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Brain Structures Involved
Individuals with blunted and/or flat affect show different regional brain activity in fMRI scans when presented with emotional stimuli compared to healthy individuals. Healthy individuals show activation in the following brain areas when shown emotionally negative pictures: midbrain, pons, anterior cingulate cortex, insula, ventrolateral orbitofrontal cortex, anterior temporal pole, amygdala, medial prefrontal cortex, and extrastriate visual cortex. Individuals with blunted affect show activation in the following brain regions when shown emotionally negative pictures: midbrain, pons, anterior temporal pole, and extrastriate visual cortex.

Limbic Structures
Flat affect patients show decreased activation in the limbic system when viewing emotional stimuli. In healthy individuals, neural processes begin in the occipitoltemporal region of the brain and go through the ventral visual pathway and the limbic structures until they reach the inferior frontal areas. Damage to the amygdala, a part of the limbic system, early in life can permanently alter affective processing. Lesioning the amygdala causes blunted affect responses to both positive and negative stimuli. This effect is irreversible; neonatal damage produces the same effect as damage that occurs later in life. The brain cannot compensate for early amygdala damage even though significant neuronal growth may occur. There is some proof that blunted affect symptoms in schizophrenia patients are not a result of just amygdala responsiveness, but a result of the amygdala not being integrated with other areas of the brain associated with emotional processing, particularly in amygdala-PFC coupling. Damage in the limbic region prevents the amygdala from being able to correctly interpret emotional stimuli by compromising the link between the amygdala and other brain regions associated with emotion.

Brainstem
Parts of the brainstem are responsible for passive emotional coping strategies that are characterized by disengagement or withdrawal from the external environment (quiescence, immobility, hyporeactivity), similar to what is seen in blunted affect. Patients with blunted affect show activation of the brain stem during fMRI scans, particularly the right medulla and the left pons, when shown sad film excerpts. The bilateral midbrain is also activated in patients with blunted affect. Activation of the midbrain is thought to be related to autonomic responses associated with perceptual processing of emotional stimuli. This region usually becomes activated in diverse emotional states. When the connectivity between the midbrain and the medial prefrontal cortex is compromised in blunted affect patients, an absence of emotional reaction to external stimuli results.

Prefrontal Cortex
Healthy individuals, as well as patients being successfully treated for blunted affect, show activation of the prefrontal cortex (PFC). Failure to activate the PFC is likely involved in impaired emotional processing in blunted affect patients. The mesial PFC is activated in healthy individuals in response to external emotional stimuli. This structure receives information from the limbic structures to regulate emotional experiences and behavior. Patients being treated for blunted affect, who show reduced symptoms, show activation in areas other areas of the PFC as well, including the right medial prefrontal gyrus and the left orbitofrontal gyrus.

Anterior Cingulate Cortex
A positive correlation has been found between activation of the anterior cingulate cortex and the magnitude of sad feelings evoked by viewing sad film excerpts. The rostral subdivision of this region is involved in detecting emotional signals. This region is impaired in blunted affect patients.

Blunted Affect vs. Flat Affect
One theory on the difference between blunted affect and flat affect does not just lie in the degree to which an individual is affected. Patients with flat affect still have reduced facial, vocal, and gestural modes of display like patients with blunted affect, but they are similar to healthy individuals in all other aspects of emotional processing. Flat affect individuals show similar levels of physiological activation compared to healthy patients and they are able to perceive emotional states of others just as well. When speaking, individuals with flat affect demonstrate less inflection than normal controls and appear to be less fluent. Normal subjects appear to express themselves using more complex syntax, whereas flat affect subjects speak with less words, and less words per sentence. Flat affect individuals use of context-appropriate words in both sad and happy narratives are similar to that of controls. It is very likely that flat affect is a result of deficits in motor expression as opposed to emotional processing. The moods of display are compromised, but subjective, autonomic, and contextual aspects of emotion are left in tact.