The goal of this research is to understand how we see what we see: how does the brain analyze the light falling on the retina of the eye to encode a world full of objects, people and things? During the past year we have focused on the perception of complex visual stimuli, in particular real-world visual scenes (Protocol 93-M-0170, NCT00001360). Perception of real world scenes: Real-world scenes are incredibly complex and heterogeneous, yet we are able to identify and categorize them effortlessly. While prior studies have identified several brain regions that appear to be specialized for scene processing, it remains unclear exactly what the precise roles of these different regions are and what information they contain. Building on a general framework for visual processing we proposed in the past few years, we have been investigating the basic visual properties of the three scene-selective regions and trying to understand how these three regions interact functionally. In particular, we have been investigating the relationship between retinotopy (point-by-point mapping of the visual field onto the cortex) and category-selectivity (differential responses to images from different visual categories e.g. scenes versus faces). We evaluate retinotopy by presenting fragments of scenes to specific portions of the visual field and measuring the response across the brain with fMRI. Similarly, we measure category-selectivity by presenting images from different categories (e.g. faces, scenes, objects, bodies) and measuring the associated response. We find that there is no simple relationship between these two different organizational principles. Category-selective regions exhibit retinotopy, but there is no simple relationship with the retinotopic maps and individual category-selective regions overlap multiple maps. These results suggests that individual category-selective regions may contain multiple sub-regions within specific retinotopic maps that perform separate computations on the images. One of these scene-selective regions is found in medial parietal cortex and is often implicated in memory functions. Our data show that there may be a gradient of function within medial parietal cortex. Posterior regions show strong retinotopy and scene-selectivity and are most strong connected with other regions of posterior visual cortex. In contrast anterior regions are much less retinotopic and scene-slective but show strong connectivity with regions of ventral temporal cortex and parietal cortex that are implicated in memory. Collectively these results provide important insights into the brain network that is involved in processing real-world visual scenes. We are currently evaluating the specific roles of these regions by using transcranial magnetic stimulation (or TMS) to temporarily disrupt their function and observe the impact on behavior. Elucidating how the brain enables us to recognize objects, scenes, faces and bodies provides important insights into the nature of our internal representations of the world around us. Understanding these representations is vital in trying to determine the underlying deficits in many mental health and neurological disorders.
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