Transcript Cognition and Perception as Interactive Activation

Cognition and Perception as
Interactive Activation
Jay McClelland
Symsys 100
April 16, 2009
What does it mean ‘to think’?
• Is it…
– To follow a set sequence of rules that
algorithmically produces an answer?
• Or is it…
– To explore alternative possibilities until you
find something that works?
‘Find something that works’
• The two string problem
– You are in an office. There is a desk with a
printer, some paper, stapler, binder clips, pens
and pencils.
– Two strings are hanging from a ceiling, about
12 feet apart in a room with a workbench.
– You can’t reach the second one while holding
the first.
– What can you do to bring them both together?
‘Find something that works (2)’
• You know how much gold weighs per cubic centimeter,
and you want to test whether the king’s golden crown is
pure gold. But you don’t know how many cc’s of gold
are in the crown. How can you find out?
• Find a word that you can combine with each of the next
three words to make a compound word:
Pine, crab, tree
• One more to work on later
S. + H. of R. = U. S. C.
Perception as ‘Finding Something
that Works’
• For the first figure,
one sees it a nothing
until the ‘solution’
emerges.
• For the second, there
are two alternative
solutions, each
involving a reinterpretation of every
part of the larger
whole.
Finding Perceptual Solutions
• It appears that our brains can search for
alternative solutions until one pops out.
• How are such solutions found?
– One answer is that the process occurs
through a gradual, noisy, interactive activation
process.
The interactive Activation Model: a Gradual Mutual
Constraint Satisfaction Process
•
Units represent hypotheses about
the visual input at several levels
and positions.
– Features
– Letters
– Words
•
Connections code contingent
relations:
– Excitatory connections for
consistent relations
– Inhibitory connections for
inconsistent relations
– Lateral inhibition for competition
among mutually inconsistent
possibilities within levels.
•
Connections run in both directions
– So that the network tends to evolve
toward a state of activation in
which everything is consistent.
Interactive Activation Simultaneously
Identifies Words and Letters
•
Stimulus input comes first to letter
level, but as it builds up, it starts to
influence the word level.
•
Letter input from all four positions
makes work the most active word
unit (there is no word worr).
•
Although the bottom up input to
the letter level supports K and R
equally in the fourth letter position,
feedback from the word level
supports K, causing it to become
more active, and lateral inhibition
then suppresses activation of R.
Goodness and Constraint Satisfaction
•
Consider a network with symmetric connections,
i.e. for all pairs of units i, j:
wij = wji
•
Provide external input ei to some of the units.
•
Define the Goodness of a state of the network as:
G = Si,jwijaiaj + eiai
•
Then as the network settles it tends toward states
of higher goodness
•
Examples:
–
–
–
Three-unit network
Necker Cube Network
Interactive activation network
•
A little randomness allows networks to ‘break
symmetry’ and jump out of local Goodness
maxima.
•
If we gradually reduce the randomness, we can
guarantee finding the best solution
Interactivity in the Brain
• Bidirectional Connectivity
• Interactions between MT
and ‘lower’ visual areas
• Subjective Contours in V1
• Distributed Constraint
Satisfaction in Binocular
Rivalry
Effect of Cooling MT
on neural activation in lower visual areas
•
Investigated effects of cooling
MT on neuronal responses in
V1, V2, and V3 to a bar on a
background grid of lower
contrast.
•
MT cooling typically produces
a reversible reduction in firing
rate to V1/V2/V3 cells’
preferred stimulus (figure).
•
Top down effect is greatest for
stimuli of low contrast. If the
stimulus is easy to see, topdown influence from MT has
little effect.
Response decrease due to
cooling in MT
Lee & Nguyen (PNAS, 2001,
98, 1907-1911)
• They asked the question:
Do V1 neurons participate in
the formation of a
representation of the illusory
contour seen in the upper panel
(but not in the lower panel)?
• They recorded from neurons in
V1 tuned to the illusory line
segment, and varied the
position of the illusory segment
with respect to the most
responsive position of the
neuron.
Response to the illusory contour is found at
precisely the expected location.
Temporal Response to Real and Illusory
Contours
Neuron’s receptive field falls right
over the middle of the real or illusory
line defining the bottom edge of the square
• The patterns seen in the
physiology are
comparable to those
seen in the interactive
activation model in that
the effect of direct input
is manifest first, followed
somewhat later by
contextual influences,
presumably mediated in
the physiology by
neurons sensitive to the
overall configuration of
display elements.
direct
context
Distributed Alternation
of Brain Activity in
Binocular Rivalry
Discussion: Conscious Vs. Unconscious
Cognition
• We’ve considered perception predominantly but we can
ask: How much of thought is like perception?
• We’ve seen that our unconscious inferences are
sometimes quite rational, though many of our conscious
inferences are not.
– What is the difference, and why are conscious
inferences so errorful?
• What have we learned, overall about whether human
cognition is rational?
Section Business
• Please attend the section you've been assigned to -your attendance will be taken there.
• Sunday, Apr 19th is the add-deadline on Axess.
• Before the 19th, ensure that you are enrolled in the
section corresponding to your section leader's name.
• Sections times and places are as follows
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Thurs 3:45-4:35, 50-51B - Anubha
Thurs 5:15-6:05, 160-318 - Rob
Thurs 6:15-7:05, 160-325 - Jason
Fri 2:15-3:05, 90-92Q - Anubha