The ecological validity of laboratory experiments on the saccadic system

J.M. Findlay

Centre for Vision and Visual Cognition, University of Durham, South Road, Durham DH1 3LE, United Kingdom (

Scientific research proceeds by investigating manageable problems and frequently this involves choosing to research simple situations. This approach is always subject to the criticism that results may not be of general applicability. In this talk, I shall consider the validity of this criticism in relation to work on the saccadic system

The most frequent use of saccades is to scan around the visual environment. However much work on the saccadic system investigates saccadic orienting to simple targets which appear suddenly. The onset visual transient of such targets will preferentially stimulate the magnocellular pathways of the visual system. Onset visual transients are recognised to have a special attentional status and also appear to have automatic and inescapable access to the saccadic system, as shown clearly in the remote distracter effect (Walker et al, 1997). Another feature of the processing of onset transients is the spatial integration demonstrated by the global effect (Findlay, 1982). Although originally found in a situation without onset transients (Coren and Hoenig, 1972), the effect has a less robust status in saccade control during the scanning of static scenes.

Visual search offers a way of investigating display scanning in a controlled situation. A typical search experiment requires subjects to search for a pre-specified target amidst a number of distracter elements. Experimental work on visual search (Findlay, 1997; Belky and Motter, 1998) has identified two factors which affect saccade landing position both of the first saccade after the display appears and of subsequent saccades. The first factor is visual similarity to the target, reflecting the operation of the search selection set adopted by the subject. The second is spatial proximity to the current fixation location. These factors are likely to be dominant ones in many scanning situations. This is illustrated by a recent study of scanpaths through a set of randomly positioned objects.

Belky E.J. and Motter B.C. (1998). The guidance of eye movements during active visual search. Vision Research, 38, 1805-1818.
Coren S. and Hoenig P. (1972). Effect of non-target stimuli on the length of voluntary saccades. Perceptual and Motor Skills, 34, 499-508.
Findlay J.M. (1982). Global processing for saccadic eye movements. Vision Research, 22, 1033-1045.
Findlay J.M. (1997). Saccade target selection in visual search. Vision Research, 37, 617-631.
Walker R., Deubel H., Schneider W.X. and Findlay J.M. (1997). The effect of remote distractors on saccade programming: evidence for an extended fixation zone. Journal of Neurophysiology, 78, 1108-1119.