Relevant and Irrelevant Stimulus Information 305This asymmetric pattern of interference has been reported
in numerous subsequent studies, including versions of the
task in which RTs to individual stimuli are recorded. An im-
portant finding is that the pattern of asymmetry is dependent
on the response mode. When the task involves pointing to a
matching color, responses to color words are delayed by in-
congruent colors, but responses to colors are not delayed by
irrelevant color words (Durgin, 2000). Similarly, in spatial
versions of the task, in which the word leftorrightis pre-
sented in left or right locations or with an arrow pointing to
the left or right, the words produce interference when the re-
sponses are made vocally, but the locations or arrows pro-
duce interference when the responses are key presses (Lu &
Proctor, 1995).
Stroop (1935/1992) showed in his Experiment 3 that a di-
mension that does not produce interference (e.g., ink colors
when the task is word reading) can be made to do so with
practice. In his experiment, subjects practiced four lists of
50 words in the color-naming task for 8 days. The average
time to read the list decreased from 50 s on the first day to
33 s on the last day, but this was still longer than the 25 s
to name a neutral list of colored swastikas. Subjects also
performed the word-reading task prior and subsequent to
practicing the color-naming task. The time to perform the
word-reading task was nearly twice as long (35 s) after the
color-naming practice as before (19 s). Thus, the practice in-
creased the strengths of the associations between colors and
names, and the colors now produced interference with read-
ing color words.
More generally, relative strength of association is a good
predictor of whether an irrelevant stimulus dimension will
affect responding to a relevant stimulus dimension. Lu and
Proctor (2001) classified the association of stimulus dimen-
sions to key presses as high if they were both conceptually
and perceptually similar (e.g., arrows are spatial and nonver-
bal, as are key presses), intermediate if they were only con-
ceptually similar (e.g., location words are spatial but verbal),
and low if they were neither (e.g., colors and color words are
not similar to key presses). Across several experiments using
various combinations of relevant and irrelevant stimulus
dimensions, the relative magnitudes of effect size were
predictable based on relative association strength. Baldo,
Shimamura, and Prinzmetal (1998) obtained similar results
varying response modalities in addition to stimulus dimen-
sions: Robust Stroop effects to location word/arrow stimuli
were observed when responding manually to location words
or vocally to arrows, but not for the reverse relations. The
results of Lu and Proctor and of Baldo et al. are generally
consistent with Kornblum et al.’s (1990) emphasis on re-
sponse activation varying as a function of dimensional over-
lap and with parallel distributed processing models of the
type proposed by Cohen, Dunbar, and McClelland (1990),
which rely on relative association strength.The Eriksen Flanker EffectAnother widely studied effect of irrelevant information is the
Eriksen flanker effect (Eriksen & Eriksen, 1974). In the typi-
cal experiment examining this effect, one or more stimuli are
assigned to left-right responses. The target letter for each trial
is presented at a known, centered location and is flanked by
instances of a distractor letter. In Eriksen and Eriksen’s ex-
periment, the letters H and K were assigned to one response
and the letters S and C to the other response. The flanking
letters could be the same as the target (HHHHHHH), the let-
ter assigned to the same responses as the target (congruent;
KKKHKKK), or a letter assigned to the opposite response
(incongruent; SSSHSSS or CCCHCCC). When the letters
were in close spatial proximity, responses were faster when
the flanking letters were identical to or congruent with the
target than when they were incongruent. This congruency
effect decreased as the spatial separation between the letters
increased.
Because distractors that are not potential targets produce
little or no interference, the results suggest that the effects
reflect response activation. That is, the flanking letters acti-
vate the response to which they are assigned, producing
response competition when that response is not the one sig-
naled by the target. This competition is evident in a tendency
for the lateralized readiness potential to show initial activa-
tion of the wrong response 150 to 250 ms after onset of the
target and incongruent distractors (Gratton, Coles, Sirevaag,
Eriksen, & Donchin, 1988). Eriksen and Schultz (1979) pro-
posed a continuous flow account of the flanker effect, much
like McClelland’s cascade model, in which stimulus informa-
tion gradually accumulates in the visual system and continu-
ously flows into the response system. Initially, a wide range
of responses is activated, but as the output from the percep-
tual system becomes more exact, the response activation be-
comes increasingly restricted to the appropriate response.
This account assumes that after a flanking letter is fully iden-
tified, it will no longer produce response activation. How-
ever, if it is assumed that fully identified flankers may still
contribute to response activation, then discrete stage models
can account for the results as well (Mordkoff, 1996).The Simon EffectThe Simon effect is another close relative of the Stroop effect
(Lu & Proctor, 1995). In the typical Simon task, stimulus lo-
cation is irrelevant and the responses, most often left-right
key presses, vary along a location dimension. The relevant