Given that we
see the world through two small, flat retinae at the backs of our eyes, it
seems remarkable that what each of us perceives is a seamless,
three-dimensional visual world. The retinae respond to various wavelengths of
light from the world around us. But that’s just the first part of the process.
Our brains have to do a lot of work with all that raw data that comes
in—stitching it all together, choosing what to concentrate on and what to
ignore. It’s the brain that constructs our visual world. Neuroscience
researchers and cognitive scientists have recently made much progress
investigating how this process works. My own research focuses on how humans
construct the visual world by selecting what visual information to pay
attention to and using visual memory to retain it over short periods of time.
There’s a lot more than simple sensory input that goes into building our
perception of the visual world we live within. The retina is a sheet of cells
at the back of each of our eyes. Some of these cells, called photoreceptors,
are sensitive to light. There are two main types: rods are sensitive to
light-dark differences and cones are sensitive to color. These photoreceptors
are most densely packed together in a small area at the center of the retina
called the fovea. It corresponds to the center of our vision, where resolution
is at its highest. Detail progressively decreases for distances further from
the center of our visual field—that is, in the periphery (hence “peripheral
vision”).
As we look
around our environment, we move our eyes. This enables us to orient the fovea
toward what we’re most interested in within the vicinity. These voluntary eye
movements are called saccades and are made about three times a second. Given
that the eyes are in constant motion, how does the picture of the world we have
in our mind remain so apparently stable? Investigating this apparent
discrepancy, neuroscientists have discovered that inputs from the eyes are
suppressed during saccades, so we don’t register the fast motion and image blur
that would otherwise occur. Furthermore, our brain corrects for movements of
the eyes using information from the eye muscles that control their movement.
Because the brain omits the information that comes in while the eyes are
moving, our visual world is perceived mostly during fixations, the short
periods of time (approximately 200-300 milliseconds long) when the eyes are
stationary. While reading for instance, our eyes are in motion only 10%-20% of
the time. During each fixation, we must select the visual information most
relevant to performing the task at hand. We have an ability to attend to or
focus on one or several sources of information while ignoring all the rest, or
at least reducing their significance. Researchers call this visual attention;
they think it’s critical for helping us bind together or integrate elementary
features (for instance, color, orientation) to form the perception of complete objects
in the environment.
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