Stimulus-driven Shifts of Spatial Attention

Turning the eyes and the head to an object when it suddenly appears in the visual f ield, producing what is called exploration behavior, is a common experience for everyone. Sokolov (1963) described this phenomenon as the orienting reflex. The purpose of the orienting reflex is to allow the central system to identify the new element that has just occurred in order to prepare the whole organism to react, if necessary, to it. This reflex is composed of a series of independent mechanisms, one of the most important of which is the orienting of attention to the region in space where the new element has appeared (see, e.g., Posner, Snyder, & Davidson, 1980). Usually, the movement of attention is accompanied by a movement of the eyes to foveate a specific area of the visual field, but as has been well established by many studies, a person can selectively attend to a region in space without making any eye movements (e.g., Eriksen & Colegate, 1971; Eriksen & Hoffman, 1972; Posner, 1980; Posner et al., 1980). When the focus of attention is correctly oriented, stimulus detection or discrimination is enhanced. This is what Posner (1980) named covert orienting. However, because objects observed in everyday life have different shapes and sizes, it is plausible that once the focus of attention has been oriented, it will also be adjusted to the size of the attended figure. That is what we call the focusing process. The main aim of the present study was to provide evidence that focusing is a different process from orienting. Focusing was investigated in isolation— that is, in the absence of orienting, which, in contrast, was involved in all of the previous studies on focusing. STIMULUS-DRIVEN SHIFTS OF SPATIAL ATTENTION