“Visual Snow” and its Possible Origins

Post by Anastasia Sares

The takeaway

Visual snow syndrome is a condition where people continually see “static” or “snow” interfering with their vision, which may be accompanied by other visual problems. Visual neurons fire randomly all the time, but for people with visual snow, it seems that the brain is amplifying this random firing, bringing it to conscious awareness.

What's the science?

Visual snow is a newly-described condition that is currently estimated to affect around 2% of the population, and there are a few hypotheses as to why it occurs. One is that the neurons in the visual system could be producing an excessive amount of noise – random fluctuations that have nothing to do with the outside world – that are then interpreted as light signals. The second, slightly more nuanced theory is that the total amount of noise in visual neurons is the same for people with visual snow and people without it, but that the gain, or amplification of this noise is especially high for people with visual snow (think of someone turning up the volume on a bad-quality radio station). This week in Brain, Brooks and colleagues pitted these two possible causes of visual snow against each other and found a victor.

How did they do it?

The authors recruited people with and without visual snow to participate in some tasks. They also recruited people in both groups with and without migraines, because these people can also have visual snow as a symptom, though it may have a different cause. One task was designed to measure the total amount of noise (random activity) in the visual system. Participants were shown two squares and had to decide which one contained a lighter-colored circle inside it. The images were made increasingly “noisier” by adding a bunch of lighter and darker pixels (the authors called this “external noise” as opposed to the “internal noise” generated by the visual system itself). By doing the experiment three times with the same stimuli, they could get an idea of a person’s consistency in their responses, which should be related to the amount of internal noise in the visual system (low internal noise allows people to make more consistent responses).

The second task was designed to measure the gain of the visual system. Participants were shown four squares and asked which one’s brightness had been different from the others. The higher the contrast between the squares and the background, the more contrast is needed to identify the odd one out—this is called contrast gain, and the researchers suspected it would be especially strong in people with visual snow.

What did they find?

When comparing the different groups of participants, the authors found no difference in the total amount of noise in the visual system. On the other hand, contrast gain was increased only in people with visual snow, regardless of migraine status. Variations in the contrast gain experiment showed that the difference was specific to neurons in the parvocellular pathway—a pathway with slower-response neurons responsible for high-resolution color vision.

What's the impact?

By comparing different clinical groups (with/without migraine and with/without visual snow), the authors showed that abnormal contrast gain is specific to visual snow. Of course, these tasks are indirect measures of what’s actually going on in the brain; other studies are needed to examine the actual neural activity involved. The better we understand this syndrome, the more likely we will be to find a treatment.

Brooks et al. Visual contrast perception in visual snow syndrome reveals abnormal neural gain but not neural noise. Brain (2021). Access the original scientific publication here

The Association Between Post-Traumatic Stress Disorder (PTSD) and Heart Disease

Post by Leanna Kalinowski 

The takeaway

There is an increased risk of heart disease among individuals with PTSD. This risk strengthens following chronic PTSD and is attributable to impaired microvascular function.

What's the science?

The human body is designed to rapidly respond to threats and stressful situations by activating the sympathetic nervous system during the stress response. This leads to an increase in cardiovascular activity, helping one to fight or flee from a stressful stimulus. This is a normal physiological response; however, it can become dysregulated in disorders such as posttraumatic stress disorder (PTSD), which is a chronic psychiatric disorder that develops in some individuals who have experienced a traumatic event. When individuals with PTSD experience a reminder of their trauma, such as a loud noise, their sympathetic nervous system is activated despite the lack of an active threat. This repeated activation is believed to cause an increased risk of heart disease among individuals with PTSD, perhaps through repeated bouts of inflammation and vascular “wear and tear”. However, the exact mechanisms underlying this risk have not been demonstrated. This week in Biological Psychiatry, Vaccarino and colleagues conducted a longitudinal twin study with war veterans to determine the mechanisms underlying the association between PTSD and heart disease.

How did they do it?

A group of 275 twins was selected from the Vietnam Era Twin Registry, which is a large national sample of adult male twins who served on active duty during the Vietnam war era. Studying twins allows for the researchers to separate out factors that are often associated with both PTSD and heart disease but do not causally link the two disorders, including genetic and environmental factors that run in families and are shared among twins. Participants each underwent two examinations that were twelve years apart, each of which included a clinical assessment of PTSD. Participants were classified into one of three groups: no history of PTSD, late-onset PTSD (i.e., not diagnosed at visit 1 but diagnosed at visit 2), and longstanding PTSD (i.e., diagnosed at both visits).

Participants also underwent myocardial perfusion at both examinations, which is a positron emission tomography (PET) imaging test that shows how well blood flows through the heart muscle. PET scans of the heart were taken before and after administration of adenosine, which is a drug that increases the workload of the heart to uncover subclinical disease. From these scans, the researchers were first able to determine whether participants lacked blood flow to the heart because of obstructive coronary artery disease, which is when plaque accumulation leads to a narrowing or blockage of the large arteries that supply blood to the heart. They were also able to assess myocardial flow reserve, which is a measure of the health of the small coronary vessels that bring blood to the heart. Unlike obstructive coronary artery disease, coronary microvascular dysfunction is caused by damage to blood vessels rather than blockage by plaque. 

What did they find?

The researchers found that PTSD is associated with coronary microvascular dysfunction, indicated by lower myocardial flow reserve. This association was particularly noted among twins with longstanding PTSD. Twins with longstanding PTSD also experienced a lower myocardial flow reserve during their second visit compared to the first, suggesting a worsening of microvascular function following prolonged PTSD. These associations persisted even after comparing twin brothers with different PTSD trajectories, ruling out shared genetic and environmental factors, as well as when accounting for other psychiatric disorders, such as depression and substance abuse. Furthermore, there was no evidence that twins with PTSD had more obstructive coronary artery disease, suggesting that the association between PTSD and heart disease is due to damage to blood vessels rather than an increase in plaque accumulation.

What's the impact?

In summary, this unique study design allowed for the researchers to examine how heart disease progresses in relation to PTSD status and duration over a 12-year period. Their findings support a link between PTSD and heart disease and suggest that microvascular function is the mechanism underlying this association. Understanding this mechanism will help in long-term efforts for risk prediction, prevention, and treatment to reduce the burden of heart disease among individuals with PTSD. 

Vaccarino et al. Posttraumatic stress disorder, myocardial perfusion and myocardial blood flow: A longitudinal twin study. Biological Psychiatry (2021). Access the original scientific publication here.

Identifying the Neural Mechanism Behind Team Flow

Post by Lincoln Tracy

The takeaway

People can get “in the zone” when playing sports, listening to music, or working — either alone or as part of a team or group. Now, researchers have identified the neural mechanism responsible for getting “in the zone” during a team-based activity.

What's the science?

“Getting in the zone”—or entering a flow state—is a psychological phenomenon characterized by intense attention and effortless reflexes, leading to a reduced sense of external awareness and a reduced sense of time. Developing a flow state can occur during individual or team-based activities, with previous research reporting the flow state from team-based activities as being more intense than individual flow states. However, the neural mechanism underlying team-based flow states is unknown. This week in eNeuro, Shehata and colleagues propose a model of these mechanisms by investigating the neural activity of partners in a team-based activity.

How did they do it?

Researchers recruited 15 participants (five males, 18-35 years) to form 10 sets of pairs—meaning some participants were paired twice. Participants played the music rhythm game “O2JAM U”, an iPad game in the same vein as Guitar Hero, under three different conditions designed to manipulate how easy it would be for participants to get “in the zone” while playing as a team. During the Team Flow condition participants played a particular song while they could see their partner and the area on the screen they had to tap to “play” the song. The Team Only condition had the same setup, but participants played a reversed and shuffled version of the song. Finally, the Flow Only condition played the same song as the Team Flow condition, but participants could see neither their partner nor the tapping area. Irrelevant beeping sounds were played throughout the songs in all conditions to test how much attention participants were paying to the game. Researchers specifically recruited people who were good at the game (i.e., they missed less than 10 cues during a song with nearly 300 cues during a practice round) and preferred playing the game with someone else, rather than by themselves.

Flow state—or how much participants felt they were “in the zone”—during the task was measured in two ways. The first was by a series of ratings that participants completed after each trial (feeling in control, enjoyment, time perception, etc.). The second was via electroencephalography (EEG) hyperscanning—where brain activity from both participants was recorded at the same time. The researchers were specifically interested in the auditory-evoked potentiations (AEP), or the brain activity that occurred in response to the irrelevant beeps played during the tasks. The more brain activity in response to the beeps, the less “in the zone” the participant was. The researchers looked at the EEG data for participants individually, as well as looking at if the level and timing of brain activity were similar between the two participants in each of the pairs.

What did they find?

First, the authors found that the AEP response was greater during the Team Only condition compared to the Team Flow and Flow Only conditions, meaning that participants were less engaged in the task during that condition. Second, they found that the AEP displayed the strongest correlation with the participant’s flow ratings during the Team Flow condition. This suggests participants were more in the zone during the Team Flow condition. Third, the authors found the beta-gamma EEG band (brain waves) had the highest power when participants were in team flow, meaning the neural signature for team flow had been identified. Finally, they found that the Team Flow condition was associated with higher interbrain neural synchrony. This means that both individuals displayed higher levels of similar brain activity when completing the task—consistent with the phenomenological experience of team flow. 

What's the impact?

This is the first study to identify an objective neural measure of team flow. These results provide a proof of concept that team flow is a distinct brain state from solo or individual flow states. The novel method used in this study will be a useful tool for future research in this area.

Shehata et al. Team flow is a unique brain state associated with enhanced information integration and inter-brain synchrony. eNeuro (2021). Access the original scientific publication here.