Sell more eye trackers and help improve science!

February 2006

"Sell more eye trackers and help improve science!" was the message we received this week from Professor Tim Jordan, Chair of Cognitive Neuroscience at the University of Leicester, UK.

Work by Professor Jordan and his group shows that experimental tasks that require accurate fixation can be done properly only by using an eyetracker. Of particular concern is that numerous studies in vision research require participants to fixate a central fixation point in order to present stimuli accurately in their left and right visual hemifields.

Unfortunately, ensuring accurate central fixation is not an easy task and many studies produce misleading data because procedures such as merely instructing participants to fixate a central point are not effective, and systematic biases in fixation location go unnoticed (see Jordan et al., 1998, 2000, 2003; Jordan & Patching, 2006).

For example, in a study using words, Jordan et al. (1998; see also Jordan & Patching, 2006) found that when participants were instructed to fixate a central point but their actual fixations were monitored using an eye tracker, central fixation occurred on just 23% of trials and most noncentral fixations fell to the right of centre, with obvious implications for performance differences between the two hemifields. These effects are not just "noise" but influential biases and even small shifts in fixation (e.g., 10 minutes of arc) from the central fixation point can have an exaggerated effect on performance because acuity is both increased for targets in the "fixated" visual hemifield and decreased for targets in the "non-fixated" visual hemifield.

To overcome these variations in fixation accuracy, researchers should ensure fixation of a fixed central point by using an eye tracker. For example, Jordan et al. (1998) used the technique of presenting a central fixation point but controlled the onset of lateralized word targets by using an eye tracker interfaced with a CRS display system. This arrangement allowed presentation of lateralized targets only when the eye tracker indicated fixation of a central fixation point and so ensured accurate target presentation on each trial.

Tim Jordan is a longstanding and valued customer of Cambridge Research Systems: back in 1998 he was investigating fixation accuracy using visual stimuli generated by our D300 Point Plotter. His more recent research uses the VSG2/5 and our top-of-the range ViSaGe Visual Stimulus Generator in conjunction with the Skalar IRIS eyetracker. (Cambridge Research Systems assumed exclusive worldwide responsibility for the supply, support and service of the Skalar Iris Infra Red Light Eye Tracker in December 2004).

• Jordan, T.R., & Patching, G.R. (2006). Assessing effects of fixation demands on perception of lateralized words: A visual window technique for studying hemispheric asymmetry. Neuropsychologia, in press.
• Jordan, T.R., Patching, G.R., & Thomas, S.M. (2003). Assessing the role of hemispheric specialization, serial-position processing and retinal eccentricity in lateralized word perception. Cognitive Neuropsychology, 20, 49-71.
• Jordan, T.R., Patching, G.R., & Milner, A.D. (2000). Lateralized word recognition: Assessing the role of hemispheric specialization, modes of lexical access and perceptual asymmetry. Journal of Experimental Psychology: Human Perception and Performance, 26, 1192-1208.
• Jordan, T.R., Patching, G.R., & Milner, A.D. (1998). Central fixations are inadequately controlled by instructions alone: Implications for studying cerebral asymmetry. Quarterly Journal of Experimental Psychology, 51A, 371-391.

To learn more about Professor Jordan's work, and why he insists on controlling his participants' fixations, visit his web page.

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