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The toolbox provides an interface between a high-level interpreted language MATLAB , a visual display programming toolbox Psychophysics Toolbox , and a video-based eyetracker Eyelink. The Eyelink Toolbox enables experimenters to measure eye movements while simultaneously executing the stimulus presentation routines provided by the Psychophysics Toolbox. Example programs are included with the toolbox distribution. Eyemovement recording is becoming a standard part of psychophysical experimentation.
Although eye-tracking techniques exist that rely on measuring electrical potentials generated by the moving eye electro-oculography or a metal coil in a magnetic field, such methods are relatively cumbersome and uncomfortable for the subject e.
We thank Eyal Reingold, Francesco Maringelli, Erin Harley, and two anonymous referees of an earlier version of this paper for commenting on the manuscript. Correspondence should be addressed to F. The Eyelink uses two high-speed cameras CCD sensors to track both eyes simultaneously. A third camera also a CCD sensor tracks four infrared markers mounted on the visual stimulus display, so that head motion can be measured and gaze position can be computed.
The cameras produce images at a sampling rate of Hz 4msec temporal resolution. The Eyelink is used with a PC with dedicated hardware for doing the image processing necessary to determine gaze position.
Pupil position is tracked by an algorithm similar to a centroid calculation, with a noise-limited resolution of 0. An optional heuristic filter Stampe, removes single-sample noise artifacts and does not affect measured saccade velocity and acceleration. Head-position data are then combined with the headreferenced data to compute the true gaze position on the stimulus display.
The optical head-tracking system has extremely low angular noise and therefore does not appreciably increase the final noise level of the system.
The Eyelink communicates via a high-speed ethernet connection with a separate computer Macintosh or PC that performs the stimulus display. Close the connection to the Eyelink PC and Gazetracker. Reports the status of the connection connected, no connection, running in dummy mode. Opens a file on the Eyelink PC to store data and events in. Closes the data file. Sends a command string to the Eyelink PC e. Sends a message to the Eyelink PC e.
Provide information about the graphics environment e. Reports which eye s is being tracked. Perform integrated tracker set-up procedure. Perform integrated drift correction procedure. Starts recording eye-movement data. Stops recording eye-movement data. Reports whether the Eyelink is recording data. Report whether a new data sample either in int or float format is available for real-time purposes. Return the most recent data sample either in int or float format. This allows the creation of near real-time gaze-contingent applications e.
The parser also provides information on fixation and saccade events and associated statistics immediately after the events have been completed. One limitation of the Eyelink is that there is considerable trial-to-trial variability in the estimate of absolute gaze position. For example, typical measurements of the standard deviations of gaze position at fixation are 0. This variability might be due to slippage of the headmounted system over time.
This method provides good relative position estimates within a trial at the cost of information about absolute position across trials. Saccade parameter and fixation position estimates of the Eyelink were highly correlated with those measured using a scleral coil system.
Stimulus Presentation Most eye-movement experiments require the display of visual targets for the subject to track or look at. Computer displays have virtually become standard equipment for conducting visual psychophysics experiments since these allow precise software specification of the stimu- lus. Although low-level programming languages such as C enable the required accuracy in control of timing, luminance, and color output of displays, they do not provide a friendly programming environment.
The Psychophysics Toolbox Brainard, ; Pelli, is a software package that adds the ability for precise stimulus specification to MATLAB, a high-level interpreted language with extensive support for numerical calculations The MathWorks, , allowing for rapid and flexible programming of psychophysical experiments. The toolbox enables one to measure eye movements while simultaneously executing stimulus presentation routines provided by the Psychophysics Toolbox as well as other MATLAB scripts.
Flags to define what data are included in each sample. Horizontal and vertical gaze position data x and y in screen coordinates pixels for the left and right eyes depending on whether they are actually being tracked.
Pupil size or area depending on settings of Eyelink of left and right eyes. Horizontal and vertical resolution of the gaze data pixels per degree for the left and right eyes. Horizontal and vertical head-referenced eye-position data for the left and right eyes. The extension, which is partly based on proprietary low-level C subroutines provided by the manufacturer of the eye tracker, can be called directly from MATLAB.
The Eyelink Toolbox provides access to all Eyelink routines. A listing of the main commands and a short description of their function is provided in Table 1. In addition, the Eyelink Toolbox provides both integrated routines for performing calibration and drift correction, as well as procedures written in native MATLAB language that allow for a large degree of customization e. To have near real-time gaze-dependent displays, the Eyelink parses the eye-movement data on line and sends gaze data, as well as other data, over the ethernet connection to the display computer.
Table 2 lists the main fields of this structure and provides a short description of its contents. Nevertheless, the Eyelink Toolbox implementation, even on a relatively old Power Macintosh with a MHz G3 processor, is fast enough to enable a near real-time gaze-dependent display.
Between eye movement and screen update there is a delay of about 20 msec,1 of which our tests indicate that almost all is due to the Eyelink hardware and software on the dedicated Eyelink PC. A millisecond or less of the delay is due to network communication between the Eyelink and display computers.
The MATLAB environment, by providing extensive support for numerical processing, in principle also allows the postprocessing of eye-movement data within the same environment.
In Step 5, data recording commences. In case the Eyelink provides no valid gaze data e. In Step 7, graphics window, data file, and tracker are closed.
The C source code of the toolbox is available though not for the proprietary subroutineson which the toolbox is based. Note that the Eyelink Toolbox is neither provided nor endorsed nor supported by the manufacturer of the Eyelink Gazetracker.
The toolbox may be used freely for teaching or research. It may not be used for commercial gain without permission of the first author of the present article. The Psychophysics Toolbox. Spatial Vision, 10, Brenner, E. Separate simultaneous processing of egocentric and relative positions. Vision Research, 40, — Cornelissen, F. Heading detection with simulated visual field defects. Visual Impairment Research, 1, The influence of artificial scotomas on eye-movements during visual search.
Manuscript submitted for publication. Huk, A. Temporal integration of visual motion information: Evidence from response times. Journal of Vision, 2 7 , a. Li, H. Systematic distortion of perceived 2D shape during smooth pursuit eye movements. Vision Research, 42, Pelli, D. The VideoToolbox software for visual psychophysics. Sheena, D. Compensation for some secondorder effects to improve eye position measurements. Fisher, R. Senders Eds. Hillsdale, NJ: Erlbaum. Stampe, D.
Heuristic filtering and reliable calibration methods for video-based pupil-tracking systems. We determined the delay from eye movement to stimulus update to be about 20 msec. The Eyelink Toolbox is compatible with both the first and the second generations of Eyelink Gazetrackers. Tant, M. Hemianopic visual field defects elicit hemianopic scanning.
The MathWorks. The MathWorks, Inc. Recording eye movements with video-oculographyand scleral search coils: A direct comparison of two methods. Journal of Neuroscience Methods, , The information is returned in a structure that also contains useful defaults and control codes e.
EyeLink Eyetracker functions
The Eyelink Toolbox: Eye tracking with MATLAB and the Psychophysics Toolbox