Mind Hacks™: Tips & Tools for Using Your Brain (14 page)

Aftereffect illusions are caused by how cells represent motion in the brain.

Watch the video of a waterfall (
http://www.biols.susx.ac.uk/home/George_Mather/Motion/MAE.HTML
; QuickTime) for a minute or so, staring at the same position, then hit pause.
You’ll have the illusion of the water flowing upward. It works best with a real waterfall,
if you can find one, although pausing at the end is harder, so look at something that
isn’t moving instead, like the cliff next to the waterfall.

The effect doesn’t work for just continuous downward motion. Any continuous motion
will create an opposite aftereffect; that includes spiral motion, such as in the Flash
demo at
http://www.at-bristol.org.uk/Optical/AfterEffects_main.htm
.

The effect works only if just part of your visual field is moving (like the world seen
through the window of a train). It doesn’t occur if everything is moving, which is why,
along with the fact that your motion is rarely continuous in a car, you don’t suffer an
aftereffect after driving.

Part of what makes this effect so weird is the experience of motion without any
experience of things actually changing location. Not only does this feel pretty funny, but
it suggests that motion and location are computed differently within the architecture of
the brain.

Brain imaging confirms this. In some areas of the visual cortex, cells respond to
movement, with different cells responding to different types of movement. In other areas
of the visual cortex, cells respond to the location of
objects in different parts of the visual field. Because the modules
responsible for the computation of motion and the computation of location are separate, it
is possible to experience motion without anything actually moving.

The other way is to be able to perceive static images but be unable to experience
motion, and this happens to some stroke victims whose motion module is damaged. Their life
is experienced as a series of strobe-like scenes, even though — theoretically — their visual
system is receiving all the information it would need to compute motion (that is, location
and time).

You don’t need brain imaging to confirm that this effect takes place at the cortex,
integrating all kinds of data, rather than being localized at each eye. Look at the movie
image of the waterfall again but with one eye closed. Swap eyes when you pause the
video — you’ll still get the effect even with the eye that was never exposed to motion. That
shows that the effect is due to some kind of central processing and is not happening at
the retina.

To understand why you get aftereffects, you need to know a little about how
information is represented in the brain. Different brain cells in the motion-sensitive
parts of the visual system respond, or “fire,” to different kinds of motion. Some fire
most for quick sideways motion, some most to slow motion heading down to the bottom left
at an angle of 27 degrees, and so on for different angles and speeds. Each cell is set to
respond most to a different type of motion, with similar motions provoking almost as much
response, and they won’t respond at all to motions with completely different angles and
speeds.

The kind of motion we perceive depends on the pattern of activation across the whole
range of motion-sensitive cells. Relative activation of the cells compared to one another
matters, not just how much each one individually is activated. But if some cells fire
continuously, their level of response drops (a process called adaptation). So as you watch
the waterfall, the cells coding for that particular motion adapt and stop firing so
much.

Pausing the waterfall means normal service is resumed but not for the adapted cells.
Relatively, they’re responding much less than the cells looking for motion in the opposite
sense, which haven’t been firing. Usually these two groups of cells should balance each
other out, but now the cells for the opposite direction are firing more. Despite a
stationary input, overall your brain interprets the response pattern as movement occurring
in the opposite direction.

Originally some people thought that adaptation in the motion aftereffect may have been
caused by simple fatigue of the motion-sensitive cells. We know now that this isn’t the
case. Instead, the mechanism is far more interesting and far cleverer. To demonstrate,
simply try the original waterfall effect, but before watching the static pattern, close
your eyes for 20 seconds. Now if the effect were due to fatigue and the effect itself
lasted for 10 seconds, a wait of 20 seconds should remove the effect completely. But
instead, you get an aftereffect nearly as long as you would have if you hadn’t waited for
20 seconds with your eyes closed. The motion-sensitive neurons should have had time to
recover — why are they still adapted?

They are still adapted because your baseline for motion perception hasn’t been reset
(because you’ve had your eyes closed). Adaptation worked as a kind of gain control,
adjusting the sensitivity of your motion perception to the new expected level of input
provided by the constant motion of the waterfall.

Aftereffects are common illusions; they don’t occur just for motion. The relative
activation and habituation of neurons are general features of the brain. The reason
aftereffects are built into neural processing is to adjust our sensations to cancel out
continuous — and therefore uninformative — information. It operates to make us sensitive to
changes around the adapted-to baseline, rather than being overwhelmed by one dominant
level of input. Think about how your eyes adjust to the dark for a good example of useful
adaptation that can result in an unpleasant aftereffect. Adaptation is discussed further
in
Get Adjusted
.

We get used to things because our brain finds consistency boring and adjusts
to filter it out.

Both relative representation and the motion aftereffect are described in
See Movement When All Is Still
. Simply put, how much “movement up” we
perceive depends on the activation of up-sensitive neurons compared against the activation
of down-sensitive neurons, not just the absolute level of activity.

Aftereffects are the easiest way to see adaptation occurring. You can have
aftereffects with most things — sounds, touch pressure, brightness, tilt, and motion are
just some. Some, like the motion aftereffect
[
See Movement When All Is Still
]
, are due to adaptation
processes that happen in the cortex. But others happen at the point of sensation. The
adaptation of our visual system to different light levels happens directly in the eyes,
not in the cortex.

To see this, try adapting to a darkened room with both eyes and then walking into a
bright room with only one eye open. If you then return to the darkened room, you will be
able to see nothing with one eye (it has quickly adapted to a high level of light), yet
plenty with the eye you kept closed in the light room (this eye is still operating at the
dark-adapted baseline). The effect is very strong as you switch between having alternate
eyes open and the whole lighting and tone of the room you’re looking at changes
instantly.

Adaptation operates for a perceptual purpose, rather than being a reflection of neural
fatigue or being a side effect of some kind of long-term memory phenomenon. It seems to be
that sensory systems contain an intrinsic and ongoing mechanism for correcting drift in
the performance of components of the system. Constant levels of input are an indication
that either some part of the neural machinery has gone wrong and is over-responding, or at
the least the input isn’t as relevant as other stuff and should be canceled out of your
sensory processing to allow you to perceive variations around the new baseline.

This relates to the idea of
channel decorrelation
¹
— that sensory channels, as far as possible, should be providing independent
evidence, not correlated evidence about the world. If the input is correlated, then it
isn’t adding any extra information, and large, constant, moving stimuli create a load of
correlation, across visual space and across time, among the neurons responsible for
responding to motion.

Not all cells adapt to all stimuli. Most subcortical sensory neurons don’t adapt.
²
Some kinds of stimuli aren’t worth learning to ignore — such as potentially
dangerous looming stimuli
[
Explore Your Defense Hardware
]
— and so aren’t adapted to.

Adaptation lets us ignore the stuff that’s constant, so we can concentrate on
things that are either new or changing. This isn’t just useful, it is essential for the
constant ongoing calibration we do of our senses. Adaptation isn’t so much a reduction in
response as a recalibration of our responses to account for the recent history of our
sensory neurons. Neurons can vary the size of their response over only a limited range.
Momentarily changing the level that the baseline of this range represents allows the
neurons to better represent the current inputs.

You can see the changing baseline easily in the adaptation of our eyes to different
levels of brightness. Perhaps more surprising is the adaptation to constant motion, such
as you get on a boat. Continuous rocking from side to side might cause seasickness on the
first day aboard, but soon adaptation removes it. Upon returning to land, many suffer a
syndrome called “mal de debarquement” in which everything seems to be rocking (no doubt in
the opposite direction, not that you could tell!).

The “deafening silence” which results from the disappearance of a constant sound is
due to auditory adaptation. Our hearing has adapted to a loud baseline so that when the
sound disappears we hear a silence more profound (neurally) than we can normally hear in
continuously quiet conditions.

Adaptation allows us to ignore things that are constant or predictable. I’m guessing
that this is why mobile phone conversations in public places are so distracting. Normal
conversations have a near-constant volume and a timing and rhythm that allow us to not be
surprised when the conversation switches between the two speakers. With a mobile phone
conversation, we don’t hear any of the clues that would allow our brains to subconsciously
predict when the other person is going to speak. Consequence: large and unpredictable
variations in volume. Just the sort of stimuli that it’s hard to adapt to and hence hard
to filter out.