300 Action Selection
are made with two fingers on the same hand (Heister,
Schroeder-Heister, & Ehrenstein, 1990). The dependence of
the effect on the spatial relations of the stimuli and responses
has led most accounts of spatial SRC to focus on spatial cod-
ing as its basis. The spatial codes are based on the task goals,
as illustrated in a study by Riggio, Gawryszewski, and
Umiltà (1986) in which subjects operated the left key with a
stick held in the right hand and the right key with a stick held
in the left hand. Even though the hands were on their normal
sides, responses were faster with the S-R mapping in which
the stimuli corresponded to the location of the response key
and not the hand used for responding.
Conceptual, Perceptual, and Structural Similarity
A variety of SRC effects in addition to spatial compatibility
have been demonstrated. Kornblum, Hasbroucq, and Osman
(1990) and Kornblum and Lee (1995) have argued that SRC
effects will occur for any situation in which the stimulus and
response sets have dimensional overlap (i.e., are similar).
Dimensional overlap is presumed to include both conceptual
and perceptual similarity. The role of conceptual similarity is
illustrated in the findings that spatial SRC effects, broadly de-
fined, occur when location words are spoken in response to
physical location stimuli, as well as when left-right key
presses are made to the words leftandrightor to left- and
right-pointing arrows. The role of perceptual similarity is
shown by the finding that SRC effects are larger within the
spatial-manual and verbal-vocal modes, that is, for physical
locations mapped to key presses and location words mapped
to naming responses, than between the modes (Wang &
Proctor, 1996).
SRC effects are also obtained when the S-R sets do not
share conceptual or perceptual similarity but have structural
similarity. When an ordered set of stimuli (e.g., A, B, C, D) is
mapped to an ordered set of responses (e.g., 1, 2, 3, 4), RT is
shorter for a mapping that preserves or reverses this order
than for one that does not. Another type of structural compat-
ibility effect occurs when a symbolic two-dimensional stim-
ulus set is mapped to index and middle finger responses on
each hand. When two letters (O, Z) of two sizes (large or
small) are mapped to the responses, the left-to-right mapping
of O, o, z, Z is easier than one of O, z, o, Z (Miller, 1982;
Proctor & Reeve, 1985). Proctor and Reeve presented evi-
dence that this difference is due to the letter identity distinc-
tions being salient for the stimulus set and the distinctions
between the two left and two right responses being salient for
the response set. Performance is best for the condition in
which the salient stimulus feature maps directly onto the
salient response feature. In other words, translation of the
specific stimulus into a response can occur more quickly
when salient features correspond. Salient features coding has
been shown to determine the compatibility effects obtained
for a variety of situations in which the stimulus and response
sets have structural similarity, but no conceptual or percep-
tual similarity (Proctor & Reeve, 1990).Compatibility Effects in Two DimensionsUmiltà and Nicoletti (1990) examined compatibility along two
dimensions in a two-choice task by varying the stimulus and
response locations for a set of trials along a diagonal (see Fig-
ure 11.3). They found that the compatibility effect was larger
for the horizontal dimension than for the vertical dimension, a
phenomenon they called right-left prevalence. Vu and Proctor
(2001) showed that this right-left prevalence effect can be re-
versed to top-bottom prevalence by increasing the relative
salience of the vertical dimension. This was accomplished by
using response sets that emphasized the top-bottom distinction.
In one experiment that showed top-bottom prevalence, subjects
responded with anatomical top-bottom effectors, a hand and
foot. In another experiment, top-bottom prevalence was ob-
tained when one hand was placed over the other so that the
top-bottom distinction was salient. Thus, although right-left
prevalence typically is obtained when left-right effectors are
used, and top-bottom prevalence when top-bottom effectors are
used, the prevalence effects do not seem to have an anatomicalFigure 11.3 Illustration of the S-R compatibility conditions and subtasks
in Umiltà and Nicoletti’s (1990) two-dimensional compatibility experi-
ments. The stimuli (depicted by circles) and response keys (depicted by
cylinders) were arranged along the same (bottom row) or different (top row)
diagonals. By varying the mapping of stimuli to responses for each of the
four cells, mappings could be generated that were compatible on both
dimensions, compatible on the vertical dimension but not the horizontal
dimension and vice versa, or incompatible on both dimensions.