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Anne-Sophie Del Vecchio, Michael W. Grünau; Comparison of two methods for equating the salience of first- and second-order motion. Journal of Vision 2003;3(9):867. doi: 10.1167/3.9.867.
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Purpose. An important issue for studies comparing attentional selection of first- and second-order motion stimuli is how to equate both components. Methods. One way is to present a series of counterphase gratings (mixture of 1st- and 2nd order), with variable luminance contrast for the 1st-order stimulus, and to derive the contrast that corresponds to the 50% threshold for perceived direction of the overall stimulus. Another method is to present a transparent plaid made-up of a mixture of a 1st- and a 2nd-order grating for a discrete time and ask the participant to continuously monitor which grating appears in front. The contrast of the 1st-order stimulus is varied to obtain the 50% threshold corresponding to equal cumulative time for both components. In the present study, both methods for equating 1st- and 2nd-order motion were compared. Since the 2nd-order grating consisted of texture modulation, random noise can be added to the 1st-order component in order to increase the similarity of the components. This effect of random noise was also investigated. Results. When noise was included in the 1st-order component, the 50 % thresholds between 1st- and 2nd-order components were not statistically different for the plaid and the grating methods. The grating method without the noise in the 1st-order component was also equivalent. Only the plaid method without noise in the 1st-order component required significantly higher luminance contrast for the 1st-order component to match the 2nd-order component. Conclusions. Both methods are acceptable in order to equate 1st- and 2nd-order components as long as noise is included in the 1st-order component.
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