Handbook of Psychology, Volume 4: Experimental Psychology

Relevant Stimulus Information 301

basis, but are by-products of the relative salience of the two
dimensions.
Compatibility effects can occur as well when the spatial
dimension along which the stimulus locations vary is or-
thogonal to that along which the response alternatives vary.
For top-bottom stimuli mapped to left-right key press or
vocal responses, the mapping of top-right and bottom-left
yields faster responding than the alternative mapping (Cho
& Proctor, 2001). A variant of salient features coding can
also explain this mapping effect (Weeks & Proctor, 1990).
Specifically, evidence indicates that the two alternatives on
the vertical and horizontal dimensions are coded asymmet-
rically, with top and right being the polar referents for their
respective dimensions. Consequently, the salient features
coding explanation is that action selection occurs faster for
the top-right/bottom-left mapping than for the alternative
mapping because it is the one for which the salient features
correspond. Adam, Boon, Paas, and Umiltà (1998) pro-
posed that this asymmetric coding is a property of verbal
codes but not spatial codes. However, Cho and Proctor
provided evidence that it is a general property of spatial
coding.
With unimanual movements of a joystick or finger, the
top-right/bottom-left mapping is also typically more compat-
ible than the alternative mapping. In this case, though, the
mapping preference is affected by the location of the re-
sponse apparatus. The top-right/bottom-left advantage is
enhanced when responding in the right hemispace, but it re-
verses to a top-left/bottom-right advantage when responding
in the left hemispace (Weeks, Proctor, & Beyak, 1995). Lippa
(1996) provided evidence that the mapping preference is also
affected by hand posture. According to her referential coding
hypothesis, the finger-to-wrist axis provides a reference
frame that allows the response set to be coded parallel to
the stimulus set. For example, when left-right responses are
made with the right hand held at a comfortable 45–90º, the
left response can be coded as top and right response as bot-
tom. Referential coding can explain many results obtained
with unimanual responses, but it cannot explain why the
mapping preferences described above occur when the
hand and finger are in a neutral posture that allows only left-
right deflections perpendicular to the sagittal body midline
(Michaels & Schilder, 1991).
Because of this deficiency of the referential coding hy-
pothesis, Lippa and Adam (2001) proposed an end-state com-
fort hypothesis. Similar to referential coding, the end-state
comfort hypothesis views orthogonal compatibility as a cor-
respondence effect. However, it assumes that the response
dimension is mentally rotated, according to relative hand pos-
ture, to bring it into alignment with the stimulus dimension.

The direction of rotation, clockwise or counterclockwise, is

determined by physical constraints of the body. The response

dimension is mentally rotated in the direction that would

yield the most comfortable end-state posture if the hand were

actually rotated (inward movement for the left or right hand

when positioned at centered or ipsilateral locations, and out-

ward movement when positioned at contralateral locations).

The end-state comfort hypothesis can account for more re-

sults obtained with unimanual responses than the referential

coding hypothesis, but both hypotheses are not directly ap-

plicable to the orthogonal compatibility effects obtained with

bimanual or vocal response sets.

Dual-Route Models

Virtually all explanations of SRC effects agree that at least

part of the difference in RT between compatible and incom-

patible mappings involves the time to translate the stimulus

into its assigned response based on the instructions provided

for the task. Translation is presumed to be fastest when an

identity rule can be applied (i.e., make the response corre-

sponding to the stimulus), intermediate when some other rule

can be used (e.g., make the response that is the mirror oppo-

site of the stimulus), and slowest when the response must be

retrieved via the specific S-R associations defined for the

task. Although some models rely exclusively on intentional

translation (e.g., Rosenbloom & Newell, 1987), dual-route

models that propose an additional direct (or automatic)

response-selection route have come to be favored (e.g.,

Kornblum et al., 1990; see Figure 11.4). The basic idea is that

when a stimulus occurs it tends to produce activation of its

corresponding response by way of long-term S-R associa-

tions, regardless of the S-R mapping defined for the task. The

resulting activation produces a benefit in responding when

the corresponding response is correct, but a cost when it is

not. The major reason that dual-route models have become

popular is that correspondence effects often occur for irrele-

vant stimulus dimensions (see Lu & Proctor, 1995), as dis-

cussed in a subsequent section.

Sequential Effects

Repetition Benefit

Bertelson (1961) was the first to formally investigate sequen-

tial effects on performance. He showed that for a two-choice

task, in which left-right stimuli were mapped compatibly to

left-right keys, the total response time for a set of trials was

less when the proportion of repetitions was .75 than when

it was .25. This repetition benefit was evident when the