“Is there something wrong with my brain?” “Does part of my brain not work correctly?” Each time we make a diagnosis of bipolar disorder in our practice, these questions inevitably and understandably come up. People want to know about their illness. This conversation is often an essential part of the treatment.
In 2012, the journal Bipolar Disorders devoted an entire issue to a review and summary of modern neuroimaging research. It included articles that presented a model of the signature disturbances found in the bipolar brain1-6. In this longer-than-average blog piece, we will present these findings. To make this understandable, we’ll begin with a short introduction about neuroimaging and the concept of functional neuroanatomy.
Introduction
Over the last 20 years, a new series of neuroimaging techniques has been developed that extended and deepened our appreciation of brain structure and function. Magnetic resonance imaging (MRI) enabled higher resolution snapshots of smaller brain structures than was possible with older computed tomography (CT) scans. The advent of functional neuroimaging – with functional MRI, positron emission tomography (PET), and single proton emission computed tomography (SPECT) scans – has allowed us to see which areas of the brain are activated when performing specific mental tasks, such as holding something in memory or looking at an emotionally charged picture. The new techniques also enable us to look beyond single region assessments. By comparing co-occurring levels of activation in different brain regions during an emotional or cognitive task, fMRI has been used plot out the functional linkage between these separate regions. We can see how much two areas work together in performing a particular job, thereby generating maps of functional neural connectivity or circuits. Diffusion tensor imaging (DTI) complements this by analyzing the integrity of the white matter pathways that connect different brain regions. The result of these new neuroimaging techniques has been the establishment of an early functional architecture of the human brain. This architecture transcends the earlier ‘brain region A correlates with mental function B’ approach, replacing it with an appreciation of dynamic neural circuits that travel through and utilize multiple brain areas to support our cognitive and affective needs. So what has this all got to do with bipolar disorder?
Using these new neuroimaging methods, psychiatric researchers have begun constructing functional models of how the brain processes emotion. Pathways involved in each of the steps of affective behavior – from the initial recognition of the emotional component of a stimulus (e.g., the anger on the face of a mad gunman), through the first emotional response (e.g., fear), the recruitment of cognitive strategies to deal with the situation (e.g., placating the gunman, negotiating, confrontation etc…), selection of the optimal approach (e.g., avoidance of the threat), and the final mobilization of adaptive behavior (e.g., flight/escape) – are being charted and refined. The result is an emerging map of what parts of the brain (which circuits) do what and how various cognitive, behavioral, and emotional responses are mediated. This is functional neuroanatomy: an engineering-like analysis of how the brain works.
With this growing knowledge of how normal human emotion is processed, the stage was set for the study of bipolar patients, to see how and where their responses and underlying neural circuit activation differs from those without this illness. Over the last 15 to 20 years, a variety of such ‘compare-and-contrast’ neuroimaging experiments have been performed. In the following sections, we present a brief overview of these findings. They contain some of the information that we provide when we attempt to answer our patient’s questions about “What is wrong with my brain?”
Brain Abnormalities Associated with Bipolar Disorder
Finding #1: The brain systems that support normal human emotional response are the ones involved in bipolar disorder. This has been both a guiding assumption, and an increasingly validated conclusion of research in this area. In other words, when researchers first began looking for neural abnormalities in bipolar disorder, they had to choose which, among the vast multitude of brain regions, to focus on. They assumed that the areas that had been found to be associated with ordinary emotional experience would be the ones most likely to show dysfunction in bipolar disorder. This assumption looks correct.
Finding #2: A circuit involving the prefrontal cortex, the amygdala, and their connecting pathways is especially implicated in bipolar disorder. The amygdala is an evolutionarily ancient cluster of neurons deep in the temporal lobe. This primitive structure is responsible for the most basic aspects of emotional experience that occur in all animal species, from reptiles through humans. This includes the rapid evaluation of emotional stimuli (e.g., is that lion friendly or looking to eat me?) and the mobilization of an initial affective response (e.g., run!). The prefrontal cortex (PFC), in contrast, is the evolutionarily most-recent, distinctly human part of the brain that sits on top of lower subcortical areas such as the amygdala. It controls much of our decision-making, planning and organizing. It has also been found to regulate, through inhibition and other mechanisms, subcortically generated emotional reflexes. The PFC and amygdala are linked through bundles of white matter fiber tracts that enable communication between the two areas. Functional neuroimaging has revealed disease-specific disturbance in each of the three areas of this brain system.
Finding #3: The functioning of the amygdala is often abnormal in bipolar disorder. This is especially true of manic states where increased activity is found. The data is less clear in states of depression and euthymia. Regarding the latter, some studies report disturbed activity during remission, others show normalized functioning. Some of these mixed results may be due to the type of task used during the fMRI. A few studies have shown increased recruitment of the amygdala even during cognitive challenges, such as calculating a sum of numbers. They imply that bipolar individuals may use the emotional part of their brain, even for more objective considerations.
Finding #4: Bipolar disorder is also associated with disturbed amygdala size. This is an example of a structure-function correlation. While less robust than the functional findings, data suggests that early in the course of illness, the amygdala is actually smaller than found in those without bipolar disorder. Interestingly, this pattern reverses with age, where bipolar adults are found to have larger amygdalae than their unaffected peers. We do not yet understand how disturbances in function affect the size of a brain area.
Finding #5: The prefrontal cortex shows reduced activation in bipolar disorder. This appears to be true across mood states (i.e., whether a person is depressed, manic or euthymic). It is most prominent during emotional tasks where the PFC fails to adequately modulate the overactive amygdala. Think of a surge protector that is not working properly: Instead of controlling and allowing the smooth flow of electricity, the device malfunctions permitting variable and excessive voltage.
Finding #6: Neuroprogression. This term refers to the concept that brain abnormalities irreversibly worsen over time similar to what occurs, for example, in Parkinson’s disease. This is a question of immense prognostic importance. To determine this, serial neuroimaging is required, i.e., testing a person in adolescence and again in adulthood, or comparing children at risk, to those in the early vs. late stage of the illness. There is less data here than with the snapshot studies comparing bipolar adults to those without the disorder at a single point in time. Nonetheless, the early research suggests that the amygdalae of individuals with bipolar disorder increase in size over time and that the PFC, in contrast, shrinks. We don’t really know what to make of these observations. Are they caused by the illness itself? By the medications used to treat the illness (lithium has actually been shown to cause the amygdala to grow)? Are the changes irreversible? Given our uncertainty about this, it may be premature to characterize bipolar disorder as neuroprogressive in nature.
Finding #7: Connectivity problems. As mentioned earlier, the most implicated circuit in manic depression includes the amygdala, the PFC, and their connections. These connections consist of white matter tracts that carry signals between these different brain regions. fMRI and DTI studies have revealed both functional and structural impairments in these white matter connections in bipolar disorder. These abnormalities have also been found before the onset of the illness (in children-at-risk, for example) and may thus represent vulnerability markers.
Finding #8: The functional and structural abnormalities mentioned above are not, by and large, found in schizophrenia. This supports the idea that bipolar disorder is a distinct illness, with its own neural substrate, and that this brain substrate is specific to the system of circuits that mediate emotional experience.
Conclusion and Cautionary Notes
The increasing clarification of the neural abnormalities associated with bipolar disorder is an exciting and potentially promising development for our field. Being able to visualize the individual structures and integrated operational circuits that underlie both normal and abnormal mood states is an enormous step forward. Appreciation of this advance, however, should be tempered by two cautionary notes. First, describing the functional brain underpinnings of a cognitive or emotional process is not synonymous with defining its cause. It simply lets us know what regions of the brain are doing at that time. Discernment of the ultimate cause of an extreme mood state may require additional genetic, cellular, epidemiologic, and psychological investigation.
Second, despite its promise, functional neuroimaging is not yet yielding current clinical application. In other words, there is no current clinical role for the type of brain scans described above in the evaluation and treatment of someone with bipolar disorder. For the moment, these tests are used exclusively in research studies. We expect and look forward to this changing in the very near future.