Visual Acuity as a Function of Retinal Eccentricity

1430 words 6 pages
Visual Acuity Measured as a Function of Retinal Eccentricity
Visual acuity is a measure of an observer’s ability to see fine spatial detail (Cavonius & Schumacher, 1966). There are a number of factors that affect visual acuity, such as illumination and contrast, and various ways to measure it (Kalloniatis & Luu, 2005). One way to measure visual acuity is through target detection which requires the perception of the orientation of a stimulus such as a Landolt C or a Snellen E (Kalloniatis & Luu, 2005). The participant in the current experiment was referred to have their acuity tested. Target detection of a stimulus was used to measure the participant’s visual acuity as a function of retinal eccentricity of the target.
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This was done to ensure that the eccentricity being tested was as close to its exact value (e.g. 20˚) as possible, therefore minimising errors. A standard tape measure was used to measure these distances at which stimuli should be presented. Central fixation was maintained throughout testing by instructing the observer to use an object (in this case a tree) situated approximately 10m straight ahead at eye level as a fixation point.
A forced choice 1down1up staircase method was used to measure visual acuity as this would determine a 50% level of performance at which the location of the gaps in the stimulus could be detected. This method seemed suitable as it is reliable and requires much fewer trials than other techniques. For each eccentricity, the stimulus was first presented around the sensory threshold at eye level six metres from the observer. The participant was required to determine which orientation they perceived the stimulus to be in out of a possible four directions and judgement was made with the right eye. Stimulus intensity was decreased with each correct response and increased with each incorrect response. The experimenter did this by stepping 30cm forward or backwards on each trial until the estimated threshold was determined. The threshold was estimated four times at each point using the mean of an even number of four reversals to prevent biasing the estimate upwards or downwards. Visual angle created by the target was then