Thus a consequence of a visual stimulus at a spatial position is to reduce the efficiency of attending to subsequent information at that location.
The second set of experiments shows that each visual stimulus causes an active orienting of attention toward its spatial position. This orienting is said to be active because it is reduced or delayed by requiring the subject to perform a secondary task (counting backwards).
Our studies indicated that the basic passive effect of a visual stimulus is to retard processing of subsequent events at that position. This spatially selective sensory inhibition must be overcome by an active process if efficient concentration is to be maintained.
An enduring problem in psychology is to understand the mechanisms by which we prepare for and select information from a source of sensory or memorial information in the presence of competing events from other sources. Most of the literature in this area has emphasized concentration upon an input or sensory channel. For example, one ear ( Broadbent, 1958), one modality ( Posner, Nissen, & Klein, 1976) or upon a location in visual space ( Posner, 1980). Eventually, however, one hopes that some of the same underlying mechanisms that allow us to concentrate upon a source of sensory information are also necessary to sustain concentration upon an area of semantic memory. If so, studies of selecting information from sensory signals may also tell us something about our ability to maintain concentration while thinking.
A major distinction in cognitive psychology is between mechanisms that are primarily passive and those which involve active or controlled processes ( Posner & Snyder, 1975; Shiffrin & Schneider, 1977). A filtering mechanism could be either active or passive, but our discussion of filtering includes the idea that the filter, although set actively, can be maintained independently of the subject's current focus of attention. It would seem important to know whether subjects are able to maintain concentration by a process which passively walls out potentially interfering information without using current processing capacity in maintaining the filter. Such a filter would be quite distinct from preparation and selectivity maintained by active orienting toward the selected source, which remains effective only while the subject was free of other tasks.
The idea of a passive filtering mechanism can be associated with selection at a very early level of the nervous system. Such mechanisms do exist physiologically. For example, Skinner and Yingling ( 1977) show the existence of a thalamic gate that can control the entry of information from sensory channels to