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The Neurology of Reasoning
Jeffrey E. Shuren, MD, JD;
Jordan Grafman, PhD
Arch Neurol. 2002;59:916-919.
INTRODUCTION
Of all the qualities of humankind, the ability to reason stands out
as our most defining characteristic. It forms the cornerstone of scientific
investigation and the expansion of human knowledge. Without the ability to
reason, the developments of modern medicine and of science in general would
never have occurred.
Although patients with isolated impairments of reasoning can perform
the simple tasks of daily life, their ability to navigate through life and
the quality of their lives are often significantly diminished. Clearly, there
is a need to attack impaired reasoning with the same fervor that we use to
treat defects in language and other cognitive functions.
This article defines reasoning as the ability to draw conclusions from
given information (premises or arguments). The conclusions reached provide
new insights. Neuropsychological investigations have traditionally divided
reasoning into 2 categories, deduction and induction, based on the type of
relationship between the premise and the conclusion. Deductive reasoning reaches
conclusions that are limited to the information contained within the original
set of premises. The premises necessarily lead to one conclusion. By contrast,
inductive (or probabilistic) reasoning reaches conclusions that extend beyond
the original set of premises. The premises give probable support only for
the conclusion drawn. Inductive reasoning creates new knowledge by using incomplete
information. This article focuses primarily on deductive-reasoning studies.
However, as noted later, the distinction between deduction and induction may
be more artificial than real.
EXPERIMENTAL TASKS
Many early investigations into human reasoning used tasks that evaluated
multiple skills. Some of these tasks, such as Raven's Progressive Matrices,
require deduction but also rely on other frontal lobe abilities such as executive
control and planning. Performance failure on such tasks does not necessarily
indicate a deficit in reasoning. However, no single task selectively assesses
reasoning; this ability requires that other cognitive skills function properly,
such as comprehension of the task and the stimuli, perception, working memory,
and in some cases mental imagery. In addition, most paradigms do not mirror
real-life situations. Therefore, poor performance on a task may not reflect
poor reasoning ability in day-to-day life, and vice versa.
Complicating matters, few measurable cognitive components of reasoning
have been identified. As a consequence, relevant neuropsychological tests
available today may signal that a deficit in reasoning exists but generally
do not identify which aspect of reasoning is impaired. Vague definitions and
the lack of units of measure also plague efforts to assess, measure, and define
reasoning.
The most frequently used tool, the Wason selection task, assesses conditional
reasoning. Conditional reasoning involves drawing (deductive) inferences from
scenarios in which the occurrence of one event is conditioned on the occurrence
of a second event. Typically, conditional relations are phrased as "If p,
then q" (eg, "If a person has not graduated from college, then he or she cannot
attend medical school"). The Wason selection task requires subjects to identify
what information is necessary to ascertain whether a conditional relation
is true or false. For example, the examiner gives subjects 4 cards, each with
a letter on one side and a number on the other. Subjects are asked to decide
which of the cards they need to turn over to find out whether a certain rule
is being followed, such as "If a card has an A on
one side, then it must have a 4 on the other side." Drawings of 4 cards follow,
showing 4 possible cases: A (p card), B (not-p
card), 4 (q card), and 7 (not-q card). The logically correct response is to select the p (A) and not-q (7)
cards. However, when presented with this task, only 4% of college students
selected these cards.1 That number increased
to 62.5% when the task used a familiar scenario.2
For example, when given the rule "If a person is drinking beer, then the person
must be older than 19 years," normal subjects select the appropriate cards
more than 60% of the time.
The gambling task assesses decision making, the ability to select a
course of action. Although not a reasoning task, studies using this test have
shed light on processes important for reasoning. In the task, subjects choose
cards from 4 decks. Selecting cards from 2 of the decks more often results
in a reward (a low immediate gain but a smaller future loss; hence, a long-term
gain), and selecting from the other 2 decks more often results in a penalty
(a high immediate gain but a larger future loss; hence, a long-term loss).
After encountering several losses, normal subjects generate an anticipatory
skin conductance response before choosing a card from a disadvantageous deck
and begin to avoid choosing from those decks.
IMAGING STUDIES
With the advent of positron emission tomography (PET), neuroscientists
have more closely scrutinized the neural basis of reasoning. Three PET imaging
studies3-5 using
syllogisms reported left prefrontal activation during both deductive and inductive
tasks. Only induction produced activation of the medial prefrontal cortex.
A fourth study6 found medial left frontal cortical
activation as well as right hemispheric involvement during deduction and induction.
The content of the stimuli in all 4 imaging studies, however, generally did
not refer to situations with which the test subjects would have had prior
experience. One possible explanation is that the left hemisphere may be more
adept than the right hemisphere at reasoning when the subject is not familiar
with the content of the reasoning task (content-independent reasoning), such
as reasoning about abstract situations.7
STUDIES IN PSYCHIATRIC PATIENTS
Deglin and Kinsbourne8 asked patients
with schizophrenia or bipolar disorder to solve syllogisms following electroconvulsive
therapy (ECT) to either cerebral hemisphere. The syllogisms contained information
that was either familiar or unfamiliar to the subjects. Prior to ECT, 86%
of subjects gave a logical answer. Following right hemisphere suppression
by ECT, the same percentage of subjects provided logical answers, but responses
came more quickly and with greater assurance. Following left hemisphere suppression
by ECT, however, 79% offered empirical answers in accordance with their own
experience. In a second experiment, subjects were asked to solve syllogisms
with familiar content but false premises (eg, "All trees sink in water; Balsa
is a tree; Does balsa sink in water or not?"). In the control condition, two
thirds of all answers were empirical. Following left hemisphere suppression
by ECT, subjects rejected false premises more frequently, with empirical answers
comprising almost 90% of the answers. In addition, after left hemisphere ECT,
subjects rejected false premises with strong emotion. Following right hemisphere
ECT, the number of logical answers almost doubled; subjects responded to false
premises calmly and appeared unaffected by the absurdity of the premises.
Deglin and Kinsbourne concluded that the right hemisphere uses acquired knowledge
to ensure that thoughts correspond to reality. In contrast, the left hemisphere
applies rules of formal logic independent of the content of the material.
STUDIES IN PATIENTS WITH BRAIN LESIONS
Golding9 evaluated responses to a Wason
selection task made by patients with unilateral brain lesions (the author
did not report the location of the lesions, however). Although no control
subjects and only 1 subject with damage to the left hemisphere selected the p card and not-q cards (the logically
correct answer), 50% of the subjects with right hemisphere damage chose both
cards. Because control subjects tended to select cards that matched the items
described in specific test sentences, Golding proposed that the perceptual
aspects of the task interfered with the control subjects' verbal reasoning
(for example, when given the sentence "Whenever there is a circle on one half
of the card, there is yellow on the other half of the card," control subjects
selected the circle or the circle and the yellow cards). According to Golding,
patients who had right hemisphere damage with impaired visual processing showed
superior verbal reasoning skills because of a lack of visual perceptual interference.
Adolphs et al10 administered a Wason
selection task using both familiar and unfamiliar stories to patients with
dorsolateral frontal lesions, those with ventromedial prefrontal lesions,
and normal controls. Subjects in all 3 groups performed equally poorly when
given unfamiliar stories. However, patients with dorsolateral lesions and
normal subjects chose the p and not-q cards (the logically correct selections) when the story involved
familiar material. In contrast, the patients with ventromedial lesions who
had damage to the medial orbitofrontal cortex (5 of 6 had bilateral damage)
did not show this facilitatory effect with familiar material. The authors
concluded that people generally reason by analogy and retrieve past experiences,
including the emotion experienced, when confronted with a familiar situation.
Patients with ventromedial prefrontal lesions may fail to retrieve or appropriately
use past experiences when reasoning.
To further explore the role of the ventromedial prefrontal (medial orbitofrontal)
cortex (VPC) in reasoning and decision making, Bechara et al11
administered a gambling task. After several trials, normal subjects generated
an anticipatory skin conductance response prior to choosing a card from the
disadvantageous decks and started to avoid selecting cards from those decks.
Patients with bilateral damage to the VPC failed to produce an anticipatory
skin conductance response and continued to select cards from the disadvantageous
decks.
Similar to their performance in laboratory studies, patients with bilateral
damage to the VPC typically make real-life decisions against their best interests
and fail to learn from their mistakes. However, their general intellect and
other executive function abilities remain intact, and they usually retain
the ability to generate options in social situations and to conceptualize
the consequences of selecting a particular option.12
In the study by Bechara and colleagues, 50% of patients with damage to the
VPC were able to recognize and identify the bad decks but still performed
disadvantageously. Therefore, the authors posited that when presented with
situations in which a decision is required, individuals generate possible
responses and the probable outcomes of those responses and recall their prior
experiences in similar situations. The VPC performs the latter function by
activating a link between factual knowledge about the situation and the type
of bioregulatory state (including the emotional state) associated with that
situation based on the individual's past experiences. When a person faces
a situation similar to one previously experienced, relevant facts are generated,
and the VPC activates linkages to reconstruct a previously learned factual-emotional
set. The VPC facilitates a person's ability to perform risk-benefit analyses
by rejecting less appropriate responses. Patients with damage to this region
of the frontal lobes do not choose advantageously because they fail to activate
a bioregulatory state appropriate to the consequences of a response. Options
and outcomes become essentially equal for the individual. Therefore, subjects
fail to respond to future consequences and are more controlled by the immediate
results. They can generate options and future outcomes but do not act on this
knowledge because they fail to activate the behavioral relevance of available
choices of action. Primate studies as well as PET imaging and electrophysiological
studies in humans support this view.13-14
In comparison, the dorsolateral prefrontal cortex may be important for generating
potential responses and their expected outcomes.
Different areas of the orbitofrontal prefrontal cortex (OPC) may perform
various functions important for reasoning. According to Elliott et al,15 the medial OPC (or VPC) monitors associations between
stimuli, responses, and outcomes and determines whether a particular response
"feels right." The lateral OPC suppresses previously rewarded responses, allowing
the individual to inhibit the current choice behavior and to modify that behavior
in response to changing circumstances.
Patients with bilateral damage to the amygdala also choose disadvantageously
on the gambling task16 and exhibit poor judgment
and decision making in their real-life behavior.17
However, the decision-making failure is likely the result of the patients'
inability to experience the emotional attributes of a situation, whereas patients
with VPC damage cannot effectively integrate all of the bioregulatory state
information provided by the amygdala and other structures. In addition, damage
to the amygdala or VPC likely impairs the ability to form associations between
complex situations and bioregulatory states.
These reports suggest that a neural network subserving reasoning that
includes the dorsolateral prefrontal cortex may identify potential responses
and their expected outcomes. A second network that includes the VPC may determine
the behavioral relevance of response options.
ROLE OF EACH HEMISPHERE
As described previously, both patients with brain lesions and PET imaging
studies suggest that each hemisphere performs different functions pertaining
to reasoning. One interpretation of these studies is that the left hemisphere
may use rules to reason independent of the content of the task. Therefore,
the left hemisphere would be more adept at abstract reasoning. In contrast,
the right hemisphere may use past experiences (factual or emotional) and thus
would be more adept when reasoning involves familiar scenarios. The ventromedial
prefrontal neural network plays a role when the behavioral relevance of possible
responses can aid the selection of the appropriate action.
A second hypothesis is that the right hemisphere holds representations
of the emotional states associated with events experienced by the individual.18 When that individual encounters a familiar scenario,
representations of related past emotional experiences are retrieved by the
right hemisphere and are incorporated into the reasoning process. In the absence
of or failure to activate such representations, the left hemisphere applies
learned rules of logic.
Asymmetric advantages in processing based on receptive field size offer
a third explanation for hemispheric differences in reasoning skills. Beeman
et al19 provide evidence that large semantic
receptive fields account for the right hemisphere's role in understanding
discourse and metaphor. A similar explanation may underlie hemispheric differences
in reasoning. Large receptive fields in the right hemisphere would permit
individuals to activate all possible relationships, local and distant, between
the items in the problem to be solved. Overlap between features from the activation
of multiple relationships would allow the right hemisphere to use past experience
to narrow possible options; patients would arrive at the appropriate conclusion
and reason by analogy. The right hemisphere would have an adaptive advantage
over the left hemisphere in analogical reasoning and reasoning involving familiar
situations. By comparison, the left hemisphere's fine coding would allow individuals
to focus on the main feature or event and on the local relationships between
the items in the problem. As a result, the left hemisphere would have an adaptive
advantage over the right hemisphere in formal logical reasoning and reasoning
involving abstract content.
FUTURE DIRECTIONS
Reasoning, like the prefrontal cortex, is a primarily human trait that
develops late in childhood.20 Reasoning deficits
can arise from various causes. For example, impaired reasoning can be an initial
symptom of frontal lobe dementia or the sequelae of frontal lobe stroke or
head trauma. Various neurological disorders can affect regions of the brain
that are important for reasoning, sometimes selectively, producing similar
clinical manifestations but requiring potentially different treatments. Regardless
of the cause, reasoning deficits can seriously affect patients' ability to
manage their daily affairs and to interact appropriately with others. As neurologists,
we must be able to detect impaired reasoning in our patients as an indication
of neurological disease, diagnose the underlying cause of that reasoning deficit,
and treat both the cause and the deficit if possible.
A better understanding of the basic mechanisms and subprocesses of reasoning
should make it feasible to evaluate reasoning abilities in different neurological
disorders. To facilitate this process, coherent nomenclature and units of
measure need to be formulated. With increasing knowledge of the specific mechanisms
that mediate reasoning will come the increasing hope of developing effective
pharmacological and behavioral therapies to treat reasoning deficits that
result from neurological damage.
AUTHOR INFORMATION
Accepted for publication March 26, 2001.
Author contributions: Study concept and design (Drs Shuren and Grafman); acquisition of data (Dr Shuren); analysis and interpretation of data (Dr Shuren); drafting of the manuscript (Drs Shuren
and Grafman); critical revision of the manuscript for important intellectual
content (Drs Shuren and Grafman); administrative,
technical, and material support (Drs Shuren and Grafman).
Corresponding author and reprints: Jeffrey E. Shuren, MD, JD, Office
of Policy, Planning, and Legislation, US Food and Drug Administration, HF-11,
Room 14-101, 5600 Fishers Ln, Rockville, MD, 20857 (e-mail: jshuren{at}oc.fda.gov).
From the Cognitive Neuroscience Section, National Institute of Neurological
Disorders and Stroke, National Institutes of Health (Dr Grafman), and the
Office of Policy, Planning, and Legislation, Office of the Commissioner, US
Food and Drug Administration (Dr Shuren), Rockville, Md.
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