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V. Lakshminarayanan, A. Raghuram, V. Rajaram; Effect of Psychophysical Procedures on Speed Discrimination . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4086.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Do the results on speed discrimination task vary significantly upon the psychophysical methodology, and are the differences significant? Method: Three psychophysical procedures were used to estimate speed discrimination threshold for a drifting grating stimuli oriented vertically. The stimulus was generated using the VSG2/5system. The speed discrimination task was assessed for simultaneously presented grating stimuli. Test distance was 1.5m. Staircase technique (ST): A pair of drifting gratings was presented on the monitor. The standard grating moved at a specific reference speed. The speed of the test grating was varied using a staircase technique. Subjects identified which grating moved faster. Method of adjustment (MOA): Subjects viewed a pair of drifting gratings and were required to match the speed of the test grating to that of the reference grating. Method of constant stimuli (MCS): Three different modes of presentation were adopted. The reference grating was made to move at 2 deg/sec in all conditions. The speed of the test grating was varied from trial to trial in a random fashion. Subjects responded whether the test grating speed was faster/slower or equal in speed to the reference grating. In the second presentation the steps were varied differently. More trials were chosen for points close to threshold. The trials were not randomized in the third presentation. The speed of the test grating was presented from a slow to a fast speed. Results: Results were obtained from 30 normal subjects. Paired T test was used to assess whether significant differences were found between the methods, on estimated threshold. There was a significant difference in the estimated threshold determined by MOA and MOC (t=3.56197; p=0.0031) Significant differences were not found between ST and MOA. Significant difference was found between the ST and the second method of presentation for the MOC (t=2.376199; p=0.032). MOC gave lower thresholds compared to other methods. Test performance for the ST and MOA had significant variability within the group. This difference could be due to the ST being more difficult due to standard and test grating positions being randomized within the trials. The results are also dependent on the observer’s criterion. If subjects look for precision in their estimation they do better in the MOA tasks. A learning curve was seen for MOA and the ST. With sufficient training performance increases with the two techniques. Conclusion: The study revealed the MOC to be more consistent in estimating the optimal threshold. However, practice improves test performance with other procedures.
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