Using the distortion of visual probes to analyse human visual pathways
Lead Research Organisation:
University College London
Department Name: Institute of Ophthalmology
Abstract
In human perceptual or 'psychophysical' experiments, we precisely define the visual stimuli that are presented to human subjects in terms of of temporal frequency, spatial frequency, wavelength and brightness. By manipulating the stimuli along some dimension, and measuring the change in the subject's response, we can infer something about the properties of the underlying visual system. Unfortunately, such measures of the intact/complete visual system do not grant us direct access to the processes that intervene between the stimulus and response. Physiologists, by contrast, have the advantage that they can make recordings within the (non-human) visual system by inserting electrodes directly into visual pathways. The use of visual nonlinearities goes some way to redress this imbalance. With them, the psychophysicist also has an 'electrode' with which to record and introduce signals within the visual pathways and so functionally 'dissect' the human visual system. In this project, we propose the novel use of visual nonlinearities to study the two primary human visual pathways, the chromatic and luminance pathways--from the inside. A visual nonlinearity is a stage in the visual pathway that distorts signals that pass through it to produce new signal components that were not present at its input. The two nonlinearities that we will study distort sinusoidal flicker signals to produce--in addition to flicker--a steady change either in colour or in brightness, which appear superimposed on the flicker and disappear when the flicker is turned off. For example, a red disc of light (c. 670 nm) can appear yellower when it is flickered (colour distortion) or a greenish-yellow disc (c. 565 nm) can appear brighter ('brightness enhancement'). Figures 1 and 3, below, illustrate these effects for flicker that slowly varies in amplitude ('amplitude-modulated flicker'). By carefully manipulating the stimulus presented to the subject, and thus the new signal components produced at the nonlinear site, it will be possible to measure separately the temporal frequency responses before and after each nonlinearity. Two nonlinearities will be investigated in the M- and L-cone pathways: one, which is compressive, causes flicker bursts to change in apparent colour and saturation, while the other, which is expansive, causes them to change in apparent brightness. Our expectation is that we will be able to determine separately the early and late properties of the two principal human postreceptoral pathways: the putative chromatic and luminance pathways, and thus provide new insights into the postreceptoral organization of the human visual system and the processing changes going on in them. The colour and brightness changes that we will investigate are distinct: first, because they can occur separately, and, second, because they are consistent with different types of nonlinearities (compressive or decelerating in the case of colour; expansive or accelerating in the case of brightness) and different percepts. Taken as a whole, these observations suggest the exciting possibility that the two nonlinearities are in the luminance and chromatic pathways, the two principal postreceptoral pathways in the human visual system. We believe that this project will provide a unique window into the internal workings of the human visual system, and will identify key signals and sites that will yield to physiological and/or anatomical study in other species.
Technical Summary
Nonlinearities revealed by the visible distortion of amplitude-modulated (or other time-varying) flicker signals will be used to functionally dissect visual pathways in the human visual system into early (pre-nonlinearity) and late (post-nonlinearity) stages. Such nonlinearities provide a landmark at which to dissect the visual pathway, because they distort input signals internally to produce new signal components within the visual system. By carefully manipulating the visual stimuli presented to the subject, we can experimentally control the new signal components produced at a nonlinear site in ways that make it possible to determine separately the properties of the visual pathway before and after the nonlinearity. We will investigate two nonlinearities in the M- and L-cone pathways. Each causes bursts of flicker to be perceived differently from a steady light of the same time-averaged intensity and chromaticity. One, which is a compressive nonlinearity, causes bursts of M- or L-cone detected flicker to change in colour and saturation. The other, which is an expansive nonlinearity, causes bursts of M- or L-cone detected flicker to increase in apparent brightness. We will use each nonlinearity to dissect the visual pathway in which it resides. In some experiments, the nonlinearity will be used to measure the temporal frequency response of the pathway before the nonlinearity; while in others it will be used to measure the temporal frequency response of the pathway after the nonlinearity. Several methods of measurement will be used. We believe that the compressive nonlinearity lies in an M- and L-cone chromatic pathway, whereas the expansive nonlinearity lies in a separate M- and L-cone luminance pathway. Thus, we will be able to study separately the early and late stages of both the chromatic and the luminance pathways. The results of the proposed experiments will provide new insights into the postreceptoral organization of the visual system.
Organisations
Publications
Petrova D
(2013)
The temporal characteristics of the early and late stages of the L- and M-cone pathways that signal color.
in Journal of vision
Petrova D
(2013)
The temporal characteristics of the early and late stages of L- and M-cone pathways that signal brightness.
in Journal of vision
Rider AT
(2018)
Harmonics added to a flickering light can upset the balance between ON and OFF pathways to produce illusory colors.
in Proceedings of the National Academy of Sciences of the United States of America
Ripamonti C
(2015)
Spectral sensitivity measurements reveal partial success in restoring missing rod function with gene therapy.
in Journal of vision
Stockman A
(2017)
Hue shifts produced by temporal asymmetries in chromatic signals depend on the alignment of the first and second harmonics.
in Journal of vision
Stockman A
(2014)
Color and brightness encoded in a common L- and M-cone pathway with expansive and compressive nonlinearities.
in Journal of vision
Stockman A
(2017)
Linear-nonlinear models of the red-green chromatic pathway.
in Journal of vision
Description | Flickering monochromatic lights near 560 nm appear brighter than steady lights of the same mean intensity, whereas lights near 520 or 650 nm appear yellower. Both effects are consistent with the visible distortion of the representation of the visual input signal at a nonlinearity within the visual pathway, but the brightness enhancement is consistent with an expansive nonlinearity whereas the hue change is consistent with a compressive one. We have manipulated the distortion products produced by each putative nonlinearity to extract the temporal characteristics of the early (pre-nonlinearity) and late (post-nonlinearity) stages of the L- and M-cone pathways signalling either brightness or colour. Contrary to our initial expectations, we find that the attenuation characteristics of both pathways are virtually identical both before and after the nonlinearity: the early temporal stage acts like a band-pass filter peaking at 10-15 Hz, while the late stage acts like a two-stage low-pass filter with a cut-off frequency near 3 Hz. We propose a physiologically-relevant model that incorporates both types of nonlinearity within a single scheme and also accounts for the shapes of the early and late filters. Modelling and analysis provided strong evidence for two nonlinearities: a rectification that gives rise to brightness enhancement, and an additional smoothly compressive nonlinearity that gives rise to changes in hue. Plausible sites for the nonlinearities are after surround antagonism from the horizontal cells. A subtractive centre-surround model of the bandpass early filter is proposed, which suggests that the filter shape results from interference between "centre" signals and more sluggish and delayed surround signals. We propose that the brightness enhancement is consistent with signal rectification, perhaps due to the segregation of the visual signal into ON and OFF streams. The compressive nonlinearity follows this rectification and affects polarised chromatic red-green signals more than achromatic ones. Comparisons with physiological recordings show a consistency with primate ganglion cell data. |
Exploitation Route | The model developed for this work is being used in our current research program. |
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 speaker, OSA Fall Vision Meeting, Reno, US |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | No |
Geographic Reach | International |
Primary Audience | Professional Practitioners |
Results and Impact | About 300 attendees. Talk stimulated considerable debate. |
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 |