WHAT CAN WE LEARN ABOUT AGING FROM THE RESTING BRAIN?

What is resting state fMRI (rsfMRI)?

It is a method of functional neuroimaging where a researcher records brain activity while a participant lies down in an MRI scanner and simply rest without engaging in any specific task.  You may think that the brain is not very active at rest. But in fact, the brain is still active and we can learn a lot from the resting brain. rsfMRI is a powerful tool to study brain function.

Why is rsfMRI so valuable?

While you are at rest, a set of brain regions fluctuates spontaneously and coherently at low frequencies – this is known as a ‘resting state network.’ Researchers describe co-activation of a set of brain regions as ‘functional connectivity’ since these regions may not be anatomically connected but functionally connected. Importantly, resting state networks closely match brain networks activated while you are engaged in a task. For example, the right fronto-parietal network fluctuates together at rest as well as when you are engaged in a cognitive task (e.g., n-back task). Thus, resting state and task-related networks go hand in hand.

How do resting-state networks change as we get older?

Previous research suggested that some resting state networks decline with age (i.e., functional connectivity gets weaker with age). This may not be surprising given that cognitive function typically declines as we get older.  But do all resting state networks decline with age, such as those associated with visual, motor and emotion functions? The answer to this question remains unclear. Particularly, little is known about how aging affects emotion networks. 

Our investigation

We examined which types of resting state networks may change with age and how it relates to performance (Nashiro et al., 2017). We were particularly interested in how aging might differently affect cognition vs. emotion networks given previous behavioral research suggesting age-related decline in cognitive domains (Park, 2000; Salthouse, 2010) but preserved emotional processing in aging (Mather, 2016).

Our findings

• Age-related decline was observed in cognitive, motor and visual networks but not in emotion networks (see A below).

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• Greater within-network connectivity was associated with better cognitive performance (see B below).

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• Relative to younger adults, older adults showed increased functional connectivity in regions outside networks (C), which was associated with poorer cognitive performance (D).

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Conclusion

What did we learn about aging from the resting brain? Our results suggest that all types of resting networks decline with age except for the emotion networks identified in our study. Age-related preserved emotional function may be able to facilitate older adults’ cognitive performance. For example, previous research suggests that mindfulness training, which focuses on improving emotional well-being, can also enhance executive function (Tang et al., 2012).

 

References

Mather, M. (2016). The Affective Neuroscience of Aging. Annual Review of Psychology, 67(1), 213–238.

Nashiro, K., Sakaki, M., Braskie, M. N., & Mather, M. (2017). Resting-state networks associated with cognitive processing show more age-related decline than those associated with emotional processing. Neurobiology of Aging, 54, 152–162.

Park, D. C. (2000). Cognitive Aging: A Primer. Taylor & Francis.

Salthouse, T. A. (2010). Selective review of cognitive aging. Journal of the International Neuropsychological Society, 16(5), 754–760.

Tang, Y.-Y., Yang, L., Leve, L. D., & Harold, G. T. (2012). Improving Executive Function and its Neurobiological Mechanisms through a Mindfulness-Based Intervention: Advances within the Field of Developmental Neuroscience. Child Development Perspectives, 6(4), 361–366.

Briana Kennedy and Kelly Durbin win NIH fellowships!

Briana Kennedy received a three-year NIA F32 postdoctoral fellowship to investigate how arousal-induced attentional mechanisms may change with age and Alzheimer’s disease (AD). Kelly Durbin received a two-year NIA F31 predoctoral fellowship to investigate how the locus coeruleus (LC) influences cognitive functioning across the adult lifespan. Congratulations to Kelly and Briana!

The Pupil: A Deeper Look

The eyes are the window to the soul. Clearly, this phrase is meant to be taken figuratively rather than literally. But is there some physiological truth behind this saying? Let’s take a closer look at how pupil physiology indexes psychology.

Of all the organs in the human body, the brain is the most complex and mysterious. How, then, do psychologists approach the daunting task of deciphering the brain? The beauty of the brain is that it is intricately connected to much of our bodily functions; it is, among many other things, a control center for our physiology. This relationship whereby the brain influences physiology is the core idea of psychophysiology, a field where scientists look at physiology to gain insight into psychology.

One increasingly popular method of psychophysiology is pupillometry – the measurement of pupil size as an indirect marker of cognitive behavior. For psychologists, pupillometry offers a number of attractive advantages. It is safe and convenient for use with human subjects. It is also well-documented as a robust measure of many cognitive functions. Its convenience and effectiveness makes pupillometry a powerful tool in the study of the brain.

Pupil Diagram - the iris dilator muscle receives sympathetic input and the iris sphincter muscle receives parasympathetic input

Pupil Diagram - the iris dilator muscle receives sympathetic input and the iris sphincter muscle receives parasympathetic input

Briefly, let us review the anatomy and physiology of the pupil. The pupil is simply an aperture that regulates the amount of light received by your retina. The size of this aperture is controlled by two antagonistic muscles, the iris dilator and sphincter muscles. These muscles are responsible for the pupillary light reflex, in which the pupil constricts in response to a light stimulus. Although the most pronounced changes in pupil diameter are caused by changes in ambient lighting conditions, psychologists are most interested in how the pupil responds to an experimental task. These task-evoked pupillary responses, which are small but detectable pupillary changes, allows researchers to infer cognitive activity.

One of the earliest studies involving pupillometry explored how the pupil tracks memory load in a digit span memory task. Kahneman and Beatty showed that the pupil dilates as participants store more and more digits in their short term memory and constricts as the participants recalls the digits (1966). With this simple experimental design, Kahneman and Beatty demonstrated that the pupil serves as a reliable marker for memory load. Since then, psychologists have demonstrated the usefulness of pupillometry in measuring many different cognitive processes, like arousal, attention, and effort.

An intriguing area of active research aims to understand the neuroanatomy that underlies the task-evoked pupillary response – how do we account for the fact that the pupil tracks so many different aspects of our psychology? Recently, compelling evidence points to the locus coeruleus (LC). The LC is a small nucleus in the brainstem that produces norepinephrine, a hormone that modulates both autonomic and cognitive activity. In general, norepinephrine interacts with α1-adrenoceptors to excite sympathetic pathways and α2-adrenoceptors to inhibit parasympathetic pathways.

Pupillary change is one of these autonomic processes – the dilator muscle receives sympathetic input and the constrictor muscle receives parasympathetic input. The LC influences this pupillary system by inhibiting the pupil’s parasympathetic input and exciting the sympathetic input (Samuels and Szabadi, 2008). Thus, the net effect of LC activity is pupil dilation. This proposed circuitry between the LC and the pupil addresses the question of how the pupil correlates with so many different cognitive functions. The LC modulates all sorts of autonomic and cognitive processes – the pupil happens to be the simplest to measure.

Pupillometry has incredible potential in the study of psychology and physiology. The good news is that eye-tracking systems are becoming more affordable and easier to implement. With the emergence of inexpensive options, like the Eye Tribe, we now have readily-accessible tools to explore the psychology and physiology beneath the pupil.

 

References:

  1. Beatty, Jackson. "Task-evoked pupillary responses, processing load, and the structure of processing resources." Psychological bulletin 91.2 (1982): 276.
  2. Samuels, E. R., and E. Szabadi. "Functional neuroanatomy of the noradrenergic locus coeruleus: its roles in the regulation of arousal and autonomic function part I: principles of functional organisation." Current neuropharmacology 6.3 (2008): 235-253.