PHYSIOLOGICAL AND NEUROLOGICAL ASPECTS OF FEAR

Physiological and Neurological Aspects of Fear

Fear is an emotion that is prototypically associated with the anticipation of danger. It can
be expressed either after conditioning or innately, and the reaction is triggered when a stimulus
predicting danger of danger itself is perceived. The role of the emotion of fear is to prepare the
body for danger. The emotion is triggered when individuals perceive a situation that is at risk for
their safety. This happens either through the interoceptive inputs (autonomic and endocrine
nervous systems) or the exteroceptive inputs. These stimuli prepare the body to face dangerous
or threatening situations in different ways. They can evoke flight, fright, fight and freeze
reactions or they may evoke tend-and-befriend responses. A person will react to danger either by
running, fighting back, or trying to lessen the danger or threat posed. The paper addresses
neurological advancements and studies made in identifying the mechanisms and circuits of fear
and how fear emotions are expressed after a situation has been evaluated.
The amygdala is the region within which the neurological processing of fear occurs. Fear
can be experienced within the normal parameters, and there are also pathological fear reactions
(Ressler & Maren, 2019). Mechanisms of fear are distinguished into specific phobias, which are
either experiential or rather mechanisms that are dependent on learning or they are
nonexperiential, implying that they are learning independent (Garcia, 2017). Poor extinction and
poor habituation are dysfunctional mechanisms of fear that lead to persistent experiential and
nonexperiential phobias. The amygdala is involved in the expression and development of
experiential or conditioned fear. Conditioned fear occurs after an initially neutral stimulus is
paired with an aversive stimulus to produce a cluster of behavioral effects.

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Fear is a phasic adaptive state among humans and animals elicited through confrontation
with danger or a threatening stimulus (Ressler & Maren, 2019). It differs from anxiety which is
a more tonic state that relates more to the preparedness and prediction of threat or danger. The
two emotions are processed in different regions within the neural system. For instance, the
amygdala is the central nucleus region that processes fear, while anxiety is processed within the
nucleus of the striaterminalis. However, these two structures (amygdala and the striaterminalis)
have dense interconnectivity, and this makes it difficult for researchers to classify them uniquely
and assign each their respective processes (Sanford et al., 2017). Thus, anxiety, fear, and panic
are all classified as three distinct types of fear that are connected with different adaptive
responses but can also be singled out into a range of imminent danger.
The content of the danger or threat can cause multiple fear circuits. Studies have shown
that there exist distinct neural systems for predators, ache, and antagonistic cognates (Ressler &
Maren, 2019). The brain processes each of these distinct causes of fear in separate sensory
channels such as visual, olfactory, and somatosensory. Further, the brain engages distinct
subnuclei in the hypothalamus and amygdala, resulting in separate responses facilitated by
ventrolateral, dorsomedial and dorsolateral, which are parts of the periaqueductal gray (PAG)
(Sanford et al., 2017). Further, there are distinct molecular markers that support some of these
differences among assumed fear subsystems. For instance, the expression of the steroidogenic
factor 1 is a marker for the subsystem that is related to predators.
Further, different fear stimuli are processed using different sets of individual neurons.
Even identical fear stimuli may be processed in different sites depending on the occasion
(Sanford et al., 2017). This occurrence does not necessarily imply that there exist fear systems
that are distinct. It can only be compared to how several distinct visual images arouse a variety of

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neural response patterns in the brain’s visual parts without necessarily implying that an
individual has different visual systems. This explains why fear is viewed as a central state. There
are many overlapping neural regions involved in processing different fear stimuli, but the
concept of fear is generally perceived as central, and reactions may almost be similar.
Sensory inputs that specify or show associations with fear are received by the basolateral
amygdala. The amygdala is an almond-shaped region located in the sight region of the brain. It is
next to the hypothalamus. The amygdala acts as the main center within which motivation,
behavior, and emotions are processed. This explains why the emotions of fear or sensory input
associated with fear are received within this region. The brain’s limbic system deals with
memory and emotions (Sanford et al., 2017). The system is made up of the hippocampus, the
hypothalamus, and the amygdala. All sensory input, apart from the olfactory input, which is
received in the medial nucleus, is sent to the amygdala. Through selective optogenetic neural
activation, any incoming sensory information within the basolateral amygdala is associated with
unconditioned fear responses (Ressler & Maren, 2019). Fear responses are mediated within the
main output regulator which is the amygdala. Mediation of fear responses is done by different
subdivisions of the amygdala. However, in some instances, the substatiainnominata, the nucleus
basalis, can inhibit the cholinergic targets or the mediation of fear responses (Gu et al., 2019).
Through projections to the PAG, the nucleus can also promote freezing. The ability of the
amygdala to flexibly modulate different downstream fear responses depends on its internal
ability to control its processes and maintain a balance.
In humans, the amygdala is involved in the identification, expression, and experience of
the emotion of fear. Further, studies have revealed that the region of the brain is not only
activated in fear and anxiety responses but also in other unpleasant stimuli such as anger and

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other pleasant stimuli such as an appetitive stimulus that has an arousal effect, for instance, a
person’s favorite song. Due to such revelations, the roles of the amygdala are broad. This region
plays an active role in fear processing, but this is not its only basic function (Sanford et al.,
2017). The region is actively involved in other abstract functions such as value, preference,
effect, vigilance, relevance, and uncertainty. The question that remains largely unexplored in the
study of the amygdala is whether each of these functions is specific to a single domain in regard
to how the stimuli are processed.
There are behavioral and cognitive responses to fear, and this has made researchers
commonly think that fear has adaptive functions. Behaviors and stimuli that are mediated by fear
have a relationship that is not only dependent on context but also one that is highly flexible
(Sanford et al., 2017). The flexibility in this relationship is one of the elements that set aside
emotions as central states that are dissociated reflexes and more akin to dispositions and
personalities. The cognitive, psychophysiological, and behavioral changes are integrated yet
highly diverse, and they also serve as guides of a central state of fear.
Facial expression is one of the prevalent behavior aspects associated with fear. According
to Darwin, adaptive functions of fear are not associated with emotional expressions. The latter
could have evolved without relying on the adaptive function of fear. Darwin refers to emotional
expressions as “serviceable associated habits.” He states that although the fear emotion could be
the most vital for human survival, it is dependent largely on heritage as opposed to experiences
(Gu et al., 2019). These were aspects of behavior that were once adaptive but have since become
vestigial. Darwin’s claim is not entirely true. However, it might hold in certain behaviors such as
alarm calls, emotional facial expressions, and body postures. These aspects are adaptive, and
their primary functionalities have evolved. Now, these aspects of fear behavior are used in

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communication; mostly socially. They, however, do not play defense and primary direct
protection roles. Still, there are residuals of most of these adaptive functions which are expressed
and enable humans to understand how these behaviors are likely to have evolved over time. For
example, facial expressions such as flared nostrils or when one opens their eyes wide are facial
expressions that are associated with fear but do not only send a message of fear to other people.
These expressions also help in better detection of olfactory cues achieved through adding the
peculiarity in the visual field of stimuli.
Human beings are relatively unique in their range or collection of emotional facial
expressions. They compare closely with animals such as chimpanzees. However, monkeys and
other animals do not have such a repertoire of facial emotional expressions (Garcia, 2017). Thus,
the fear emotion is most developed among humans. Fear has an impact on almost all other
aspects of cognition. The effects range from decision-making, judgment to memory. Studies on
the adaptive nature of emotions have revealed the impact of emotional states on a person’s
ability to make decisions (Fentaw, 2017). Putative fear has systematic effects on choice, and this
has been observed even on bees.
Remarkably, different instances of fear tend to feel similar among humans beings, and
they are categorized and verbalized as similar. Theories in neurobiology and psychology have
made effort to explain the range of behaviors, stimuli, and circumstances, states or situations
associated with fear. According to appraisal theorists, emotional responses are elicited after an
individual has evaluated a situation and determined its importance to their well-being (Braem et
al., 2017). The quality and intensity of the emotional response do not depend on the situation
itself but rather on the individual evaluation of the situation in regard to its appraisal dimensions.
In reference to fear, a person will react to a situation after evaluating it and realizing that it is a

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threat to their well-being. For instance, when a person sees a snake, they will not run because of
the snake itself but will run because after evaluating the animal, they will realize that it is a threat
to their safety. This explains why an infant may not run away from a snake, neither will they
have any facial expressions to show fear, because they do not have the cognitive ability to
evaluate the situation and identify it as dangerous.
Different theorists hold different assumptions about the physiological arousal and
emotional responses in relation to fear. According to James-Lange theory, emotions arise or are
experienced due to physiological arousal (Fentaw, 2017). Thus, an emotional response, such as
fear, can only occur after physiological arousal. Thus, according to James-Lange, when a person
is in a threatening situation, such as when they see a snake, they will only experience an
emotional reaction after the physiological arousal has taken place. They will first experience an
increase in perspiration and a heart race, then an emotional reaction. However, the Cannon-Bard
theory holds a different proposition. The theorist believes that an emotional experience occurs
simultaneously but independent of physiological arousal (Fentaw, 2017). Thus, when one
perceives danger, they are likely to run, fight, and feel fear all at the same time. The synthesis of
the emotion of fear starts with an affective state, then incorporates somatic and interceptive
knowledge of a person’s actions and body and the situation that is dependent on the context.
Knowledge on which fear reactions are based is stored in one’s memory, in language, and is
acquired within one’s cultural context.
Overall, fear is a complex psychic phenomenon that is crucial to the survival of human
beings. It is an emotion that is basically processed within the amygdala. Fear, just like other
emotions, is of a high diagnostic value; its intensity and quality reveal the ability of a person to
evaluate a situation as either threatening to the well-being or not. A person’s emotional reaction

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depends on their evaluation of the situation. More research, however, is necessary to explain how
anxiety and fear are processed and how the overlapping functions of the brain in the processing
of these emotions are distinct.

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References

Braem, S., De Houwer, J., Demanet, J., Yuen, K. S., Kalisch, R., & Brass, M. (2017). Pattern
analyses reveal separate experience-based fear memories in the human right
amygdala. Journal of Neuroscience, 37(34), 8116-8130.
Fentaw, T. D. (2017). Autonomic Specificity for Basic Emotions: A Review of the Implication
of James–Lange theory of Emotion for the Autonomic Specificity Model of Emotion.
International Journal of Science and Research (IJSR) ISSN (Online): 2319-7064
Garcia, R. (2017). Neurobiology of fear and specific phobias. Learning & Memory, 24(9), 462-
471.
Gu, S., Wang, F., Patel, N. P., Bourgeois, J. A., & Huang, J. H. (2019). A model for basic
emotions using observations of behavior in Drosophila. Frontiers in psychology, 10, 781.
Ressler, R. L., & Maren, S. (2019). Synaptic encoding of fear memories in the
amygdala. Current opinion in neurobiology, 54, 54-59.
Sanford, C. A., Soden, M. E., Baird, M. A., Miller, S. M., Schulkin, J., Palmiter, R. D., … &
Zweifel, L. S. (2017). A central amygdala CRF circuit facilitates learning about weak
threats. Neuron, 93(1), 164-178.