Professor Dave here, let’s talk about feelings.
All the creatures in the animal kingdom exhibit certain types of behavior, but only a small
portion of those creatures experience what we would describe as emotions.
These are instinctive or intuitive states of mind that are symptoms of complex cognitive
function.
Many or most mammals display emotional states, and humans are no exception.
We can be angry, joyful, depressed, or fearful, and these states of mind are not always direct
reflections of reason.
Why do we feel feelings?
If everything about our anatomy and physiology makes some amount of sense in an evolutionary
context, what is the benefit of possessing emotions?
What purpose do they serve in the sense of survival?
An analysis of emotional responses in animals goes back all the way to the time of Darwin
himself.
He compared such responses in a variety of species, and proposed that expressions of
emotion evolved in an organism so as to signal to other organisms what it was about to do next.
For example, when an animal is feeling threatened, it may display a stance of aggression.
It may bare its teeth, it may lean forward and make aggressive sounds.
All of this behavior is to indicate the potential for violent behavior, and it does well in
intimidating some percentage of other organisms nearby, which may then retreat in fear without
necessitating actual combat.
Thus, this type of behavior has a quantifiable impact on the survival of the organism, and
therefore becomes a preferable trait within the gene pool.
Submission is the opposite of aggression, and thus is expressed in the opposite manner
physically, with downward posture, an aversion of the gaze, and other behaviors that communicate
the lack of a physical threat.
This can also serve a survival purpose, in reducing the chances of a confrontation.
Emotional states catalyze these behaviors, and it is in this context that we can understand
how emotions must have evolved in animals.
How does this work in the brain?
The first theory that attempted to explain this was the James-Lange theory, in the late
19th century.
This states that sensory stimuli are received by the cortex, which in turn activate both
the somatic nervous system and autonomic nervous system, and these responses in turn trigger
the experience of emotion in the brain.
This means that physiological states create emotion.
This was sort of the opposite of common sense, which would seem to dictate that the emotion
produces the physical response.
Nevertheless, the theory maintained that it is autonomic activity, like change in heart
rate and other involuntary responses, that trigger the emotional state.
So whereas common sense would suggest that when perceiving danger, like a bear, one would
feel fear, which would cause physiological reactions, instead the James-Lange theory
would propose that seeing the bear would trigger the physiological reactions first, which then
trigger the feeling of fear.
A bit later, the Cannon-Bard theory proposed instead that the stimuli trigger both the
physical expression of the emotion and the emotional state itself, as parallel processes.
This would mean that seeing the bear would trigger both the physical and emotional response
simultaneously and independently.
However, modern studies have shown that neither of these models can be totally accurate, and
it would seem that perception, experience of the emotion, and the physical response
can all influence one another, which means a diagram like this is more reasonable.
Another important theory relates to the limbic system.
This is comprised of the amygdala, mammilary body, hippocampus, fornix, cortex of the cingulate
gyrus, septum, olfactory bulb, and hypothalamus.
This theory proposes that emotional states are expressed when these structures act on
the hypothalamus, and are then experienced through their action on the cortex.
Surprisingly, certain aspects of emotional response are consistent across not just humans
but even other primate species.
An excellent example of this is facial expressions.
It is proposed that there are six primary expressions that correlate with anger, disgust,
fear, happiness, sadness, and surprise, and that any other expression is some mixture
thereof.
The facial feedback hypothesis even proposes that simply enacting a particular facial expression
can influence the onset of the corresponding emotion, and there exists some evidence to
support this.
When it comes to studies of emotion, most research focuses on fear.
This is the emotional reaction to any kind of immediate threat.
It is the easiest emotion to study because it is simple to infer from the behavior of
many different species.
Fear tends to produce either defensive behaviors, meant to protect the organism from harm, or
aggressive behaviors, meant to threaten or harm the threat itself.
These are instinctual behaviors that have evolved for survival purposes, and they take
on a variety of forms in different mammalian species, though they often obey the target-site
concept.
This means that the behaviors are meant to attack specific places on a threatening organism
while protecting certain vulnerable areas of oneself.
Aggressive behaviors can be predatory, for eating, or social, so as to establish and
maintain a social hierarchy, typically among males, hence the so-called alpha male that
exists in communities of certain animals.
Defensive behaviors are more varied.
They can be intraspecific, defending against social aggression.
Freezing and flight involves staying perfectly still when a threat is perceived, until a
certain safety zone is penetrated, at which point the organism will attempt to flee rapidly.
Animals engage in risk assessment, scanning the environment defensively, some burrow into
the ground and hide, and of course there are maternal defensive behaviors, whereby a mother
will instinctively protect their young, at times in so violent a manner that it resembles
male social aggression.
Social aggression in most species is exhibited very prominently in males, typically much
more so than in females, with the exception of the aggression associated with a mother
protecting her young.
This is because the period in which testosterone is released in males around birth seems to
have an effect on the organization of the nervous system, which is reinforced by further
testosterone release around the time of puberty.
However, the correlation between testosterone and aggression is still being studied, and
is far from conclusive.
Our understanding of fear on a mechanistic level is quite a bit more sound, due to studies
utilizing fear conditioning on rats, which is the establishment of fear in response to
what should be a neutral stimulus.
This is a type of classical conditioning, which we learned about when we discussed memory
and learning.
An example in this context would be the production of an auditory tone just before an electric
shock is administered.
The rat eventually responds to the tone itself with a variety of defensive behaviors and
nervous system responses.
The neural mechanisms that produce these responses can then be mapped through studies involving
lesions in the auditory pathways.
These studies suggest that signals such as the tone that induces fear must reach the
medial geniculate nucleus, and then travel from there to the amygdala, as lesions to
either of these regions render fear conditioning ineffective.
This makes sense in the context of our understanding of the amygdala as the region that receives
sensory input and assigns emotional significance based on survival-relevant information.
A number of pathways carry signals from the amygdala to structures in the brainstem that
control the emotional response, like the hypothalamus which directs the sympathetic response, and
the periaqueductal gray matter of the midbrain that directs the behavioral response.
We should of course note that the amygdala is not a singular structure, it is actually
a cluster of nuclei, which is why we sometimes refer to it as the amygdaloid complex.
Each of these regions serves a different function, as has been evidenced by studies on fear,
demonstrating that the lateral nucleus is involved with conditioned fear, while the
central nucleus controls defensive behavior, and so forth.
From the standpoint of cognitive neuroscience, there is still much work to be done surrounding
the brain mechanisms of human emotion.
It appears that the brain activity associated with each individual emotion is not highly
localized, but is rather diffuse.
Each emotion, or even the experience of empathy when observing emotional states of others,
is accompanied by activity in the motor cortex and sensory cortex.
And finally, brain imaging studies show that similar patterns of brain activity are enacted
whether a person experiences an emotion, imagines that emotion, or observes someone else experiencing
that emotion.
These realizations strongly inform current neuroscience research surrounding emotion
and the brain.
Finally, let’s briefly discuss the stress response.
When the body is exposed to excessive harm or threat, this initiates physiological changes
known as the stress response.
These are adaptive changes in response to the stressor that are beneficial in the short
term, but eventually have adverse effects on an organism.
This response is centered around the anterior pituitary adrenal cortex system, better known
as the HPA axis.
Stressors stimulate the release of adrenocorticotropic hormone from the anterior pituitary, which
triggers the release of glucocorticoids from the adrenal cortex, and these are the signaling
molecules that produce many of the physiological effects of the stress response.
The sympathetic nervous system is also involved, in the way of the release of epinephrine and
norepinephrine from the adrenal medulla.
The combination of these two systems is called the two-system view of the stress response.
Stress responses are complex, and they vary from person to person, but we do know that
certain common psychological stressors relating to jobs, relationships, or other social obligations,
can have effects that are similar to physical stressors.
In fact, studies have shown that social evaluative threat, which represents a context in which
the self could be judged negatively by others, is the most reliable stimulus for inducing
an HPA axis response, which makes sense given the evolutionary importance of close relationships.
Excessive amounts of stress can result in certain disorders, such as the development
of gastric ulcers, which are lesions in the stomach lining that can be very painful due
to the high acidity of the gastric acid in the stomach, as well as cardiometabolic disorders.
Stress can even have an impact on immune function, in that brief stressors can actually improve
innate immune function, while chronic stressors have an adverse effect on adaptive immune function.
The mechanisms by which this occurs are not well-understood, but it seems clear that the
two systems outlined in the stress response can influence immune response, as T cells
and B cells have receptors for glucocorticoids, and lymphocytes have receptors for these too,
in addition to epinephrine and norepinephrine, so the link does not seem farfetched.
But further investigation into psychoneuroimmunology will have to wait for another time.
For now let’s move on and examine a few disorders of the brain.