Two Visual Systems and Their Eye Movements: a Fixation-Based Event-Related Experiment with Ultrafast fMRI Reconciles Competing Views
Studies of active vision in naturalistic scenes show the existence of two classes of eye movements manifested in ambient and focal visual fixations. This finding seems to corroborate with the anatomical separation of two “streams” of visual processing related to localization (dorsal system) or to identification of objects (ventral system). Direct verification of this connection proved to be difficult due to an insufficient resolution of the conventional noninvasive brain-imaging methods. Another hypothesis recently attributed the same observation to the lateralization of global and local attention modes in the right and left hemispheres, correspondingly. Thus, there are two tentative explanations for the brain mechanisms of the same eye movement patterns in free image viewing. Our study aimed at resolution of this controversy. Materials and Methods. 13 healthy subjects (age 21 to 31 years, right handed, 8 females) with normal or corrected to normal vision and without known history of neurological diseases participated in this experiment. Using a combination of ultrafast multi-band fMRI scanning with the fixation-based event-related (FIBER) paradigm of data collection, we measured the brain functional activity in its relation to tasks, a semantic category of the inspected object (houses or faces), brain regions as well as ambient and focal visual fixations during free viewing of complex images with an unprecedently high temporal and spatial resolution. Results. The results unexpectedly showed that both competing hypotheses are confirmed. In line with our early proposal, ambient fixations were accompanied by activation of structures traditionally associated with the dorsal visual pathway, while focal fixations correlated with that of the ventral pathway. At the same time, the second hypothesis also proved to be correct: the activated structures of the dorsal pathway were localized in the right hemisphere and those of the ventral brain networks mainly - albeit not exclusively - in the left hemisphere. Conclusion. The present study for the first time demonstrates pronounced lateralization of both basic brain mechanisms in charge of visual perception and eye movement control in free processing of complex images. This conclusion poses a number of further questions about a possible relation between two modes of active vision and other forms of asymmetries found at different levels of human brain organization.