Does the 'P300' speller depend on eye gaze?

Abstract

Many people affected by debilitating neuromuscular disorders such as amyotrophic lateral sclerosis, brainstem stroke or spinal cord injury are impaired in their ability to, or are even unable to, communicate. A brain-computer interface (BCI) uses brain signals, rather than muscles, to re-establish communication with the outside world. One particular BCI approach is the so-called 'P300 matrix speller' that was first described by Farwell and Donchin (1988 Electroencephalogr. Clin. Neurophysiol. 70 510-23). It has been widely assumed that this method does not depend on the ability to focus on the desired character, because it was thought that it relies primarily on the P300-evoked potential and minimally, if at all, on other EEG features such as the visual-evoked potential (VEP). This issue is highly relevant for the clinical application of this BCI method, because eye movements may be impaired or lost in the relevant user population. This study investigated the extent to which the performance in a 'P300' speller BCI depends on eye gaze. We evaluated the performance of 17 healthy subjects using a 'P300' matrix speller under two conditions. Under one condition ('letter'), the subjects focused their eye gaze on the intended letter, while under the second condition ('center'), the subjects focused their eye gaze on a fixation cross that was located in the center of the matrix. The results show that the performance of the 'P300' matrix speller in normal subjects depends in considerable measure on gaze direction. They thereby disprove a widespread assumption in BCI research, and suggest that this BCI might function more effectively for people who retain some eye-movement control. The applicability of these findings to people with severe neuromuscular disabilities (particularly in eye-movements) remains to be determined.

Figure 1. The effects of the distance to the center (eccentricity) on visual acuity

This figure shows the degradation of visual acuity at increasing angles from a centered focus point. The visual acuity (expressed as the Snellen fraction, i.e., 20/20 equals 100%) quickly declines to approximately 20% at 10 degrees eccentricity. (Modified from Westheimer 1965.)

Figure 2. Experimental setup

Subjects were presented with a matrix on a computer screen. There were two experimental conditions. In condition 1 (“letter”), the subject was free to gaze at the target (e.g., the letter F). In condition 2 (“center”), the subject was asked to gaze at a fixation cross in the center of the matrix. Fixation was verified in real time by an eye tracker.

Figure 3. Electrode montage for groups A and B

EEG from group B was recorded from the 64 locations shown here (extended 10–20 montage (Sharbrough et al. 1991)). EEG from group A was recorded from an optimized subset of 8 electrodes (shown in blue) (Krusienski et al. 2006, Krusienski et al. 2008).

Figure 4. Distributions of the distance of eye gaze from the center during the two conditions

The traces show the distributions of the horizontal (for the six columns) or vertical (for the six rows) distances of gaze location from the fixation cross for the “letter” condition (red) and the “center” condition (blue). Shading shows standard deviation across subjects.

Figure 5. Classification accuracy as a function of the number of stimulus repetitions

As expected, classification accuracy steadily increases with number of stimulus repetitions. Accuracy is substantially greater for the “letter” condition than for the “center” condition.

Figure 6. Accuracy as a function of the distance of eye gaze from the center

Red and blue traces show results for Conditions 1 and 2, respectively. Shading indicates standard deviation across subjects.

Figure 7. Accuracy versus stimulus repetitions for the 8-channel (red) and 64-channel (blue) montages for Condition 1 (down-pointing triangles) and Condition 2 (up-pointing triangles) for the subset of subjects with 64-channel recordings

Figure 8. Average traces and topographies for the two tasks

Average signed r² traces (A) and wave forms (B) for the two tasks. The traces show negative early VEP components around 180 ms post stimulus for the “letter” task (red traces). P3 components appear to be delayed and smaller in amplitude for the “center” task (blue traces). (C,D): Topographies show an early VEP component (topographies at 180 and 300 ms) for the “letter” task that is absent for the “center” task. Topographies also show a classical P300 response (topographies at 420 ms).