From visual stimulus to perception
How dynamic patterns of brain activity lead to conscious perception
Prof. Cyriel Pennartz and Jorrit Montijn, MSc, of the Swammerdam Institute for Life Sciences (SILS) have discovered an important pattern in the brain mechanism that allows for the conscious perception of visual objects. A mechanism in the collective activity of brain cells, responsible for the processing of visual information, can predict whether or not an object is consciously perceived. The results were recently published in the scientific journal eLife.
Processing visual stimuli is not the same as perception, when an object is detected and interpreted by the brain. Things can be in your field of view without perceiving them as such, for example when you're daydreaming with your eyes open. Many scientists have studied how the brains react differently when an object is actively detected or not. But so far, only small differences in the brain have been found in the acute response to visual stimuli.
No average activity
Many of the techniques for studying brain activity only observe the average activity of a group of neurons – the brain cells that process information. PhD student Jorrit Montijn and colleagues led by Cyril Pennartz at the Swammerdam Institute for Life Sciences of the University of Amsterdam have therefore investigated whether the observation of an object will depend on subtle patterns or complex interactions in the brain’s activity that have little effect on the average activity. To this end, mice were trained to respond when they saw a particular picture. With a technique called "two-photon calcium imaging" called, they measured the activity of tens to hundreds of neurons at the same time while the mice carried out the sensing task.
The researchers found that the average activation of a group of neurons showed only a weak correlation with whether the mouse had seen the picture. But, at the moment that the scientists looked at the differences in the activation force between the neurons, these were found to correlate strongly with the moment of observation. When the mouse had perceived the picture there was a heterogeneous activity in the group of neurons of the visual cortex. The diversity of neural responses also related to the rate of reaction by the mouse, and could even be used for a few seconds before the picture was shown, to predict whether the mouse was going to perceive it. These differences in activity can not be picked up with techniques which depend on the average activity of many cells, such as an electroencephalogram (EEG) or fMRI, which explains why these effects were not previously noticed.
These new findings are another step towards explaining visual awareness. Further research should determine whether similar mechanisms are also found in non-visual areas.
Montijn, JS, Goltstein PM, and Pennartz, CMA (2015). “Mouse V1 Population Correlates of Visual Detection Rely on Heterogeneity within Neuronal Response Patterns.” eLife, December, e10163. doi:10.7554/eLife.10163.