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Rebecca Champion, Tom Freeman; Access to retinal image movement during pursuit eye movement is only direct at high motion coherence. Journal of Vision 2008;8(6):1028. doi: https://doi.org/10.1167/8.6.1028.
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Observers can directly access retinal motion signals prior to the stage that combines retinal movement with eye-velocity information (Freeman, Sumnall & Snowden, 2007, Perception, 36S, 4). We investigated whether this remains true for motion stimuli decoded by the dorsal pathway, primarily because this region also receives extra-retinal input. Speed discrimination thresholds were measured for low and high motion coherence using different mixtures of pursuit eye movement. On each trial, observers pursued a target dot for 2s. Midway through the pursuit, a large dot pattern was presented for 0.25±0.01s, during which the target disappeared. The stimulus consisted of signal and noise dots. The retinal speed of the signal dots varied according to a staircase around a pedestal of 4deg/s. Retinal motion was always in the opposite direction to pursuit. The retinal speed of noise dots was constant at 4deg/s. In the ‘homogeneous’ condition, intra-interval pursuit target motion was constant (4,4; 8,8; 12,12deg/s). In the ‘heterogeneous’ condition, intra-interval pursuit varied (4,8; 8,12; 4,12deg/s). These were crossed with coherence levels of 100, 50 and 35.4% signal. Psychometric functions were constructed either as a function of objective speed or retinal speed. The latter was based on eye movement recordings. Results showed no difference between homogeneous and heterogeneous retinal thresholds for low noise conditions. Observers therefore had direct access to the relevant retinal signal. In support of this, the ratio of retinal to objective threshold was less than one in the heterogeneous condition. However, in the high noise condition, the homogeneous retinal threshold was about half that of the heterogeneous threshold. Moreover, the ratio of retinal to objective thresholds rose to around 4 in the heterogeneous condition. These results suggest that when the relevant retinal signal resides in the dorsal pathway, extra-retinal signals are automatically combined and direct access is prevented.
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