving dots and also when the dots moved randomly, helped people correctly perceive when the dots were moving from one side to the other. Hearing enhanced seeing, even though the added sense couldn't help them make the choice.
"Most of us understand that smell affects taste. But people tend to think that what they see is what they see and what they hear is what they hear," Kim said.
The findings of this study offer "further evidence that, even at a non-conscious level, visual and auditory processes are not so straightforward," she says. "Perception is actually a very complex thing affected by many factors."
"This study shows that at least in regards to perception of moving objects, hearing and sight are deeply intertwined, to the degree that even when sound is completely irrelevant to the task, it still influences the way we see the world," Shams said.
The study has been published in Psychological Science.