Andrew Stockman, Ph.D. - US grants

Color vision in humans depends on particular receptor cells called cones in the retina of the eye. One population of cones, known as the short-wavelength cones or S-cones is most sensitive to light in the blue region of the spectrum; others are the medium-wavelength or M-cones, and long-wavelength or L-cones, most sensitive to red. Cones contribute to two conceptual channels of information leading to visual perception. Intensity of light stimuli is signalled in the luminance channel, and color of light is signalled in the chromatic channel. Sorting out the relative contributions of the different kinds of receptors to these different channels has been an important problem. This project will examine the temporal properties of the chromatic and luminance contributions. A human subject will set flicker of a colored test light to be at just the threshold of visible flickering. The frequency or the intensity of the flicker, or the background, or a second stimulus that tends to mask or augment the flicker of the first, can all be varied. Using very intense lights, this lab already discovered that S- cones can contribute to the luminance channel. Fast responses to light intensity apparently use a brisk pathway to a luminance channel, while slower responses to color use a sluggish pathway to a chromatic channel. The current project will asses the relative prominence and the temporal properties of the two pathways at more normal light levels, and try to determine if the S-cone contribution to luminance is important at these lower levels. This work uses innovative techniques and methodology for delivering and analyzing pulsing stimuli, and results from this work will be very important to theories of color vision, to commercial engineering measurements of colored lights, and to understanding human vision in general.

The optics of the eye focus an image of the world onto the retina which contains four types of photoreceptors: blue, green and red cones and a single type of rod. Experiments are being conducted to learn how the signals from the three cone photoreceptor types of the retina are organized and how the information about color is organized and processed by the early stages of the visual system. Conventional wisdom holds that information from photoreceptors is transmitted by two main visual pathways in the brain: one is sluggish and transmits information about color; the other is rapid and transmits information about intensity. The specific studies being conducted in this research project use flickering light of different colors and flicker interactions to address specific questions about the nature of these two visual pathways, as well as how their signals change when the visual system adapts to increasingly intense lights. This research is providing clear evidence that the early stages of the visual system are more complex than the prevailing psychophysical models suggest.

A series of experiments is proposed that will use various non-linearities revealed by time-varying visual stimuli to analyze the human visual pathway. When an input signal passes through a non-linear site, new signal components are produced that were not present at its input. By carefully manipulating the visual stimuli that provide the input signal to the non-linearity, it is possible to distinguish the temporal properties of the visual pathways before and after each non-linear site. Three non-linearities will be studied. Each causes a burst of flicker to be perceived differently from a steady light of the same time-averaged intensity and chromaticity. The first, a compressive non-linearity, causes S-cone detected flicker to change in color; the second, also a compressive non-linearity, causes bursts of M- of L-cone detected flicker to change in color and saturation; and the third, an expansive non- linearity, causes bursts of M- or L-cone detected flicker to increase in apparent brightness. The primary aim of the proposed experiments is to use each non-linearity to dissect the early visual pathway. In some experiments, the non-linearity will be used as a recording electrode -- to measure the temporal frequency response of the visual system before the non-linearity; and in others it will be used as a stimulating electrode -- to measure the temporal frequency response of the visual system after the non-linearity. Preliminary evidence suggests that the temporal frequency response after each compressive non-linearity is characteristic of a chromatic pathway, while the response before each compressive non-linearity may more closely reflect the temporal frequency response of the receptors that the results of conventional temporal sensitivity measurements. It is proposed that the expansive non- linearity that affects M- and L-cone flicker is in a separate luminance pathway. The results of the proposed experiments will provide new insights into the postreceptoral organization of the visual system. Each non-linear site can provide a landmark at which psychophysical and physiological results can be compared. In the long-term, these experiments also may have important clinical implications, since it will be possible to use each non-linear site to localize and damage or deterioration that results from degenerative disease.

The goal of this project is to use flicker and flicker interactions to understand more about vision and visual perception. A common theme throughout is the combination of measures of flicker sensitivity and flicker delay (between signals generated by different receptor types or by the same receptor types but in different eyes) to reveal the inner workings of the visual system. Two related areas of study are proposed: (I) Organization of the early visual system The phase delays and amplitudes of the middle-wavelength (M) and long- wavelength (L) cone inputs to the channel(s) that signal flicker will be systematically analyzed under various adaptational conditions. Preliminary evidence shows that the data can be modeled by assuming that slow (+MS-LS or +LS-MS) as well as fast (+Mf+Lf) cone signals feed into the achromatic channel ( - means inverted in sign, + non-inverted, f fast and s slow). (II) Light adaptation of the visual system Measures of the delay between flicker signals generated by the two eyes under different states of adaptation will be combined with measures of flicker sensitivity to characterize the effects of light adaptation more completely than has been done before. Armed with this newly won information, we will critically evaluate current models of light adaptation and develop new ones. Our preliminary results have led to the development of an elegant model that requires just two parameters to account for light adaptation over a range of luminances of greater than 105. Measurements will be aimed first at adaptation in M and L-cone pathways, and then at adaptation in short-wavelength (S) cone and rod pathways. Our results, we believe, will: (I) force a radical reappraisal of the way in which color-opponent and luminance signals are assumed to interact, lead to a more realistic psychological model of the organization of the early visual system, and require modifications to current models of the luminance channel; and (II) allow a critical reassessment and reformulation of models of light adaptation. In each case, we will relate our results to, and be guided by, the underlying retinal physiology and anatomy. Present knowledge of physiology and anatomy reveals a more complicated system than the canonical psychophysical model of luminance and chromatic channels. Our work, which identifies complex flicker interactions between fast and slow signals from the three cone types that are also seen in records from macaque ganglion cells will provide a link to current physiological and anatomical studies, and, we hope, motivate new ones.