UC Berkeley

The ability to discriminate individual items in the presence of flankers is necessary for tasks such as reading and driving. The reduction in the ability to identify an item in peripheral vision due to the presence of flankers is called crowding. Crowding tasks provide a measure of the spatial resolution of visual perception. Previous studies have found better performance on crowding tasks in the lower visual field than in the upper visual field, and this effect is known as the lower visual field advantage. However, when the size of the upper and lower visual field extent are taken into account by placing stimuli at locations with the same percentage of visual field extent, the lower visual field advantage goes away.

Spatial working memory (SWM) is the short-term storage of locations of items not currently visible in the environment for immediate use. Manipulation of cholinergic or dopaminergic signaling alters the spatial tuning of macaque prefrontal cortical neurons during the delay period of a SWM task and can improve SWM performance in primates. Moreover, increasing synaptic levels of acetylcholine reduces the excitatory receptive field size of neurons in marmoset primary visual cortex and sharpens the spatial tuning of visual cortical fMRI responses in humans. These results point to cholinergic enhancement leading to increased spatial resolution and cholinergic and dopaminergic enhancement improving spatial working memory.

In chapter one of this dissertation, I discuss the relationship between crowding and visual field shape. I show that for radially configured flankers there is a lower visual field advantage for critical spacing (the minimum distance between a target and its flankers that is required for a certain level of performance on a crowding task) when crowded stimuli are placed at the same eccentricity. This suggests that the spatial resolution of perception is better in the lower visual field than in the upper visual field. When stimulus locations are matched for the same percentage of visual field extent, however the lower visual field advantage for critical spacing is no longer present. We did not observe a lower visual field advantage for stimuli with a tangential configuration.

In chapter two I present a study that examined the effects of cholinergic and dopaminergic enhancement on spatial working memory. I found that pharmacological enhancement of the cholinergic system (using donepezil) and the dopaminergic system (using levodopa/carbidopa) did not result in improved spatial working memory precision, which we defined as subjects being able to notice smaller changes in the spatial location of a remembered stimulus.

The first study demonstrates that spatial resolution is different in the lower and upper visual field when stimuli are oriented radially. The second study concludes that the precision of spatial resolution in working memory is not limited by the spatial resolution of perception and that cholinergic or dopaminergic enhancement through an acute dose of donepezil or levodopa/carbidopa, respectively, does not improve spatial resolution in working memory.