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Perception, learning
and memory
T
he human brain is a highly
complex organ shaped by hundreds of millions of years of evolution. It has evolved to detect
meaningful patterns, to learn,
memorize and recall them, and to adapt.
Our neural networks can produce and decode communication signals, extract and
process useful features from the environment, and produce vital innate behaviours
such as eating, fleeing and mating.
Amazingly, this specialized structure selfassembles, growing from one cell to tens
of billions, and each developing brain incorporates both hidden biases shaped
through natural selection, and the means
with which to sculpt itself throughout its
lifetime as the individual encounters new
experiences and sensations.
Brain Basics
Our brain contains billions of neurons,
which are specialized cells that process and
transfer information, and are arranged into
complex cellular circuits. These cells communicate via synapses, which are junctions
that allow the transfer of chemical or electrical information from one neuron to the
next (Fig. 1).
Neurons are the most diverse cell type in
the body. They are usually polarized with
specialized projections for receiving (dendrites) and relaying (axons) information
(Fig. 2). Sensory neurons convert external
stimuli, such as light, sound or pressure, into
electrical signals, whereas motor neurons
use electrical signals to control muscles. A
third, more abundant, type of neuron lie
between these inputs and outputs.
Non-neuronal cells, called glia, play
fundamental roles in the development,
support and plasticity of neural circuits;
however, neurons and their synapses
remain the focus of learning and memory
research. Changes in neuronal activity and
synaptic strength are thought to underpin
learning and memory. Moreover, neuronal
loss and synaptic malfunctioning have
been implicated in various neurological disorders that involve learning and memory
deficits.
change. Proteins are continually recycled
and replaced, and new proteins are required for learning and memory to occur2.
So, how can some memories remain stable
when so many of the underlying components are constantly changing?
Unravelling perception,
learning and memory
The study of perception, learning and memory offers many challenges and research opportunities. Our technical toolbox means
it is now possible to catalogue and describe
the constituents of the brain and its neural
circuits — an essential step towards understanding the brain. All that is needed is the
time and optimized methods to help handle huge data sets.
The development of high-resolution
serial electron microscopy, super-resolution
light microscopy3,4 and multicoloured
genetic tools for neuronal labelling5, and
the increased affordability of immense
computing power, make it possible to imagine a day when the connection matrix of
a small-to-medium-sized brain (perhaps
that of a fly or a mouse) will be known with
a reasonable degree of accuracy. This endeavour requires the ability to handle enormous data sets, and a multi-disciplinary
approach incorporating molecular biology,
genetics, electrophysiology, imaging, electronics, nanotechnology, mathematics,
computer science and nonlinear dynamics.
It will breed a new type of cooperation
between areas of science that have often
worked separately.
Despite intense efforts to understand
perception, learning and memory, there
are still huge gaps in our knowledge.
We have yet to pinpoint the neuronal
mechanisms that underlie perception.
Millions of neurons in structurally diverse
networks activate for us to perceive even
the simplest of objects, and teasing apart
the neural circuits that are responsible is
no small achievement.
Perception and memory are intimately interlinked — perceiving an object
would be meaningless without the ability
to recall and link it to corresponding
memories. Although perception, memory
formation and recall are likely to rely on
interlinked mechanisms and substrates,
we have yet to understand them fully, or
to decipher the effects of sleep, attention
and other ill-understood processes on
learning and memory.
We do know that memory is a spatially and temporally dynamic process. As
memories are stored and consolidated,
they are shifted from one part of the brain
to another in a process that can take weeks
and appears to be dependent on brain
activity during certain phases of sleep1.
Memory-related proteins, synapses, neurons and neural networks are also dynamic. Neurons die off as part of normal
ageing, yet for the most part we notice no
T
he dynamic and coordinated behaviour of neurons in the brain
can be detected in brain oscillations that occur at a variety
of frequencies (for example, 2–200 hz). a recent study by
researchers at the max planck institute for Brain research found
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Research Perspectives of the Max Planck Society | 2010+
mapping the Brain
neUral networking
A major challenge for modern neuroscience
is to explain perception and behaviour
in terms of neural activity. Given the size
of the brain, the number of neurons and
that when memory-related neurons in the brain fire synchronously
with brain waves at the theta frequency (2–8 hz) during learning, the
resulting memories are stronger than if this synchronization does not
occur (Rutishauser, U. et al., Nature 464, 903–907, 2010).
Biology and Medicine
perception, learning and memory are interconnected processes controlled
by the coordinated activity of molecules, synapses, cells and neural networks
within the brain.
although we know much about the activity of individual neurons and
synapses, we know far less about how these components interact.
Top right Images: courtesy of the Schuman laboratory. Reprinted with permission from Macmillan Publishers Ltd: Nature Neuroscience 13, 897 - 905 (2010).
neuroscience techniques must evolve to realize a new era of multidisciplinary
research studying networks of interacting elements.
the distributed nature of neural activity6, it
is increasingly clear that traditional
methods will yield limited results. Patch
clamping, for example, can record the
activity of single cells at high resolution,
but tells us nothing of how these cells
contribute to larger circuits. Functional
magnetic-resonance imaging offers a broad
view of brain activity on a large scale, but
lacks the resolution to reveal the activity of
individual neurons (Fig. 3).
Much is known of the functioning of
individual neurons and synapses, but
much less about their coordinated action
in ensembles of millions. The brain derives
its magic from coordinated activity on the
large scale and high degrees of specialization on the small scale7.
Networks, neurons and molecular constituents need to be studied in combination rather than in isolation, and experimental techniques traditionally used to
study individual elements need to evolve
towards this. One new approach involves
light-activated genetic switches that
control the activity of specific, discrete
neuronal populations8,9. This technique —
‘optogenetics’ — is already bearing fruit
and it is thought that such studies will
help reveal cell function within the context of neural circuits.
Neural activity needs to be sampled at
an intermediate scale: that of networks of
interacting elements. Rather than studying a handful of cells in a handful of
animals, studies should focus on the population level, with high-sampling density
and mobile animals. This will be technically challenging, and will rely on major
developments in the fields of optics,
microelectronics, nanoelectronics and
computer science.
The rewards will be great. Deciphering
the neural basis of perception, learning
and memory is a fundamental part of
understanding how the brain functions in
health, ageing and disease. Teasing apart
the contributory mechanisms might offer
us the chance to influence and improve
these most human of skills.
➟ For references see pages 38 and 39
Fig. 1 | Neuron
communication
Fig. 2 | Neuron structure
The intricate, branching dendrites
of a cultured neuron can be
visualized by labelling them with the
fluorescent-tagged marker protein
microtubule-associated protein 2
(MAP2).
Neurons (labelled here with a pink
fluorescent-tagged marker protein)
communicate with each other
via specialized junctions called
synapses (labelled here with a green
fluorescent-tagged marker protein).
»
Much is known of the functioning of individual
neurons and synapses, but much less about their
coordinated action in ensembles of millions.
Fig. 3 | Levels of understanding
Magnetic-resonance images can currently resolve certain brain structures, whereas
implanted electrodes (arrows) are needed to reveal the electrical activity of
individual neurons10.
2010+ | Research Perspectives of the Max Planck Society
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