Many visual tasks, such as the resolution of a letter, show a linear decrease in performance with increasing eccentricity away from the central visual field (fovea). This performance loss results directly from changes in the density of retinal ganglion cells. For a given task, the rate of decline in performance is characterized by calculating the parameter E2 – the eccentricity where stimulus size must double to match performance at the fovea. This gradient is also used as a behavioural measure of the spatial grain of the cortex. Psychophysical estimates of E2 show good agreement with anatomical and physiological measures of cortical magnification. One potential problem is that these estimates are usually derived from observations that are limited in the extent of the visual field tested (typically 0-50 deg.). In this experiment, we will measure the performance loss to see if there is any significant deviation from the predicted pattern as we approach the edge of the visual field. There is now good anatomical evidence to suggest an abrupt change in receptor type, density and relay neuron (ganglion cell) as the edge of the field is approached. This may produce a discontinuity in the performance-eccentricity function and would have implications for models of cortical magnification.
In this lab rotation, you will become familiar with the use and calibration of large-field visual displays. You will gain experience in experimental design, programming, visual psychophysical methods, data analysis and data visualisation. You will gain a solid understanding of how the human retina samples visual space and how this representation is dramatically altered in the visual cortex.
The lab rotation will be delivered as follows:
The final results will be presented at a meeting of the Visual Neuroscience Group.
In primate retina, photoreceptors have a highly distinct geographical distribution that supports the duplex nature of vision. The central retina (fovea) is dominated by retinal cones that are tightly packed forming a foveal mosaic and endowing the central visual field with a high-resolution ‘spotlight’. As retinal eccentricity increases, cone density declines dramatically and the number of rod receptors, which are completely absent at the fovea, increases. In the mid-peripheral retina rods outnumber cones by a factor of 25:1, which increases the retina’s ability to capture light and endows the retinal periphery with vastly superior light sensitivity. The dominance of rods over cones was thought to extend to the very edge of the sensory retina. However, histological evidence from human retinal tissue has revealed that the ratio of cones to rods rises sharply towards the edge of the retinal surface (ora serrata), and is particularly pronounced in nasal retina. Indeed, this cone-enriched rim at the edge of the retina contains more than three times as many cones as the fovea. At present, the functional role of this rim is unknown, although several theories have been advanced. These include alerting and orienting, encoding optic-flow from locomotion, estimation of colour illuminants and circadian regulation. However, there is no convincing empirical data to support any of these hypotheses.
Psychophysical experiments at the edge of the visual field are technically challenging and consequently little data currently exist. This behavioural project will use a unique display system to identify the functional correlates of the cone-enriched rim in human vision. Nottingham hosts the NITES (Nottingham Integrated Transport Environment Simulators) facility which has a large, horizontally circular (180 degree) display (radius 2.5 meters). The display is spanned by 3 separate high-resolution digital light processing projectors. The projectors are controlled by rendering software (Sol7) that is used to geometrically warp images for projection to the circular surface and remove luminance artifacts from areas where the projection areas overlap. This system allows us to generate a high resolution continuous large-field display, which can be used to deliver visual stimuli for human psychophysical experiments. This system is unique in the UK. By using this display we will be able to present visual stimuli at the very edge of the visual field, whilst monitoring eye position centrally (via video-based eye tracking). This will allow us to test each of the functional accounts proposed for retinal cone-enriched rim. This is an area of growing interest. It has recently been proposed that the extreme retinal periphery projects to a single morphological class of ganglion cells, unlike the rest of the retina, and the output of these ganglion cells project to specific region in the limbic cortex – termed the prostriata. This unique physiological arrangement suggests that the extreme retinal periphery serves a particular function not fulfilled by more central retinal structures and signals from this area can rapidly access multiple brain systems.
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