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Visual perception - part 4

Microsaccades và trôi dạt có thể, tuy nhiên, di chuyển hình ảnh võng mạc nhanh hơn nhiều so với ngưỡng tốc độ mà tại đó người ta sẽ phát hiện chuyển động đối tượng bên ngoài (Nakayama và Tyler, năm 1981;. McKee et al, 1990), và trong thực tế, các tế bào thần kinh thị giác trong khu vực | 195 1999 . Microsaccades and drifts can however move the retinal image much faster than the threshold speed at which one would detect external object motion Nakayama and Tyler 1981 McKee et al. 1990 and in fact visual neurons in area V1 and higher do respond to image oscillations originating in such eye movements Bair and O Keefe 1998 Leopold and Logothetis 1998 Martinez-Conde et al. 2000 2002 Snodderly et al. 2001 . It may be the case that image slip by a microsaccade goes unnoticed because of saccadic suppression i.e. transient decrease of displacement detectability as seen in large-scale saccades for review see Ross et al. 2001 . But eye drifts cannot be cancelled the same way because they occur constantly unlike saccades and microsaccades. Since the retinal image motions due to small eye movements are indeed registered in early cortical representation some computation must be needed to cancel them. For large-scale eye movements such as smooth pursuit the visual system may use extraretinal estimation as to how the eye is currently moving efference copy of oculomotor commands Helmholtz 1866 and proprioceptive signals from extraocular muscles Sherrington 1918 . By subtracting the estimated eye-movement vectors from retinal image flow image motion components due to eye movements would ideally be cancelled out. For fixational eye movements however it is doubtful that these extraretinal signals work effectively. First cancellation of random image oscillation would require temporally precise and accurate synchronization between visual and extraretinal processes but actually this may be difficult because each of them or at least visual processing may be running on its own clock with poor temporal resolution and wide latency fluctuation Murakami 2001a b Kreegipuu and Allik 2003 . Second eye drifts are partly derived from random neuronal activities in the periphery of the oculomotor system Cornsweet 1956 Eizenman et al. 1985 to which the efference-copy system may be .