298 Action Selection
electroencephalograms (EEG), which measure voltage
changes in the brain over time from electrodes placed on the
scalp. Of particular concern are event-related potentials
(ERPs); these are voltage changes in the EEG elicited by a
specific event (e.g., a stimulus onset), averaged across many
trials to remove background EEG activities. One reason for
the popularity of ERPs is that, while a task is being per-
formed, they provide continuous measures of brain activity
presumed to be systematically related to cognitive processes.
By comparing the effects of task manipulations on various
ERP components, their onset latencies, and their scalp distri-
butions, one can make relatively detailed inferences about the
cognitive processes. These inferences can be used, along with
behavioral measures, to evaluate alternative information pro-
cessing models.
There are a number of different ERP components, or fea-
tures, that are indicators of different aspects of processing.
These are labeled according to their polarity, positive (P) or
negative (N), and their sequence or latency. Early compo-
nents such as P1 and N1 (the first positive and negative com-
ponents, respectively) are associated with early perceptual
processes. They are called exogenous componentsbecause
they occur in close temporal proximity to the stimulus event
and have a stable latency with respect to it. Later components
such as P3 (or P300) reflect cognitive processes such as at-
tention. These components are called endogenousbecause
they are a function of the task demands and have a more vari-
able latency than the exogenous components. For example,
when an occasional target stimulus is interspersed in a stream
of standards, the P3 is observed in response to targets, but not
to standards.
A measure that has been used extensively in studies of ac-
tion selection is the lateralized readiness potential (LRP;
Eimer, 1998), mentioned previously. This potential can be
recorded in choice-reaction tasks that require a response with
the left or right hand. It is a measure of differential activation
of the lateral motor areas of the visual cortex that occurs
shortly before and during execution of a response. The asym-
metric activation favors the motor area contralateral to the
hand making the response, because this is the area that con-
trols the hand. Of importance, the LRP has been obtained in
situations in which no overt response is ever executed, allow-
ing it to be used as an index of covert, partial response
activation. The LRP is thus a measure of the difference in ac-
tivity from the two sides of the brain that can be used as an in-
dicator of covert reaction tendencies, to determine whether a
response has been prepared even when it is not actually exe-
cuted. It can also be used to determine whether the effects of
a variable are prior or subsequent to response preparation,
as Osman et al. (2000) did. Falkenstein, Hohnsbein, and
Hoormann (1994) suggested that the latency of the LRP is
linked most closely to central decision processes (i.e., action
selection), whereas the peak is more closely related to central
motor processes.
Electrophysiological measurements and recordings of
magnetic fields do not have the spatial resolution needed to
provide precise information about the brain structures that
produce the recorded activity. Recently developed neuroimag-
ing methods, including positron-emission tomography (PET)
and functional magnetic resonance imaging (fMRI), measure
changes in blood flow associated with neuronal activity in dif-
ferent regions of the brain. These methods have poor temporal
resolution but much higher spatial resolution than the electro-
physiological methods. Combined use of neuroimaging and
electrophysiological methods provides the greatest degree of
both spatial and temporal resolution (Mangun, Hopfinger, &
Heinze, 1998).RELEVANT STIMULUS INFORMATIONUncertainty and Number of Alternatives:
The Hick-Hyman LawMerkel (1885), described in Woodworth (1938), provided
the initial demonstration that RT increases as a function
of the number of possible alternatives. In Merkel’s experi-
ment, the Arabic numerals 1–5 were assigned to the left hand
and the Roman numerals I–V to the right hand, in left-to-right
order. Results showed that when the number of alternatives
increased from 2 to 10 choices, mean RT increased from ap-
proximately 300 ms to a little over 600 ms.
Contemporary research dates from Hick’s (1952) and
Hyman’s (1953) studies in which the increase in RT with
number of alternatives was tied to information theory,
which quantifies information in terms of uncertainty (forN
equally likely alternatives, the number of bits of informa-
tion is log 2 N). The stimuli in Hick’s study were 10 lamps
arranged in an irregular circle, and responses were 10 keys
on which the fingers of the two hands were placed. In
Hyman’s study, the stimuli were eight lights corresponding
to the eight corners of inner and outer squares, and each
light was assigned a spoken name. In both studies, RT in-
creased as a logarithmic function of the number of alterna-
tives. Moreover, RT also varied systematically as a function
of the relative proportions of the stimulus-response (S-R)
alternatives, the sequential dependencies, and speed-
accuracy trade-off, as expected on the basis of informa-
tion theory. This relation between RT and the stimulus