Monday, December 20, 2010

A Theory of Mind III: Emotions

In laying out the essential ingredients for a theory of mind, I previously elaborated on the crucial roles of language and memory with illustrative examples. A befitting saying proclaims that a stool needs three legs to stand. Emotions represent another important ingredient of our mind. They seem to narrow our choices by focusing or diverting our attention, thus limiting our choices and profoundly affecting our behavior. They strengthen our memory of particularly intense moments. Plenty important decisions in our lives are being made because of the emotions we attach to a loved one. Therefore, emotions constitute a worthy third leg for the theory I strive to construct. Multifarious types of emotions have been recognized. We may distinguish the simple emotions of “affect programs”, like fear and aversion, from the complex, more “cognitively penetrable” ones (see de Sousa, 2003 for review).

Complex Emotions
Complex emotions have been recognized since the middle of the 19th century, when the tragic fate of the railroad construction worker Phineas P. Gage in New England brought the fundamental role of emotions in defining our personality and their roots in the brain to public attention.

Phineas Gage and his tamping iron
(from the collection of Jack and Beverly Wilgus)
Phineas miraculously survived a grave accident at age 25 which destroyed primarily his left frontal lobe. On Sep. 13, 1848, he was tamping blasting powder down into a borehole with a solid iron rod that weighed 13.25 lbs and was 1.25 inches in diameter and 3 feet and 7 inches long. A spark triggered an explosion and the rod shot straight up like a missile, tapered end first, entirely penetrating Phineas' skull. The iron was found later 80 feet away from the site of the accident. Astoundingly, Gage reportedly did not lose consciousness. He was brought home where Dr. John Martyn Harlow found him sitting up and talking. Harlow was an observant physician who diligently recorded Gage's condition and the progress of his recovery. Phineas, by then a cause célèbre, was a changed man. Before the accident, he used to conduct himself in an even-keeled, considerate manner fit to work as a foreman for the railroad. After the accident, his behavior had become more irritable and irate, and he struggled with a lack of focus. Yet, he managed to hold down a job as a stage coach driver in Chile for several years and lived for another twelve. At the age of 37, he developed severe seizures and passed away within a few months.

Informed of Phineas' death, Harlow sought out the family and was able to procure the tamping iron. He gained permission for a brief exhumation of Phineas' remains during which he retrieved the skull. Harlow published his observations on Gage in a scientific journal (Harlow, 1848) as one of the first documented neurological case histories that related anatomical with behavioral findings. Skull and iron are in the possession of the Warren Anatomical Museum at Harvard Medical School today, and several attempts have been made to reconstruct the damage to Phineas' brain. Certainly, his left frontal lobe, but likely also a part of the right frontal lobe, were severely compromised by the rod's impact.

Phineas' terrible accident kindled intense scientific interest in the role that the frontal lobes play in our emotions and specific frontal lobe areas have been associated with distinct aspects of emotional behavior. For example, the anterior cingulate cortex near the juncture of the cortical hemispheres and the frontal orbital cortex, that is the very nose-ward aspect of the cerebral cortex, seem engaged in focus, decision making and judgment, notably of pain and danger, and are considered part of our attention and risk assessment mechanisms. In addition, anterior cingulate cortex influences autonomic functions like blood pressure and heart rate.

The anterior cingulate cortex sends output to insular cortex wedged between the frontal and the temporal lobe under the operculum, and frontal orbital cortex receives input from this region. Insular cortex receives sensory as well as visceral input, integrating information from inside and outside our body. Von Economo first discovered spindle cells there. Spindle cells are a particular morphological type of nerve cell peculiar to a select number of cortical areas of high-level functional specialization among which we also find anterior cingulate cortex. I have written about these cells in my post with the title "Constantin von Economo's Spindle Cells & The Mind" dated Aug. 21, 2009. Moreover, a special functional type of nerve cells, known as mirror neurons, have been found in anterior cingulate cortex (Rizzolatti and Sinigaglia, 2008). They have been implicated in empathy. I have written about them in my post with the title "fMRI III: Religiosity & Brain Activation" published Mar. 31, 2009. Anterior cingulate, frontal orbital and insular cortex are considered extended parts of the limbic system.

Simple Emotions
Simple emotions are processed by members of the limbic system that are phylogenetically older brain structures than the cortical regions discussed above. We widely share them with less developed vertebrates (Ebner, 1969), inspiring Paul McLean's hypothesis of the triune brain (McLean, 1990). Structures laying down long-term episodic memory have been included here. They comprise the hippocampus, composed of dentate gyrus, fornix, fimbria, and subiculum, as well as the parahippocampal gyrus, divided into perirhinal and the entorhinal cortex.  Particularly, the amygdalae, almond-shaped structures composed of histologically and functionally distinct, interconnected subregions at the bottom of the medial temporal lobe of cerebral cortex, are instrumental in startle and fear. They are strongly influenced by the neurotransmitter dopamine (Reynolds, 1983) and known to play an instrumental role in the influence of emotional arousal on the strength of the memory for our experiences (LeDoux, 1998).

The amygdalae provide output to the reticular formation, instrumental to alert and arousal, the striatum, involved in motor control, as well as structures in the brainstem, mesencephalon and diencephalon that control visceral, gustatory, nociceptive, motor and humoral functions. Moreover, they project to hippocampus, entorhinal cortex, prefrontal cortex, sensory cortical areas of all modalities and multimodal sensory association cortex. While the outputs can be either inhibitory or excitatory, inputs to the amygdalae are predominantly excitatory, utilizing the neurotransmitter glutamate. Fear conditioning in rats has been shown to increase their synaptic strength through a plastic mechanism known as long-term potentiation (Paré and others, 2003).

The inputs to the amygdala originate in most cortical and subcortical areas the structures project to, plus the olfactory bulbs (McDonald, 1998). Moreover, in opossums (Kudo and others, 1986), rats (Ottersen and Ben-Ari, 1979; Doron and Ledoux, 2000) and cats (Ottersen and Ben-Ari Y, 1979), the amygdalae also receive multimodal sensory direct input from the diencephalon, notably the visual (lateral posterior nucleus equivalent to the pulvinar in us) and auditory thalamus (medial geniculate body). The medial geniculate body is a relay station of the ascending auditory pathway. The direct connection with the amygdalae facilitates rapid responses to unexpected menacing stimuli. Alas, the input from the auditory thalamus does not appear to exist in us (Munoz-Lopez and others, 2010), and the one from the pulvinar seems to exert less impact on the amygdalae than in rodents (Pessoa and Adolphs, 2010). This paucity may manifest itself in a less abrupt startle reflex, because cortical processing is involved. Regardless, we immediately seem to duck when we hear a loud bang. When others take cover or flight, we do not hesitate either, as the video of this horrific incident shows. We only need to watch the beginning of the clip. Amygdala in action is visible at 10 seconds:

A story on National Public Radio's Talk of the Nation Science Friday with the title "No Fear" broadcast Dec. 18, 2010, informs us about patient S.M. whose amygdalae have been damaged on both sides (Feinstein and others, 2010). She literally knows no fear. The insights she provides may invaluably help advance treatments for people with anxiety and post-traumatic stress disorder.

Perhaps the founder of human ethology Irenäus Eibl-Eibesfeldt most strikingly revealed the prevalent and universal role of emotions in our lives. He developed camera lenses with a mirror that permitted him to record the facial expressions of unsuspecting bystanders unnoticed. By filming people around the world through these right-angle lenses, he and his colleagues were able to identify archetypal face expressions commonly used to communicate emotions across diverse cultures (Eibl-Eibesfeldt, 1989). The raising of the eye brows to signal readiness for social interaction constitutes one impressive example. Pictures of this behavior are shown on the home page of the Film Archive of Human Ethology. It is a wonderfully emotional scene.

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