Somatic marker hypothesis

The somatic-marker hypothesis (SMH) proposes a mechanism by which emotional processes can guide (or bias) behavior, particularly decision-making. This hypothesis has been formulated by Antonio Damasio.

Hypothesis
Real-life decision making usually involves assessment, by cognitive and emotional processes, of the incentive value of the various actions available in particular situations. However, often situations require decisions between many complex and conflicting alternatives, with a high degree of uncertainty and ambiguity. In such situations, cognitive processes may become overloaded and be unable to provide an informed option.

In these cases (and others), somatic markers can aid the decision process. In the environment, reinforcing stimuli induce an associated physiological affective state. These types of associations are stored as somatic markers, possibly in the ventromedial prefrontal cortex (VMPFC; a subsection of the orbitomedial PFC). In future situations, these somatic-marker associations are reinstated physiologically and bias cognitive processing. In cases where complex and uncertain decisions need to be made, the somatic markers from all reward- and punishment-associated experiences with the relevant stimuli are summed to produce a net somatic state. This overall state is used to direct (or bias) the selection of the appropriate action. This biasing process may occur covertly (unconsciously), via the brainstem and ventral striatum, or overtly (consciously), engaging higher cortical cognitive processing. Somatic markers are proposed to direct attention away from the most disadvantageous options, simplifying the decision process.

The amygdala and OMPFC are essential components of this hypothesised mechanism and therefore damage to either structure will disrupt their proposed action in mediating the development and action of somatic markers. A major source of supporting evidence for this theory is provided by experiments using the Iowa gambling task.

Experiments
In a quest to produce a simple neuropsychological tool that would adequately assess the obvious deficits in emotional processing, decision-making, and social skills of OMPFC lesioned individuals Bechara et al. 1994 created the Iowa gambling task. Their aim was to produce "[a] task which simulates in real time, personal real-life decision-making relative to the way it factors uncertainty of premises and outcomes, as well as reward and punishment" (p. 8). The task measures a form of learning that has been defined as emotion-based learning. Studies using the Gambling Task have found deficits in various neurological (e.g. amygdala and OMPFC) and psychiatric populations (e.g. schizophrenia, mania, drug abusers), providing support for the SMH.

Criticism
Some researchers believe that the use of somatic-markers (i.e., afferent feedback) would be a very inefficient method of influencing behaviour. Damasio's notion of the as-if experience dependent feedback route, whereby bodily responses are re-represented utilising the somatosensory cortex (postcentral gyrus), also proposes an inefficient method of affecting explicit behaviour. Rolls (1999) stated that; "it would be very inefficient and noisy to place in the execution route a peripheral response, and transducers to attempt to measure that peripheral response, itself a notoriously difficult procedure" (p73). Reinforcement association located in the orbitofrontal cortex and amygdala, where the incentive value of stimuli is decoded, is sufficient to elicit emotion-based learning and to affect behaviour via, for example, the orbitofrontal-striatal pathway. This process can occur via implicit or explicit processes.