Interactive Activation: Behavioral and Brain Evidence and the Interactive Activation Model
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Interactive Activation: Behavioral and Brain Evidence and the Interactive
Activation Model
PDP ClassJanuary 10, 2011
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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
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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.
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Critique of Fodor’s appeal to ‘Reflexes’
• Reflexes are not encapsulated.
• Instead they involve at least a synapse between the sensory and motor neuron or even an interneuron (as shown).
• Additional inputs either to the interneuron or the motor neuron can modulate the response.
• The cat’s paw reflex makes clear that context can modulate reflexes.
• They are still fast and (pretty) direct but still highly context sensitive.
• Influence of subsequent context requires more complex mechanisms.
Illustration fromMcGraw Hill OnlineLecture on Motor Control.
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Critique of the Marr’s Modular Approachin Vision (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).
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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.
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Findings Motivating the IA Model
• The word superiority effect (Reicher, 1969)– Subjects identify letters in words better than single letters or letters in
scrambled strings.• The pseudoword advantage
– The advantage over single letters and scrambled strings extends to pronounceable non-words (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:– Used pairs of 4-letter words
differing by one letter READ ROAD
– The ‘critical letter’ is the letter that differs.
– Critical letters occur in all four positions.
– Same critical letters occur alone or in scrambled strings _E__ _O__ EADR EODR
W PW Scr L
Perc
ent C
orre
ct
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READREAD
_E__ O
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The Contextual Enhancement Effect
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The Interactive Activation Model• 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:
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How the Model Works:
Words vs. Single Letters
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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.
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Simulation of Contextual Enhancement Effect
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Role of Pronouncability vs. Neighbors
• Three kinds of pairs:– Pronounceable:
SLET-SPET
– Unpronouncable/good:
SLCT-SPCT
– Unpronouncable/bad:
XLQJ-XPQJ
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Autonomous vs. Interactive Approaches
visual or auditory feature level
Letter/phoneme identification
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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
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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.
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Interactivity in the Brain
• Bidirectional Connectivity• Interactions between V5 (MT) and V1/V2:
Bullier• Subjective Contours in V1:
Lee and Nguyen• MEG Evidence:
Bar et al (2006)
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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.
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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.
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Response to the illusory contour is found at precisely the expected location.
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Temporal Response to Real and Illusory Contours
Neuron’s receptive field falls rightover the middle of the real or illusoryline defining the bottom edge of the square
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Feedback loop between OFC and Fusiform Gyrus indicates top-down contribution to object
recognition
Bar, M., Kassam, K., Ghuman, A., Boshyan, J., Dale, A., Hämäläinen, M., Marinkovic, K., Schacter, D.L., Rosen, B., and Halgren, E. (2006). Top-down facilitation of visual recognition. Proceedings of the National Academy of Sciences, 103(2), 449-54.