Interactive Activation

Cognitive Core ClassMay 2, 2007

What is Interactive Activation?

• It is the idea that our perception, cognition, attention, and action arise out of a mutual constraint satisfaction processes involving bi-directional influences among active representations via bi-directional connections.

• Interactive is used in the sense of mutual engagement and influence:– activity, interaction, sentiment (G. Homans)

• NOT in the sense of statistical interaction

The interactive activation modelRumelhart & McClelland (1981)

• Units stand for hypotheses at different levels of representation.

• Activation is the system’s only currency.

• Mutual excitation between mutually consistent hypotheses.

• Mutual inhibition between mutually inconsistent hypotheses.

• System settles to a stable state across all parts given inputs of any type.

The Interactive Activation Model as implemented for perception of letters

• Feature, letter and word units.

• Between-layer connections were + or -; only inhibitory connections within.

– Later the between layer inhibition was deleted

• Activation follows the ‘iac’ function.

• Response selected from the letter units in the cued location :

• Strength s is a monotonic function of each unit’s activation.

Findings Addressed by 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 non-words.

• 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.

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.

The Contextual Enhancement Efffect

The TRACE model of Speech Perception

• Extends the IA model directly to speech.

• Units representing hypotheses are reduplicated across spatial positions.

• Competition is proportional to degree of overlap.– Results in segmentation as

well as selection.• Top-down effects from word to

phoneme level explain: – lexical effects on phoneme

identification• Top-down effects from

phoneme to feature level explain– The “perceptual magnet

effect”

Lexical Effect on Word Segmentation

Discrimination and identification of

stimuli varying on a single cue

TRACE vs. MERGE

Compensation for Coarticulation

• Value of the dimension ‘acute’ that signals ‘g’ (or other phoneme) depends on what comes after it.

• In Elman & McClelland (1986) we proposed that phoneme units in one position can modulate connections from feature to phoneme units in other positions.

• This led to the idea: Maybe top-down effects can trigger compensation for co-articulation.

Lexically-mediated compensation for co-articulation (LMCC)

• ‘sh’ lengthens oral ‘tube’, ‘s’ shortens it.

• This colors a following /t/or /k/.• Perceivers compensate for this

as illustrated in the bottom left panel.

• Q: Can lexical information that disambiguates a sound half-way between ‘s’ and ‘sh’ also produce this effect?– ChrismaX -> ‘s’– fooliX -> ‘sh’

• Yes, the effect does occur, though it is weaker than the acoustically mediated effect, as predicted by the simulation.

Other ‘knock-on’ effects

• LM Selective adaptation– Hearing an ambiguous segment in a context causes

adaptation consistent with the contextually determined identity of the sound: E.g. Right after hearing X in ‘fooliX’ you’re less likely to identify an ambiguous sound as ‘sh’.

• LM Retuning of speech perception– Hearing an ambiguous segment in a context causes

retuning consistent with the contextually determined identify of the sound: E.g. After hearing X in contexts like ‘fooliX’ interleaved with clear ‘s’ in contexts like ‘Christmas’ you will tend to identify the ambiguous sound as ‘sh’.

Interactivity in the Brain

• Bidirectional Connectivity• Interactions between V5 (MT) and V1/V2:

Bullier• Subjective Contours in V1:

Lee and Nguyen• Distributed Constraint Satisfaction in Binocular

Rivalry: Logothetis

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.

• Cooling typically produces a reversible reduction in firing rate to the cell’s optimal stimulus.

• 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 rightover the middle of the real or illusoryline defining the bottom edge of the square

Figure shows a V1/V2 neuron that showed strong modulation in firing around epochs in which the monkey perceives the cell’s preferred stimulus.

From Leopold and Logothetis, 1996.

Top: psth’s show strong orientation preference.

Bottom: When both stimuli are presented simultaneously, neuron is silent just before a response indicating perception of the null direction, but quite active just before a response (t < 0) indicating perception of the preferred direction.

Leopold and Logothetis (Nature, 1996, 379, 549-553) found that some neurons in V1/V2 as well as V4 modulate their responses in concert with Monkey’s percept, as if participating in a massively distributed constraint-satisfaction process. However, some neurons in all areas do not modulate their responses. Thus the conscious percept appears to be correlated with the activity of only a subset of neurons. The fraction of neurons that covary with perception is greater in higher areas.

Interactive Activation and ‘Seeing Black’

Eberhardt et al (2005)*

• Black faces prime perception of crime objects.

• Crime or basketball words increase attention to black faces and the locations in which they occur.

• There is a stereotypicality effect in face memory that is accentuated by crime primes.

Guilt by association in an interactive activation model

??