Only a fraction of the stimuli surrounding us reaches our conscious perception at any moment. How does our brain switch from one stimulus to the other? And which forces influence that process? Associate professor Tomas Knapen studies these questions at VU university in Amsterdam and received a grant from NWO to combine forces with the Chinese lab of Peng Zhang at the Chinese Academy of Sciences.
In order to reliably study conscious perception, Knapen executes his research in a strongly regulated lab setting. He uses binocular rivalry, an optic illusion for which participants wear special glasses which have one red and one green lens. Observing a red-green figure, this means one eye is confronted with the green shape, while the other eye observes a red figure in the same location. In human perception, this results in the observer being solely aware of a red figure at one point, while a moment later the image shifts and the observer only registers the green figure.
Apparently, the brain contains a mechanism that causes regular shifts in awareness between the streams of information from the left and the right eye. Knapen wants to understand how this mechanism works, also because he suspects a similar principle to be at play in switching perception in daily life: from the conversation you are in to a more interesting one at the adjacent table, or from the book you are reading to the screen of your phone. Knapen: ‘In the real world, the amount of possible switches is unlimited and the stimuli also constantly change, with passers-by that enter and leave your field of vision. In this experiment, we limit the possibilities to two consistent stimuli. That enables us to really gain understanding of the brain mechanisms at play in selecting the input that reaches consciousness.’
So what causes the shift? Is it top-down regulation from the frontal cortex or other higher level visual brain areas that are bored with the information from one eye and ‘request’ a change? Or, alternatively, is it processes of local habituation in the primary visual cortex that causes the signal from one eye to extinguish and for the other signal to take over in the information flowing to higher visual areas? Or is it some form of interaction between these two scenario’s?
To shed light on these questions, Knapen uses a 7 Tesla fMRI-scanner. That gives him the resolution to distinguish between top-down and bottom-up information streams in the different layers of the primary visual cortex. In Peking, Knapens colleague Zhang uses a MEG-scanner, which has a high temporal resolution, to differentiate between the two types of information streams. Eventually, the two teams will combine their data, making use of the good temporal and spatial resolution of the different techniques to learn about the mechanisms at play in switching attention between stimuli.
Simultaneously, Knapen will conduct a series of sub-experiments to control for biases as a result of the research instructions. ‘I find it important to be sure that we really measure what we assume to measure. Therefore, I deliberately try to control for factors that might skew the experiment.’ In the main experiment, participants receive instructions to report on their perception by pushing buttons corresponding to the green and the red image. But doing so triggers fluctuations in attention and other cognitive processes and these could influence perception. When the subject reports perceiving a green image by pushing the corresponding button, this reporting in and by itself gives the green image more valence, and that might influence the switch to the red image, Knapen theorizes. Therefore, he will compare this research setup to one in which participants don’t have to report their perception in real time, but are asked to report afterwards how many switches of the image they perceived.
Knapen hopes his experiments will help him understand conscious perception and the way in which our actions influence that perception. Knapen: ‘Ultimately I hope this type of research adds to our understanding of how people move through the interactive film that is life.’
Text: Marieke Buijs