Neural correlates of behavior and stimulus sensitivity of individual neurons and population responses in the primary visual cortex
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The overall goals of this dissertation were 1) to understand the role that neurons in primate primary visual cortex (V1) play in the detection of small visual stimuli, and 2) to understand the quantitative relationship between the responses of individual neurons and neural population responses in V1. These goals were addressed in experiments with awake, behaving macaque monkeys using electrophysiological and imaging techniques. Initially, I employed ideal observer models to assess V1 neural detection sensitivity in a reaction-time visual detection task and found it to be comparable to the monkey's detection sensitivity. Using the same detection task, I found weak, but significant, correlations between V1 neural activity and the trial-by-trial behavior of monkeys (choice and reaction time). The conclusion of these studies is that the monkey's behavior in the detection task was likely mediated by large neural populations. Voltage-sensitive dye imaging (VSDI) is a powerful imaging technique that is well suited for assessing the link between the activity of large neural populations and behavior. VSDI measures changes in membrane potential over a cortical area of 1-2 cm² with high spatial and temporal resolutions. Using position tuning experiments with VSDI and electrophysiology, I described the relatively unknown quantitative relationships between spiking activity, the local field potential, and VSDI. These relationships were well captured by non-linear transfer functions. Lastly, these experiments also revealed important new findings about the representation of visual space by populations of neurons in V1. In particular, we resolved a long standing debate regarding the size of the cortical point image (CPI), the area of cortex activated by a single point stimulus. We found that the CPI is constant across eccentricity in parafoveal V1, suggesting that each point in space activates an approximately equivalent amount of cortical tissue. In conclusion, the results and analyses described in this dissertation contribute to our understanding of the role that neural populations in V1 play in mediating visual detection, reveal important properties of the representation of visual space by populations of neurons in V1, and provide the first analysis of the quantitative relationship between VSDI and electrophysiological signals.