Air Pollution is Associated with Increased Amyloid-β Plaques in Older Individuals with Cognitive Impairment

Post by Amanda McFarlan

What's the science?

Alzheimer’s disease is a neurodegenerative disease associated with the accumulation of amyloid-β plaques and neurofibrillary tangles. It has been suggested that environmental factors like air pollution may contribute to the development of Alzheimer’s disease. Indeed, human and animal studies have provided evidence that exposure to air pollution may lead to increased production and deposition of amyloid-β in the brain. This week in JAMA Neurology, Iaccarino and colleagues investigated whether increased levels of air pollution are associated with the accumulation of amyloid plaques in the brains of older individuals with cognitive impairment.

How did they do it?

The authors obtained data from the Imaging Dementia—Evidence for Amyloid Scanning (IDEAS) study which assessed amyloid-β accumulation in over 18 000 participants with cognitive impairment using positron emission tomography (PET). They also obtained demographic information as well as residential zip code data for all participants in the study. Then, they used the Downscaler model provided by the United States Environmental Protection Agency to gather data on the air quality, as measured by fine particulate matter (particles with a diameter less than 2.5 μm) and ground-level ozone, from 2002-2003 (~14 years prior to the PET scan) and from 2015-2016 (~1 year prior to the PET scan). Using this data, the authors performed statistical analyses to determine whether participants living in areas with higher levels of fine particulate matter and ground-level ozone were more likely to be positive at the amyloid-PET scan, indicating brain amyloid-β accumulation. 

What did they find?

The authors found that the probability of detecting amyloid-β in the participants’ PET scans was significantly increased in areas with higher concentrations of fine particulate matter. They used marginal effects analyses to show that for every 1 μg/m3 increase in fine particulate matter, the probability of having a positive amyloid PET scan was +0.5% in 2002-2003 and +0.8% in 2015-2016. The association between positive amyloid PET scans and fine particulate matter remained statistically significant after adjusting for covariates including sex and US Census tract (distinct geographic regions that are used for the exchange of geographic and statistical data) random effects. Higher concentrations of ground-level ozone, on the other hand, were not associated with positive amyloid PET scans.

What’s the impact?

This study shows that the regional concentration of fine particulate matter in the air is associated with increased accumulation of amyloid-β plaques in the brains of older individuals with cognitive impairment. These findings suggest that airborne pollutants may be associated with Alzheimer’s disease pathology. These findings can inform public health policy decisions, as pollution levels may contribute to an individual’s risk of developing Alzheimer’s and dementia.

Iaccarino et al. Association Between Ambient Air Pollution and Amyloid Positron Emission Tomography Positivity in Older Adults With Cognitive Impairment. JAMA Neurology (2020). Access the original scientific publication here.

Close Relationships Facilitate Efficient Performance of Social Goals

Post by Lincoln Tracy

What's the science?

Humans are inherently social creatures with complex social skills. Neuroscientific models hypothesize that our brain has undergone substantial changes as we have evolved to become more social. The extent to which social behaviors are embedded in the human brain suggests that the brain operates by responding to natural and dynamic social exchanges to a greater degree than static or non-social stimuli. Consequently, social neuroscience research is focusing more on how brains synchronize during naturalistic social moments to achieve specific goals. Specifically, modern research utilizes hyperscanning, a method where data is collected simultaneously from two or more brains to measure synchronous activity. This week in NeuroImage, Amir Djalovski, A Ph.D. student at Ruth Feldman's lab sought to test how different types of relationships affect neural synchronization, by using electroencephalography (EEG) hyperscanning while partners completed various tasks. 

How did they do it?

The authors recruited 158 adults (79 male-female pairs) organized into one of three groups: couples (long-term romantic partners who had been living together for at least a year), friends (close friends), or strangers (demographically matched individuals who did not know each other). Both participants had an EEG cap placed on their heads for simultaneous recording of brain activity. Participants then engaged in two naturalistic interactions: a motor task and an empathy giving task. First, participants were given an “Etch a Sketch” and asked to draw predefined pictures while only being allowed to twist one nob each. Then, participants took turns sharing a distressing or troubling experience. After completing the tasks, participants rated how comfortable they felt in each task, and how empathic and helpful they felt their partner was. EEG data was bandpassed to assess brain synchrony in the alpha, beta, and gamma frequency bands. Electrodes on the EEG cap were divided into pre-defined brain areas of interest: left temporal, right temporal, and central. The level of behavioral synchronicity (i.e., social interactions) was assessed for behaviors such as a positive and relaxed mood and the reciprocity of interaction.

What did they find?

Couples displayed higher behavioral synchronicity, higher interbrain synchronicity, and task performance on the motor task compared to friends and strangers. Importantly, the relationship between behavioral and neural synchrony was only moderated by attachment bonds in couple pairs. That is, the greater the behavioral synchrony the higher the neural synchrony (in couples). In the empathy giving task, couples again displayed higher behavioral synchrony compared to strangers. Couples showed the lowest interbrain synchrony during the empathy giving task, while strangers exhibited the highest. However, strangers felt much less supported than couples or friends during the empathy giving task.

What's the impact?

This study shows that natural social processes in the brain are shaped by attachment bonds and sustained by behavioral coordination. Specifically, romantic partners who lived together displayed the most efficient two-brain-two-behavior balance toward best performance. This highlights how attachment bonds shape interpersonal brain processes, and that differences between romantic partners and strangers are not simply the result of familiarity. Rather, other aspects of long-term romantic love contribute to these differences. While the hyperscanning literature is rapidly growing and more research is needed, this study underscores the importance of looking at interbrain processes in order to understand how two brains synchronize during real-life social interactions.   

Djalovski et al. Human attachments shape interbrain synchrony toward efficient performance of social goals. NeuroImage (2021). Access the original scientific publication here.

Behavioral and Neural Correlates of Stress in Response to Threat In Virtual Reality

Post by Cody Walters 

What’s the science?

It has been challenging to study fear and anxiety in humans owing to the limited number of experimental paradigms that mimic naturalistic threat scenarios in a lab setting. This week in Current Biology, Balban et al. utilized a novel virtual reality task to study the behavioral, physiological, and neural correlates of visually evoked threat in human subjects.

How did they do it?

Subjects wore a virtual reality (VR) headset that realistically depicted the laboratory room where the experiment was being conducted. While wearing the headset, subjects were instructed to complete a cognitive task using a virtual panel on the wall, then 1 minute into the task the panel would move to the other end of the virtual room. At the moment of the panel transition, the VR room would undergo one of two possible modifications: a ‘no heights’ stimulus (during which the walls and ceiling were removed to reveal a gray background) and a ‘heights’ stimulus (during which the walls, ceiling, and parts of the floor were removed to reveal a 150 ft above-the-ground skyscraper landscape). 

During the heights stimulus, participants navigated across a plank to reach the panel on the other side of the room and complete the cognitive task, eliciting a height-induced visual threat response. While subjects performed the task the authors recorded their heart rate, skin conductance levels, eye movements, and respiration. Additionally, the authors ran a similar experiment in a cohort of epilepsy patients fitted with intracranial electroencephalography (iEEG) for seizure localization, thus allowing for the recording of neural data in tandem with skin conductance and eye tracking metrics in this group. 

What did they find?

The authors found that the participants’ skin conductance, heart rate, and respiration all increased upon seeing the heights stimulus. During the heights condition, visual scans surrounding the plank positively correlated with skin conductance levels and latency to step out onto the plank (a measure of behavioral inhibition). These data indicate that the virtual heights stimulus triggered a physiological threat response. The authors observed that participants in the control condition exhibited less physiological arousal, fewer visual scans, and a reduction in the latency to approach the opposite side of the room compared to the heights condition. Individuals with anxiety disorders experience heightened levels of physiological arousal, yet it remains unclear how it affects their threat reactivity. To explore this question, the authors recruited a cohort of subjects who scored high in generalized or trait anxiety to perform the task. Relative to control subjects, anxious individuals (1) experienced an elevated skin conductance response (the fast-changing component of the galvanic skin response), (2) performed more visual scans, and (3) exhibited a significantly greater latency to step out onto the plank. Interestingly, there were sex differences among the anxious subjects, with females exhibiting a greater degree of behavioral inhibition than males. 

In order to relate these behavioral and peripheral measures of arousal with neural signals, the authors recorded iEEG data from the insula and orbitofrontal cortex in epilepsy patients while they performed the task. They found that gamma activity in the insula positively correlated with skin conductance response during the heights stimulus, with high gamma activity preceding an elevated skin conductance response by approximately 8 seconds. Additionally, the authors found that there was elevated gamma activity in the insula during moments of threat-induced visual scanning. In the orbitofrontal cortex, on the other hand, theta activity negatively correlated with skin conductance response during the heights stimulus.

What’s the impact?

This study showed that increased visual scanning is a behavioral correlate of anxiety, and that brain activity is altered prior to sympathetic arousal induced by visual threat. Altogether, this study used a novel virtual reality paradigm to identify behavioral, physiological, and neural correlates of human stress in response to a semi-naturalistic threat scenario. 

Balban et al. Human Responses to Visually Evoked Threat. Current Biology (2020). Access the publication here.