Although work on visual selectivity has for some years been rather less popular than work on auditory selectivity, considerable progress has been made. The paucity of research may be related to the technical and conceptual difficulties involved, but even so it is rather surprising when one considers that probably the most common of all instruction in visual experiments is an attentional instruction such as: 'Fixate the dim red spot, and pay attention to the stimuli which will appear just to its left.' It is curious that there has been, very little modern work, if any, on the importance and effect of this instruction. Obviously it ensures that the observer will at least be looking in the right direction when the signal is presented and, as we shall see from the work of Sanders (1963), this can be expected to have a marked effect. There are a number of studies on the effect upon the accuracy of perception of the angular distance from the fovea at which the stimuli are presented, but they are scattered and unsystematic. Only a few recent papers have attempted to investigate directly the effect of attentional factors (e.g. Grindley and Townsend, 1968) and they found quite substantial effects. Grindley and Townsend compared the accuracy of reports of the occurrence and orientation of a small geometrical figure in the periphery of the visual field by subjects who were with and without knowledge of which quadrant was to contain the signal.