Brain Rules (Updated and Expanded)

First sensing and routing, then perceiving
No matter which model is eventually declared the winner, the
underlying processes are the same, and they operate in the same order:
sensing, routing, and perceiving. Sensing involves capturing the energies
from our environment that are pushing themselves into our orifices and
rubbing against our skin. The brain converts this external information into a
brain-friendly electrical language. Once the sensory information is encoded,
it is routed to appropriate regions of the brain for further processing. As we
discussed in the Wiring chapter, the signals for vision, hearing, touch, taste,
and smell all have separate, specialized places where this processing occurs.
A region called the thalamus—a well-connected, egg-shaped structure in
the middle of your “second brain”—supervises most of this shuttling.
The information, dissected into sensory-size pieces and flung widely
across the brain, next needs to be reassembled. Specialized areas throughout
the brain take over from the thalamus to make this happen. They are not
exactly sensory regions, and they are not exactly motor regions, but they are
bridges between them. Hence, they are called association cortices.
(“Cortices” is the plural of “cortex.”) As sensory signals ascend through
higher and higher orders of neural processing, the association cortices kick
in.

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The association cortices employ two types of processors: bottom up and
top down. Let’s walk through what they might be doing in your brain as you
read the next sentence—a randomly chosen quote attributed to author W.
Somerset Maugham.
The rank and file make a report
“There are only three rules for writing a novel,” Maugham once said.
“Unfortunately, nobody knows what they are.”
After your eyes look at that sentence and your thalamus has routed each
aspect of the sentence to the appropriate brain regions, “bottom-up”
processors go to work. The visual system is a classic bottom-up processor.
It has feature detectors that greet the sentence’s visual stimuli. These
detectors, working like auditors in an accounting firm, inspect every
structural element in each letter of every word in Maugham’s quote. They
file a report, a visual conception of letters and words. An upside-down arch
becomes the letter 
U
. Two straight lines at right angles become the letter 
T
.
Combinations of straight lines and curves become the word “three.” Written
information has a lot of visual features in it, and this report takes a great
deal of effort and time for the brain to organize. It is one of the reasons that
reading is a relatively slow way to put information into the brain.
Higher-ups interpret the report
Next comes “top-down” processing. This can be likened to a board of
directors reading the auditor’s report and then reacting to it. Many
comments are made. Sections are analyzed in light of preexisting
knowledge. The board in your brain has heard of the word “three” before,
for example, and it has been familiar with the concept of rules since you
were a toddler. Some board members have even heard of W. Somerset
Maugham before, and they recall to your consciousness a movie called 
Of
Human Bondage
, which you saw in a film history course
.
Information is
added to the data stream or subtracted from the data stream. In plenty of
cases, as we saw in the McGurk effect, the brain resorts to making
something up.
At this point, the brain generously lets you in on the fact that you are
perceiving something.

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Given that people have unique previous experiences, they bring
different interpretations to their top-down analyses. Thus, two people can
see the same input and come away with vastly different perceptions. It is a
sobering thought. There is no one accurate way to perceive the world.