Neural Mechanisms of Visual Crowding

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Project Summary Visual perception is an active process of information selection. Two critical components of this selection process are the mechanisms of spatial attention and saccadic eye-movements. Attention allows us to deploy limited neural resources for privileged processing of salient stimuli; saccades allow us to bring stimuli of interest to the center of gaze for detailed inspection. The dynamic interplay of attention and saccades is particularly prominent in peripheral vision where inappropriate integration of contextual information has been proposed as one of the underlying mechanisms of visual crowding. Visual crowding is the inability to identify stimuli amidst clutter and is a fundamental bottleneck to object recognition in peripheral vision. It is particularly detrimental in the case of patients with central vision loss such as macular degeneration (MD), since these individuals rely on a peripheral retinal locus (PRL) for object recognition tasks. Crowding zones, the area in visual space over which distractors affect target identification, are markedly elongated along the radial axis aligned with the fovea in normal vision. Crowding zones undergo systematic reorganization in MD patients with an elongated component along the radial PRL axis. Despite decades of research, the neural underpinnings of crowding and that of the elongated shape of crowding zones remain unknown. In this proposal we will bridge these knowledge gaps in the context of the activity of six neural sub-populations – two cell classes (inhibitory and excitatory neurons) in three layers (superficial, input, deep) – that are fundamental components of the sensory laminar cortical circuit. Our broad hypothesis is that sub-population specific activity dynamics form the substrate for contextual interactions underlying crowding and for the formation and re-organization of crowding zones. We propose to answer several key questions regarding this hypothesis. What are the laminar mechanisms of contextual modulation in crowding zones (Aim 1)? What the spatio-temporal dynamics of peri- saccadic activity in the laminar cortical circuit (Aim 2)? What are the neural mechanisms of crowding zone reorganization following central vision zone (Aim 3)? We will achieve these aims using high-density laminar neural recordings in visual area V4 of awake non-human primates. V4, a critical node in the object recognition pathway, that also receives strong attention related and oculomotor signals, is ideally suited for these investigations. The results of these investigations will uncover the neural mechanisms of crowding, provide a broad framework for visual form processing in the periphery and inform possible future intervention studies in central vision loss.