The
ability of birds to navigate their way back to their nests, often more
than thousands of miles each year, is fascinating. Arctic terns, which
have the longest migration distance known, travel from their Arctic
breeding grounds to overwinter on the Antarctic coast: an annual
roundtrip of over 40,000 miles (>64,000 km).
Homing pigeons are of course famous for their extraordinary
navigation abilities. They can find their way home from new locations up
to 1100 miles (1800 km) from their nests. There are several theories
that attempt to explain the navigational and migratory skills of birds,
and homing pigeons are a useful model to help us understand these extraordinary abilities.
The olfactory navigation hypothesis
suggests that homing pigeons use an olfactory map to orient themselves.
This is based on a study led by Silvano Benvenuti in 1972, which showed
that pigeons who had their olfactory nerves sectioned could not find their way home when released from unfamiliar locations.
The magnetic map theory poses that birds get their sense of direction from the Earth’s magnetic field.
The Earth’s magnetic field becomes stronger the further you get from
the equator, and homing pigeons may be sensitive to these minute changes
in the magnetic field strength.
The use of the sun as a compass, a bird’s circadian rhythm and the possession of an internal map
are other theories that try to explain bird navigation. However, at the
moment there is no clear evidence that supports any of these theories
over the others, and how these internal cues might work together is also
poorly understood. Sadly, the mapping skills of birds remain largely
unexplained.
How do we find our way home?
Although we still don’t know the complete story, there are some very
strong clues to how birds navigate. But how mammals (including humans)
navigate is relatively unexplored. Recent research is however giving
some very interesting insights.
The hippocampus is a sea horse-shaped structure located in the centre
of the brain. It is well known for its role in memory formation, but
also is important to our sense of direction and navigation. In 1971,
John O’Keefe discovered the first element of a mammalian “inner-GPS”
when he observed a subtype of neurons
in the hippocampus that became active whenever a rat moved to a
particular area of a room. These neurons, now known as “place cells” are
thought to help rats – and us – build a picture of where we are in our
environment.
In 1984, James Ranck made another step in explaining spatial
representation in our brains. He found that mammals have “head-direction
cells”: cells that fire in response to the specific angle and direction
of your head’s orientation in space.
Has our sense of direction been found?
These findings have been important for starting to uncover the neural basis of human navigation (O’Keefe won a Nobel Prize for his research), and a study published just after Christmas has continued to help unravel the mystery of how we navigate.
Martin Chadwick and his colleagues at UCL asked participants to
navigate around a simple virtual environment on a computer and gave them
target destinations to move towards.
Using functional magnetic resonance imaging, the researchers were
able to pinpoint a population of neurons that signalled the direction in
which participants needed to go to their target destinations – and, the
varying strengths of brain signals from these cells were able to
predict subjects’ navigational ability.
Does this explain why some people have a better sense of direction than others?
These specialised neurons were found in the enterohinal cortex, a
region next to the hippocampus. But although the study shows that the
enterohinal cortex has greater activity in those better at navigating,
the results do not really explain why this may be.
In other words:
Are you a better navigator because you have higher activity in your enterohinal cortex?
OR
Is the activity in your enterohinal cortex higher because you are a better navigator?
What is really driving this relationship? When we misinterpret
correlation, we can find ourselves, just for example, assuming that chocolate consumption might improve cognitive function in entire populations.
Use it or lose it: why iPhones may be disrupting your navigation skills
When people use GPS-like instructions to reach a destination rather
than navigating on their own, their brains in general are found to be much less active.
Perhaps the less you use your phone’s GPS to help you navigate, and the
more often you learn to make it on your own, the stronger the activity
in your hippocampal region – and so the better a navigator you become.
In Chadwick’s study, the stronger neural activity amongst the better
navigators may have been a result of them having better developed their
navigation skills throughout their lives.
Chadwick explains that future research will be needed to support the
case for a causal relationship between neural activity and our mapping
skills. He would like to see similar studies carried out in real life
environments to demonstrate that these neurons extend their functions
beyond those described in the virtual environment simulated for his
study.
If you feel lost after reading this post, perhaps eat some chocolate –
it might improve your brain power. Or, if you will, take a stroll
somewhere new and try to find your way back home – without your GPS app!
Chadwick MJ et al. A Goal Direction Signal in the Human Entorhinal/Subicular Region. Curr Biol 2015;25:87–92.
Source: http://www.theguardian.com
