Multiple signals interact in flicker: recursive surround networks

Before visual stimuli arriving at the eye are consciously perceived they are transformed and encoded by multiple circuits in the intervening visual pathways. These processes subtly alter our perception of visual stimuli. Many of the processes cannot be directly observed - they are carried out by very small neurons with a very large number of even smaller connectors and it is the whole nest of these complex interconnections that determine the way the nest behaves. One way to determine some of the characteristics of these inaccessible processes is to study behaviour that depends on them. Our aim is to determine how the circuitry of the human visual system works through careful perceptual measurements of flicker perception. We rely on the fact that interactions between different circuits within the visual system alter visual signals in characteristic ways. Typically, these interactions cause flicker to be relatively less visible at some frequencies but easy to see at other frequencies. By looking for those characteristic effects in perceptual measurements, we can tease apart the different underlying visual circuits and their contributions to perception. In previous work, we discovered a surprisingly complex circuitry that uses the signals from the individual light receptors-cones-in several different and unexpected ways. In this project, we propose to monitor and model those signals under a variety of new conditions to understand more about how the eye and brain work. We confidently expect to provide a greater understanding of vision and visual processing, which will yield important benefits in other areas of vision research, and will help to bridge the widening gap between relatively simple models of the visual pathways and what we are now learning about the complex structure and function of the visual system from other disciplines.

The primary aim is to elucidate the functional circuitry of the human visual system through psychophysical experiments. Our approach is based on the prediction that different visual circuits produce signals with amplitudes and delays that vary with temporal frequency in characteristically different ways. We successfully used this approach to identify-in addition to the expected, fast positive M- and L-cone inputs-sluggish middle(M)- and long(L)-wavelength-sensitive-cone inputs of both positive and negative sign into the luminance or achromatic pathway. The measurements were made on deep-red (658-nm) adapting fields. We propose to make extensive measurements using other adapting field wavelengths to determine the influence of wavelength and intensity on the relative strengths of the various cone inputs into the human luminance channel. As before, a vector model will be used to analyse the data and identify the signs and strengths of the various inputs. Preliminary evidence shows sluggish inputs are clearly evident on adapting fields of short and middle-wavelengths. We confidently expect to produce a more realistic model of the human visual system and human visual processing, and, by characterizing the signals that are accessible by observers and evident in their behaviour, help to provide new links between behavioural and physiological experiments.

The primary goal of this research project is the elucidation of fundamental properties of the human and primate visual system. Consequently, the initial beneficiaries of this research will be other scientists studying the visual system. These will include not just visual psychophysicists like the PI, but also sensory physiologists, electrophysiologists and cognitive neuroscientists working on visual processing in the human and primate visual system. The initial impact will be, we expect, the provision of a model of early visual processing that identifies the visually significant signals that should also be evident using other objective measurements. Many other scientists, who are less directly involved in vision research, including some geneticists, anatomists working on the early stages of the visual system, and brain-imaging specialists will also potentially benefit from this research. At the more applied level, this work may also benefit engineers interested in the ergonomics of man-machine interfaces. Such interfaces and displays include videos, films, mobile phones and other dynamic presentation devices.
Longer term benefits will include clinical applications of the research. Once we have defined and modelled the characteristics of visual processing in normal observers, we can make comparable measurements in patients with specific clinical visual deficits. As we have found with other similar paradigms, this can be very fruitful in way of understanding the nature of the clinical deficit. We already have several ongoing collaborations with our sister institution Moorfields Eye Hospital.
Given the wide range of interest in vision research 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.