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Isabelle Legault, Rémy Allard, Jocelyn Faubert; Detecting curvature in first and second-order periodic line stimuli. Journal of Vision 2005;5(8):464. doi: 10.1167/5.8.464.
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It has been suggested that second-order processes may have coarser orientation tuning functions than first-order mechanisms. The purpose of the present study was to determine if differences between these two classes of stimuli are evident when processing periodic line stimuli of different frequencies and to further determine whether periodic line discriminations solicit oriented receptors. The stimuli were composed of D4 luminance or contrast defined lines that were distorted with sinusoidal curvature modulations. The curvature modulations varied in frequencies between 1/8 and 1 cycle per deg and the total image size was 8x8 deg. The lowest spatial frequency was such that a minimum of one full cycle was visible. Five young healthy observers participated in the study. Individual contrast thresholds were obtained for the first and second-order stimuli to adjust for stimulus visibility. The thresholds were obtained with a temporal forced choice paradigm where the subject had to indicate whether the stimulus was present in the first or second presentation for contrast detection, or whether curvature was present in the first or second stimulus for the curvature amplitude measurements. The results show that, when the visibility is individually adjusted, there is no difference between first and second-order class stimuli for this type of task. This suggests that the mechanisms involved in detecting curvature in periodic line stimuli are common for both first and second-order processing mechanisms and probably minimally solicit oriented receptive fields which would make this processing analogous to a Vernier alignment hyperacuity type task.
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