Space Perception is Hard
Foundations Of Mind
•We perceive a stable, continuous, 3D spatial layout
•Perception seems immediate, effortless & nearly error-free
Perceptual Knowledge:
•The mechanisms of perception are complex and puzzling
Lecture #2
Space
Objects
Slide# 1
All senses operate on contact
(e.g. surface touches skin, molecules interact with tastebuds)
But, Perception brings us knowledge of things at a distance
How?
Slide# 2
Vision: first steps
space perception in other modalities:
Hussain ibn Abdullah ibn Hassan ibn Ali ibn Sina
Latinized: “Avicenna” (980-1037 CE) (av′i sen′ə)
(1) audition
overturned the common idea that
somehow the eye emitted rays that
bounced off an object being viewed
(note: modern invented image, 1976)
(2) touch
Slide# 3
Slide# 4
Descartes (La Dioptrique, 1637)
(Traité de l’Homme, 1664)
Some problems of visual space perception:
(1) depth
René Descartes (1596-1650)
(2) size
Slide# 5
Slide# 6
Visual space perception: depth cues
(3) position and motion
Object motion
Interposition
Observer motion
Linear perspective
Slide# 7
Slide# 8
2 Depth Cues
Discussed by Descartes and Berkeley
Question
What makes us able to interpret these cues so as to gain
knowledge of the surrounding spatial layout?
Accommodation
NEAR in focus
Two different answers
1. We are endowed with special-purpose cognitive
systems for interpreting these cues in the right ways:
core knowledge of space
nativism
FAR in focus
Convergence
Slide# 9
2. We are endowed with a general-purpose capacity to
learn about our surroundings, and this learning leads us to
construct appropriate interpretations of spatial cues.
no core knowledge of space
empiricism
Slide# 10
Two Theories of Space Perception
Nativism (Descartes):
The human mind and brain are built to infer space automatically
.
Perceptual processes are like reasoning processes:
geometric inferences
Perception of depth involves internal innate unconscious
computations
Empiricism (Berkeley):
The human mind and brain are built to sense only
impinging stimuli (light, muscular effort)
.
We learn through experience to interpret these stimuli in
terms of depth, by association with touch: no reasoning
involved.
Perception of depth involves rote learning, associative pairing
of convergence & accommodation with the sense of touchSlide# 11
Two Theories of Space Perception
The prejudice shared by Rationalism and
Empiricism is that man does not know things
directly but grasps only their impressions (sense
data).
Both Rationalism and Empiricism needed a
new method; the former adopted mathematical
deduction, the latter scientific induction.
Slide# 12
A Nativist Theory of Space Perception
Descartes Theory of Vision
René Descartes (The Optics, 1637)
philosopher, mathematician, scientist
Analogy: the blind man
demo
Slide# 13
Descartes’ theory is:
• mechanistic (light, refraction, nerves, signals to brain)
• computational (geometric inferences)
• rationalist and nativist (“natural geometry”)
Slide# 14
Berkeley’s critique and alternative theory
(1709)
“I appeal to any one’s experience, whether, upon sight of an
object, he compute its distance by the bigness of the angle made
by the meeting of the two optic axes? … In vain shall all the
mathematicians in the world tell me, that I perceive certain lines
and angles which introduce into my mind the various ideas of
distance; so long as I myself am conscious of no such thing.”
Method: introspection
Primitives: sensations (e.g., muscular effort)
Interpretive process: learning to associate vision & touch
“Less effort”
Berkeley’s theory is:
• empiricist
• associationist
Berkeley’s “effort”
Accommodation
NEAR in focus:
FAR in focus:
“More effort”
more effortful
less effortful
Convergence
Slide# 15
Slide# 16
Helmholtz on the origins of space perception:
Hermann von Helmholtz
(1850s) on the origins of
space perception:
Helmholtz’s next-best strategy: Study the modifiability of space
perception in adults.
--alter the spatial relationship between the perceiver and the
layout so that visual space perception is systematically in error.
--observe whether the perceiver adapts to the altered relationship
& corrects the error.
Q: Do we perceive depth by innately structured computational
mechanisms or by associative learning?
A: To answer, must study space perception in infants and
children.
But we cannot do this!
Next best thing: screw around with adult perception
Slide# 17
Helmholtz’s studies of adaptation to visual rearrangements:
(1) Natural observation: walking in the forest with new glasses
(2) Experimentation: prism adaptation
If adaptation occurs:
adults are able to learn to perceive space correctly.
most reasonable to suppose babies learn too.
If no adaptation occurs:
the mechanisms of space perception are fixed in adults.
most reasonable to suppose they are fixed in infants.
Slide# 18
Perceptual change or conscious decision? aftereffects
Pointing with prism
Pointing without prism
Pointing with prism
Before adaptation
Before adaptation
After adaptation
Perceptual change or conscious decision?
Slide# 19
After adaptation
Before adaptation
After adaptation
Conclusions:
space perception is modifiable in adults
on grounds of plausibility, probably learned in children.
Slide# 20
More on prism adaptation: Harris (1960s)
Prism adaptation: conclusions
(1) Where does the perceptual change occur: in vision or in
touch? (if vision learned by association with touch, as Berkeley
claims, then vision should change in adaptation experiments).
(1) mechanisms of space perception are modifiable, but touch is
more modifiable than vision.
Adaptive Shift In Degrees
10
(2) modifications depend on active, self-produced motion.
8
6
4
2
0
-2
Visual
Targets
Auditory “Straight
Targets Ahead”
(2) how does the perceptual change occur? By associative pairing
or active motion? (if Berkeley’s associative theory is correct, then
associative pairing should suffice)
Slide# 21
Helmholtz said:
We can’t study babies directly
Tomorrow
Spelke et al. Say: Oh yes we can
Slide# 23
If studies of prism adaptation in adults sheds light on visual
space perception in infants, then initially Helmholtz’s studies
show some support for Berkeley, but the further studies of
Harris show that the phenomenon more accurately offers some
support for Descartes’ nativist theory and no support for
Berkeley
--touch doesn’t teach vision
--learning is not by passive association
Slide# 22