YOUR SENSE OF THE WORLD AND MOVEMENT WITHIN ITyour brain. Another branch, instead of going to the thalamus, routes
direct to a sub-cortical structure known as the superior colliculus – this
is an evolutionarily old visual pathway that allows you to respond rapidly
without engaging the thinking parts of the brain.
The bulk of visual processing goes on in the visual cortex where there
is a hierarchical division of labour – early regions process the most
basic aspects of the incoming signal, such as line orientation, while
later regions focus on characteristics like colour and motion. A famous
case study, first described in the early 1980s, involved a German woman,
known as L.M., who suffered damage to the part of the visual cortex
specifically devoted to processing motion, following a stroke. Though
she could still recognize things, she could no longer see them move.
Water poured from a kettle appeared frozen like an iceberg and crossing
roads became a serious hazard.
After the visual cortex, the visual pathway branches in two. One branch
travels over to the temporal cortex, near the ears, is involved in object
recognition and similar processes, and is known as the “what pathway”.
The other branch takes the high road to the parietal cortex, near the
crown of the head. This branch is involved in processing spatial infor-
mation and is known as the “where pathway”. This dual-pathway account
How to visit the toilet in the dark
Learning how vision works can have its advantages. Take the process of
light adaptation. This is the way that our vision gradually adjusts to the
dark. But does this adaptation occur in the eyes, or later on in the visual
pathway, in the brain? You can test this by sitting in a dark room and
adjusting both your eyes to the darkness. Next, cover one of your eyes
with your hand, and then turn the lights back on full. Wait a minute or
so, so that adaptation occurs through your open eye. Now you need to
turn the lights off again and switch your hands around, so that you’re
now covering the eye that was exposed to the light. If adaptation
occurs centrally in the brain, you should be blinded by the darkness.
After all, you spent a minute or so with light entering your brain via
your one open eye. However, hopefully you’ll find that you can see just
fine in the dark with the eye that you kept covered. That’s because light
adaptation doesn’t occur in the brain, it occurs locally in each eye. You
can use this technique when visiting the toilet in the night. When you
get to the toilet and turn the light on, keep one eye closed. When it’s
time to return back to bed, simply close your light-adjusted eye and
navigate your way with the eye that you kept dark adjusted. Hey presto- no more tripping up in the night!