Extreme precision needed to accurately represent the slowly changing visual world
have long attempted to unravel the cryptic code used by the neurons of
the brain to represent our visual world. By studying the way the brain
rapidly and precisely encodes natural visual events that occur on a
slower timescale, a team of Harvard bioengineers and brain scientists
from the State University of New York have moved one step closer
towards solving this riddle. The findings were reported in a September
6th Nature article.
"Visual perception is limited by the relatively slow way in which the
neurons in our eyes integrate light. This is why, for example, a
Hollywood movie consisting of a series of flickering images appears to
us as seamless motion," explains Garrett Stanley, Associate Professor
of Biomedical Engineering at the Harvard School of Engineering and
Applied Sciences. "However, when the brain responds to some kind of
visual event, such as a ball bouncing, the activity of the neurons
responsible for sending information can be precise down to the
millisecond, despite the fact that the motion of the ball is much
To determine why the brain might encode visual
information with such precision, the researchers relied on data
obtained by directly recording neuronal activity in animals while they
viewed natural scene movies. Doing so enabled Garrett and his
colleagues to pinpoint the pattern of neuronal firings in cells that
respond to form and motion.
Their analysis of the data suggests
that the brain's timescale depends on the nature of the visual
stimulus. In other words, the precise timing of the neurons (i.e. their
internal clock) changes relative to the timescale of the visual scene.
For example, a faster bouncing ball results in more precise brain
activity than a slower one. In each case, however, the precision of the
neurons' activity was several times that of the speed of the bouncing
It turns out that the extreme precision of the brain's
neural response to visual stimuli is, paradoxically, necessary to
accurately represent the more slowly changing visual world. The
neuron's response must be more precise to recover the important aspects
of the visual environment.
"We believe that this type of
relative precision may be a general feature of sensory neuron
communication," says Stanley. "You can think of it like digital
sampling used for audio recordings. The brain 'digitizes' the visual
stimulus. As with digital audio recordings, for clear and
representational 'playback', the encoding frequencies must be at least
double that of the signal information."
In future research, the
researchers plan to further clarify why and how the brain encodes
visual information across larger networks of cells and across
functional units of the brain. They also will investigate how the
visual pathway of the brain adapts to changes in the visual scene. They
believe cracking the neural code will help other scientists and
engineers better "communicate" with the brain. Understanding the speed
at which the brain encodes information is critical for designing
interfaces such as neural prosthetics, that seek to augment or replace
brain function lost to trauma or disease.
Harvard University. September 2008.