Using eye movements to study scene-based change detection
Our Visual environment is in a constant state of flux, with objects routinely appearing and disappearing from view due to object motion and changing retinal location as we make eye, head, and body movements. Change detection refers to the perception of these Visual discontinuities, and change blindness refers to the frequent cases when these changes go undetected, as if we were blind to these Visual events. Change blindness has been the focus of considerable research interest in recent years, largely due to demonstrations that even substantial changes to a scene often go undetected (Rensink, 2002). The underlying causes of the change blindness phenomena, however, remain a mystery. Logically, the process of detecting a change between two real-world scenes requires encoding the objects from one scene into memory, then comparing these objects to those appearing in a second scene. The goal of this project was to use an eye movement methodology to study these encoding and comparison operations. Specifically, encoding operations might be revealed by the object fixations made while inspecting the pre-change scene, and comparison operations might be revealed by preferential fixation patterns to objects in the post-change scene. The results indicated that the comparison operation does not require fixation on the post-change object, suggesting that the Visual routine responsible for comparing the two scenes may be using a parallel process (Zelinsky, 2001b). Moreover, change detection difficulty increased with the number of objects in the scene. Such a change detection set size effect suggests a serial encoding operation, as well a potential theoretical connection to the search literature.
Research Philosophy
Each time we engage in a moderately complex task, we likely enlist the help of an untold number of simpler visuo-motor operations that exist largely outside of our conscious awareness. Consider for instance the steps involved in preparing a cup of coffee. For the sake of simplicity, assume that the coffee has already been brewed and is waiting in the pot, and that all of the essential accessories, an empty cup, a spoon, a carton of
cream, and a tin of sugar, are sitting on a countertop in front of you. What is your first step toward accomplishing this goal? The very first thing that you might do is to move your eyes to the handle of the coffee pot, followed shortly thereafter by the much slower movement of your preferred hand to the same target. Because the coffee pot is hot and the handle is relatively small, this change in fixation is needed to guide your hand to a safe and useful place in which to grasp the object. After lifting the pot, your eye may then dart over to the cup. This action is needed, not only to again guide the pot to a very specific point in space directly over the cup, but also to provide feedback to the pouring operation so as to avoid a spill. After sitting the pot back on the counter (an act that may or may not require another eye movement), your gaze will likely shift to the spoon. Lagging shortly behind this behavior may be simultaneous movements of your hands, with your dominant hand moving toward the sugar tin and your non-preferred hand moving to the spoon. The spoon is a relatively small and slender object that again requires assistance from foveal vision for grasping; the tin is a rather bulky and indelicate object that does not require precise Visual information to inform the grasping operation. Once the spoon is in hand and the lid to the tin is lifted, gaze can then be directed to the tin in order to help scoop out the correct measure of sugar. To ensure that the spoon is kept level, a tracking operation may be used to keep your gaze on the loaded spoon as it moves slowly to the cup. After receiving the sugar, and following a few quick turns of the spoon, your coffee would finally be ready to drink (see Land et al., 1998, for a similarly framed example).
eye movements and visual cognition