Researchers have new insight about what causes distractions.
Three decades ago, Dr. Francis Crick proposed that the thalamus “shines a light” on regions of the cortex, which readies them for the task at hand, leaving the rest of the brain’s circuits to idle in darkness.
Senior author Dr. Michael Halassa, PhD, from New York University’s Langone Medical Center, explains that people use a very small percentage of incoming sensory stimuli to guide their behavior, successfully filtering out what is unimportant.
In many neurological disorders, this filtering function may be broken, leading to a lack of control over sensory input so that the brain becomes overloaded.
Neuroscientists have long believed that the prefrontal cortex (PFC), an area at the very front of the brain, selects what information to focus on, but how this happens remains unknown.
One theory is that neurons in the PFC send signals to cells in the sensory cortices, located on the outer part of the brain.
PFC neurons may send signals to location deep inside the brain
However, Dr. Halassa’s team believes that instead, PFC neurons may send signals to inhibitory thalamic reticular nucleus (TRN) cells, located deep inside the brain.
To investigate this, they designed a test that challenged mice to focus and ignore distractions.
They trained mice to use either a light or a sound to discover which of two doors hid a milk reward. Before each decision, the mice heard a noise telling them to anticipate the light or the sound that would lead them to the correct door. They had to use the correct cue and ignore the irrelevant one to get their reward.
The researchers used genetically modified mice in which specific neurons could be activated or inhibited with rays of light.
The mice made more errors when neurons in the PFC were silenced during anticipation of the cue. They chose the wrong door in response to the light or sound cue, implying they could not concentrate when the PFC neurons were silenced.
In contrast, silencing the neurons of the visual cortex, the part of the brain that processes visual information, at the moment of anticipation, had no effect on attention.
The mice chose the correct door in response to a light cue. Contrary to previous beliefs, connections between PFC and sensory cortical neurons would seem not to be involved with this type of attention.
They then tested whether the TRN cells played a critical role.
When they switched on the TRN neurons involved in vision during anticipation of the light cue, the mice struggled to concentrate on the light. When the TRN vision circuit was turned off, it had the opposite effect. Now the mice struggled to focus on the sound, but not the light.
The team interpreted this to mean that inactivating the visual TRN makes irrelevant visual input more distracting.
Prefrontal cortex and thalamus interact to prevent distraction
They also observed that when mice needed to focus on the light, activity dropped in the visual TRN and increased in the part of the thalamus that processes visual inputs, called the lateral geniculate nucleus (LGN).
In contrast, when the PFC was inactivated, those changes did not happen.
These findings suggest that the PFC modifies activity in the thalamus in order to shift attention toward visual information.
To test whether fluctuations in the TRN and LGN were linked, a new technique was developed, called chloride photometry.
This allowed researchers to directly monitor how much chloride was entering LGN neurons in real time, and to see how circuit problems in the mouse thalamus may lead to problems with concentration.
The more chloride ions flowed into a neuron, the more inhibited the mice became. More chloride entered and inhibited the LGN during trials that required mice to ignore the light and focus on the sound.
James Gnadt, PhD, program director at the National Institutes of Health’s (NIH) National Institute of Neurological Disorders and Stroke (NINDS), says:
“We are constantly bombarded by information from our surroundings. This study shows how the circuits of the brain might decide which sensations to pay attention to.”
Medical News Today recently reported on a novel magnetic resonance imaging (MRI) technique that highlighted characteristics of the brain in people with autism.
Written by Yvette Brazier