Francolini and Egeth (1980) reported findings that are consistent with the
conclusions of Johnston and Dark (1986). Subjects were presented with a circular
array of red and black letters or numerals. Their task was to count the number
of red items and to ignore the black items. Performance speed was reduced
when the red items consisted of numerals conflicting with the answer, but there
was no distraction effect from the black items. These findings suggest that there
was little or no processing of the to-be-ignored black items.
Subsequent research by Driver (1989) contradicted this conclusion. He used
the same task as Francolini and Egeth (1980), but focused on whether or not
conflicting numerical values had been presented on the previous trial. He found
that there was an interference effect, and that this interference effect was of
comparable size from red and black items. The fact that performance on trial
nwasaffectedbythenumericalvaluesofdistractingitemspresentedontrial
n1 means that those items must have been processed.
Driver’s (1989) findings demonstrate the phenomenon ofnegative priming.In
this phenomenon, the processing of a target stimulus is inhibited if that stimu-
lus or one very similar to it was an unattended or distracting stimulus on the
previous trial. For example, Tipper and Driver (1988) found that having a pic-
ture as the unattended stimulus on one trial slowed the processing of the cor-
responding word on the next trial. The details of the processes producing this
negative priming effect are not known, but it is clear that the meaning of the
unattended picture must have been processed.
Section Summary
The fact that processing and responding to attended visual stimuli are often
unaffected by the nature of distracting or unattended stimuli has suggested to
many theorists that there is very little processing of unattended stimuli. How-
ever, the phenomenon of negative priming indicates that this conclusion is un-
warranted. It is probable that there is generally at least some processing of the
meaning of unattended visual stimuli, but that this processing often does not
disrupt responding to attended stimuli.
Visual Search
Sofarwehaveconsideredsomeofthegeneralcharacteristicsoffocusedvisual
attention. In so doing, we have not discussed in detail the various underlying
processes involved in focused attention. Some progress in identifying these
processes has been obtained from the use of visual search tasks. In such tasks,
subjects are presented with a visual display containing a variable number of
stimuli. A target stimulus (e.g. red letter G) is present on half of the trials and
absent on the other half, and the subjects’ task is to decide as rapidly as possi-
blewhetherthetargetispresentinthedisplay.Theeffectsofvariationsinthe
nature of the target and the nature of the non-targets on the speed of response
are observed.
Perhaps the most influential theory based on visual search is thefeature inte-
gration theoryproposed by Treisman (1988, 1992). This theory has been criti-
cised by various theorists including Duncan and Humphreys (1989). Duncan
and Humphreys (1992) proposed an alternative explanation of the visual search
372 Michael W. Eysenck and Mark T. Keane