The neural basis of task-set preparation.

Task-switching has recently become a popular paradigm for studying executive
control. Participants are required to switch back and forth between two choice
reaction time (RT) tasks afforded by the same class of stimulus. For example,
participants might have to switch between classifying colored shapes by color
or by shape, or between classifying digits as odd/even or high/low. The task to
be performed on a given trial is determined by a visual cue presented prior to
each stimulus. The cue-stimulus interval provides a prepara¬tion interval
during which, on a switch trial, we might expect participants to attempt to
reconfigure their cognitive processes for the changed task. The most basic
observation is that changing tasks incurs a switch cost: mean RT is longer (and
error rate usually greater) when the task changes than when the same task is
performed as on the previous trial (Allport et al., 1994; Rogers & Monsell,
1995). Of particular interest here is the effect of the preparation interval on
the switch cost. As the preparation interval increases up to about half a
second, there is typically a substantial reduction in switch cost -- the
preparation effect (Meiran, 1996; Nieuwenhuis & Monsell, 2002).
What mental process does the preparation effect reflect? In other
words, what do subjects do during the preparation interval that causes them to
be more prepared when the stimulus arrives? Two classes of theory have been
offered. According to Rogers & Monsell*s (1995) task-set reconfiguration
theory, the preparation effect is an index of an endogenously-triggered control
process of task-set reconfiguration*a sort of mental *gear changing**carried
out by the subject prior to the stimulus onset. Task-set reconfiguration can
include shifting attention between stimulus attributes or elements, retrieving
goal states (what to do) and condition*action rules (how to do it), enabling a
different response set and adjusting response criteria. Task-set
reconfiguration may well involve inhibition of elements of the prior task set
as well as activation of the required task set.
According to priming theory (Logan & Bundesen, 2003; Schneider & Logan,
2005), subjects can perform task-switching experiments using a single, general
task set, and switch costs reflect cue-encoding benefits, not endogenous
task-set reconfiguration. This theory exploits a peculiar feature of cued
task-switching procedures: The cue and the imperative stimulus can be treated
as a compound stimulus that uniquely determines the correct response. For
example, the cue Odd-Even and the stimulus 7 map uniquely onto one of the
response buttons. From this perspective, there is no endogenous act of control
that prolongs RT on task switch trials. Instead, there may be a benefit from
repeating the cue*part of the compound stimulus*on task repetition trials.
Priming theory explains the benefit from repeating the cue in terms of memory
retrieval and the principles that govern it, including repetition priming. The
preparation effect reflects the operation of these elementary memory processes
during the preparation interval.
It has proven hard to distinguish between the task-set reconfiguration
theory and priming theory on the basis of behavioral experiments (e.g., Monsell
& Mizon, in press). However, the two theories make distinct predictions with
regard to patterns of brain activity during the preparation interval. Task-set
reconfiguration theory predicts that the preparation interval should be
characterized by preparatory activity in the brain areas associated with the
relevant task set, that is, the various perceptual, cognitive, and motor
components of the upcoming task. Furthermore, this preparatory activity should
be predictive of subsequent task performance. Priming theory denies the
importance or existence of such task-specific preparatory activity, and instead
posits that the same elementary memory processes are active during the
preparation interval, regardless of which task has to be performed. The
proposed research attempts to test between these predictions using
event-related fMRI.

The primary objective of this study is to understand the neurocognitive basis
of task-set preparation. To this end, we will acquire fMRI data and behavioural
responses of healthy adults (aged 18-30 years).

Participants will perform a simple task-switching experiment while fMRI data is
acquired. The study has the purpose to understand how preparation for a
particular task is reflected in functionally distinct brain areas (see protocol
for details)

There are no known risks associated with participating in an fMRI study. This
is a noninvasive technique involving no catheterizations or introduction of
exogenous tracers. Numerous children and adults have undergone magnetic
resonance studies without apparent harmful consequences. Some people become
claustrophobic while inside the magnet and in these cases the study will be
terminated immediately at the subject's request. The only absolute
contraindications to MRI studies are the presence of intracranial or
intraocular metal, or a pacemaker. Relative contraindications include pregnancy
and claustrophobia. Subjects who may be pregnant, who may have metallic foreign
bodies in the eyes or head, or who have cardiac pacemakers will be excluded
because of potential contraindications of MRI in such subjects. Although there
is no direct benefit to the participants from this proposed research, there are
greater benefits to society from the potential knowledge gained from this
study. An important factor of successful human functioning is the ability to
rapidly shift between various tasks, an ability that if often compromised in
older adults and other neurologically impaired groups. The proposed research
will lead to a better understanding of task-set preparation and its
implementation in the brain.