Inhibitory component of externally controlled covert orienting in visual space.

Four experiments are reported that investigate an inhibitory effect associated with externally controlled orienting and first identified by Posner and Cohen (1980, 1984). The effect takes the form of an inability to respond quickly to a stimulus appearing in the same location in the visual periphery as a previous one that produced covert orienting. Several characteristics of the effect are revealed that eliminate possible explanations in terms of response inhibition, masking, and sensory habituation. The inhibitory component of orienting occurs whether or not the first stimulus requires a response (Experiment 1), lasts at least a second (Experiments 1, 2, and 3), affects not only the originally stimulated location but also nearby locations (Experiment 2), is determined by environmental coordinates (Experiment 3), and occurs both in the periphery and at the fovea (Experiment 4). It is concluded that inhibition may act together with an early facilitatory component (Posner & Cohen, 1984) in directing the attention and eye movement systems in order to maintain efficient spatial sampling.