Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Science

Program

Neuroscience

Supervisor

Dr. Matthew Heath

Affiliation

Western University

Abstract

A statistical summary representation (SSR) is a phenomenon wherein a target property (e.g., size) is encoded based on the average of the stimulus-set to which it belongs. Here, I examined method of adjustment (MoA), visually and memory-guided grasping tasks in separate blocks in which differently sized targets (i.e., 20 30 and 40 mm) were presented with equal frequency (control weighing condition) and when the 20 mm and 40 mm targets were asymmetrically presented (i.e., small-target and large-target weighting conditions). The weighting conditions were used to determine whether the different tasks are influenced by an SSR. In the MoA task, responses for the small- and large-target weighting conditions were biased in the direction of the most frequently presented target in the stimulus-set. In contrast, grip apertures for visually and memory-guided grasps were refractory to the different weighting conditions. Accordingly, an SSR influences perceptions but not goal-directed grasping.

Summary for Lay Audience

When scanning our visual environment, the judgments we make about the properties (e.g., size, shape) of a target object are influenced by non-target items. In fact, several studies have shown that when we view a display including a 'target' circle and differently sized non-target circles (i.e., a stimulus-set) our perception of the size of the target circle is biased by the average of all circles (i.e., target and non-target) – a phenomenon referred to as a statistical summary representation (SSR). The majority of the SSR research has focused on visuoperceptual judgments; however, it is unclear whether the phenomenon influences grasping movements directed to three-dimensional target objects. This represents a notable question because functionally and anatomically distinct visual pathways are thought to mediate perceptions and actions. To address this issue, participants were presented differently sized three-dimensional targets (20, 30, and 40 mm in width) and were asked to complete a classic perceptual judgment that involved adjusting the size of a line appearing on a computer monitor until it matched the size of the target object (i.e., method of adjustment task). As well, participants completed grasps of the same targets in conditions with (i.e., visually guided) and without (i.e., memory-guided) vision during the response. The method of adjustment, visually and memory-guided grasping tasks were completed in separate blocks and in conditions wherein the different targets were presented with equal frequency (i.e., control weighting) and when the 20 and 40 mm targets were presented five times as often as the other targets in the stimulus-set (i.e., small-target weighting and large-target weighting). Results showed that responses in the method of adjustment task were biased in the direction of the most frequently presented target in the stimulus-set; that is, responses were influenced by an SSR. In contrast, the transport and aperture shaping components of visually and memory-guided grasps were not influenced by the different target weighting conditions. Accordingly, results show that SSR influences perceptual judgments but does not influence goal-directed actions.

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