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Recent science has shown the fruitfulness of taking the brain
to be the seat of all those mental faculties medieval thinkers,
such as Thomas Aquinas, had attributed to the soul. Therefore,
we consider here a variety of results from neuroscience which
make it appear that the various human capacities once attributed
to the soul are better understood as capacities of the human brain.
One sort of research concerns the localizing of various cognitive
and affective functions in specific regions or distributed systems
of the brain. This research began by studying victims of brain
damage, correlating lost faculties with localized damage discovered
during autopsies. With the development of CAT scans (computerized
axial tomography), it has become possible to study correlations
between structural abnormalities and the behavior of people while
they are alive. Further, MRI scans (magnetic resonance imaging)
now provide quite detailed pictures of the brain, more easily
revealing locations of brain damage. And PET scans (positron emission
tomography) allow research correlating localized brain activity
with the performance of specialized cognitive tasks.
These varied techniques have allowed for the localization of
a vast array of cognitive functions. To show the extent to which
current science now studies the capacities once attributed to
the soul, let us consider in more detail the account developed
by Thomas Aquinas of the hierarchically ordered faculties, or
powers, of the soul.
Vegetative Faculties
The `lowest' powers of the human soul, shared with plants and
animals, are the vegetative faculties of nutrition, growth,
and reproduction. All of these processes are now fairly well understood
in biological terms, especially since the discovery of DNA. The
brain is significantly involved here, in that neurochemicals play
a large role in appetite and sex drive; while pituitary hormones
control growth.
Sensitive Faculties
Next higher are the sensitive faculties, shared with
animals but not plants. They include the exterior senses of sight,
hearing, smell, taste, and touch, as well as the four "interior
senses," called sensus communis, phantasia (imagination),
vis aestimativa, and vis memorativa (memory). The
sensus communis is the faculty that distinguishes and collates
data from the exterior senses. An example of this faculty would
be associating the bark and the brownness of the fur with the
same dog. The vis aestimativa allows for apprehensions
that go beyond sensory perception. Here, an example would be apprehending
the fact that something is useful or useless; friendly or unfriendly.
This sensitive level of the soul also provides for the power of
locomotion and for lower aspects of appetite -- the ability to
be attracted to sensible objects. This appetitive faculty is further
subdivided between a simple tendency toward or away what is sensed
as good or evil, and a more complex inclination to meet bodily
needs or threats with appropriate responses: attack, avoidance,
or acquiescence. Together, these appetitive faculties (all still
at the sensitive level) provide for eleven kinds of emotion: love,
desire, delight, hate, aversion, sorrow, fear, daring, hope, despair,
and anger.
Locomotion is now known to be controlled by the motor cortex
-- running across the top of the brain -- and by the efferent
nervous system.
Great progress has been made in tracing the processes involved
in sensation. For example, signals are transmitted from
two different kinds of light-sensitive cells in the retina, through
a series of processors, and on to the visual cortex. Smell involves
the sending of signals from six different kinds of receptor cells
to the olfactory lobes.
The task Aquinas assigned to the "interior sense"
sensus communis -- the ability to synthesize input from
the various external senses -- is now studied by neuroscientists
as "the binding problem."
The "interior sense" of memory, identified by Aquinas,
has also been researched a great deal. Long-term memory is now
understood to arise from patterns of connections within the neural
network. Short-term memory is believed to be enabled by a system
of "recurrent pathways," such that information is processed,
recycled, and then fed into the process again. The hippocampus
is involved in converting short-term into long-term memory, but
how this happens is not yet known.
One of the most interesting findings involves the localization
of specific sorts of memory. Paul Churchland presents a map of
the brain showing regions involved in language memory, with different
locations being responsible for verb access, proper name access,
common noun access, and color terms. The parietal lobes
are an example, as they are involved in our memory of faces.
PET scans make it possible to record localized elevations of
neuronal activity. Paul Churchland reports an experiment in which
his wife, Patricia, was asked to perform a task involving her
visual imagination. The activity in her visual cortex was
elevated exactly during the time she was doing the exercise, but
not to the same extent as when she received external visual stimulation.
Paul Churchland hypothesizes that visual imagination involves
the systematic stimulation of the visual cortex "by way of
recurrent axonal pathways descending from elsewhere in
the brain."
The vis aestimativa of Aquinas included the ability
to distinguish between the friendly and the unfriendly, the useful
and the useless. One clear instance of this is our ability to
read others' emotions. While there does not seem to be a single
location responsible for this capacity, there are patients whose
brain damage has resulted in its loss. For instance, Churchland
describes the patient "Boswell," who suffers from extensive
lesions to the frontal pole of both temporal lobes, and to the
underpart of the frontal cortex. One, among many, of his mental
deficits is the inability to perceive emotion. Churchland reports:
I watched as Boswell was shown a series of dramatic posters
advertising sundry Hollywood movies. He was asked to say what
was going on in each. One of them showed a man and a woman, in
close portrait, confronting one another angrily. The man's mouth
was open in a plainly hostile shout. Boswell, without evident
discomfort or dismay, explained that the man appeared to be singing
to the woman.
The sensitive appetite postulated by Aquinas was responsible
for emotions such as desire, delight, sorrow, and despair. Studies
of the etiology of mental illnesses involving inappropriate affect
have shown a significant role for neurotransmitters such as serotonin.
Rational Faculties
The rational faculties described by Aquinas are distinctively
human: passive and active intellect and will. The will is a higher
appetitive faculty whose object is the good. Since God is ultimate
goodness, this faculty is ultimately directed toward God. The
two faculties of the intellect enable abstraction, grasping or
comprehending the abstracted universals, judging, and remembering.
Morality is a function of attraction to the good, combined with
rational judgment in reference to what the good truly consists.
These higher mental faculties Aquinas attributed to the rational
soul are further from being understood. However, all of them involve
language. Even if we do not understand how these mental
faculties depend on brain functioning, we know that they
do because of the close association of linguistic abilities with
specific brain areas, especially Wernicke's area and Broca's area.
Summary
To review, a variety of results make it appear that the various
human capacities Aquinas had attributed to the soul are better
understood as capacities of the human brain. In fact, these capacities
are attributable to specific regions of the brain.
These conclusions are not uncontroversial. First, there is
the argument within neuroscience over specialization versus globalism.
That is, many would argue that each of the mental capacities listed
above is much more a result of global functioning of the brain,
not localized functioning. We need not get into this argument;
all that needs to be pointed out is that the regions cited above
are involved in the specified functions, since all we know
is that, if a region is damaged by illness or injury, a corresponding
function is lost. Second, there are still some philosophers and
scientists who maintain a dualist account of the mind and brain.
They point out that however precise science may become in associating
mental functions with the brain, science will never prove it is
the brain performing the functions. It may simply be the case
that functions performed by an independently existing mind, or
soul, are just highly correlated with brain functions.
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| Contributed by: Dr. Nancey Murphy
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