The development of neural prostheses that can efficiently stimulate sensory or motor nerve pathways by controlled amounts of electric charge is a new area of research with important clinical applications. While many studies have investigated the response characteristics of peripheral nerves to neurostimulation, relatively few studies have addressed the question of how cerebral neuron populations respond to local stimulation. In this proposal, we will systematically explore the electrical requirements for cortical cell activation using microelectrodes of specified size and configuration, and stimuli that are charged balanced to avoid neural damage. Using the cat visual cortex as a model system, we will study how local intracortical microstimulation can be used to selectively activate discrete cell populations involved in spatial pattern recognition. For single neurons identified by their visual receptive field properties, we will measure the threshold for neural activation as a function of stimulation depth and distance and note the patterns of excitation and/or inhibition that result when such stimuli are applied. Evidence from our preliminary studies have demonstrated that intracortical stimulation evokes powerful inhibitory as well as excitatory effects on single visual cortex neurons. These inhibitory interactions will be examined carefully since current hypotheses suggest that inhibitory pathways within the cortex may play a key role in establishing the orientation, direction and spatial frequency selective characteristics of simple and complex cells as well as the length-selectivity of hypercomplex cells. We will also investigate the visual and electrical response properties of a new cell-type (tentatively termed recruitment-response cells) that may play an important role in both normal and artificial (phosphene) vision. Intracellular recordings will be carried out from these cells and intracellular staining with horseradish peroxidase (HRP) will be used to identify the morphological features of this cell class. In sum, we will first determine the optimum strategies for achieving cortical cell activation using local neurostimulation, and then use this technique to investigate the intracortical interactions that shape the receptive field characteristics of neurons that are critical to visual pattern recognition.