Does the sluggishness of colour perception reflect a limitation in the maximum (slew) rate at which chromatic pathways respond to changes in colour?

Lead Research Organisation: University College London
Department Name: Institute of Ophthalmology

Abstract

At higher temporal frequencies, the appearance of flickering lights is dominated by a steady average, with some residual disturbance at the flicker frequency. The average appearances of flickering lights that turn rapidly off and slowly on and those that turn slowly on and rapidly off can be very different even when their mean colours and intensities are the same. (We refer to these waveforms as 'sawtooth' stimuli.) These differences, which are unexpected, are important because they can provide unique insights into the underlying processing of visual signals that encode colour. Our initial hypothesis is that the differences are due to chromatic mechanisms being better able to follow slow colour changes than fast ones. As a result, their mean outputs become skewed in the direction of the slow change. For example, a yellow light that repeatedly changes slowly to red but then rapidly to green looks on average more red than a light that changes rapidly to red but slowly to green, which looks more green. This imbalance is inconsistent with many current models of chromatic processing, but can be explained by our model. Specifically, we propose that the rate at which colour mechanisms can signal changes in colour is limited to a particular level. If the rate of change exceeds that level, the output varies only at the limiting rate of change. We call this the 'slew-rate' model. Although the idea of a slew-rate limit is familiar in electronics (for example in the specification of operational amplifiers), it is as far as we know novel in the field of vision research. We propose a series of experiments that are designed to test and develop the slew-rate model. Measurements of chromatic detection and chromatic discrimination will be made to characterize the effects of changing the slopes of the sawtooth waveforms as a function of various stimulus parameters. At each stage, these results will be compared with predictions generated by computer simulations of the slew-rate model. Our goal is to critically evaluate the slew rate model. To what extent can the model account for the data? Can simple modifications to the model better account for the data? Can other types of waveforms be used to critically test the model? Can computer simulations suggest other experiments? In addition to causing changes in the mean colour of sawtooth waveforms, a limiting slew-rate should also make us less sensitive to flicker. Thus, the well-known sluggishness of colour perception may be due to the slew-rate limit rather than the more conventional explanation that colour signals are averaged over time. We will also test this prediction by comparing flicker sensitivity measurements with the sawtooth measurements. Testing the slew-rate model per se is not necessarily the most critical feature of this proposal. Equally critical is the fact that we are generating a new dataset using unconventional stimuli and an unconventional model and approach. This new dataset will be made available to other researchers, and can be used by others in the field as well as ourselves to generate new models of how colour vision works. The primary goal of this proposal is to understand more about how chromatic signals are processed in the human brain and in particular why chromatic signals are processed much more slowly than achromatic ones. Despite the many recent advances in related fields, details about how colour processing in humans work remains relatively little understood.

Technical Summary

In general, we are relatively less sensitive to high temporal-frequency chromatic flicker than we are to high temporal-frequency achromatic flicker. The relative loss of chromatic sensitivity is usually taken as evidence that the chromatic response is somehow 'slower' than the achromatic one. But in what way is colour processing 'slow'? In the linear-systems view, the additional slowness of chromatic signals is attributed to chromatic mechanisms having longer integration times, either because of additional filtering stages and/or stages with longer time constants. In this proposal, wWe should like to take advantage of a novel chromatic phenomenon that is inconsistent with this type of model: The new phenomenon is that the mean colour appearance of rapidly-on and slowly-off sawtooth flicker |\|\|\|\|\ can be markedly different from that of slowly-on and rapidly-off sawtooth flicker /|/|/|/|/|, even though the waveforms have the same mean luminance and chromaticity. Although inconsistent with conventional models, the phenomenon is consistent with a simple model in which the chromatic mechanisms are assumed to be limited by the rate at which they can signal changes in colour (slew-rate limitation). Thus, the chromatic mechanisms are better able to track the slowly changing part of the sawtooth than its quickly changing part, so that their mean output will always be skewed in the direction of the slow change. In this project, we propose to develop and test 'slew-rate-limit' models. We will use objective two-alternative forced choice (2AFC) measurement techniques to systematically characterize the phenomenon as a function of various stimulus parameters. In parallel with these measurements, we will generate computer simulations of the effects of a limiting slew-rate in order to produce testable predictions. The model will be adapted or revised as necessary. This project will, we believe, shed light on fundamental aspects of human colour vision.

Planned Impact

The research will directly benefit vision scientists, sensory physiologists, and cognitive neuroscientists working on chromatic processing in the human and primate visual system in several ways. First, it will benefit vision scientists and psychophysicists by providing a better understanding of how chromatic signals are processed and limited by postreceptoral processing and what makes them slower than achromatic signals. Second, and in parallel, it will benefit retinal physiologists by providing them with behaviourally-measured baselines and new techniques and stimuli to evaluate and guide their single-cell recording experiments. The approach of viewing their results as slew-rate limited may lead to exciting and fruitful new avenues of research. Third, it will provide cognitive neuroscientists with new information about the types and nature of chromatic signals that are likely to be encountered in the human cortex. Many other scientists, who are less directly involved in colour vision research, including some geneticists, anatomists, brain-imaging specialists, and electrophysiologists will also potentially benefit from this research. At the more applied level, better understanding of chromatic processing will be useful in machine-computer interfaces. Knowing the limits of chromatic processing will help to define the capabilities required of such interface systems. For example, if the colour mechanisms are limited by a particular slew rate, there is no need for the interface device or other display device to exceed that rate. Such interfaces and displays include videos, films, mobile phones and other dynamic presentation devices. The work may also provide important insights into efficient algorithms for extracting chromatic information in artificial vision systems. Given the wide range of interest in colour vision across many disciplines it will be important that our results and models are widely disseminated. We will publish our work in open source journals and present the data at international conferences. As part of this project, we also propose to make our data available at the Colour and Vision Research Laboratory (CVRL) website run by the PI at http://www.cvrl.org This resource is well-known and widely used in colour research both by academics and in industry. We propose to develop this resource further to provide general information on vision.

Publications

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Conway BR (2018) A tour of contemporary color vision research. in Vision research

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Dev Borman A (2012) Early onset retinal dystrophy due to mutations in LRAT: molecular analysis and detailed phenotypic study. in Investigative ophthalmology & visual science

 
Description When alternated at low temporal frequencies pairs of coloured lights usually appear to vary in hue at their alternation frequency. As the frequency is increased, the hue variation fades to a steady mixed-hue appearance at frequencies well below the flicker fusion frequency, but a luminance variation of the mixed-hue remains (unless the alternating lights are luminance-equated).
Traditionally, this loss of hue variation at relatively low frequencies has been assumed to reflect chromatic mechanisms' having greater temporal integration than the luminance mechanism. For several years, our group has been accumulating evidence for an alternative model that postulates that the hue variation is partly limited by the rate at which chromatic mechanisms can signal changes in hue (i.e., they are a limited by a "slew" rate).
Our approach has been to vary the temporal waveform of flicker in ways that give rise to predictable changes in either the mean ("DC") hue appearance or the time-varying ("AC") hue variation that should result from a slew limit. These waveforms include sawtooth stimuli that vary in their on- and off-slopes from rapid-on to triangular to rapid-off, square-wave stimuli that vary in duty-cycle, combinations of 1st and 2nd harmonic sinusoidal flickering lights that vary in phase, and other more complex waveforms. The objective psychophysical tasks have included detection and matching of the DC and/or AC components.
We have compared these psychophysical data with simulations. We find that a model in which the slew rate limit follows an expansive nonlinearly that accelerates input signals above and below the mean can account for many of the phenomena we have encountered in terms of changes to both the DC and the AC. Moreover, slew-rate models in which different unipolar or bipolar hue mechanisms have different slew rates may provide the basis for explanations of pattern induced flicker colours.
Exploitation Route We have developed a completely new and exciting model from this work that has resulted in several new papers published this year. Instead of a slew rate, we now think the colour change is related to the balance of signals in ON and OFF pathways.
Sectors Aerospace, Defence and Marine,Other

 
Title CVRL database 
Description This web resource provides an annotated database of downloadable standard functions and data sets relevant to colour and vision research and to colour technology, as well as providing information about the research outputs of our group. Updated frequently. 
Type Of Material Database/Collection of data 
Year Produced 2006 
Provided To Others? Yes  
Impact Widely used in science and industry, the site started at UC San Diego in 1995 and moved to UCL with the PI in 2001. 
URL http://www.cvrl.org
 
Description BBC World Service, CrowdScience participant. 
Form Of Engagement Activity A broadcast e.g. TV/radio/film/podcast (other than news/press)
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact CrowdScience participant as an expert on colour vision.
Year(s) Of Engagement Activity 2018
 
Description Invited speaker, 16th International Symposium on the Science and Technology of Lighting, Sheffield 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact 100 attendees and the talk sparked discussion about human vision and lighting.
Year(s) Of Engagement Activity 2018
 
Description Invited talk, Department of Psychology, University of Washington 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Professional Practitioners
Results and Impact Seminar presentation. Talk stimulated useful and interesting discussion.
Year(s) Of Engagement Activity 2018
 
Description Lecturer, ICVS summer school, Oxford. 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact International school on colour vision held every two years bu the International Colour Vision Society. Educationally important.
Year(s) Of Engagement Activity 2018
 
Description Ophthalmology Grand Rounds talk, Department of Ophthalmology and Visual Sciences, University of British Columbia 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Clinical vision talk that was broadcast around British Columbia. Significant positive feedback and interest.
Year(s) Of Engagement Activity 2018
 
Description PI was chair and co-chair of the Colour Group GB 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact The Colour Group GB organizes public meetings, school lectures and events on the broad topic of colour.

Wider interest and appreciation of the scientific and artistic aspects of colour.
Year(s) Of Engagement Activity 2009,2010,2011,2012,2013,2014
URL http://www.colour.org.uk
 
Description Short course instructor, 26th Color and Imaging Conference, Vancouver. 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Class and workshop in colour and colour vision for people in Colour and imaging. Important for forging links with industry.
Year(s) Of Engagement Activity 2018
 
Description UC Davis, Vision Sciences seminar speaker 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Professional Practitioners
Results and Impact Local seminar. Produced good discussion. Lab visits there were very useful and interesting.
Year(s) Of Engagement Activity 2018
 
Description UC San Diego, Department of Psychology Colloquium speaker 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Professional Practitioners
Results and Impact Colloquium audience at UC San Diego. Sparked questions and discussion,
Year(s) Of Engagement Activity 2018
 
Description Visual Cognition talk, Department of Ophthalmology and Visual Sciences, University of British Columbia 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact About 60 attendees enjoyed a talk on colour vision that led to useful discussions and feedback.
Year(s) Of Engagement Activity 2018