The brain maintains a fluctuating background activity even when it is not stimulated. This is what functional MRI teaches us. For instance, the visual areas of the brain of an individual whose eyes are closed, in other words an individual who is not receiving any visual stimuli, show fluctuating activity. However, when MRI collected data is analyzed, the variations of so-called spontaneous activity are considered as background noise without any importance and are ignored.
To determine whether the spontaneous fluctuations in specialized brain areas have an impact on perception, researchers belonging to a joint CEA-I2BM/INSERM team at NeuroSpin subjected 12 people to a functional MRI. During the test, they briefly (150ms) and repeatedly showed them an ambiguous stimulus, i.e., an image representing either two faces or a vase, at irregular intervals of at least 20 seconds. During the trails, half the subjects perceived the vase whereas the other half perceived the faces. The detailed study of MRI findings led the scientists to observe that, in the tests where the subjects perceived faces, spontaneous activity level in a brain area that is greatly involved in face recognition called FFA (Fusiform Face Area) was higher than in the test where the subjects perceived a vase.
In other words, the more the spontaneous activity in the area is high before the stimulus is shown, the higher the probability of seeing faces rather than a vase. So based on the study of a subject's spontaneous activity, the way the individual will perceive the stimulus may be deduced long before the stimulus is shown. Therefore, the brain is not silent when there is no stimulation and does not react reflexively to external stimuli. Thus, the fluctuations of spontaneous activity correspond to intrinsic active dynamics of the brain that, in a variable manner, keeps generating hypotheses to interpret the outside world, thus determining its constructive interaction with its environment.
The findings challenge the 'behaviorist' view of the brain. Theoretically, this school considers that thought operates like a kind of automatic mechanism that passively records data from the outside environment and responds by a combination of reflex actions.
- CEA - Damien Larroque - email: damien.larroque@cea.fr - Hesselmann G, Kell C, Eger E, Kleinschmidt A (2008). Spontaneous local variations in ongoing neural activity bias perceptual decisions, PNAS, REF Proc.Natl.Acad.Sci. USA, 105, 10984-10989 : http://redirectix.bulletins-electroniques.com/s6M4e
