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The "aperture problem" indicates that a local reading of the velocity of an oriented contour is inherently ambiguous, insufficient by itself to recover the velocity of image points. In Wallach's "barber pole" display consisting of moving diagonal lines within an elongated rectangular aperture, it has been suggested that the unambiguous motion of(More)
Velocity discrimination is not affected by random changes in contrast or temporal frequency. Observers judged the relative velocity of a moving sinusoidal grating when target contrast was varied randomly from trial-to-trial over the range from 5 to 82%. The Weber fraction for the random mixture of interspersed contrast levels was about 0.06, comparable to(More)
Image regions corresponding to partially hidden objects are enclosed by two types of bounding contour: those inherent to the object itself (intrinsic) and those defined by occlusion (extrinsic). Intrinsic contours provide useful information regarding object shape, whereas extrinsic contours vary arbitrarily depending on accidental spatial relationships in(More)
There exists a class of two-dimensional figures (including cumulative gaussian waveforms) whose contours have a limited range of orientations. These figures can appear as highly nonrigid if they undergo pure translation in the image plane. In the case of the cumulative gaussian waveform, it is the region around the inflection point that appears as nonrigid.(More)
The upper displacement limit (Dmax) or Braddick limit was measured in random dots. We then interposed a variable duration pause at half the distance jumped. Of interest was to see the shortest time (delta t) which would yield a value of two times Dmax, thus indicating the time required to process one additional Braddick limit. A surprisingly short interval(More)
To examine how local velocities from different regions of the visual field combine to form a coherent motion percept, we subjected a sinusoidal line stimulus to translational motion. Horizontal movement of a sinewave line along its axial direction is perceived as nonrigid if the angle at the zero crossing is smaller than a critical angle of about 15 deg.(More)
Treisman and others have reported that the visual search for a target distinguished along a single stimulus dimension (for example, colour or shape) is conducted in parallel, whereas the search for an item defined by the conjunction of two stimulus dimensions is conducted serially. For a single dimension the target 'pops out' and the search time is(More)
Vertical sine-wave gratings of varying spatial frequency were stepped instantaneously to the right or to the left at differing phase angles (theta). Separate paradigms measured the contrast threshold for the detection of such a step and for the discrimination of the direction of the same step. By considering the grating before and after its displacement as(More)
The sensitivity of the visual system to motion of differentially moving random dots was measured. Two kinds of one-dimensional motion were compared: standing-wave patterns where dot movement amplitude varied as a sinusoidal function of position along the axis of dot movement (longitudinal or compressional waves) and patterns of motion where dot movement(More)
Objects occlude other objects in natural scenes, and this occlusive relationship increases the spatio-temporal complexity of sensory inputs to the two eyes, especially when objects are moving. We ask whether the visual system can employ clever strategies which make use of real-world constraints on inputs to the eyes to determine the depth of objects.(More)