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Tom C. A. Freeman; Pursuit eye movement, motion adaptation and two types of velocity aftereffect. Journal of Vision 2006;6(6):78. doi: 10.1167/6.6.78.
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© ARVO (1962-2015); The Authors (2016-present)
One means of judging velocity during pursuit is to compare signals encoding retinal motion with information about the eye movement. Some authors argue the latter is based exclusively on extra-retinal signals originating from the motor-control system. Others argue for a compound reference signal comprising extra-retinal and retinal components. Recent support for the compound reference comes from Haarmeier & colleagues. They demonstrated that simultaneously adapting to pursuit and retinal motion changed perceived motion during pursuit, even when net adaptation directions were counterbalanced across trials (Haarmeier et al, 2001, Neuron, 32, 527). They confirmed that counterbalancing eliminated classical motion aftereffect (MAE) and suggested simultaneous adaptation therefore recalibrates a compound reference as opposed to changing retinal motion encoding. Here I question this conclusion. Counterbalancing direction eliminates MAE but not the way retinal speed is encoded (the velocity aftereffect, VAE). I suggest that simultaneous adaptation in fact induces two types of VAE, one retinal and one extra-retinal, which compete to determine perceived velocity during pursuit. I first show that retinal VAE persists following counterbalanced simultaneous adaptation, tested using a speed-matching technique. I then show that Haarmeier et al's original effect can be predicted by separately adapting to pursuit or retinal motion. This is tested using their motion-nulling method, in which stimulus velocity is adjusted post-adaptation until it appears head-stationary during pursuit. Preliminary data also suggest that the reported direction-selectivity is specific to retinal VAE only. The results provide little evidence in favour of a compound reference.
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