EEG Effects of Time Intervals in Natural Scene Perception

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    EEG effects of repetition interval

    lengths during natural scene

    perception

    Hielke Prins

    Iris Groen

    Steven Scholte

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    When participants perceive natural images, their EEG

    response correlate with Weibull distribution parameters

    Weibull distributions have two free parameters (beta andgamma) and describe distributions of low-level features

    in the stimuli (here: contrast values)

    For contrast features, distributions vary from power lawlike to Gaussian depending on the amount of clutter in

    the stimuli

    Background: image statistics

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    Background: image statistics

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    A model (Scholte et al. 2009) describes processing of

    the stimuli by the brain

    This model effectively calculates the two parameters of

    the Weibull distribution

    Explained variance between the model and the EEG

    response increases with the number of presentations

    (Ghebreab et al., in submission)

    Background: stimuli repetition

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    Background: stimuli repetition

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    Question

    How long does the effect of repetition lasts?

    Goals1. Confirm the effect of a previous encounter of the

    stimulus on the EEG response of participants perceiving

    the stimuli

    2. Determine whether the length of the interval between

    successive repetitions of the stimuli affects the response

    Experimental question / goal

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    16 participants

    1 hour of EEG acquisition per session

    4 conditions reflecting different repetition intervals

    Short Intermediate

    Long

    Over-night

    2 sessions on the same time on 2 subsequent days

    400 stimuli per condition

    All 4 stimulus sets have equal spread in beta-gamma

    values

    Experimental design: numbers

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    Experimental design: intervals

    Set Repetition Interval Number of Stimuli

    I 4-8 minutes 2 * 200

    II 12-20 minutes 2 * 200

    III > 30 minutes 2 * 200

    IV 23-24 hours 400

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    Participants have to categorize the stimuli in man-made

    and natural environments according to their first

    impression

    A small questionnaire serves as a pilot to determine

    whether an influence of long-term previous visual

    experience can be determined

    Experimental design: task

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    Computing the explained variance between model

    Weibull parameters and early deflections in the ERP

    Comparison of the explained variance between the firstand second presentation of the stimuli for each repetition

    interval

    Data analysis

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    Analysis of differences in the effect of repetition between

    the repetition intervals provides information about the

    duration of the effect

    Long lasting effects require different mechanisms of

    plasticity then fading shorter ones

    An answer on the research question might thus provideclues on how to improve the model to account for the

    observed effect of repetition on the explained variance

    Conclusions under various outcomes

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    40 blocks of 40 stimuli

    Short, intermediate and long blocks repeated once

    Over-night blocks repeated in second session

    Discussion

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    Discussion

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    How to counter-balance stimuli in order to minimize theeffects of processing history or fatigue?

    Is it possible to quantify the best solution?

    Discussion