306 Action Selection
stimulus dimension typically involves a distinction other than
location (e.g., color or letter identity). The Simon effect is
that responses are faster when the location of the stimulus
and response correspond than when they do not. The effect
typically is larger when responses are fast than when they are
slow, implying that activation of the location information oc-
curs quickly and then decreases because it is irrelevant to the
task (Hommel, 1993b). Consistent with this view, when
the correct response is not the one that corresponds with the
location of the stimulus, the lateralized readiness potential
shows evidence of slight, initial activation of the spatially
corresponding response, which then shifts to activation of the
correct, noncorresponding response (De Jong, Liang, &
Lauber, 1994).
Considerable research on the Simon effect has focused on
why stimulus location is coded when it is irrelevant to the
task. Stoffer and Umiltà (1997) attribute the Simon effect to
shifts of attention associated with eye movements. According
to them, the position of the object attended at stimulus onset,
typically a fixation point, provides a frame of reference. The
location of the stimulus relative to the focus of attention is
coded only when attention is shifted to the stimulus. This
code specifies the direction and amplitude of the saccade pro-
gram to shift fixation to the stimulus. The types of evidence
they have presented in support of the attention-shifting hy-
pothesis are that the Simon effect is absent when attention
shifts are prevented by the need to report a stimulus presented
at fixation and reversed when an attention shift back from the
stimulus location to the fixation point is required.
Hommel (1993b) has argued instead that spatial coding
occurs with respect to various frames of reference, of which
the focus of attention may be one. Perhaps the best evidence
for his referential coding hypothesis is that the Simon effect
can vary as a function of multiple frames of reference. In a
procedure used by Lamberts, Tavernier, and D’Ydewalle
(1992) and Roswarski and Proctor (1996), a stimulus can
occur in one of eight locations, four to the left of fixation and
four to the right. Initially, four boxes appear to one or the
other side to designate the possible locations for that trial.
Then the two left or two right boxes disappear, and the im-
perative stimulus is presented in one of the remaining boxes.
In this case, a Simon effect occurs with respect to three
frames of reference: Left-right side of fixation; two left ver-
sus two right on a side; and the left-right location within the
final pair. The largest difference between corresponding and
noncorresponding responses occurs when the stimulus is in
the far left or far right location, for which all three spatial
codes are in agreement (e.g., all left or all right).
As with compatibility for relevant stimulus information,
the Simon effect varies as a function of task goals. Hommel
(1993a) had subjects respond to a high or low pitch tone, pre-
sented to the left or right side, by pressing a left or right key.
The key closed a circuit that lit a light on the opposite side.
When instructed to press the left key to the high pitch tone
and the right key to the low pitch tone, a typical Simon effect
occurred. However, when instructed to turn on the right
light to the high pitch tone and the left light to the low pitch
tone, the Simon effect was a function of light location. That
is, in this case, responses were faster when the stimulus was
on the side opposite the responding hand, rather than on the
same side. Guiard (1983) obtained a similar finding in an ex-
periment in which subjects responded to tone pitch by turning
a steering wheel clockwise or counterclockwise. In the con-
dition of most interest, the subject’s hands were placed at the
bottom of the wheel, and a clockwise turn moved a cursor
to a right target location and a counter-clockwise turn moved
it to the left. Because of the hand placement, when the
wheel was turned clockwise the hands moved to the left, and
vice versa when the wheel was turned counter-clockwise.
A Simon effect was obtained as a function of the direction
of wheel rotation, rather than as a function of the direction
in which the hands moved.
Another goal-related phenomenon is the Hedge and
Marsh (1975) reversal, in which the Simon effect reverses to
favor noncorresponding locations when the response keys are
labeled according to the same dimension as the relevant stim-
ulus information, and subjects are instructed to respond in an
incompatible manner (e.g., press the green key to the red
stimulus and vice versa). The explanation proposed by Hedge
and Marsh, and which has continued to be the most widely
accepted, is that of logical recoding. The basic idea is that
arespond oppositerule is applied both to the relevant stimu-
lus dimension and, inadvertently, to the irrelevant location
dimension, leading to activation of the noncorresponding
response.Negative PrimingFor the Stroop color-naming task, and related tasks with ir-
relevant stimulus information, the target stimulus value on a
trial can not only be a repetition or nonrepetition of the rele-
vant value on the previous trial, but also the same as the value
of the irrelevant information. When the value of the relevant
stimulus dimension is the same as that of the irrelevant di-
mension on the preceding trial, an effect called negative
priming is often observed. This effect was first demonstrated
by Dalrymple-Alford and Budayr (1966) for the Stroop
color-naming task. Subjects had to name the ink colors for
lists of Stroop color words that differed in the relation
between successive stimuli. For the control list, there was no