Transcript PDP_01_08_10_InteractiveActivation

Interactive Activation: Behavioral and
Brain Evidence and the Interactive
Activation Model
PDP Class
January 8, 2010
Overview
• Modular approaches: Marr and Fodor
• A critique of modular approaches in vision
• The word superiority effect and the interactive
activation model
• Interactivity in the brain
– Anatomy and neurophysiology
Marr’s Modular Approach
• Divide and conquer approach to vision:
– E.g., Marr suggests studying shape from shading, shape from
motion, shape from depth as separate computations.
– A key motivation is that sometimes just one type of information is
enough (crumpled newspaper example).
– Forces close attention to just how much can be done with each
source of information alone, and a careful consideration of how it
might be done.
• Similar to Fodor’s modular approach in Modularity of
Mind.
– Fodor assumes that specific input and output systems are
encapsulated (insensitive to inputs from other sources) and
reflex-like.
– The suggestion is that in order for them to be reflex-like they
must be very narrow in the range of considerations they take into
account.
– If in fact the brain were modular this would be nice, for then
function would be considerably easier to analyse.
Critique of the Modularity Approach
(Bulthoff and Yuille, 1996)
• Provides no insight into what to do when
different modules both provide inconclusive
(possibly conflicting) evidence.
• More generally, it punts on how we
successfully integrate multiple sources of
information, as we clearly do much of the
time.
• Ignores the fact that one source of information
can change the way we use information from
another source (see next slide).
Gilcrest, A. L. Perceived lightness depends on perceived
spatial arrangement. Science, 1977, 195, 185-187.
• Experiment shows that subjects assign a surface a ‘color’
based on which other surfaces they see it as co-planar with.
Thus color depends on perceived depth, violating modularity.
Findings Motivating the IA Model
• The word superiority effect
(Reicher, 1969).
– Subjects identify letters in
words better than letters in
scrambled strings or single
letters.
• The pseudoword advantage.
– The advantage over single
letters and scrambled strings
extends to pronounceable nonwords (e.g. LEAT / LOAT…)
• The contextual
enhancement effect.
– Increasing the duration of the
context or of the target letter
facilitates correct identification.
• Reicher’s experiment:
– Words are constructed in pairs
differing by one letter.
– E.g. READ vs ROAD.
– The ‘critical letter’ is the letter
that differs between members
of the pair.
– Critical letters occur in all four
positions.
– Display is followed by a mask
made of lines or letter
fragments.
– The critical letter and the
alternative then appear as
choice alternatives, with dashes
indicating the tested position.
– There are trials with scrambled
letter strings and single letters,
with the same critical letters
used.
The Contextual Enhancement Effect
The Interactive Activation Model
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Feature, letter and word
units.
Between-layer connections
were + or -; only inhibitory
connections within.
Activation follows the ‘iac’
function.
Response selected from the
letter units in the cued
location according to the
Luce choice rule:
How the Model
Works:
Words vs. Single
Letters
Word and Letter Level Activations for
Words and Pseudowords
Idea of ‘conspiracy effect’ rather than consistency with rules as
a basis of performance on ‘regular’ items.
Simulation of Contextual Enhancement
Effect
Role of Pronouncability vs. Neighbors
• Three kinds of pairs:
– Pronounceable:
SLET-SPET
– Unpronouncable/good:
SLCT-SPCT
– Unpronouncable/bad:
XLQJ-XPQJ
Autonomous vs. Interactive Approaches
Letter/phoneme
identification
visual or auditory
feature level
Can the Models be Distinguished?
• Attempts to support IA models seek to
demonstrate ‘knock-on’ effects influencing the
phoneme level inputs to the word level.
– Lexically-Triggered Compensation for Coarticulation
– Selective Adaptation
– Tuning Phoneme Boundaries
• Basic logic:
– Use context to determine identity of an ambiguous
segment
– Show that the contextually-determined segment identity
triggers a phenomenon that affects phoneme identification
(on the way to lexical access)
• See TiCS paper in readings for details
Tuning Phoneme Boundaries
• Present ambiguous (s/f) segment in a context where
lexical information determines its identity as ‘f’, while
presenting normal ‘s’ segments:
– Consider / Con(s/f)use
• Later, test for identification of the ambiguous segment,
and identification of words where both interpretations
are possible.
– (f/s)ear
• Participants identify ambiguous segment and the word
containing it as though they hear it now as an ‘f’.
• This can be explained by assuming participants use the
top-down signal to adjust the connection weights
mapping features onto the ‘f’ sound.
Interactivity in the Brain
• Bidirectional Connectivity
• Interactions between V5 (MT) and V1/V2:
Bullier
• Subjective Contours in V1:
Lee and Nguyen
Hupe, James, Payne, Lomber, Girard & Bullier (Nature,
1998, 394, 784-787)
• Investigated effects of
cooling V5 (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’ optimal
stimulus (figure)
• Top down effect is greatest
for stimuli of low contrast.
If the stimulus is easy to
see when it is not moving,
top-down influences from
MT have little effect.
• Concept of ‘inverse
effectiveness’ arises here
and in many other related
cases.
*
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