EEG was used in combination with a visual task to detect how the neurons in the visual part of the brain keep track of past images. Credit: janiecbros/ E+/ Getty Images.
Appearances can be deceiving. A study by scientists at the International School for Advanced Studies (SISSA), in Trieste, shows that our ability to process visual information and produce the image of an object is partly conditioned by the visual stimuli we encountered immediately before. For example, an object may seem bigger to us if we have just observed a large object1.
The authors enrolled 63 volunteers who were asked to look at sets of objects on a screen, such as simple black and white dots, in different sequences. The participants were asked to evaluate the number of objects in each sequence, their size and the duration of their appearance on the screen, which was in the order of hundreds of milliseconds. While judging the objects, the researchers performed an electroencephalogram (EEG), a test to assess the electrical activity of the brain.
The results show that the participants were conditioned by what they had just seen: they judged some sets as more numerous or containing larger objects when the preceding sequences had those properties. Furthermore, the EEG showed that neurons in the visual part of the brain remained active and kept track of past images: the larger these traces, the greater the perceptual bias in the images displayed subsequently. The authors explain that, when we approach a new visual scene, the brain does not build it from scratch but compares the new stimulus with the trace of past information and obtains an ‘intermediate’ representation. This process seems to occur when the image begins to take shape in our mind, rather than later, when memories are formed.
Although preliminary, the study provides one more piece of the puzzle to understand the mechanisms underlying vision and sensory processing. In the future, research in this area could also help to better characterize conditions such as autism spectrum disorders and visual hallucinations. “According to a hypothesis still to be explored,” says Michele Fornaciai, a researcher at SISSA and co-author of the work, “some people with autism spectrum disorders could present a deficit in the use of past information, including visual information”. For those suffering from hallucinations the problem would be the opposite, Fornaciai explains. “In this case the hypothesis is that the brain might rely too much on internal predictions based on past visual perceptions".
