1287_ch10.pdf

(588 KB) Pobierz
Motor Cortex in Voluntary Movements
Section III
Motor Learning and Performance
Copyright © 2005 CRC Press LLC
 
10
The Arbitrary Mapping
of Sensory Inputs
to Voluntary and
Involuntary Movement:
Learning-Dependent
Activity in the Motor
Cortex and Other
Telencephalic Networks
Peter J. Brasted and Steven P. Wise
CONTENTS
0-8493-1287-6/05/$0.00+$1.50
© 2005 by CRC Press LLC
Copyright © 2005 CRC Press LLC
 
ABSTRACT
Studies on the role of the motor cortex in voluntary movement usually focus on
, in which movements are directed toward sensory
cues. Sensorimotor behavior can, however, show much greater flexibility. Some
variants rely on an algorithmic transformation between a cue’s location and that of
a movement target. The well-known “antisaccade” task and its analogues in reaching
serve as special cases of such
transformational mapping
, one form of
nonstandard
. Other forms of nonstandard mapping differ from both of the above: they
are arbitrary. In
, the cue’s location has no systematic
spatial relationship with the response. Here we explore several types of arbitrary
mapping, with emphasis on the neural basis of learning these behaviors.
arbitrary sensorimotor mapping
10.1 INTRODUCTION
Many responses to sensory stimuli involve reaching toward or looking at them.
Shifting one’s gaze to a red traffic light and reaching for a car’s brake pedal exemplify
this kind of sensorimotor integration, sometimes termed
standard sensorimotor
Other behaviors lack any spatial correspondence between a stimulus and
a response, of which Pavlovian conditioned responses provide a particularly clear
example. The salivation of Pavlov’s dog follows a conditioned stimulus, the ringing
of a bell, but there is no response directed toward the bell or, indeed, toward anything
at all. Like braking at a red traffic light, Pavlovian learning depends on an arbitrary
.
1
Copyright © 2005 CRC Press LLC
standard sensorimotor mapping
mapping
mapping
Some forms of arbitrary mapping involve
choosing among goals or actions on the basis of color or shape cues. The example
of braking at a red light, but accelerating at a yellow one, serves as a prototypical
(and sometimes dangerous) example of such behavior. In the laboratory, this kind
of task goes by several names, including
.
1
conditional motor learning
,
conditional
. One stimulus provides the con-
text (or “instruction”) for a given response, whereas other stimuli establish the
contexts for different responses.
, and
stimulus–response
conditioning
Arbitrary mapping enables the association of any
dimensions of any stimuli with any actions or goals.
The importance of arbitrary sensorimotor mapping is well recognized — a great
quantity of animal psychology revolves around stimulus–response conditioning —
but the diversity among its types is not so well appreciated. Take, once again, the
example of braking at a red light. On the surface, this behavior seems to depend on
a straightforward stimulus–response mechanism. The mechanism comprises an
input, the red light, a black box that relates this input to a response, and the response,
which consists of jamming on the brakes. This surface simplicity is, however,
misleading. Beyond this account lies a multitude of alternative neural mechanisms.
Using the mechanism described above, a person makes a braking response in the
context of the red light regardless of the predicted outcome of that action
2
3
and
,
but experts use this term with varying degrees of rigor. Experiments on rodents
sometimes entail the assumption that all stimulus–response relationships are habits.
2
Such behaviors are often called
habits
4,5
But other possibilities exist. Braking at a red light could reflect a voluntary decision,
one based on an attended decision among alternative actions
2
and their predicted
In addition, the same behavior might also reflect high-order cognition,
such as a decision about whether to follow the rule that traffic signals must be obeyed.
Because the title of this book is
, this
chapter’s topic might seem somewhat out of place. However, the motor cortex —
construed broadly to include the premotor areas — plays a crucial role in arbitrary
sensorimotor mapping, which Passingham has held to be the epitome of voluntary
movement. In his seminal monograph, Passingham
Motor Cortex in Voluntary Movements
defined a voluntary movement
as one made in the context of choosing among alternative, learned actions based on
attention to those actions and their consequences. We take up this kind of arbitrary
mapping in Section 10.5 , in which we discuss the premotor areas involved in this
kind of learning. In addition, we summarize evidence concerning the contribution
of other parts of the telencephalon — specifically the prefrontal cortex, the basal
ganglia, and the hippocampal system — to this kind of behavior. Because of the
explosion of data coming from neuroimaging methods, Section 10.5 also contains
a discussion of that literature and its relation to neurophysiological and neuropsy-
chological results. Before dealing with voluntary movement, however, we consider
arbitrary sensorimotor mapping in three kinds of involuntary movements — condi-
tioned reflexes ( Section 10.2 ) , internal models ( Section 10.3 ), and habits ( Section 10.4 ) .
Finally, we consider arbitrary mapping in relation to other aspects of response
selection, specifically those involving response rules ( Section 10.6 ) . For a fuller
consideration of arbitrary mapping, readers might consult Passingham’s monograph
2
2
Copyright © 2005 CRC Press LLC
relationship between a response and the stimulus that triggers it. That is, it depends
on
arbitrary sensorimotor mapping
discrimination
without any consideration of alternatives.
outcomes.
3
and previous reviews, which have focused on the changes in cortical activity that
accompany the learning of arbitrary sensorimotor mappings,
6
the role of the hippoc-
ampal system
7,8
and the prefrontal cortex
9
in such mappings, and the relevance of
arbitrary mapping to the life of monkeys.
10
10.1.1 T
YPES
OF
A
RBITRARY
M
APPING
10.1.1.1 Mapping Stimuli to Movements
Pavlovian conditioning is rarely discussed in the con-
text of arbitrary sensorimotor mapping. Also known as classical conditioning, it
requires the association of a stimulus, called the conditioned stimulus (CS), with a
different stimulus, called the unconditioned stimulus (US), which is genetically
programmed to trigger a reflex response, known as the unconditioned reflex (UR).
Usually, pairing of the CS with the US in time causes the induction of a conditioned
response (CR). For a CS consisting of a tone and an electric shock for the US, the
animal responds to the tone with a protective response (the CR), which resembles
the UR. The choice of CS is arbitrary; any neutral input will do (although not
necessarily equally well). The two types of Pavlovian conditioning differ slightly.
In one type, as described above, an initially neutral CS predicts a US, which triggers
a reflex such as eye blink or limb flexion. This topic is taken up in Section 10.2.1 .
In another form of Pavlovian conditioning, some neural process stores a similarly
predictive relationship between an initially neutral CS and the availability of sub-
stances like water or food that reduce an innate drive. Unlike the reflexes involved
in the former variety of Pavlovian conditioning, the latter involves the triggering of
consumatory behaviors such as eating and drinking. For example, animals lick a
water spout after a sound that has been associated with the availability of fluid from
that spout. This kind of behavior sometimes goes by the name Pavlovian-approach
behavior (a topic taken up in Section 10.2.2 ) . Both kinds of arbitrary sensorimotor
mapping rely on the fact that one stimulus predicts another stimulus, one that triggers
an innate, prepotent, or reflex response.
Stimuli can also be arbitrarily mapped to motor pro-
grams. For example, Shadmehr and his colleagues (this volume
) discuss the evidence
for internal models (IMs) of limb dynamics. These models involve predictions —
computed by neural networks — about what motor commands will be needed to
achieve a goal (and also about what feedback should occur). The IMs are not
examples of arbitrary sensorimotor mapping per se. Arbitrary stimuli can, however,
be mapped to IMs, a topic taken up in Section 10.3 .
11
When animals make responses in a
given stimulus context, that response is more likely to be repeated if a reinforcer,
such as water for a thirsty animal, follows the action. This fact lies at the basis of
instrumental conditioning. According to Pearce,
12
many influential learning theories
have held that after consistently making a response
in a given stimulus context, the expected outcome of the action no longer influences
an animal’s performance. The instrumental conditioning has produced an involuntary
movement, often known as a habit or simply as a stimulus–response (S–R) association.
13–15
Copyright © 2005 CRC Press LLC
Stimulus–Reflex Mappings.
Stimulus–IM Mappings.
Stimulus–Response Mappings in Habits.
of the past 100 years or so

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika:

Zgłoś jeśli naruszono regulamin