Psychobiology of Emotion

Right in the beginning of this unit we extensively discussed the concept of conditioning. The procedures of classical and instrumental conditioning create multiple representations of the value of the stimuli in the brain. Motivational and emotional states are determined by the assessment of this value. Nucleus accumbens mediates the motivational effect of emotionally significant stimuli. Amygdala plays significant role in human emotional responses. Conditioning studies of emotional responses have examined freezing and fear-potentiated startle in rats. Freezing is considered a species-specific response to threat that makes the animal motionless. In fear-potentiated startle the presence of threat stimuli increases startle reflex. Central nucleus and basolateral amygdala (BLA) are the two functional units of amygdala that regulates emotional processes. BLA has reciprocal projections with polysensory neocortex and frontal lobe. It projects to the ventral striatum and the central nucleus. BLA is supposed to store the association that allows CS to retrieve the affective or motivational value of the US. This, in turn, controls central nucleus, hypothalamus, midbrain and brainstem to induce affective response. Freezing and fear-potentiated startle are examples of such affective response.

Brain damage and lesion studies also confirm these findings. A lesion of central nucleus impairs freezing conditioning. Damage to amygdala in human beings increases the threshold of perception and expression of emotion. Lesions in amygdala adversely affect emotional learning and memory of emotional events. Human beings with damaged orbito-frontal cortex damage resemble those with amygdala-lesion. They have impairment of reactions to emotional stimuli and decision making.