“…One such method is to have an observer watch subjects' eyes. Trials on which, in the opinion of the observer, subjects were not fixating centrally are rejected or re-presented at a later time (see, e.g., Berlucchi, Tassinari, Marzi, & Di Stefano, 1989;Maddess et al, 1973;Mohr, Pulvermiiller, & Zaidel, 1994;Nicholls & Atkinson, 1993;Schwartz, Montagner, & Kirsner, 1987). For example, Maddess et al described a method in which fixation location could be monitored by watching subjects' eyes through a monocular lens, whereas Berlucchi et al used video camera to project an image of subjects' eyes onto a television screen by which fixation location could be monitored.…”