Interactions of local and global judgements in object

interaction

Nick Davis*Bangor University

*and Martyn Bracewell and Helen Morgan1

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Introduction

•Efficient manipulation of objects is crucial for most behaviours

•People spontaneously pick up objects by grasping across the centre of mass (CoM), and are very sensitive to displacements in CoM (Lederman & Wing 2003; Endo et al submitted)

•People are also sensitive to the properties of the surface under the fingertip (Flanagan et al 2006)

•In this experiment we combined global (CoM) and local (surface) judgments in a lifting task

Introduction

•We believe that local and global judgements should be subserved by different brain areas

•If so, it may then be possible to manipulate these areas separately

•For example, disrupting activity in the “local” area should reduce accuracy in judging surface textures, but not global structure

•Disruption would be evidence of machine-brain interaction

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Apparatus

•Six wooden blocks with dowels pushed through

•Dowel was either in the centre or was off-centre

•Ends of the dowels were covered with three surfaces• Rough sandpaper (P100)

• Medium sandpaper (P150)

• Smooth card

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Method

•10 subjects were recruited• 8 female, mean age 28.6, all right-handed

•In separate blocks subjects were asked to judge either the structure of the object (centre or off-centre) or the surface of the object (rough, medium, smooth)

•Six observations of each condition were taken

•The main measure was accuracy as a percentage

•Data were analysed by analysis of variance (Anova).

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Results

•Subjects achieved a mean accuracy of 73.9% overall

•Accuracy was not different when asking about the surface versus asking about the structure

•There was no difference in judging between on- and off-centre objects

•However there was a difference in accuracy across surfaces, with smooth surfaces being easier to detect than rough (p<0.01)

•The global structure affects performance differently for global versus local questions (p=0.027)

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Results – local judgment

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Results – global judgment

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Results – global/local interaction

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Results – local confusion matrix

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Conclusions

•Subject accuracy is high in both global and local judgments

•Smoother surfaces are recognised more readily than rougher surfaces

•An additional result is that the global and local judgements are helped by different global structures

•Possibly this is due to the integration of small rotations of the object into the judgment, which are easier to detect for rougher surfaces

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Future work

•Subject accuracy on this task with the current apparatus is good for further work (i.e. not at ceiling or floor)

•If asked to perform the task in an MRI scanner we predict that different activation patterns will be seen for the two judgements

•Further work could then selectively alter performance in one or other condition by disrupting these areas using TMS.

•Selective disturbance would be a proof of machine-brain interaction

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