Maintenance of stable central eye fixation is crucial for a variety of behavioral, electrophysiological, and neuroimaging experiments. Naive observers in these experiments are not typically accustomed to fixating, either requiring the use of cumbersome and costly eyetracking or producing confounds in results. We devised a flicker display that produced an easily detectable visual phenomenon whenever the eyes moved. A few minutes of training using this display dramatically improved the accuracy of eye fixation while observers performed a demanding spatial attention cuing task. The same amount of training using control displays did not produce significant fixation improvements, and some observers consistently made eye movements to the peripheral attention cue, contaminating the cuing effect. Our results indicate that (1) eye fixation can be rapidly improved in naive observers by providing real-time feedback about eye movements, and (2) our simple flicker technique provides an easy and effective method for providing this feedback.
In many visual experiments, maintenance of central eye fixation is crucial. Some experiments require that stimuli be presented at specific locations on the retina (e.g., to investigate hemispheric or eccentricity-based differences in visual processing). Other experiments aim to manipulate only covert visual attention and need to minimize overt eye movements. Eye movements also cause artifacts in EEG recordings and can lead to loss of trials. Eye fixation can be improved by monitoring eye movements with an eyetracker and providing feedback when eye movements occur (e.g., Steinman, Haddad, Skavenski, & Wyman, 1973). Our goal was to devise an easy and effective method that does not require the use of an eyetracker and that rapidly trains naive observers to maintain central eye fixation, even under challenging conditions.
We took advantage of an eye-movement-contingent visual effect. Individual flashes of a rapidly flickering dot, which are too fast to resolve when the eyes are stationary, are seen as a spatially displaced array of flashes during the fast flight of a saccadic eye movement, since each flash is painted at a different location on the retina (e.g., Hershberger, 1987). We adapted this phenomenon into a display that generated a clear visual effect whenever observers made even a small eye movement (e.g., a saccade, a mechanically induced [e.g., by tapping on the head or the side of the eye] movement of the eye, a blink).
Our display consisted of a fine-grained random dot pattern (with 50% black and 50% white pixels) that flickered rapidly (37.5 Hz, the fastest rate possible using a typical 75-Hz monitor) in counterphase (i.e., all black pixels became white and all white pixels became black in alternate frames) (see Figure 1). When the eyes were stationary, the display appeared uniformly gray, because the black and white pixels were perceptually averaged at each location (except for some graininess perceived because of the visual system's sensitivity to borders defined by regions flickering in opposite phases; e.g., Forte, Hogben, & Ross, 1999).
As soon as the observer made even a small eye movement, the precise temporal averaging of the black and white pixels was disrupted, because individual frames fell at shifted locations on the retina, and the observer saw a clear black-and-white random dot pattern momentarily popping out from the gray field. Because naive observers are often unaware of their eye movements and blinks, we thought that this eye-movement-contingent effect might provide a useful form of feedback for training naive observers to maintain steady central eye fixation by making them aware of their unintended eye movements in real time.
We determined whether a few minutes of training with this flicker display substantially improved central eye fixation in naive observers. We measured the effect of this training using a typical spatial attention cuing task (see Funes, Lupiáñez, & Milliken, 2005, for a review), in which a peripheral onset cue was flashed to the left or right of a central fixation marker, and the subsequent target stimulus was presented on the cued or opposite side (Figure 2). …