Dissertation

Timecourse of a perceptual judgment and factors affecting it

Shankar, Swetha

2012

Overview

Perceptual decisions are extremely fast and take on the order of milliseconds. The time taken to view the stimuli during these choices, termed processing time (PT), is an important quantity because it defines how informed, and thereby accurate, the choice is. Typically, the PT has been estimated from the reaction time (RT), which is the total amount of time taken to report a choice, starting from stimulus onset. The issue here is that the RT reflects not only the PT, but also related motor and other non-decision delays involved in a choice. To resolve this, the compelled-saccade (CS) task was developed. The CS task is a two-alternative forced-choice task with a crucial difference: the instruction to initiate a saccade (GO signal) precedes cue presentation, which is provided a variable delay later. This design achieves a separation between the perceptual and non-perceptual processes that constitute a choice, which then provides a means to extract PTs and study the speed of perceptual processing. To aid in this effort, the tachometric curve - a psychophysical metric that plots accuracy as a function of PT - was generated. Three features of the tachometric curve, namely, center-point, slope and maximum height help quantify the onset, speed and efficiency of perceptual processing, respectively. Prior to using the tachometric curve to characterize perceptual capacity, it was ensured that it reflected perceptual processing alone. The validation process was two-fold. First, motor behavior was altered using a directional bias instituted by differentially rewarding correct saccades to the two target locations. While this produced dramatic differences in the saccade metrics to the two locations, the tachometric curves in the two conditions were virtually identical, thus proving that the tachometric curve was not influenced by non-perceptual factors. Second, stimulus characteristics were altered in three different ways to show that this did, indeed, change the tachometric curve. First, a motivational bias was induced by differentially rewarding target color. While the change in saccade metrics was small and subtle, the tachometric curves showed a more robust change reflecting altered perception as a result of the motivational bias. Next, physical features of the stimulus were altered. This was done by (1) lowering the saturation of the color stimuli and (2) changing the relevant discrimination feature from color to shape. With both these manipulations, overall RTs did not change substantially but the tachometric curve parameters indicated significantly different perceptual processing between conditions. Finally, the tachometric curve was used to examine the perceptual effects of learning, which is known to lead to higher accuracy and shorter RTs. As the subject learned the task, they not only became faster and more efficient at it, but also started stimulus processing earlier. All of these factors went into improving the subject's overall performance. The tachometric curve has thus been used to study perceptual processing and the ways it influences behavior. The separation of perceptual and non-perceptual processes that the CS task enables will help parse out the neural basis of decision-making and the specific contributions of brain areas to different aspects of the phenomenon.

• Publisher: ProQuest Dissertations & Theses
• Subject: Neurosciences
• ISBN: 9781267626486, 1267626488