Mind Hacks™: Tips & Tools for Using Your Brain (19 page)

Following seemingly identical objects around with your eyes isn’t an easy job.
Concentrating, it’s possible, and the brain can even track objects when they momentarily
pass behind things and disappear, but only in certain circumstances.

Watch the sequence of
multiple object tracking
(MOT)
demonstrations at Dr. Zenon Pylyshyn’s Visual Attention Lab (
http://ruccs.rutgers.edu/finstlab/demos.htm
).
¹
Multiple object tracking is a class of experiment based around trying to
keep track of many objects (small circles in the first demonstration) simultaneously, as
they jiggle about. It tests the limits of your attention and specialized tracking
skills.

Start with the General MOT experiment (
http://ruccs.rutgers.edu/finstlab/motMovies/mot.mov
; QuickTime;
Figure 3-4
). In
this demo, you’re required to track four of the eight circles as they move around; you’re
told which four as they flash briefly at the beginning of the movie.

The point of this demonstration is simply to point out that you can indeed attend to
more than one object at a time. It’s not a trivial matter to follow all four circles
around simultaneously, but you’ll find that you can gaze at the center of the screen and
track your four chosen circles fairly easily, without even having to stare directly at
each in turn.

In the Occluder task (
http://ruccs.rutgers.edu/finstlab/motMovies/mot-occocclusion.mov
; QuickTime;
Figure 3-5
), the
circles have been replaced by identical white squares, and now they disappear occasionally
behind bars placed over the field of movement.

Aside from the newly introduced bars, the experiment is the same as the General MOT
experiment; four of the eight squares flash at the beginning, your task being to track
those four for the duration of the movie. This certainly isn’t as easy as the general MOT
experiment and may take a couple of attempts, but you should be able to complete the
task.

The Virtual Occluder MOT movie (
http://ruccs.rutgers.edu/finstlab/motMovies/mot-occ-virtocc.mov
; QuickTime;
Figure 3-6
), on
the other hand, requires serious concentration. Now instead of sliding behind visible
bars, the white squares momentarily vanish. The occluders — the bars that occlude, or hide,
the squares behind them — are now the same color as the background and have become
invisible. Tracking the four white squares that flash at the beginning for the duration of
the whole experiment is now a real challenge. You have to give the task your full
concentration, and any distraction will cause you to confuse one of your chosen white
squares with one of the other, apparently identical, distracter objects. It doesn’t help
that they all keep disappearing and reappearing — the two smaller white rectangles in
Figure 3-6
are just reappearing from behind
one of the invisible virtual occluders.

Figure 3-4. You have to track four of these circles as they move around the screen

While it is still possible to do a reasonable job of tracking the four targets, we’ve
now reached the limits of your attention. But there’s one more movie to go: the
Implode/Explode task (
http://ruccs.rutgers.edu/finstlab/motMovies/mot-occ-implosion.mov
; QuickTime). The single difference between this experiment and the previous
(Virtual Occluder) is that here the squares shrink down to a dot when they encounter one
of the invisible black bars instead of slipping behind it. On the other side of the bar,
they grow back from a dot into a square instead of appearing from underneath the bar’s
edge.

Figure 3-5. Tracking the moving shapes becomes harder when they periodically move behind the
black bars (outlined in white)

Give it a whirl. It’s not just difficult this time, it’s more or less
impossible. You can’t complete this multiple object tracking task at all.

The ability to perform multiple object tracking (MOT) is the skill we get by giving
attention to objects: without attention, you can’t keep track of which object is which
(let alone more than one at a time). Attention is both a mechanism the brain uses for
giving some objects more processing time and what you feel is an extra layer on your
visual perception. Despite all eight circles in the General MOT experiment having the same
visual appearance, you perceive four of them as somehow different, just because they
flashed at the beginning. That’s the feeling of attention feeding into your visual
perception.

If you had followed around only a single circle in that first movie, you would have
very easily been able to distinguish it from all of the others. But you wouldn’t have been
able to distinguish the other seven from one another. That’s what attention does.

Figure 3-6. Tracking gets harder still: the slimmer white shapes are squares half-hidden behind
invisible obstructing bars

Although in this case you’ve applied attention voluntarily to certain objects,
it’s actually a semiautomatic process. Attention can be captured
[
Grab Attention
]
by sudden movements, for example. And
it doesn’t always feel as obvious as “I am now able to distinguish these objects” — you
momentarily give your attention to every single car that passes you when you’re waiting to
cross the road, but not in the same way as you give attention to these multiple object
tracking demonstrations — it’s not a concentrated kind of attention, just an awareness that
you’ve seen it.

Attention is something that can be allocated piecemeal. You can choose to notice
certain colors, for example, or look out for particular movements. Conversely, you can
choose to suppress attention for those features (that’s what negative priming
[
The Brain Punishes Features that Cry Wolf
]
is
all about). In this case, you’re choosing to allocate attention to collections of features
that appear to move round together: blue-ness, circle-ness, move-at-a-certain-speed-ness.
We tend to refer to bundles of features as
objects
. (It’s possible
that attention has a role to play in bundling the features together.)

We deal with objects in our attention in a special way, setting up
object files
that can persist through time. The brain automatically
says, “This is an object I need to remember” and sets up a
file
(a
kind of invisible sticky tag on the object) to do it. Imagine if you used your finger to
point at something as it moved about, like running your finger along a row in a table of
figures. It helps you remember which row, out of all the similar-looking rows, was the one
you were following. Object files are like finger pointing, but using attention instead.
That’s how you know that the circle at the end of the task is the same as the one you
identified at the beginning: your brain set up a file about it — an index to the bundle of
features that are appearing in your visual perception — and kept that for as long as your
attention was on that object.

Given this, the brain must have some automatic processes to reclaim attention as soon
as it’s no longer needed. One way of doing this would be to close the object file as soon
as the object disappears. That would often be a hindrance however — imagine if you, as an
early human living on the African savannah, lost track of a predator every time it went
behind a bush.

That’s what the Occluder demonstration shows. Object files are kept open when it looks
as if the object being tracked is slipping behind some other object in the visual field,
in this case black bars. The Virtual Occluder demo is hard because you’re maxing out your
attentional resources tracking four objects (four or five objects is about the maximum we
have room for) and relying entirely on your automatic processes to imagine where the
objects are even when they’re hidden and reattach the object file when the squares
reappear from behind the invisible bars. But it’s still possible because this is the kind
of situation visual perception needs to deal with: an animal darting around in a forest
will keep disappearing behind branches or greenery, and the dense foliage in the
foreground has the same pattern as everything else and is basically invisible against the
background.

Here’s the trick your object files system is using to know not to shut down the file:
As the squares disappear behind the bars — that is, as they are occluded — they disappear line
by line. They vanish from one edge. That’s the cue your brain uses to know occlusion is
occurring.

The final demo, the Imploder/Exploder, is a bit of a cheat. It’s supposed to be
impossible. Tracking that many objects is deliberately hard so it fills your voluntary
attentional capacity and forces you to rely on your automatic brain functions. That’s a
way of getting the automatic functions to reveal themselves.

In this case, the demonstration disrupts the occlusion cue. Shrinking down to a dot,
the squares instead present the cue that they’re moving away into
the distance. Thinking the object has disappeared from the vicinity and is no
longer important, your brain immediately swings in and closes the object file, reclaiming
the attention for use elsewhere. When the square reappears from a dot a moment later, it’s
as if it’s coming back from a long way away. But the object file has already been closed,
it’s as if it’s a different white square entirely.

It’s cues as small as how objects disappear behind other objects that your brain uses,
even in cartoons such as these movies (which don’t have shadows, or perspective, or even
3D depth — anything that we’d usually say makes a scene feel real or physical), to figure
out what to track and what to give attention to.