Parametric measurements of optic flow by humans
Optic flow and beyond
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Recent neurophysiological evidence (e.g., Graziano, Andersen, & Snowden, 1994) suggested that some cells in the medial superior temporal area (MST) of the Old World monkey are sensitive to complex motions such as those brought about by a surface moving in depth or rotating. Two important findings were that these cells show position invariance (i.e., their preferred stimulus does not change across the receptive field), and that some cells were selective for “spiralling” stimuli rather than pure rotations or pure expansion/contractions. This paper attempts to provide evidence for similar processes in the human visual system by employing the technique of selective adaptation. We have simulated surfaces undergoing a motion in depth (div) or a rotation (curl), but have removed any cues that are not related to global motion. After adapting to a large pattern undergoing, say, an expansion, an aftereffect that contained an element of contraction could be elicited by placing small test patterns anywhere in the adapted area. This suggests that the global structure of the motion field must have been encoded as well as the local motion. Likewise thresholds for detecting motions similar to the adapting motion were elevated across the adapted area, while thresholds for other motions were not. Hence the effects of adaptation are both selective and show a degree of position invariance. Adaptation to pure div or pure curl stimuli was compared with adaptation to spiralling stimuli. Threshold elevation was always selective for the adapting motion and the shape and broadness of tuning did not vary. In simulations we could not reproduce our results using a model that had only div and curl detectors, but we could reproduce them if we allowed for detectors tuned for a broad range of spiral pitches. Our results suggest that humans encode the complex motion of surfaces by detectors tuned to many different types of motion and that the detectors are invariant across space in their properties.