Evolution of human emotion: a view through fear

Abstract

Basic tendencies to detect and respond to significant events are present in the simplest single cell organisms and persist throughout all invertebrates and vertebrates. Within vertebrates, the overall brain plan is highly conserved, though differences in size and complexity also exist. The forebrain differs the most between mammals and other vertebrates. The classic notion that the evolution of mammals led to radical changes such that new forebrain structures (limbic system and neocortex) were added has not held up nor has the idea that so-called limbic areas are primarily involved in emotion. Modern efforts have focused on specific emotion systems, like the fear or defense system, rather than on the search for a general purpose emotion systems. Such studies have found that fear circuits are conserved in mammals, including humans. Animal work has been especially successful in determining how the brain detects and responds to danger. Caution should be exercised when attempting to discuss other aspects of emotion, namely subjective feelings, in animals since there are no scientific ways of verifying and measuring such states except in humans.

Fig 1. Pavlovian Fear and Avoidance Conditioning Circuits

Fear is the emotion most thoroughly understood in the brain. Much of the progress made has involved studies of Pavlovian fear conditioning in rats. During conditioning the conditioned stimulus (CS), usually a tone, and the unconditioned stimulus (US), usually a footschock, converge in the lateral nuclecus of the amygdala (LA) to induce synaptic plasticity of the CS inputs (CS–US convergence not shown). The CS is then able to flow through amygdala circuits to the central nucleus (CE) to control the expression of hard-wired, automatic, defensive reactions (freezing behavior, autonomin nervous system, ANS, activity, and hormonal release). CE outputs also activate networks in that control the release of neuromodulators, such as norepinephrine (NE), dopamine (DA), acetylcholine (ACh), and serotonin (5HT) throughtout the brain. These, like hormonal feedback, help add intensity to and prolong the duration of the aroused state. In addition to these various automatic responses controlled by CE, the LA also sends information, via the basal nucleus (B) to the ventral striatum, especially the nucleus accumbens. The latter connections are likely to be involved in the invigoration of goal-directed behaviors that allow the organism to act in certain ways on the basis of past instrumental learning or on-the-spot decisions about how to cope with the threat. Other abbreviations: ITC, intercalated nuclei of the amydala; CG, central gray; LH, lateral hypothalamus; PVN, paraventricular nucleus of the hypothalamus; VP, ventral pallidum.