When to look: three experiments on the timing of eye movements

Introduction

From tracking the arc of a free-throw or tennis serve to shooting flirtatious glances across a room, eye movements are an important and ubiquitous part of our daily lives, allowing us to visualize the world around us and communicate our intensions to others.

Generally speaking, eye movements can be described in two ways: where to look, which comprises both the direction and extent of the motion, as well as when to look. Given these variables, psychologists have attempted to make computational models which accurately describe and predict human eye movements.

When it comes to quantifying when we look, researches have relied on a measurement called the pro-saccade reaction time (PSRT). A saccade refers to any ballistic eye movement: that is, an eye movement which jumps from one target to another, as opposed to moving smoothly. A pro-saccade is a saccade made in the direction of a target, as opposed to an anti-saccade, which is directed away from a target. Taken together, then, the PSRT is simply the total amount of time it takes us to make a saccade towards a target.

Experiments on PSRTs have revealed a consistent finding known as the vertical visual field asymmetry (VVFA). This asymmetry refers to the observation that the PSRT for saccades…