Limitation of the eye-tracker

A major limitation when eye-tracking is that the full circle of the pupil and the Purkinje images (eye reflections) must be clearly visible within the video image at all times. Loss of data is inevitable, but measures can be taken to reduce this. Critical to the development of the eye-tracking tasks is appreciating the constraints of the equipment and factoring this into the task programming and collection procedures. Both the participant and the eye-tracking equipment need to be managed to optimise the chances of obtaining good quality data.

Image loss of the pupil or Purkinje images can occur for a number of reasons. The eyelids or eyelashes can obscure the image; particularly problematic if the

participant has long eyelashes or wears mascara, if the eye lid is drooping because the participant is sleepy, or if the pupil is much dilated. To reduce data loss

participants were asked not to wear mascara. Care was taken to ensure that the participant’s pupil was located in the centre of the video screen before starting the calibration process. Participants were encouraged to ensure they were well rested before turning up for data collection. Pupil dilation was reduced by having the room well illuminated and by not having the background images on the monitor screen too dark (note however that too much room illumination can interfere with the Purkinje reflections). The centre of the monitor screen was raised up slightly relative to eye- level which can help to prevent the eyelid clipping the top-most part of the pupil. This does introduce some bias into the data as the automatic calculations used to determine the direction of gaze require that the eye be in line with the centre of the monitor screen (dependent on the spatial accuracy requirements this may be an acceptable trade-off). In this instance this was considered a reasonable trade-off. The image is of course lost when a participant blinks but the eye-tracker can

typically relocate the pupil very quickly. The wearing of glasses or contact lenses was not a problem. I ensured that participants were wearing their glasses or lenses in

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order that they could see the screen clearly when gathering data. Occasionally light reflections on the surface of glasses were problematic. In such cases changing the orientation or location of the eye-tracker with respect to the light source remedied the problem. Participants will have a natural tendency to move their head in addition to their eyes when making a saccade. While the chin rest reduces this tendency, it can prove to be problematic with children below the ages of 7. Participants were reminded not to move their head when making a saccade. I avoided having stimuli at the extreme bottom corners of the screen. If the eye-tracker was having trouble picking up the pupil or was taking excessively long to do so after a blink, then the focus and aperture of the camera were altered via the rings above the camera. Slightly reducing the aperture of the camera (which should by default be fully opened) was found to be particularly useful, and while the image of the eye observable on the control monitor often appeared less clear when the size of the aperture was reduced, the performance of the software identifying the pupil was often improved.

Another approach for improving the quality of the data collected is to use gaze contingent eye-tracking paradigms. One of the benefits of gaze contingent tasks is that both the experimenter and the participant will be made aware of poor image quality and data loss. For example, in the current training and assessment program is an algorithm that requires that a clear image of the participant’s pupil and Purkinje Images be present before the start of each trial. The program algorithm will wait until the pupil can be identified, and will also wait until both the X and Y coordinates are less than 3 cm form the centre of the screen, before starting a trial. While the pupil image and Purkinje Image can be lost once the trial is initiated, this algorithm helps to identify a problem within a single trial. It also serves to remind participants to keep their eye opened sufficiently wide. The alternative is for the task to passively continue to present stimuli regardless of whether a clear image of the eye has been identified.

When it comes to processing and interpreting the data collected it is important to consider the loss of data that has occurred. The loss of data may contain a direction bias common for all participants, particular to one participant, one session, or one block. A directional bias in the loss of data is typically seen for saccade to targets on

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the bottom of the screen. This is due to the fact that downward saccades tend to result in the lowering of the eye-lid which can obscure the pupil. This can impact on the calculation of the RTs. If after making a saccade the pupil is lost and then relocated a moment later this can produce an apparently longer RT than the actual RT (dependent on the method used to identify saccades). Additionally, if a

participant has shifted their head slightly left or right with respect to their initial set up position a horizontal bias can be introduced as their pupil can move out of the image if they make a sideward saccade in the same direction as the head shifts.

A large amount of data loss can distort the results in a number of ways. In a stop signal task the loss of the pupil during a saccade to a target on a stop trial can result in the appearance of a successful inhibition of a saccade. Conversely, the eye-tracker can fail to identify a saccade and the participants will receive immediate negative feedback despite having made a saccade. Equipment failure such as this can bias task strategies. Participants can become frustrated if gaze continent tasks are not working well due to the loss of data. In such cases the distortion of their

performance resulting from frustration will be an idiosyncratic distortion that does not reflect their ability to perform the task or learn, but which reflects dispositional factors related to frustration and equipment limitation. It can also lead participants to engage in behaviour that they feel may aid the eye-tracker, such as forcefully widening their eye, which can also distort data. Poor data generally leads to ambiguity as to whether a successful or failed trial was due to the participant’s on task behaviour or directly or indirectly due to the eye-tracker losing the pupil. Some individuals will be more prone to provide poor quality data than others independent of their ability to complete the experimental task or follow instructions. It is

important that these issues are minimised and considered by maximising the quality of the data collected, factoring these issues into the programming of the tasks, and being aware of the potential for bias during data processing and analysis.

The eye-tracker has limitations with regards to gaze location accuracy. Slight shifts in head position can lead to inaccurate calculation of the eye-gaze direction.

Attempting to have a high spatial accuracy threshold increases the need to conduct frequent recalibration procedures. For this project it was felt that a high accuracy threshold would be particularly problematic for an ADHD population, therefore our

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tasks set a low accuracy threshold. This is not seen as a limiting factor in the task design as the focus of the intervention necessitates precise temporal accuracy and only a degree of spatial accuracy. Having a reduced spatial accuracy threshold avoided problems such as the need for frequent recalibration and also most likely avoided participant frustration.