When people are confronted by a typical, crowded visual scene, attentional mechanisms are needed to limit visual processing to objects that are currently relevant to behavior. An understanding of the neural basis of these attentional mechanisms will ultimately help in developing a visual prosthesis to aid people with severe visual impairments, and will also help in developing treatments for people with brain disorders such as attention deficit hyperactivity disorder. The goal of this proposal is to understand how spatial attention is mediated by neurons in the ventral processing stream important for object recognition, which progresses through areas V2, V4, and inferior temporal (IT) cortex. Recent work in the Desimone lab has demonstrated that, when an animal attends to a location in space, neurons in area V4 with receptive fields that include the attended location show increases in both the firing rate and synchrony of spiking. Because cells have limited synaptic integration times, increases in both firing rate and neural synchrony may work together to enhance the effectiveness of attended stimuli in driving postsynaptic cells. Some of the critical gaps in our current knowledge of the mechanism of attention include (1) the generality of attentional effects on firing rates and synchrony in the ventral stream, and (2) the anatomy of the attentional mechanism, including the direction of attentional flow across areas. This proposal addresses these gaps. The experiments are designed to test the hypothesis that attentional effects on firing rates and neural synchrony are not only present in area V4, but also in IT cortex, the highest processing area in the ventral stream. Furthermore, the experiments will address the question of the anatomical "direction" of attentional effects across V4 and IT, i.e. whether the influence of attention begins first in IT cortex and is communicated "backwards" through the ventral stream, or whether it begins first in V4 and is then relayed "forward" to IT cortex. Together, the experiments will advance our understanding of the neural mechanism by which attention gates visual processing, and ultimately inform efforts to devise effective treatments for cognitive disorders such as ADHD and schizophrenia.