Stephen A. Engel - US grants

The goal of this project is to understand the nature of color opponent processing in human visual areas. Color perception results from a sequence of neural transformations of a signal originally encoded by the photoreceptors. The first stages of this sequence are fairly well understood, but the neural basis for the second perceptually identified stage of processing, opponent color mechanisms, remains unclear. A long history of perceptual experiments indicate that a second stage encodes color as three combinations of cone signals, encoding the relative local amounts of red and green, blue and yellow, and light and dark. While parvocellular neurons in the lateral geniculate nucleus (LGN) receive opponent inputs from cones, these cells differ from perceptual mechanisms in important ways. LGN neurons appear to jointly encode red-green and light-dark, they respond well to high temporal frequencies, and they over-represent red-green relative to blue-yellow. The work proposed here attempts to localize cortical areas whose response properties more closely match perceptual mechanisms. We will measure the responsiveness of human visual areas to stimuli of many colors. Functional MRI (fMRI) data will be acquired while subjects view large colored patterns. Responses to each stimulus will be averaged within the LGN and visual areas VI-V4. Color tuning is a measure of responsiveness as a function of color. A series of experiments will measure the color tuning of visual areas under conditions that reveal identifying features of perceptual mechanisms. First, the linearity of color tuning will be tested. Next, color tuning will be measured at a variety of temporal and spatial frequencies. Then it will be measured in the presence of adapting stimuli. Finally, color tuning will be measured in the visual areas of dichromatic observers. All of these conditions effect perceptual opponent color mechanisms in distinctive ways. Visual areas whose response properties resemble perceptual mechanisms are likely to be important stages in the neural pathways that give rise to color perception. Differences in the response properties of areas will help reveal the sequence of neural transformations that results in perceptual color opponency.

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Data sets we have obtained from previous studies suggest a large (>200 msec) difference in BOLD onset timing due to differences in stimulus strength: stronger responses may begin earlier than weaker ones. Although stimulus strength (such as contrast amplitude) is thought to result in small onset timing differences in the neural response in the LGN (Hartveit and Heggelund 1992), V1 (Reich et al 2001), and other visual areas (Gawne et al 1996), these small timing differences (10's of msec) are an order of magnitude smaller than what we have seen in the BOLD fMRI response. We propose to make fine measurements of the timing of the BOLD response using an fMRI sequence with a short TR, gathering one volume every 250 milliseconds. Should timing differences as a function of response strength exist, they would have a substantial impact in the field, as an increasing number of studies are investigating timing of neural response using fMRI. The timing differences may also impact work using dynamic causal modeling (DCM), in which timing is used to infer causal links between cortical areas (Stephan et al 2008, Mechelli et al 2003). The data obtained here will be key for several grant proposals which propose to use fMRI and joint fMRI/EEG to study timing of perceptual processes. Engel, Olman, Kersten, and He all have interests or proposals under development in which such measurements of timing may play an important role.

The brain has a remarkable ability to change its own function. In the visual system, such plasticity is evident when children learn to read, when the elderly adjust to impaired vision, and when new drivers master navigating traffic. The mechanisms that produce long-lasting plasticity in the visual brain remain unclear, however. With support from the National Science Foundation, Dr. Stephen Engel of the University of Minnesota is carrying out research to elucidate mechanisms of long-term visual plasticity. The work builds upon effects of short-term plasticity that are relatively well-understood. For example, when observers view a bright pattern of stripes for a few minutes, their visual systems adapt, and fairly dramatic consequences can result: Faint patterns of similar stripes that were previously apparent can be briefly rendered completely invisible. But few laboratory studies have examined perceptual adaptation over the long term, primarily due to methodological limitations. This project uses recently developed technology to overcome those roadblocks. Participants view video on a head-mounted display that originates in a head-mounted camera and is image-processed by a lap-top computer. The system allows participants to live in a visual world that has been digitally altered. Four experiments use this system to test hypotheses about long-lasting plasticity in the visual system. The first hypothesis is that longer duration adaptation will produce longer lasting plasticity. Participants are placed for up to five days in environments similar to those that produce short-term perceptual adaptation effects. The persistence of adaptation is measured using traditional tests of perceptual abilities. The second hypothesis is that the visual system adapts to discount noisy, uninformative visual input, as suggested by preliminary results. Experiments in this project measure the strength of this novel form of adaptation over the long term. The third hypothesis is that both kinds of adaptation effects arise in early visual cerebral cortex. To test this, the neural bases of long-term adaptation is measured with functional magnetic resonance imaging (fMRI). Together, these experiments can strongly constrain both empirical and theoretical accounts of long-lasting visual plasticity.

This research is advancing understanding of how the human visual system can modify its own operation. This ability, visual plasticity, underlies the acquisition a wide array of human skills, from learning to read, to learning to hit a baseball, to learning to see patterns in satellite imagery. The project uses novel technology to place observers in a digitally altered world for up to 5 days, and measures how their visual perception and their visual cortex adapt to this challenge. Results can help identify specific factors, for example the length and kind of visual stimulation, which lead to long-lasting changes in visual performance. These in turn should have important applications in a diverse array of fields where visual plasticity is critical, from education to the military to public health.

Project Summary Age-related macular degeneration (AMD) typically produces extensive central vision loss, leading to difficulty in reading, navigating, and other critical tasks. Current assistive devices for vision with central scotomas, while promising, are limited in effectiveness. This proposal aims to fill this gap by advancing understanding of remapping, a method that improves the effectiveness of patients? residual vision by shifting information from inside the central blind spot (scotoma) to intact locations in the visual field. Only a handful of past studies have examined remapping, including just a single study of patients. Our group recently demonstrated the method?s promise, showing that it can improve reading substantially in observers with simulated field loss. Critically, patients differ widely in the shape of their scotomas, and the quality of their vision across the visual field. This variability imposes severe limitations on the ?one size fits all? remapping approaches used in prior work, that simply shift the image away from the scotoma center. If, for example, a patient?s preferred retinal location (used for high acuity tasks) is near the lower left edge of their scotoma, shifting text upward or rightward may not aid reading as much as shifting it downward, as the latter will place the text in regions of best visual acuity. This proposal will test the value of personalized remappings, which shift the image in a way that is optimized for each patient?s residual vision. The remappings are constructed using a novel letter recognition perimetry task, which measures performance across the visual field: The personalized remappings are constructed to shift text to maximize observers? total letter recognition ability. Proposed work will first test the hypothesis that personalized remapping can improve single word recognition. Single word reading will be measured with a variety of scotoma sizes, shapes, and PRL locations. Performance with personalized remapping will be compared to that with traditional remapping and no remapping in both control observers with simulated scotomas and people with macular degeneration. Proposed work will also test the hypothesis that personalized remappings can improve free reading. Reading speed, error rates, and eye movement patterns will be measured in a sentence reading task, and in free reading of natural images containing text. Preliminary data support the value of remapping generally, and the potential of the letter recognition perimetry task for building personalized remappings. Results of this proposal will provide the first thorough testing of remapping, and will also inform models of peripheral reading in patients and controls. The proposed studies will develop and evaluate a novel method for assisting people with low-visions, tailored to their individual residual performance. Personlaized remapping may also be incorporated into practical visual aids to improve daily visual function and quality of life.