PAIN
IS A PSYCHOLOGICAL EXPERIENCE THAT IS
SEPARATE FROM BEHAVIORAL REACTIONS TO
INJURIOUS STIMULI
It
has become very clear that pain is a
psychological experience with both a
perceptual aspect and an emotional aspect.
The perceptual aspect tells us that we
have been injured, like the first
sensation when you hit your thumb with a
hammer. The emotional aspect is separate
as in the suffering that follows after we
are first aware of hitting our thumb. But,
injurious stimuli do not always lead to
the experience of pain. Think of a trip to
the dentist.
When a dentist injects a local
anesthetic into your jaw to block nerve
conduction, some of your teeth and a part
of your mouth feel numb. When a tooth is
then drilled, the sensory nerve cells in
the tooth that would normally trigger pain
are still excited, but the nerve block
prevents activity in these receptors from
being sent to the brain, so pain is not
felt.
In addition, a person’s
behavioral reaction to pain is separate
from pain experience. We see this
separation when a person endures pain
without showing any discomfort. On the
other hand, people sometimes react
behaviorally to injury without any feeling
any experience of pain or suffering. This
kind of separation between behavioral and
psychological responses to injury results
from certain forms of damage of the brain
or spinal cord. Because the experience of
pain is separate from the behavioral
response to injury, the term nociception
is used to refer to detection of injury by
the nervous system (which may or may not
lead to pain).
Injurious stimuli that usually lead
to pain experience are called nociceptive
stimuli.
The term pain
should be used only to refer to the
unpleasant psychological experience that
can result from a nociceptive stimulus.
REACTIONS
TO INJURY ARE PRESENT IN ALL FORMS OF
ANIMAL LIFE BUT THESE REACTIONS DO NOT
MEAN THAT PAIN IS EXPERIENCED-IT IS NOT
NECESSARY FOR A NOCICEPTIVE STIMULUS TO BE
CONSCIOUSLY EXEPERIENCED FOR A BEHAVIORAL
REACTION TO OCCUR
In
humans, reactions to nociceptive stimuli
are usually associated with feelings of
pain. Consequently, humans often assume
that reactions by animals to nociceptive
stimuli mean that these animals experience
similar pain. In reality, reactions to
nociceptive stimuli are protective
responses that can occur in forms of life
that are incapable of perceiving pain. The
ability to detect and react to nociceptive
stimuli is a widespread characteristic of
animal life. Single-celled creatures such
as an ameba will move away from irritating
chemical or mechanical stimuli. These
reactions are automatic and because the
ameba doesn’t have a nervous system, it
has no ability to actually sense the
stimulus that causes its reaction or to
feel pain. There are many other
invertebrate organisms (animals without
backbones) that also react to nociceptive
stimuli, but with somewhat more complex
patterns of escape than an ameba. For
example, starfish have a primitive nervous
system that interconnects sensory
receptors detecting injurious stimuli with
muscle cells that cause movements,
enabling the starfish to slowly move away
from a nociceptive stimulus. The starfish’s
nervous system has only a small number of
nerve cells. It has no brain, so like the
ameba, its reactions are not very precise
or complex and it can’t experience, in
the way of humans, the stimuli that
trigger its reactions. Thus, protective
reactions don’t require very complex
nervous systems and can occur in animals
incapable of perceiving, that is being
aware of, the stimuli that cause such
reactions.
IN
VERTEBRATES, REACTIONS TO INJURIOUS
STIMULI ARE CONTROLLED BY THE SPINAL CORD
AND BRAINSTEM
Vertebrates
generally have more complex nervous
systems than invertebrates and vertebrates
have a clearly developed brain. This brain
receives information from the spinal cord
about nociceptive stimuli that contact the
body surface. Working together with the
spinal cord, the brain generates rapid,
coordinated responses that cause the
organism to escape these stimuli. These
automatically generated responses include
withdrawal of the stimulated body part,
struggling, locomotion and in some
animals, vocalizations. All of these
responses are generated by the lower
levels of the nervous system, including
the brainstem and spinal cord.
HUMAN
EXISTENCE IS CEREBRALLY-DOMINATED- A FISH’S
EXISTENCE IS BRAINSTEM DOMINATED
Human
existence is dominated by functions of the
massively developed cerebral hemispheres.
Fishes have only primitive cerebral
hemispheres and their existence is
dominated by brainstem functions. The
brains of vertebrate animals differ
greatly in structural and functional
complexity. Cold-blooded animals, such as
fish, frogs, salamanders, lizards and
snakes, have simpler brains than
warm-blooded vertebrates, the birds and
mammals.
Fish have the simplest types of
brains, of any vertebrates, while humans,
have the most complex brains of any
species. All mammals have enlarged
cerebral hemispheres that are mainly an
outer layer of neocortex. Conscious
awareness of sensations, emotions and pain
in humans depend on our
massively-developed neocortex and other
specialized brain regions in the cerebral
hemispheres. If the cerebral hemispheres
of a human are destroyed, a comatose,
vegetative state results. Fish, in
contrast, have very small cerebral
hemispheres that lack neocortex. If the
cerebral hemispheres of a fish are
destroyed, the fish’s behavior is quite
normal, because the simple behaviors of
which a fish is capable (including all of
its reactions to nociceptive stimuli)
depend mainly on the brainstem and spinal
cord. Thus, a human’s existence is
dominated by the cerebral hemispheres, but
a fish is a brainstem-dominated organism.
The
capacity to perceive and be aware of
sensory stimuli, rather than just react to
such stimuli requires a complex brain. In
humans, the cerebral hemispheres,
especially the neocortex, is the
functional system that allows us to be
aware of sensory stimuli. If
the cortex of the human brain is damaged
or made dysfunctional, we lose our
awareness of sensations.
For example, damage of the visual
part of the cortex causes blindness, even
though vision-related sensory activity is
still occurring in subcortical parts of
the brain. If the neocortex is widely
damaged we lose our capacity to be aware
of our existence in general. This loss of
awareness occurs in spite of the fact that
the levels of our nervous system below the
cerebral hemispheres, the brainstem and
spinal cord, can still be functioning and
processing signals from sensory stimuli,
including injurious stimuli. In a fish,
“seeing” is performed by the brainstem
and occurs automatically without
awareness. Consequently, a fish’s visual
behavior is quite normal if the small
cerebral hemispheres are removed, but a
human is blind if the visual cortex region
of the cerebral hemispheres is destroyed.
This is because our visual behavior
depends greatly on conscious awareness of
visual sensations.
In
spite of our unawareness of brainstem
functions, the brainstem and spinal cord
contain programs that control our more
automatic behavioral functions. Smiling
and laughter, vocalizations, keeping our
balance, breathing, swallowing and
sleeping are all processes that are
generated by these lower, brainstem and
spinal cord programs.
FISH
DO NOT HAVE THE BRAIN DEVELOPMENT THAT IS
NECESSARY FOR THE PSYCHOLOGICAL EXPERIENCE
OF PAIN OR ANY OTHER TYPE OF AWARENESS
The
experience of pain depends on functions of
our complex, enlarged cerebral
hemispheres. The unpleasant emotional
aspect of pain is generated by specific
regions of the
human cerebral hemispheres, especially the
frontal lobes. The functional activity of
these frontal lobe regions is
closely tied to the emotional aspect of
pain in humans and damage of these brain
regions in
people eliminates the unpleasantness of
pain. These regions do not exist
in a fish brain. Therefore, a fish doesn’t
appear to have the neurological capacity
to experience the unpleasant psychological
aspect of pain. This
point is especially important, because
some opponents of fishing have argued that
fish are capable of feeling pain because
some of the lower, subcortical nervous
system pathways important for nociception
are present in fish.
Obviously this argument has no
validity because without the special
frontal lobe regions that are essential
for pain experiences, lower pathways alone
can’t produce this experience. The
rapid, well-coordinated escape
responses of a fish to nociceptive stimuli
are generated automatically at brainstem
and spinal cord levels but, if a fish’s
brainstem and spinal cord work like a
humans (and it is very likely that they
do) there is no awareness of neural
activity occurring at these levels.
It
might be argued that fish have the
capacity to generate the psychological
experience of pain by a different process
than that occurring in the frontal lobes
of the human brain, but such an argument
is insupportable. The capacity to
experience pain, as we know it, has
required the massive expansion of our
cerebral hemispheres, thus allocating
large numbers of brain cells to the task
of conscious experience, including the
emotional reaction of pain. The small,
relatively simple fish brain is fully
devoted to regulating just the functions
of which a fish is capable. A fish brain
is simple and efficient, and capable of
only a limited number of operations, much
like a 1949 Volkswagen automobile. By
comparison, the human brain is built on
the same basic plan as that of a fish, but
with massive expansions and additional
capacities. The human brain is more like a
modern luxury car with all-wheel drive,
climate control, emission controls,
electronic fuel injection, anti-theft
devices and computerized systems
monitoring. These refinements and
additional functions couldn’t exist
without massive additional hardware. The
massive additional neurological hardware
of the human cerebral hemispheres makes
possible the psychological dimension of
our existence, including pain experience.
There
are also huge differences between mammals
in the degree of complexity of cerebral
hemisphere development, especially within
the frontal lobes. The brains of predatory
mammals are typically larger and more
complex than brains of their prey. For
example, the brains of sheep and deer have
a tiny fraction of the frontal lobe mass
that is present in humans, making it
probable that the kinds of psychological
experience of these animals, including
pain, is quite different from human
experience.
THE
REACTIONS OF FISH TO NOCICEPTIVE STIMULI
ARE SIMILAR TO THEIR REACTIONS TO
PREDATORS AND OTHER NON-NOCICEPTIVE
STIMULI
When
a fish is hooked by an angler, it
typically responds with rapid swimming
behavior that appears to be a flight
response. Human observers sometimes
interpret this flight response to be a
reaction to pain, as if the fish was
capable of the same kind of pain
experience as a human. From the previous
explanation, it should be clear that fish
behavior is a result of brainstem and
spinal patterns of activity that are
automatically elicited by the stimulation
of being hooked, but that fish don’t
have the brain systems necessary to
experience pain.
It is very important to note that
the flight responses of a hooked fish are
essentially no different from responses of
a fish being pursued by a visible predator
or a fish that has been startled by a
vibration in the water. These visual and
vibratory stimuli do not activate
nociceptive types of sensory neurons so
the flight responses can’t be due to
activation of pain-triggering neural
systems. Instead, these flight responses
of fish are a general reaction to many
types of potentially threatening stimuli
and can’t be taken to represent a
response to pain. Also, these flight
responses are unlikely to reflect fear
because the brain regions known to be
responsible for the experience of fear,
which include some of the same regions
necessary for the emotional aspect of
pain, are not present in a fish brain.
Instead, these responses are simply
protective reactions to a wide range of
stimuli associated with predators or other
threats, to which a fish automatically and
rapidly responds.
Although
fish don’t have the capacity to
experience human-like pain or suffering,
their reactions to nociceptive stimuli or
capture are still important because these
reactions include the secretion of stress
hormones.
These stress hormones can have
undesirable health effects on fish if they
are secreted in large amounts over a long
period of time.
So, it’s important when
practicing catch-and-release fishing to
observe the usually recommended procedures
of landing a fish before it is exhausted
and returning it to the water quickly.
The
facts about the neurological processes
that generate pain make it highly unlikely
that fish experience the emotional
distress and suffering of pain. Thus, the
struggles of a fish don’t signify
suffering when the fish is seized in the
talons of an osprey, when it is devoured
while still alive by a Kodiak bear, or
when it is caught by an angler.
FURTHER
READING
Much
of the evidence supporting the conclusions
of this paper is found only in specialized
neuroscience literature.
The references designated by * are
more appropriate for the interests of
non-specialist readers wishing to know
more about species differences in brain
function or the bases of conscious
awareness of experiences, including pain.
*Allman,
J. 1998. Evolving brains. Scientific
American Library: New York
Butler,
A. B. and Hodos, W. 1996. Comparative
vertebrate neuroanatomy: Evolution and
adaptation. Wiley-Liss: New York.
Craig,
K.D. 1994. Emotional aspects of pain. In
P.D. Wall & R. Melzack (Eds.),
Textbook of Pain, 3rd Ed (pp. 261-274).
Churchill Livingston: Edinburgh.
Davis,
R. E. and Kassel, J. 1983. Behavioral
functions of the teleostean telencephalon.
In R. Davis and G. Northcutt (Eds.) Fish
neurobiology. Vol. 2. Higher brain
functions. (pp. 237-264). University of
Michigan Press: Ann Arbor.
*Donald,
M. 1991. Origins of the modern mind.
Harvard University Press: Cambridge.
Jouvet,
M. 1969. Coma and other disorders of
consciousness. In P.J. Vinken & G.W.
Bruyn (Eds.) Handbook of Clinical
Neurology, Vol. (3) (pp. 62-79). New York:
Elsevier Science Publishers.
Kandel,
E. R., Schwartz, J. H. and Jessel, T. M.
2000. Principles
of neural science. McGraw-Hill: New
York.
*Kennedy,
J. S. 1992. The new anthropomorphism. Cambridge
University Press: Cambridge.
*Kolb,
B. and Whishaw, I. Q. 1995. Fundamentals
of human neuropsychology. W. H. Freeman:
New York.
Libet,
B. 1997. Consciousness: neural basis of
conscious experience. G. Adelman and B. H.
Smith (Eds.), The encyclopedia of
neuroscience. Elsevier: Amsterdam. CD ROM
*Macphail,-Euan-M.
1998. The evolution of consciousness.
Oxford University Press: New York.
Melzack,
R., and Fuchs, P.N. 1997. Pain, general.
In G. Adelman & B. Smith (Eds.)
Encyclopedia of Neuroscience. Elsevier:
Amsterdam. CD ROM.
Nieuwenhuys,
R., ten Donkelaar, H. J. and Nicholson, C.
1998 The central nervous system of
vertebrates. Springer: Berlin
Overmier,
J.B. and Hollis, K. 1983. The teleostean
telencephalon and learning. In R. Davis
and G. Northcutt (Eds.) Fish neurobiology.
Vol. 2. Higher brain functions. (pp.
265-284). University of Michigan Press:
Ann Arbor.
Preuss,
T. M. 2000. What’s human about the human
brain? In
M. S. Gazzaniga (Ed.), The new cognitive
neurosciences. (pp. 1219-1234). MIT Press:
Cambridge.
Treede, R.-D., Kenshalo, D. R.,
Gracely, R. H. and Jones, A. K. P. 1999.
The cortical representation of pain. Pain.
79/105-111.
Wall,
P.D. 1987. Pain: Neurophysiological
mechanisms. In G. Adelman & B. Smith
(Eds.) Encyclopedia of Neuroscience.
Elsevier: Amsterdam. CD ROM.
Wulliman,
M. F. The central nervous system. 1998. In
D. H. Evans (Ed.) The Physiology of
Fishes. (pp. 245-282). CRC Press: Boca
Raton
Young,
G. B., Ropper, A. H. and Bolton, C. F.
1998. Coma and impaired consciousness.
McGraw-Hill: New York.
Xu,
X., Fukuyama, H., Yazawa, S., Mima T.,
Hanakawa, T., Magata, Y., Kanda, M.,
Fujiwara, N., Shindo, K., Nagamine, T.,
and Shibasaki, H. 1997. Functional
localization of pain perception in the
human brain studied by PET. NeuroReport,
8/ 555-559.
