Relevant Stimulus Information299informationthatistransmittedintheresponsesisknownas
theHick-Hymanlaw:
RTabHT,whereais basic processing time and bis the amount that RT
increases with increases in the amount of information trans-
mitted (HT; log 2 Nfor equally likely S-R pairs with no errors).
The slope of the Hick-Hyman function is negatively
correlated with measures of intelligence, which several re-
searchers have claimed to reflect ability to process informa-
tion rapidly (see Jensen, 1980). However, the fact that the
slope of the function is highly dependent on the amount of
practice (described later) and other factors severely limits
any conclusions that can be drawn from the negative correla-
tion with intelligence tests. Arecent study by Vickrey and
Neuringer (2000) showed that the Hick-Hyman function has
a lower slope for pigeons than for humans, even when they
are tested in similar circumstances, which, if the relation to
intelligence were accepted, would imply that pigeons are
more intelligent than humans.
OnecriticismoftheHick-Hymanlawisthatthefunction
relatingRTtonumberofalternativesisnotlogarithmic.
Kvälseth(1980)introducedavarietyoflaws,includinga
powerlawforthecaseofequallylikelyalternativesandan
exponentiallawforcasesinwhichthealternativesarenot
equallyprobable.Longstreth,El-Zahhar,andAlcorn(1985)
claimedthatthespecificpowerlaw,RT=a+b(1 – N–^1 ),
provides a better fit to data for equiprobable alternatives than
the logarithmic function. Longstreth et al.’s main argument
for the power law is that as the number alternatives increases
beyond 8, the function is no longer linear with respect to the
logarithm, but becomes curvilinear (see Longstreth, 1988).
Although theoretically derived from an attentional model,
Longstreth et al.’s power law is a special case of the more gen-
eral power law proposed by Kvälseth (1980). In addition,
Kvälseth (1989) and Welford (1987) pointed out that
Longstreth et al.’s power law has several problems. Kvälseth
(1989) captures the status of the Hick-Hyman law, stating,
“Although, on purely empirical grounds, Hick-Hyman’s law
may not be uniformly superior to other lawful relationships, it
has been clearly established that it does provide a good sum-
mary description of a substantial amount of data” (p. 358).
Stimulus-Response Compatibility
Stimulus-response compatibility (SRC) is one of the princi-
pal factors affecting efficiency of action selection. SRC refers
to the fact that performance is better with some mappings of
stimuli to responses than with others. SRC effects are ubiqui-
tous and occur with a variety of stimulus and response sets,
although much of the research has focused on spatial SRC
effects.Spatial Compatibility EffectsPaul Fitts is given credit for formalizing the concept of
SRC. Fitts and Seeger (1953) examined performance of
eight-choice tasks using all combinations of three stimulus
arrangements and three response arrangements. They found
that responses were faster and more accurate when the stim-
ulus and response arrangements corresponded spatially than
when they did not. Fitts and Deininger (1954) showed that for
conditions in which the stimulus and response arrangements
were the same, responses were much slower with an arbitrary
mapping of S-R locations than with one in which the corre-
sponding response was made to each stimulus. Even more
interesting, performance was also much better with a mirror-
reverse mapping of stimulus locations to response locations
than with a random mapping, although performance was still
inferior to that of the spatially corresponding mapping.
The spatial SRC effect is robust in that it is obtained with
auditory and tactual stimuli and with key presses, joystick
movements, and unimanual aimed movements (see Proctor &
Reeve, 1990, and Hommel & Prinz, 1997, for edited volumes
on SRC). The slope of the function for the Hick-Hyman law,
relating RT to the number of alternatives, is inversely related
to SRC (Smith, 1968), approaching zero for highly compati-
ble S-R mappings (Teichner & Krebs, 1974). In other words,
SRC effects increase in magnitude as the number of S-R al-
ternatives increases.
Many studies have used a two-choice task in which a left
or right key press is made to a left or right stimulus. In two-
choice tasks, responses are typically 50–100 ms faster when
the S-R mapping is spatially compatible than when it is not,
regardless of whether the stimuli are visual or auditory.
Moreover, PET scans show increased bloodflow for incom-
patible mappings compared to compatible mappings in the
same brain regions (left rostral dorsal premotor and posterior
parietal areas) for both visual and auditory modalities
(Iacoboni, Woods, & Mazziotta, 1998). This spatial SRC
effect is a function of relative position of the stimuli and
responses: It occurs even when the stimulus display or hands
are shifted to the left or right of center (Nicoletti, Anzola,
Luppino, Rizzolatti, & Umiltà, 1982). Moreover, the SRC
effect is found when the hands are crossed so that the left
hand operates the right key and the right hand the left key
(Roswarski & Proctor, 2000), as well as when the responses