Natural visual scenes contain numerous objects, but gaze can be directed to only one at a time. This project has provided numerous insights into the neural mechanisms in the frontal lobe contributing to this essential ability. This application proposes 3 aims designed to investigate significant questions about frontal cortical circuitry that have been exposed by insights gained through the previous grant periods. Results demonstrate that the frontal eye field (FEF) consists of a diversity of functional types of neurons including visually responsive neurons that contribute to saccade target selection and the allocation of attention and presaccadic movement neurons that contribute to the control of saccade initiation. Likewise, the supplementary eye field (SEF) consists of a diversity of functional types including visually responsive neurons that do not contribute to saccade target selection and other neurons that monitor task performance by signaling when conflicting responses are planned, when errors are produced, and when reinforcement is earned. Like all cortical areas, the diversity of functional types of neurons in FEF and SEF are arranged in embryologically and morphologically distinct layers with different inputs and outputs. Relative to the detailed information about functional architecture of primary visual cortex, very little is known about the laminar organization of FEF and nothing at all about SEF nor about the laminar organization of interactions between FEF and SEF. An innovative combination of techniques will be used to characterize the time and magnitude of modulation in ensembles of neurons and synaptic potentials recorded simultaneously across the layers of FEF and SEF of macaque monkeys performing visual search tasks. These tasks will require monkeys to locate a target among an array of distractors. The neural signals will be sampled using combinations of multi-contact linear electrode arrays and conventional metal microelectrodes. Neural signals will be assigned to cortical layers through high- resolution coregistration of stereoscopic digital X-rays, computed tomography and magnetic resonance. Preliminary data demonstrate the feasibility of this combination of techniques.
Aims 1 and 2 will characterize the diversity of functional neuron types and describe the laminar microcircuitry of FEF and SEF, respectively.
Aim 3 will describe the laminar organization of interactions between FEF and SEF. Information derived from these experiments is significant for several reasons. First, without it, the next generation of models of the functional architecture in two major frontal visual areas controling visual search cannot be formulated. Second, it will provide new insights into the hierarchical interactions between granular prefrontal and agranular premotor cortical areas. Third, it will show whether the canonical cortical microcircuit formulated to explain primary visual cortex can be applied to these two distinct areas in the frontal lobe.
Clinical neuroscience has progressed by determining the nature of interactions between neurons and the functional architecture of cortical areas. Clinical researchers studying ADHD, Parkinson's disease, schizophrenia, and traumatic brain injury cite the publications derived from this research program as evidence for a dissociation of the mechanisms of overt and covert orienting, the diagnostic usefulness of the stepping task and the role of the frontal eye field in the guidance and control of voluntary saccade production. The new information about two areas in the frontal lobe that control visual search behavior that will be obtained through the proposed research plan is essential to more effectively diagnosis and eventually treat disorders of visual attention, orientation and mobility and for the design and implementation of effective prosthetic devices.
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