The Colour Group (Great Britain)Meetings 2005-2006

MEETINGS FOR 2007-2008

Wednesday 9 JANUARY 2008

VISION MEETING & PALMER LECTURE
10.00 - 17.30 hrs Wednesday 9 January 2008
Lecture Room, Institute of Ophthalmology
11-43 Bath Street, London, EC1V 9EL

NOTE: Space is limited, early registration is essential

10.00	COFFEE & REGISTRATION
10.30	Colour slew-rate: are chromatic pathways limited by a maximum rate at which they can signal changes in colour? Andrew Stockman, Institute of Ophthalmology, London
11.00	Palmer Lecture 2007 History and theory of the chromatic Stiles-Crawford effect Pieter Walraven, The Netherlands
12.00	Understanding color categories, color constancy, color induction, and lightness perception from information theory Li Zhaoping, University College London
12.30	LUNCH
13.30	Cambridge Research Systems Sponsored Lecture 2008 Calibrating Colour Vision Michael Webster, University of Nevada, Reno, USA
14.30	Moving Colours Annette Werner, Institute for Ophthalmic Research, University of Tübingen
15.00	GRANVILLE TEA
15.30	DOUBLE FEATURE: Neural substrates involved in Instantaneous Colour Constancy Computations Karoline Spang, Human Neurobiology, Bremen University, Bremen Dichoptic Studies on Instantaneous Colour Constancy in Human Vision John Barbur, Applied Vision Research Centre, City University, London
16:30	Colour constancy and memory colours of familiar objects Milena Vurro, Institute of Neuroscience, Newcastle University
17:00	Bridging the Gap between Simultaneous Contrast and Colour Constancy - Simple Spatial Filters J Kraft, University of Manchester

REGISTRATION. The fee, including lunch is £20 for Colour Group members, and £25 for non-members.
The fee without lunch is £10 for members and £15 for non-members.
Cheques made to The Colour Group (GB). Send with a completed registration form to:
Sophie Wuerger
School of Psychology
Eleanor Rathbone Building
Bedford Street South
University of Liverpool
Liverpool L69 7ZA
so that it arrives by January 4th.

For further information contact Sophie Wuerger. e-mail: s.m.wuerger@liverpool.ac.uk Tel: +44 (0) 7905 609 408

ABSTRACTS

Colour slew-rate: are chromatic pathways limited by a maximum rate at which they can signal changes in colour?
Andrew Stockman, Institute of Ophthalmology, London
At low to moderate flicker frequencies (4 to 10 Hz), the overall mean colour appearances of M- and L-cone-isolating sawtooth
stimuli depend on whether the direction of the sawtooth is rapid-on (|\|\|\|\) or rapid-off (/|/|/|/|). Rapid-on-L-cone and rapid-off-M-cone
sawtooth stimuli appear greener, while rapid-off-L-cone and rapid-on-M-cone sawtooth stimuli appear redder, even though they have
the same mean chromaticities. These changes can be explained by supposing that chromatic mechanisms are better able to track the slowly
changing phase of the sawtooth than its fast phase. Colour, in other words, may be "slew-rate" limited.

Understanding color categories, color constancy, color induction, and lightness perception from information theory
Li Zhaoping, UCL
I explore an understanding of colour appearance predicated on the brain's mapping sensory inputs into discrete categories conveying the
maximum bits of Shannon information about the input. Under sufficiently high (but not infinite) signal-to-noise ratio, when an input ensemble
contains the usually large dynamic range, an information maximizing mapping from the contrast-gain-controlled photoreceptor inputs to, e.g., six,
categories typically carves the input space into regions that correspond to the perception of white, black, red, green, blue, and yellow colour
categories. This input-to-category mapping corresponds to another mapping from surface reflectance to category of colour appearance.
Illumination changes that sufficiently preserve signal-to-noise can alter the input-to-category mapping but leave the reflectance-to-category mapping
almost unchanged, achieving colour constancy. This hypothesis of informationally optimal colour boundaries, when applied to small input
ensembles made of inputs from a single or a part of a scene, can account for various colour illusions in particular color induction, and, under
achromatic inputs, typical phenomena in lightness perception. It does not at present accommodate the spatial configuration factors that influence
colour/lightness appearance.

Calibrating Color Vision
Michael Webster, University of Nevada, Reno, USA
To reliably signal properties of the world, color vision must be compensated for variations in the spectral sensitivity of the visual system. We have examined the nature of these compensatory processes and how they correct for the perception of white across the visual field, the perception of contrast across different directions in color space, and the perception of hue as saturation varies. Each of these cases suggests that color appearance is calibrated to match the color statistics of the environment, and point to the possible stimulus attributes that color conveys.

Moving Colours
Annette Werner, Institut for Ophthalmic Research, University Tübingen
Traditionally colour research has focused on static stimuli. But recently, visual motion has prooven to be an important factor influencing the
processing of colour signals, mostly with an enhancing effect on perception. I will review the latest work on colour motion interactions and present
new research on the role of attention in the enhancement of moving colour.

DOUBLE FEATURE:
Neural substrates involved in Instantaneous Colour Constancy Computations
Karoline Spang¹, Manfred Fahle^1,2 & John Barbur² ¹Human Neurobiology, Bremen University, Bremen, Germany; ² Applied Vision Research Centre, City University, London, UK
We measured the Bold response to variations of chromatic context, using a Mondrian stimulus. Our aim was first to identify instantaneous colour
constancy (ICC) mechanisms compensating for sudden changes of illuminant. Second, we wanted to measure the activation of these mechanisms
through either illuminant or material changes. The third aim was to establish whether they respond specifically to illuminant changes or play a more
generic role in colour-perception.
Mondrian stimuli changed in both luminance and chromaticity to produce a range of chromatic interactions during passive viewing in 8 subjects.
Random luminance changes and other stimulus manipulations were employed to ensure that the difference between test and reference conditions
maximised either the extraction of colour, colour constancy or both. The experiments involved: 1) simulated "global" changes of illuminant consistent
with the "classic" definition of ICC; 2) "local" changes of illuminant; 3) "global" changes of chromaticity as dictated by a change of illuminant with "local"
changes of luminance; 4) random, reassignment of chromaticities and luminances amongst Mondrian patches. Surprisingly, the different conditions
elicited similar activations of early visual areas (V₁ to V₄). Hence "local" colour-generating interactions play an important role in defining the perceived
colour of objects. The findings of this study suggest that “local” processing of signals in V1 plays an important role in instantaneous colour constancy
when coloured objects are seen in context.

Dichoptic studies of instantaneous Colour Constancy in human vision
John Barbur¹, Manfred Fahle^1,2 & Karoline Spang² ¹Applied Vision Research Centre, City University, London, UK; ² Human Neurobiology, Bremen University, Bremen, Germany
The contribution the primary visual cortex and extrastriate visual areas make to instantaneous colour constancy (ICC) was investigated in dynamic
colour-matching experiments under sequential changes of illuminant using monocular, binocular and dichoptic Mondrian stimuli. The importance of immediate
surround was investigated in both binocular and dichoptic conditions. Localisation of brain activity for stimulus conditions that drive ICC mechanisms was also
measured using fMRI. We wanted to establish the extent to which ICC depends only on the monocular processing of chromatic context (that precedes the
conscious perception of colour). The latter condition was achieved in dichoptic viewing by producing changes of illuminant that could be either in phase or in
counterphase in the two eyes. The counterphase modulation condition is perceptually equivalent to the Mondrian being illuminated by a steady, unchanging illuminant.
The strength of the ICC index achieved in such dichoptic, colour-fusing experiments remains completely independent of the “perceived” changes of illuminant
colour in the Mondrian surround and depends only on local, monocular processing of chromatic context.

Colour constancy and memory colours of familiar objects
Milena Vurro, Yazhu Ling and Anya Hurlbert, Institute of Neuroscience, Newcastle University
Colour constancy is a complex phenomenon underpinned by multiple mechanisms, at the sensory, perceptual and cognitive levels. In this study, we explore the contribution of memory colour and object shape to colour constancy, using a set up designed to preserve the natural 3D shapes of real objects while allowing the free adjustment of their apparent colours. In a forced-choice task, observers were required to report , as quickly as possible, whether particular colours were “appropriate” for particular familiar objects (e.g. banana, carrot, and courgette). Illumination and object shape were varied systematically. The range of “appropriate” colours, as well as reaction times, vary significantly across illumination and geometric configurations. The results demonstrate that the colour constancy of memory colours for familiar objects depends on object shape as well as the particular illumination conditions.

Bridging the Gap between Simultaneous Contrast and Colour Constancy - Simple Spatial Filters
J Kraft, Y Ouyang, University of Manchester
A wide variety image statistics provide information useful for determining reliable, accurate colour appearance. A system using such statistics to establish the appearance of a surface might preferentially consider light falling on areas of the retina (or other image sensor) adjacent to light from the surface under consideration, implying a spatial filter. Previous studies of simultaneous contrast often found greater chromatic contrast for larger surrounds, but we have observed surprisingly little evidence for simple spatial filters in simultaneous contrast with relatively large illuminated stimuli in simple configurations. Our recent experiments using more 'Mondrian-like' stimuli have revealed behaviour consistent with finite, localized spatial filters, however. Four observers made achromatic settings in the context of illuminated posters displaying small coloured patches. The patches were drawn from 18 reflectances chosen to be consistent with two illuminants, one real and one simulated by a consistent reflectance shift. Annular surrounds of five sizes were defined by groups of patches corresponding to one illuminant on a background of patches corresponding to the other illuminant. Surround borders coincided with edges of coloured elements and did not contain x-junctions which would have been consistent with illumination boundaries. Fitting a model indicated that for these stimuli, the extent of the spatial filter was such that half of the simultaneous contrast effect was observed at a surround width of between 1.0 and 1.4 degrees visual angle.

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