One of the most challenging tasks performed by our visual system is the rapid and accurate identification of visually similar objects. This is particularly important for recognition of individual faces, where subtle differences are behaviorally crucial. On one hand, face recognition is highly precise, as it entails discrimination of very similar visual stimuli. On the other hand, it is flexible, as we can effortlessly and rapidly recognize a specific face in spite of considerable variations in its retinal image. Somehow, the cortical mechanisms that underlie face recognition (as well as recognition of other specific objects) are simultaneously both strict and tolerant. Most of our detailed knowledge about how the human brain recognizes faces and other objects comes from blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI). In particular, fMRI has identified a specific area in the ventral part of the temporal cortex, the fusiform face area (FFA) that responds much more strongly to faces than to any other category of visual object. Despite its name, the precise role of the FFA in recognizing faces is unclear, partly because of the indirect relationship between neural activity in the FFA and the BOLD fMRI signal. To allow for direct measurement of neural activity in the FFA, our experiments will be conducted using electrodes implanted in patients for the clinical evaluation of epilepsy. These implantations offer a unique and safe opportunity to directly record electrophysiological from human cortex in a way that is otherwise not possible. Although the BOLD response in the FFA to faces in general is greater than the response to other objects, the response in FFA to different individual faces is similar, likely because the BOLD response averages the response of many neurons over space and time. However, using direct recording of activity in the FFA, we will determine if stimulus-evoked local field potentials recorded from FFA in single trials can discriminate between two different faces. The PI will also use these recordings to determine if neural activity in the FFA underlies our ability to recognize a face in spite of variations in its size and position. Finally, we will test whether that the FFA is critical to judgments about face identity. A key test of the importance of a neuronal population for behavior is the connection between its activity and behavioral performance on a trial-by-trial basis. We will examine correlations between FFA activity and behavior across individual trials as a subject recognizes morphed faces. These correlations would provide powerful evidence that FFA is involved in discrimination of individual faces. The proposed experiments examine a fundamental question in neuroscience (what is the neural basis of human visual perception?) that is also clinically relevant. Impairments in visual perception are a frequent and significant cognitive deficit in victims of acquired brain injuries due to trauma and stroke, which in turn are major problems facing U.S. veterans today. A better understanding of how visual objects are processed in the human brain may provide important insights into the pathophysiology and rehabilitation of these disabling impairments.
Many of the U.S. veterans with acquired brain injuries due to head trauma or stroke suffer a loss in their ability to cognitively process the word around them. Little is know about the brain allows us to process our surroundings, and there is currently very little that can be done to rehabilitate or treat patients with disabling deficits in mental processing. Our project uses a unique opportunity to directly observe the human brain's activity in order to study how the brain enables us to recognize specific objects. A better understanding how the brain is miraculously able to process visual information should ultimately help us better treat patients with injured brains.