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Ikuya Murakami; A Filehne illusion at equiluminance. Journal of Vision 2007;7(9):985. doi: 10.1167/7.9.985.
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© ARVO (1962-2015); The Authors (2016-present)
Equiluminance is known to compromise several aspects of temporal vision including perceived speed. However, most studies have been conducted with fixating eyes, making it difficult to infer what happens in more naturalistic viewing conditions with moving eyes. During smooth pursuit eye movements, the visual image retinally moves in the opposite direction. To compensate for such retinal image slip, the visual system is arguably comparing two velocity signals, one from visual inputs and the other from extraretinal information such as the efference copy of oculomotor commands. If both signals were obtained with the gain of unity, externally stationary objects would be perfectly stable in the visual world as well. However, a stationary object usually appears to move in the opposite direction to pursuit (the Filehne illusion), indicating that the gain of extraretinal information may be less than one. The question of this study is, what kind of motion is perceived if the stationary object is equiluminant? The prediction is that, if the input gain of retinal velocity is lower at equiluminance, the Filehne illusion will be reduced or even reversed to the same direction as pursuit. Our observations revealed that it is actually the case, though within a limited range of pursuit speed. This reversed Filehne illusion was more pronounced in peripheral viewing, in which an equiluminant stationary object sometimes appeared to be glued to the moving gaze. These results suggest that speed reduction at equiluminance takes place in an early processing level of retinocentric coding, in accord with current neurophysiological knowledge about the parvocellular-blob information stream, but that the successive stage of velocity comparison with extraretinal information can result in overestimation of the external velocity of the equiluminant object, so color-defined things can look faster. This is also consistent with classical claims about unstable, “jazzy” impressions of equiluminant stimuli.
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