A Rose by Any Other Name: PTSD and the Olfactory System

In this post, I review and discuss an article on aggression and impulsivity in combat veterans with PTSD tied to Olfactory Identification Dysfunction (OID) (see also Dileo, Brewer, Hopwood, Anderson, & Creamer, 2007). While the human olfactory system has the second highest number of sensory receptor cells – after the visual system (Carlson, 2010, p. 256), is the oldest sensory system evolutionarily speaking (Zillmer, Spiers, & Culbertson, 2008, p. 186), and is the least understood (p. 186), it also has axons (in the olfactory tract of the olfactory bulbs, where the glomeruli synaptically connect to the mitral cells) that project directly to the amygdala and to two other limbic structures called the piriform cortex and the entorhinal cortex (Carlson, 2010, pp. 256-7). This is important for a couple of reasons. First, because the olfactory system does not pass through the thalamus before reaching the cortical (conscious) regions (i.e., Orbitofrontal cortex, OFC, PFC, etc.), rather it links directly to the subcortical limbic system (Zillmer, et al., 2008, p. 189). Second, the olfactorily enervated limbic system, a system largely preconscious, has a direct and practically immediate effect on emotion, mood, and even memory (see also Zillmer, et al., 2008, p. 189). When these systems are disordered, things like Anosmia (think: a-NOSE-mia, loss of smell), dysosmia (distorted smell sensation), phantosmia (experience of a phantom/hallucinatory smell), and hyposmia (diminished taste sensation) are prevalent (Zillmer, et al., 2008, p. 189).

Even Freud was interested in the role of smell and psychological disorders, yet the idea was mostly abandoned until much more recently when scientists began exploring the olfactory correlates of Alzheimer’s Disease (AD), Parkinson’s Disease (PD), and even schizophrenia (i.e., hyposmia and dysosmia as early markers of AD and PD; phantosmia in schizophrenia) (p. 189). Fortunately, the neuropsychological community now has a method of testing for some of these early symptoms, by way of the Pennsylvania Smell Identification Test (UPSIT) – a scratch and sniff test – (Doty, Shaman, & Dann as cited in Zillmer, et al., 2008, p. 189), which Dileo and colleagues also used in their PTSD research (2007).

The deep connectivity to the “smell brain” (or “reptilian brain,” as it is sometimes called) enjoys a burgeoning evidence base and has been found to augment PTSD research and treatment. Dileo and colleagues (2007) add to this research by taking the thinking one step further with the combat veteran population (Vietnam era, outpatient, n=31, with community controls) (p. 523). The authors reasoned that since the complex functioning of the Obitoprefrontal Cortex (OFC) is often impaired in those suffering from PTSD, and since this area of the brain executively applies inhibitory responses, and the OFC is highly enervated to the basolateral amygdala, the OFC is very involved in emotional modulation (2007, pp. 523-4). Aggressiveness and impulsivity is thought to occur due to a failure of medial prefrontal structures to assert executive control over subcortical, limbic, amygdalic firing, the natural question, because of the direct link to the olfactory system, was to test these veterans who are often aggressive and impulsive due to their diagnosis, to determine the correlation of OID. As one might expect, and as other research demonstrates (see also Dileo, et al., 2007, p. 524), the OID is now thought to be a useful measure of OFC integrity and thereby a good predictor of PTSD-associated aggressivity and impulsivity (p. 524). In fact, the “OFC mediates odor identification [and refining of smells] as found via lesion and neuroimaging…[and because the OFC is connected with the limbic system and parallels the circuits for smell identification] OID’s have been strongly associated with impaired inhibition of affect, delusions, and maladaptive behavior (Martzke, Kopala, & Good, as cited in Dileo, et al., 2007, p. 524) and are found in combat veterans with PTSD (p. 528). Most importantly, the study concluded that the presence of OIDs are an important diagnostic indicator in predicting aggression and impulsivity, though as of yet, it remains difficult to say how effective OIDs can be in explaining the complexity of the OFC (p. 529).



References

Carlson, N. (2010). Physiology of behavior, (10th ed.). Boston: Allyn & Bacon.

Dileo, J. F., Brewer, W. J., Hopwood, M., Anderson, V., & Creamer, M. (2008). Olfactory identification dysfunction, aggression and impulsivity in war veterans with post-traumatic stress disorder. Psychological Medicine, 38(4), 523-531. doi:10.1017/S0033291707001456

Zillmer, E., Spiers, M., & Culbertson, W. (2008). Principles of neuropsychology, (2nd ed.). Belmont, CA: Thomson Wadsworth.