MedicalResearch.com Interview with:
Michael M. Halassa, MD, PhD, Assistant professor
Departments of Psychiatry and Neuroscience and Physiology
The Neuroscience Institute Depts. of Psychitatry
Langone Medical Center
New York, NY 10016
Medical Research: What is the background for this study? What are the main findings?
Dr. Halassa: Attention is a vital aspect of our daily life and our minds are not merely a reflection of the outside world, but rather a result of careful selection of inputs that are relevant. In fact, if we indiscriminately open up our senses to what’s out there, we would be totally overwhelmed. Selecting relevant inputs and suppressing distractors is what we call attention, and as humans we are able to attend in a highly intentional manner. Meaning, we choose what to pay attention to, and we do so based on context. If you’re driving and getting directions from your GPS, you’ll be intentionally splitting your attention between your vision and hearing. Now, in one context, you might have just updated the GPS software, so you know it’s reliable; this would allow you to intentionally pay attention more to the voice coming from the GPS. In another context, the GPS software may be outdated making voice instructions unreliable. This context would prompt you to direct your attention more towards using visual navigation cues and less to the GPS voice. How the brain intentionally and dynamically directs attention based context is unknown.
The main strength of our study is that we were able to study context-dependent attention in mice. Mice are unique models because they provide genetic tools to study brain circuits. Meaning, we can turn circuits on and off very precisely in the mouse, and in a way we cannot do in other experimental animals. The inability to do these types of manipulations has been the major roadblock for progress in understanding what brain circuits mediate attention and its intentional allocation.
Because we couldn’t train mice to drive and listen to the GPS, we decided to do something much simpler. Based on context (the type of background noise in the experimental enclosure), a mouse had to select between conflicting visual and auditory stimuli in order to retrieve a milk reward. Mice love milk; it turns out, and will work tirelessly to do well on getting it. Each trial, the mouse is told ‘you need to pick the light flash’ or ‘you need to pick the auditory sweep’; these stimuli appeared on either side of the mouse randomly so the animal really had to pay attention in order to get its reward. It also had to take the context into account. We found that mice did this task, and as humans would do, they were reliant on the prefrontal cortex for determining the appropriate context. The major finding was that the prefrontal cortex changed the sensitivity of the brain to incoming stimuli (meaning, made the visual stimulus brighter when the mouse cared about vision and made the auditory stimulus louder when the mouse cared about hearing), by influencing activity in the thalamus. The thalamus is the major early relay station in the brain. The prefrontal cortex does that by instructing the brain’s switchboard, known as the thalamic reticular nucleus (TRN) to control how much visual or auditory information the thalamus was letting through. So in a sense, we discovered that executive function, represented by the prefrontal cortex, can talk to ‘attentional filters’ in the thalamus to determine what ultimately is selected from the outside environment to build our internal world.
Medical Research: What should clinicians and patients take away from your report?
Dr. Halassa: There are a couple of take home messages from this work.
First, the brain invests quite a bit of effort in blocking out distractors. The thalamic reticular nucleus (TRN), or the attentional filters according to our study, are inhibitory neurons that seem to be quite active at rest. They actually become less active when we need to pay attention to something, and only let the information we care about go through. This is an important clinical insight, which tells us that the default mode of operation of the brain is to block sensory input. Otherwise, I guess we would all be overloaded.
Being overloaded is particularly relevant to many neurodevelopmental disorders, but particularly to ADHD, schizophrenia and autism. Many patients with these disorders frequently report a feeling of sensory overload. In fact, some even report that the reason why they socially withdraw is because they feel overloaded I actually see that in my own practice, when I notice that one of my patients is uncomfortable and distracted by the sound of my computer fan or some construction outside. To me, and other more fortunate people, we can effortlessly filter out these sensory inputs, but for some of my patients it really gets in the way of their quality of life. This is a huge motivation for my lab, and I finally feel that we have some clue on what circuits to go after in order to target such symptoms.
Second, it is important to know the limits of multitasking. Yes, there is a lot to pay attention to out there, and the demands of modern society continue to increase. However, our brains have limits on how much we can do at any one point in time, and how thinly we can spread our attention across multiple domains. Making transitions between objects that we pay attention to is also essential, and it seems that the brain circuits we identified maybe also important for minimizing interference between sensory inputs, making the transitions cleaner and multitasking easier.
Third, the TRN may protect the brain from sensory inputs during sleep, so it’s a unique circuit that links how well we sleep at night with how well we pay attention and multitask during the day. This may represent a unique way to fix attention and sleep problems in patients who have problems with both.
Medical Research: What recommendations do you have for future research as a result of this study?
Dr. Halassa: I would tell young people who are interested in neuroscience, attention and the thalamus is where it’s at! There is so much to learn about how the brain selects what to pay attention to and there are so many exciting discoveries to be made by studying the type of problems we’re studying. We’re always looking for colleagues to make a dent in these big problems, and I hope that the next big discovery will come from someone who is listening to this show!
Ralf D. Wimmer, L. Ian Schmitt, Thomas J. Davidson, Miho Nakajima, Karl Deisseroth, Michael M. Halassa. Thalamic control of sensory selection in divided attention. Nature, 2015; DOI: 10.1038/nature15398
Michael M. Halassa, MD, PhD (2015). Brain Circuits Limit Our Ability To Multitask