Components of orienting in visual space
Experiments were conducted to investigate the alignment of attention with a location in visual space as the result of either an internal search plan or an external stimulus event; namely internally- and externally-controlled orienting. First, if an informative cue is presented to fixation indicating the probable location of a subsequent target to appear in the visual periphery within the next 1000 msec, the detection response is faster to a target appearing in that location them to one appearing elsewhere, even in the absence of eye movements (internally-controlled covert orienting). Second, a brief non-informative cue presented in the visual periphery also speeds the detection response to a target if it appears 100 msec after and in the same location as the cue, compared to a different location (externally-controlled covert orienting). In addition to the facilitation of manual responses, a target from the same location in the periphery as the cue appears to occur earlier than one from a different location, for intervals between the cue and the target of up to 500 msec. Although temporal judgments are unaffected at longer cue-target intervals, both manual and ocular responses are slower to a target appearing between 300 and 1300 msec after a cue in the periphery and in the same location than to one in a different location. This inhibitory effect requires that the cue and the target share environmental, but not necessarily retinal, co-ordinates and occurs regardless of whether or not the first event (the cue) requires a response. Externally-controlled orienting is a necessary but insufficient condition to produce inhibition, as the alignment of either the attention or the eyes with the previously-stimulated location can overcome the effect. However, not every event in the visual periphery results automatically in externally-controlled covert orienting. It can be reduced, delayed or even prevented by additional information present in the visual field, or by the requirements of secondary tasks. The facilitatory and inhibitory components of externally-controlled orienting appear to act together to direct the eye movement system and to maintain selectivity in visual space.