Global shape processing involves feature selective and feature agnostic coding

mechanisms

Jason BellMimosa ForsythDavid R. BadcockFred A.A. Kingdom

An object’s shape is a strong cue for its recognition.

Experimentally: Attneave, 1954

…and even earlier…

Specialised shape detectors in area V4:macaque

Gallant et al. (1993, 1996): global form detectors in macaque V4

Neuron 1: concentric & spiral Neuron 2: Radial

Specialised shape detectors in area V4:Humans

Wilkinson et al. (2000): shape detectors in human V4

Shape coding modelsPasupathy & Connor, 2002; Poirier & Wilson, 20062 critical attributes of a shape1) Curvature of features2) Angular position of curved

featuresStep 1: sample curvatureStep 2: code curvature x angleResulting shape code is unique

Are global shape detectors selective for shape orientation?

To test this we use:a) Radial Frequency (RF)

patterns: widely used to study global shape processing

b) Adaptation and after-effects

The RF Amplitude After-Effect(RFAAE) Bell & Kingdom, 2009

Measuring shape orientation selectivity of the RFAAERF3 rotated through 60 degrees:

RF4 rotated through 45 degrees:

4 observers & 6 runs of each condition

Shape orientation tuning? Yes and no

Not simply curvature adaptation

Feature invariant processingThe results of Experiment 1 suggest two distinct mechanisms1) Shape orientation selective

mechanism2) Shape orientation invariant

mechanism

Is this invariant mechanism insensitive to other shape attributes?

For example, luminance polarity

Luminance polarity selectivity?Yes and no

All Observers Polarity

Adaptor Rotation Difference

RFAA

E

-0.05

0.00

0.05

0.10

Adaptor

Test

(a) (b)Interaction Between Lum inance Polarity

and Shape Orientation

Adaptor Rotation DifferencePo

larity

Transfer

0.0

0.5

1.0

1.5

black/whitewhite/white

Summing up: two stage global shape coding

Global shape mechanism A: orientation selective and luminance-polarity selective: as the models predict.

Global shape mechanism B:insensitive to orientation and luminance polarity

A A A

A

B

A A

Feature invariant processingCue convergence in object sensitive LOC. 1) Luminance, motion and texture-defined form: Grill-

Spector et al., 1998; 2) Contrast invariant activation to attended object:

Murray & He, 2006

fMRI studies show RF patterns are processed in V4 and the LOCBetts et al. 2008; Rainville et al., 2005; Wilkinson et al., 2000

Consistent with two stage processing of RF shapes

Thank you for your time

Questions??

A A A

A

B

A A