Exploring the Science of Sight

I like to think that I’m
pretty good at sensing the world around me. My body is covered with specialized
sense organs (e.g. eyes and fingers), products of millions of years of
natural selection. Sometimes I think about the ancient humans and
primates who died grisly deaths at the claws of terrifying predators,
thereby continuing the process of natural selection that produced my
exquisitely designed ear canals.

These
organs, paired with some pretty funky neurons, allow me to access a ridiculous
amount of information about my environment—how it smells, tastes, feels,
sounds and, most importantly, how it looks. Every conscious moment of my existence
is filled with sensations, and there’s not much I can do about it. When I shut
my eyes I’m forced to see the pitch-black interior of my eyelids, and all my
other senses are heightened. A lot of mental effort can tune out certain sounds or feelings, but there’s no way to close my ears or turn off my skin.

In some ways, I use my senses as
tools to navigate, understand and generally survive in my immediate
environment. They can’t be described as instruments for my
use because they are fundamentally me.
With no senses, I’d have no clue what my body looked like, or that I even have a
body at all. I might have some sort of mentally derived internal dialogue, but
it’s unclear if that’d be possible had I never heard or seen any form of
symbolic language.

So everything I experience is rooted
in my sensation of the outside world. That sounds fine as long as I can trust
my senses. Unfortunately, as anyone who’s seen an optical illusion knows, we often can’t. The reason our senses can be fooled is because our big, complex
brains handle most of our day-to-day needs unconsciously, analyzing
information from two million visual neurons every instant to determine our
environment’s shape, color and motion.

If the brain wasn’t so helpful, we’d likely be very bewildered animals without
the slightest idea of what was happening. I certainly don’t think I
could constantly analyze two million visual pixels while instantaneously
integrating that information with the billions of neurons firing from my ears,
mouth, nose and skin—not to mention the emotions, beliefs and memories conjured with everything I’m sensing.

Our brains are extraordinarily fast
and accurate at doing all of this, which is important because these abilities keep
us alive and give our consciousness space to focus on things like calculus
homework. But too much unconscious, ‘top-down’ processing can run us into
trouble. Take our ability to process faces, a task that was so evolutionarily
important that it has its own brain region, the fusiform face area (FFA). Due to the FFA’s
prowess, faces are seen in nearly anything that could loosely be construed as one. Others’
emotions are often obvious to us through tiny changes in their facial
composition, and we don’t have to think about
these things for them to happen.

The brain’s ability to see faces is great, unless the FFA is damaged or underdeveloped, which causes a disorder
called prosopagnosia, or face
blindness. Face blindness is characterized by a total or partial inability to
recognize faces. In severe cases, people shown images of their parents
or children have no idea who they’re looking at. Their brain simply can’t put the
face together in a way that it can understand, and there’s no way for people to
consciously solve this problem. The only solution is to rely on other cues that
identify people such as their hair, gait or voice.

Victims
of brain damage to other regions give us insight into the brain’s unconscious
world. People with injuries, or lesions, on the upper right side of their brain
completely lose awareness of the left half of their visual field, despite a
normal left eye. We know this by asking such people, who have hemispatial neglect, to draw the numbers
of a clock within a pre-drawn circle. Those with hemineglect get to 6, at the
bottom of the clock, then stack the rest of the numbers vertically up to 12,
completely ignoring the left half of the clock. Perhaps most surprising, they
have no idea what they’ve done, known to psychologists as a lack of insight.  

Other
perceptual defects include a lack of motion vision (e.g. objects in motion
disappear) or the inability to see more than one object at time (so one could
see only a wine bottle or a corkscrew
before them, unless the corkscrew is put into the wine bottle).

I
describe these rare deficits in vision to illustrate the point that our sensory
systems are incredibly adept at analyzing billions of pieces of information to
be presented instantaneously to our conscious awareness. To do this, the brain
must make millions of tiny ‘decisions’ every second to make sense of our
environment because it cannot simply give an objective view of the world like a
video camera can objectively record a scene—this would leave far too much
information for our consciousness to sort out. The brain’s perceptual adeptness
therefore comes at a cost: It can be tricked quite easily, and damage to the intricately
designed system can have strange and dramatic consequences.

I
have focused on vision for many reasons, including its essential role as a
sensory modality and its extensive scientific characterization. Other examples
of the brain’s perceptual overreach exist, however, like the phenomenon of
phantom limb pain. In these cases, people with an amputated body part report
feelings of ‘phantom’ pain in the lost limb, which should be impossible with no
sensory input from that region.

These
examples are not meant to question our ability to trust our senses, though they
should be questioned readily, but rather to celebrate the extremely complex
process of sensation. Our sensory systems have developed in a way that was most
suited to keep us alive and reproducing, but evolution is a delicate tug-of-war
of trade-offs.

When
it comes to vision and our other senses, we’ve traded an objective view of the
world for high-speed systems that promote our survival. The trade-off is that
our experience is defined by imperfect sensations over which we have no
control. But that doesn’t worry me too much—in the end, we’re
only humans: easily fooled, rather limited, and far from perfect.

Warren Szewczyk PO ’15 is a neuroscience major who also co-hosts the radio program Reality Check, which explores the intersection of science and the spirit. In his spare time, he is an avid writer of spoken-word poetry. 

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