Noise disturbs the brain's compass
Identifying causes for troubles in spatial navigation

Date:
June 10, 2020
Source:
DZNE - German Center for Neurodegenerative Diseases
Summary:
Our sense of direction tends to decline with age. Researchers
report on new insights into the causes of this phenomenon. These
study results could contribute to the development of diagnostic
tools for early detection of dementia.



FULL STORY ==========================================================================
Our sense of direction tends to decline with age. In the scientific
journal "Nature Communications," researchers from the German Center for Neurodegenerative Diseases (DZNE) and experts from the USA report on new insights into the causes of this phenomenon. According to their findings,
the main source of errors in determining spatial position and apparently
the cause of age-related orientation problems is a "noisy" and therefore imprecise perception of the speed at which one is moving. These study
results could contribute to the development of diagnostic tools for
early detection of dementia.


==========================================================================
From visual stimuli to muscle feedback and signals relayed by the
vestibular system -- the human brain uses a wide range of sensory inputs
to determine position and to guide us through space. An essential part
of the necessary information processing happens in the "entorhinal
cortex." In this area, which is present in both brain hemispheres,
there are special neurons that generate a mental map of the physical environment. Thus, information on real space is translated into a "data format," which the brain can process. "The human navigation system works
quite well. But it is not without flaws," explained Prof. Thomas Wolbers, principal investigator at the DZNE, Magdeburg site. "It is well known
that there are people with good orientation skills and those who find
it harder to find their way around. This ability usually diminishes
with age, because older people generally find spatial orientation
more difficult than younger individuals, especially in unfamiliar
surroundings. Therefore, the chances of getting lost increase with age."
Study in virtual space To understand the causes of this decline, DZNE scientists led by Thomas Wolbers, in collaboration with experts from the
US Massachusetts Institute of Technology and the University of Texas at
Austin, designed a specific experiment: A total of about 60 cognitively
healthy young and older adults who were fitted with "virtual reality"
goggles had to move and orient themselves - - separately from each other
-- within a digitally generated environment.

Simultaneously, participants also moved physically along convoluted
paths. They were assisted by an experimenter who led the individual
test person by the hand. In doing so, real locomotion led directly to
movements in virtual space.

"This is an artificial setting, but it reflects aspects of real
situations," said Wolbers.

During the experiment, participants were asked several times to estimate
the distance and direction to the starting point of the path. Because
the virtual environment offered only a few visual cues for orientation, participants had to rely mainly on other stimuli. "We looked at how
accurately participants were able to assess their position in space
and thus tested what is known as path integration. In other words, the
ability to determine position based on body awareness and the perception
of one's own movement. Path integration is considered a central function
of spatial orientation," explained Wolbers.

"Noisy" model Just as important as the experimental setup was the
mathematical modeling of the measured data. This was based on an approach
to describe interfering effects on position determination as noise. "The
human body and its sensory organs are far from perfect. Information
processing in the brain is therefore affected by glitches, which can be interpreted as noise. This is similar to a radio broadcast, where noise
can superimpose the actual signal," said Wolbers.

"With the help of our mathematical model, we were able to unravel the contributions of various sources of error and identify what distorts
position tracking the most and what has little effect. Such sources of
error have never been investigated at this level of detail."


==========================================================================
For example, data evaluation showed that body rotation in the direction
of the path's starting point was consistently quite accurate. And memory
errors played virtually no role. "To determine the location in space
while you are moving, you have to constantly update your position in your
mind. This requires you to remember where you were moments before. In
this respect, our analysis found only minimal errors," said Wolbers.

A matter of velocity The research team's conclusion: Errors in path
integration are mainly caused by "accumulating internal noise" in
information processing -- and this phenomenon is probably a consequence
of inaccuracies in the perception of movement speed.

"It should be noted that humans intuitively estimate distances covered on
the basis of how long and how fast they were previously travelling. Yet,
our study suggests that the critical source of error for determining
position is not time perception, but apparently random fluctuations in
the speed information that gets to the brain," said Wolbers.

This source of error was dominant both in the younger (average age
22 years) and in the older adults (average age 69 years). "The young
subjects were generally better at orientation than the older study participants. Critically, the accumulating internal noise increased with
age. This phenomenon is apparently the main cause of deficits in path integration and probably also the trigger for age-related orientation
problems. However, we do not yet know the exact origin of this noise
and why it increases with age," said Wolbers.

Early detection of dementia In previous studies, Wolbers and other
DZNE researchers had found that in cognitively healthy, older adults,
certain neurons of the entorhinal cortex - - so-called grid cells -,
which are essential for spatial navigation, fire irregularly: Their
activity is unstable. This was related to age-related difficulties in orientation. The current results suggest that these instabilities are
not due to malfunctions of the grid cells themselves, but are caused
by noise from outside. The problem is therefore not in the grid cells
but in the flow of information that reaches the entorhinal cortex. This
points to a possibility for the early diagnosis of Alzheimer's.

"Alzheimer's disease is associated with damage to the entorhinal cortex
at an early stage. It is therefore reasonable to assume that orientation disorders such as those that manifest in Alzheimer's originate in this
area of the brain.

Unlike age-related orientation difficulties, as our current study
suggests," explained Wolbers. "This could provide an opportunity to
distinguish normal age-related orientation problems from those caused by Alzheimer's. In the long term, our aim is to develop diagnostic methods
that detect Alzheimer's at an early stage. This might be possible using technology such as virtual reality.

We are currently preparing clinical studies on this."

========================================================================== Story Source: Materials provided by DZNE_-_German_Center_for_Neurodegenerative_Diseases.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Matthias Stangl, Ingmar Kanitscheider, Martin Riemer, Ila Fiete,
Thomas
Wolbers. Sources of path integration error in young and aging
humans.

Nature Communications, 2020; 11 (1) DOI: 10.1038/s41467-020-15805-9 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200610112105.htm

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