The brain uses the same neural networks to engage in conscious and
unconscious learning
Nov. 5, 2002
Bethesda, MD - How do we learn? At the same time, when learning is
conscious, does the brain engage in learning based on experience? Many
scientists have believed that the two processes are independent of each
other. Now, new research findings published in the current edition of the
Journal of Neurophysiology, suggest otherwise.
Procedural learning, such as perceptual-motor sequence learning, is
thought to be an obligatory consequence of practiced performance and to
reflect adaptive plasticity in the neural systems mediating performance.
Prior neuroimaging studies, however, have found that sequence learning
accompanied with awareness (declarative learning) of the sequence activates
entirely different brain regions than learning without awareness of the
sequence (procedural learning). However, conflicts between imaging and
behavioral studies have not resolved whether true independence exists
between the two brain functions.
The Study
A breakthrough imaging study has created conditions that allow for such
direct comparison of simultaneous procedural and declarative learning. A
team of physiologists used an MRI to discover whether declarative learning
does or does not prevent learning in procedural memory systems. They created
conditions in which subjects were simultaneously learning different
sequences under implicit or explicit instructions.
The authors of "Direct Comparison of Neural Systems Mediating Conscious
and Unconscious Skill Learning," are Daniel B. Willingham, from the
University of Virginia, Charlottesville, VA; Joanna Salidis, from Stanford
University, Stanford, CA; and John D.E. Gabriel, representing both
institutions. Their findings appeared in the September 2002 edition of the
Journal of Neurophysiology, a journal of the American Physiological Society
(APS).
Methodology
Ten males and nine females, all right-handed, participated in the study.
Participants ranged in ages from 19 to 30 years old.
The serial response time task (SSRTT) paradigm circle appeared in one of
four squares, arranged horizontally in the middle of the computer screen.
Subjects pressed the response key (in a row of 4) with the index and middle
finger of both hands, each finger mapped to a key. Each stimulus stayed on
the screen for 600 ms with a 250-ms interstimulus interval. Sequences (each
12-units long) were randomly chosen for each subject from a corpus of 576
sequences, each of which followed the following constraints: equal frequency
of each position, no direct repetitions, and no runs (e.g., 1234) or trills
(1212) of more than three positions in a row. Stimuli were presented in
blocks of 24 with a 2.2's inter block interval. Each block started with a
520-ms fixation mark (a cross) between the middle two boxes.
Subjects were explicitly instructed that red circles denoted a repeating
sequence of locations and that black circles denoted a random ordering of
locations. Prior to scanning, subjects responded to a single repeating
sequence that always determined the location of the red circles. This
sequence constituted the "explicit-overt" condition because subjects were
aware of the repeating sequence appearing in red.
Prior to scanning, subjects also responded to black circles. Unbeknownst
to subjects, some black circles actually appeared in a second repeating
sequence (the others appeared in random locations). This sequence
constituted the "implicit" condition because subjects were unaware that
there was a repeating sequence for black circles. Thus, prior to scanning,
subjects simultaneously learned one sequence explicitly and another sequence
implicitly.
Results
The behavioral results demonstrate that: (1) subjects were conscious of
the explicit sequence; (2) unconscious of the implicit sequence; and (3)
unconscious of the explicit sequence when it appeared covertly in black.
Subjects were aware of the sequence in the explicit-overt condition.
Throughout scanning, they performed it faster than the random or implicit
sequences. They also learned it declaratively, indicated by the fact that
they selected it among the distracters (random and implicit sequences) in
the postscan recognition test as a sequence they had seen before. The
subjects also learned the sequence procedurally in the implicit condition.
They responded faster to the implicit sequence than to the random sequences,
but slower to it than to the explicit sequence.
Nevertheless, even at the end of the experiment, they failed to recognize
the implicit sequence above chance. The postscan recognition test was
designed to be highly sensitive to any awareness of the sequence: a graded
rating scale was used so subjects could show even partial declarative
knowledge. Furthermore, subjects made the recognition judgments
simultaneously with performing the sequences, showing a concurrent
dissociation between their procedural (RTs faster than random) and
declarative knowledge (no difference from random sequences). Finally,
subjects were not aware of the explicit sequence in the explicit-covert
condition.
Conclusions
The behavioral and neuroimaging results from this study demonstrate that
procedural learning in this paradigm is an obligatory consequence of
performance. In the present paradigm, procedural memory (implicit greater
than random condition) activated left prefrontal cortex, left inferior
parietal cortex, and right putamen. The same regions were also active in the
explicit-covert condition in which the sequence had been declaratively
learned. Although the degree of activation differed in some of these
structures, the neural network that enhanced performance for the implicit
and for the explicit-covert conditions was virtually the same. The explicit
covert activation, therefore, documents procedural modulation that occurred
under conditions of declarative learning and awareness in the prescan skill
learning session.
These findings suggest a more refined interpretation of the parietal
cortex's role in spatial attention in this task. Spatial attention may
facilitate orienting to targets in either an externally or internally driven
fashion. In the implicit and explicit covert conditions, orienting is
externally driven by the appearance of the target. In sum, the role of
cognitive load in procedural learning is not yet clear, and may differ
across different varieties of procedural knowledge such as motor skill,
classification, and classical conditioning.
The present findings indicate that when awareness and performance are
well controlled, modulation occurs in the same neural network for procedural
learning whether that learning is or is not accompanied by declarative
knowledge. Declarative learning, however, activates many additional brain
regions. This conclusion suggests an integral role for the procedural system
in some skills requiring physical practice regardless of whether learning
occurs with or without declarative memory.
From the September 2002 edition of the Journal of Neurophysiology, a
journal of the American Physiological Society (APS).
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