1Rotman Research Institute, 2Department of Psychology, University of Toronto, 3Department of Psychiatry, University of Toronto
This article is a part ofJoVE Neuroscience. If you think this article would be useful for your research, please recommend JoVE to your institution's librarian.Recommend JoVE to Your Librarian
Current Access Through Your IP Address
Current Access Through Your Registered Email Address
Ryan, J. D., Riggs, L., McQuiggan, D. A. Eye Movement Monitoring of Memory. J. Vis. Exp. (42), e2108, doi:10.3791/2108 (2010).
Equipment used during data acquisition
The eye tracker used in the current protocol is an EyeLink II system (SR Research Ltd; Mississauga, Ontario, Canada). This head-mounted, video-based eye tracker records eye position in X, Y-coordinate frame at a sampling rate of either 500 or 250 Hz, with a spatial resolution of < 0.1°. One camera is used to monitor head position by sending infrared markers to sensors placed on the four corners of the display monitor that is viewed by the participants. Two additional cameras are mounted on the headband situated below each of the eyes, and infrared illuminators are used to note the pupil and corneal reflections. Eye position may be based on pupil and corneal reflections, or based on the pupil only. The padded headband of the eye tracker can be adjusted in two planes to comfortably fit the head size of an adult participant. Most eyeglasses and contact lenses can be accommodated by the eye tracker.
Two PCs are used to support eye movement recording. One computer serves as the display computer, which presents the calibration screens, necessary task instructions and the images used in the experimental paradigm to the participants. The display computer details the data collection parameters that are then governed by the second, host computer. The host computer calculates real-time gaze position and records the eye movement data for later analysis, as well as any button press or keyboard responses made the participant. Participant setup and operations of the eye tracker are performed via the host PC.
In this protocol, the timing and order by which experimental stimuli are to be presented to the participants, and the manner in which eye position is to be collected by the host PC are programmed through Experiment Builder, a software program specifically developed by SR Research Ltd to interface with the eye tracker host computer. However, stimulus presentation can also be conducted through other software programs (e.g., Presentation, Neurobehavioral Systems; Albany, CA). Conversion of the eye movement data to a series of fixation and saccade events that are time-locked to stimulus presentation (or another external event) is achieved through the host computer and can be interrogated with Data Viewer, a software program developed by SR Research Ltd; however, again, other programs can be used to derive the required eye movement measures. Here, the detection of fixations and saccades are dependent on an online parser, which separates raw eye movement samples into meaningful states (saccades, blinks and fixations). If the velocity of two successive eye movement samples exceeds 22 degrees per second over a distance of 0.1°, the samples are labeled as a saccade. If the pupil is missing for 3 or more samples, the eye activity is marked as a blink within the data stream. Non-saccade and non-blink activity are considered fixations.
Eye tracking procedures.
Below, we detail the procedures for obtaining eye movement recordings for each participant.
Multiple measures can be derived from the eye movement recordings, including measures that describe overall viewing to the image (including the characteristics of each fixation/saccade), and measures that describe the pattern of viewing that has been directed to a particular region of interest within an image 3. Measures of overall viewing to an image may include (but are not limited to): the number of fixations and the number of saccades made to the image, the average duration of each fixation, and the total amount of viewing time that was spent fixating on the image. Measures that describe the pattern of viewing to a particular region of interest may include (but are not limited to): the number of fixations made to the region of interest, the amount of time spent within a region of interest, and the number of transitions made into/out of a region of interest. Further, measures may be derived from the eye movement recordings that outline the timing by which (i.e., how early) a particular eye movement event has occurred, such as when, following stimulus onset, the eyes fixate on a specific region of interest, when the first saccade is made on an image, and the entropy (constraint/randomness) inherent in the sequence of eye movement patterns.
For any given image, eye movement measures can detail where the eyes were fixated, when and for how long. To obtain an index of memory, we can give viewers different types or amounts of exposure to distinct sets of images, and then compare viewing patterns across those sets and across participants. For instance, to probe memory for repetition of an image, scanning patterns can be contrasted between novel images and images that have been viewed multiple times throughout a testing session. Viewing images repeatedly throughout a testing session results in a decrease in overall viewing of the image 2, 4-5, 8. This can be seen in Figure 2. In this representative result from Riggs et al. 11, participants viewed novel pictures of faces once in each of five testing blocks; with increasing exposure, there was a decrease in the amount of fixations that viewers make to the faces. To probe memory for particular details of an image, scanning patterns can be contrasted between images that have been repeatedly viewed in their original, unaltered form (repeated images) and images that have similarly been viewed repeatedly throughout a testing session, but a change has been introduced to some element within a scene during the final exposure (manipulated images). In such cases, scanning patterns are attracted differentially to the region that has been altered within a manipulated image compared to the same, unaltered region of repeated images 2-3, 7-8. However, such eye movement indices of memory are not present in certain populations, such as when healthy older adults and patients with amnesia due to medial temporal lobe damage are assessed for their memory of the spatial relations among objects in scenes, as outlined in Figure 3 2, 8. Therefore, findings from eye movement monitoring can be used to contrast memory among groups of participants with differing neuropsychological status 2-3, 8-10.
Figure 2. Eye movements reveal memory for repetition. In this representative result from our laboratory , participants viewed faces across 5 study blocks; as the number of viewings increased (from 1-5), the number of fixations to the faces decreased.
Figure 3. Eye movements reveal memory for changed details. Younger adults [2 (Experiments 1, 2), 8 (Free Viewing Condition)] directed a greater proportion of their total eye fixations to a critical region in a manipulated image that has undergone a change from a prior viewing compared to when the region has not undergone a change, as in novel and repeated images. Such effects of memory were absent in healthy older adults [8 (Free Viewing Condition], and in amnesic patients .
Eye movement monitoring is an efficient, useful tool with which to assess memory function in a variety of populations. This protocol describes the use of a head-mounted video-based eye tracker, but the protocol can be easily adapted to the use of remote eye tracking devices, as remote eye trackers remove the need for helmet adjustment and simplify the camera adjustments. However, with a remote eye tracker, head movement must be constrained to maintain accuracy of eye recordings. Accurate calibration of the eye movements is paramount for obtaining useful, and interpretable, data.
Indices of memory obtained through eye movement monitoring obviate the need for acquiring explicit (i.e., verbal) reports of memory, which may be advantageous for rapid investigation of memory in populations with compromised communication skills. Eye tracking may also be used in concert with explicit reports to determine whether there is information that is maintained in memory but is not available for conscious introspection. Additionally, eye movement patterns can be probed to determine when the influence of memory induces a change in those patterns. All together, when compared to explicit reports, measures derived from eye movement monitoring provide more comprehensive detail regarding what is maintained in memory, and when it is accessed 2-3.
Comparing eye movement patterns across population groups provides insight into how the integrity of memory function may change with age, and/or altered neuropsychological status. Interrogating eye movement indices of memory in individuals with lesions to particular areas of the brain can reveal those neural regions that are critical for forming and maintaining particular kinds of information 2-3, 9-10. With further research that examines the reliability of obtaining indices of memory for individual participants with minimal trials outside the laboratory environment, eye tracking may become a useful methodology to monitor and validate memory in training environments, clinical settings and/or law-enforcement situations, such as in eyewitness identification procedures 12.
No conflicts of interest declared.
|Eye Tracker||SR Research Ltd.||Eyelink II|
|Experimental Control Software||SR Research Ltd.||Experiment Builder|
|Eye Movement Analysis Program||SR Research Ltd.||Data Viewer|
Formal Correction: Erratum: Eye Movement Monitoring of Memory
Posted by JoVE Editors on 09/16/2010. Citeable Link.
A correction was made to Eye Movement Monitoring of Memory. There was an error in the author, Douglas A. McQuiggan's, name. The author's name has been corrected to:
Douglas A. McQuiggan