Title: Chapter 1: What Is Vision Science The Thinking Eye, The Seeing Brain by James T. Enns
1Chapter 1 What Is Vision Science?The
Thinking Eye, The Seeing Brain by James T. Enns
2What vision is not
- Folk Psychology and Commonsense intuitions are
wrong - Four Myths
- The myth that seeing provides a faithful record
of the world in front of us eye as a camera - The myth that seeing occurs automatically and
without any thoughtful activity on our part
passivity of sense - The myth that our eyes are responsible for our
sight windows of the soul - The myth that we can think without using our
senses immutability of soul
3Slogans
eye as a camera
passivity of sense
4Figure 1.1. The myth of vision as a faithful
record
Concentric circles or continuous
spiral? The pattern of light is of
concentric circles Human vision sees a
continuous spiral
5Figure 1.2. Vision as a useful construction of
the mind
Which two tabletops have identical
dimensions on the page? Tabletops A and B
are identical in size on the page except for
rotation. Human vision sees tabletops B
and C as similar because of a three-dimensional
interpretation of the world that is built into
our brains and through which we see.
6The Myth of seeing as a faithful record
- Tables (a) and (b) are invariant in 3D spatial
Manifold - Fraser Spiral is really a series of concentric
circles that we see as a continuous spiral. Why
is that?
7The Myth of vision as a passive process
- The Grand illusion of complete perception
- (1) Vision is not rich in detail
- the size of a thumbnail at arms length gets
processed - (2) Attention is limited the law of ONEs
- vision sees one object, one event, one location
- These two factors are illustrated by
- Impossible triangle
- Escher drawings
8Brains construct a well-behaved 3-D world so we
cannot experience a world that is not. Here we
see an ordinary triangle and building with normal
corners and angles instead of the shocking
reality. Why? (p. 9)
Go to http//www.michaelbach.de/ot/cog_imposs1/in
dex.html
9The Myth of the Seeing Eye
False perception only occurs when the images
recorded by the eyes are sent to the brain.
- Software becomes hardware
- the eye of frogs and rabbits perform a
sophisticated analysis of visual motion, an
analysis that occurs in only the higher levels of
the brain in monkeys and humans.
10The Myth of Imageless thought
- sensory imagination is active
- Answer the following
- When you approach the front door or the main
entrance, how many windows are visible - helicopter approach, floating air-car approach
- eye movements mimic the real situation
- Where are your hands?
- Experiments show that physically similar images
increase response time for similar questions. - Do some dogs have tails? and Do some cats have
tails? trigger faster response than a pair with
dissimilar physical characteristics. Why?
11The Myth of objective vision
- Necker cube
- Brains see random variation between limited
alternatives (see next slide) - Geometrically, many views are possible
- humans only see two possibilities
- selection of view is random
12Figure 1.5. Subjective perceptions are not
necessarily arbitrary perceptions
Brains see two instead of all of these
interpretations? Why not? Humans bring
shared assumptions to the vision project, (1)
that objects are generally convex, (2) that
straight lines in a picture represent straight
edges in an object, and (3) that three-edge
junctions are generally right-angled corners.
13Figure 1.4. Subjective perceptions are not
necessarily arbitrary perceptions
A perceptually ambiguous wire
cube How many different interpretations
can you see?
Go to
http//mindbluff.com/necker.htm
14vision myths
- myth vision as passive
- what is vision for?
- survival not to reproduce
- intersubjective, objective, arbitrary
- tables, Necker cubes
- faithful reproduction less important than fast
response
15Measurement in Vision Science
- light measured in nanometers (nm)
- 1nm1billionth meter
- eyes sense from 360nm to 780nm on the infinite
scale of wavelength nothing. - light is whatever energy falls in that range -
light by definition, is anthromorphized. - 1st feature of light
- wavelength480blue, 540green, 565yellow,590red
nanometer (nan'?-me't?r)
16The Spectrum of Visible Light The visible part of
the spectrum may be further subdivided according
to color, with red at the long wavelength end and
violet at the short wavelength end, as
illustrated (schematically) in the following
figure.
How Roy G. Bv Lost a Vowel The sequence of colors
red, orange, yellow, green, blue, and violet may
be remembered by memorizing the name of that fine
fellow "ROY G. BV". This was originally "ROY G.
BIV", because it used to be common to call the
region between blue and violet "indigo". In
modern usage, indigo is not usually distinguished
as a separate color in the visible spectrum thus
Roy no longer has any vowels in his last name.
17The notation "eV" stands for electron-volts, a
common unit of energy measure in atomic physics.
Each color has its energy. A graphical
representation of the electromagnetic spectrum is
shown in the figure below.
Thus we see that visible light and gamma rays
and microwaves are really the same things. They
are all electromagnetic radiation they just
differ in their wavelengths.
18measurement
- 2nd feature of light
- intensity or amount, photon catch the number of
photons that actually make contact with light
sensitive receptors in the eye - candelaintensity of light emitted from a candle
onto a square meter at one meter, 1/1000 watt
bulb with a peak wavelength of 555 nm. - Lightnessthe degree of surface reflectance (24),
- brightnessthe intensity of the source of light
19measurement
- Visions various jobs
- determine the combination of brightness and
lightness that generates the total photon catch
at the eye - is it a black paper illuminated by bright light,
or white paper under dim light? - Both pieces of paper may reflect similar amounts
of light to the eye but usually the black
reflects more so why arent we fooled into seeing
it as white? - Space
- candela is defined in terms of space
- the angle formed by rays extending from the lens
to the object equals the angles formed by rays
extending from the lens to the retina. - visual angle is measured in degrees, minutes,
seconds (see next)
20Figure 1.6 The 2-D image size (on the retina) is
a function of S. Visual sizes and distances are
measured in terms of degrees of visual angle
(hunk of an arc) for convenience
21measurement
- time
- the amount of light shining on the eye depends on
how long the eyes sits still. - a single cycle of neural activity in response to
a signal takes between 1-10 milliseconds, 1
millisecond1/1000 second. Between the retina and
the cerebral cortex, there are 4 cycles, or 40 ms
of a lapse time. the minimum time for a motor
response is 200ms, a fifth of a second. - neurons can track pulsating light only up to 60
cycles, 8 ms on and 8 ms off. Individual movie
frames change every 30 ms (33 cycles per second)
and we cant see them. - Eyeblink duration is around 100 ms.
22measurement
- cones vision under bright illumination
- Cone time constant100ms, 1,000 photons can be
presented in a brief period (say, 1000 photons
within a 1 ms period) or over a long period (such
as 200 photons in each of five 20 ms period) for
the same visual effect. The cones cannot tell the
difference. - Rod time constant400 ms, meaning the photon
catch extends over a longer time interval working
like a slow shutter speed on a camera (well,
cameras once had shutters that had a click that
digital cameras artificially reproduce even
though there is nothing to click any more.)
23measurement
- limitations
- space constants of the eye are limited by the
size of, and interaction among, cones and rods. - constant, the point at which the eye cannot
distinguish whether the number of photons were
concentrated in a small region of the retina or
were distributed over a large region - space and time constants are limited by the size
and interaction among cones an rods - the closer to the center the eye, the smaller the
constant (the eye can see more in daylight)
(27).
24behavior
- psychophysical function
- measurement of a stimulus that is too weak,
- measurement of a stimulus that is too strong,
- measurement of the threshold halfway between too
weak and too strong
25Figure 1.7. The visual threshold
How much of a stimulus is needed for
detection? When a stimulus can be seen
50 of the time, it is said to be at the
threshold of the visual system.
26Figure 1.8. Visual discrimination and
identification
It is always possible to trade response
accuracy for response speed. Tasks A and
B are therefore equally difficult increased
accuracy in Task A has only come about through a
reduction in response speed. Task C is
easier than either Task A or B it is performed
faster than Task A at the same level of accuracy,
and it is performed faster and more accurately
than Task B.
27 28Visual mapping of the idea and brain
- Close your right eye and fix your gaze on the dot
at the center of figure 2.3b. - hold the book about 6 inches from your face.
- Follow the rest of the directions.
- Perform the second experiment
- Perform the third experiment
29Analysis
- We do not see the gap in the bars or the hole in
the center donut. The bars become continuous and
the donut becomes a solid disk. - These experiments illustrate that the brain can
generate experience in a location of space even
when no sensory input takes place
30Thought experiment
- If the bar was plaid, would the filled in segment
be plaid? - what would this suggest about first person
reports that the person sees the bar to be
solid?