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Murthy Bhavaraju, Ennio Mingolla; Speed perception across variations in spatiotemporal frequencies in apparent motion stimuli. Journal of Vision 2007;7(9):35. doi: https://doi.org/10.1167/7.9.35.
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Many pairs of spatial and temporal frequencies in a motion display that result in the same stimulus speed for a moving object can produce different speed percepts (Priebe NJ et al., J Neurosci. 2003, 23(13): 5650–61). We previously reported that judgments of the speed of an object depend on the spatiotemporal frequency of the moving pattern in an inverted-U function, peaking at a specific spatial and temporal frequency combination [http://www.journalofvision.org/4/8/84/]. The location of this peak is largely independent of the size and shape of the object. In the present series of experiments, with the use of high coherence dot motion stimuli, we investigated the dependence of perceived speed on both spatial and temporal frequencies. The perceived speed of the stimulus was estimated using a 2AFC paradigm with interleaved QUEST staircases; subjects were asked to pick the faster of the two spatially separated [6 deg eccentricity] patches of dots moving in opposite directions. We systematically varied the speed of the dots from 2 to 32 deg/sec such that there was an overlap of the spatial and temporal frequency components across trials with different speeds. Results indicate that observers are less sensitive to biases in speed estimation caused by variations of spatial frequency at moderate velocities than at the extremes. Also, as the actual speed increases, the stimulus that appears faster tends to move toward lower spatial and higher temporal frequencies. Moreover this shift is gradual over a range of speeds. These results are in line with speed and spatial frequency correlation of MT neuron responses (Priebe NJ et al., J Neurosci. 2004, 24(8): 1907–16) and predictions from multi-channel models of speed estimation, where larger cells with bigger receptive fields encode higher velocities (Chey J et al, Vision Res. 1998, 38(18): 2769–86).
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