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An orientational difference of only 0.3-0.5 deg can be discriminated between two gratings or two lines, although psychophysical channels and cortical cells both have comparatively broad orientation bandwidths of 10-25 deg. One proposed explanation for the fineness of orientation discrimination is that, while detection is determined by the most excited(More)
We measured the accuracy with which subjects judged that a square or circle was perfectly symmetrical i.e. that aspect ratio (a/b) was exactly unity (where a and b were, respectively, the vertical and horizontal dimensions). Errors were remarkably small, ranging from 0.7 to 0.4% for the judgement of squareness and from 1.4 to < 0.1% for the judgement of(More)
We measured both the just-noticeable difference in time to collision (TTC) with an approaching object, and the absolute accuracy in estimating TTC in the following cases: only binocular information available; only monocular information available; both binocular and monocular information available as in the everyday situation. Observers could discriminate(More)
Inspecting a target whose size oscillates about a constant mean value selectively depresses visual sensitivity to oscillating size. The effect transfers from positive to negative contrast and vice versa. This depression cannot be attributed to fatigued movement detectors. We propose that there are, in the human visual system, channels in which information(More)
It is well known that, if a rigid sphere is moving at constant speed towards the eye along the line of sight then, for small values of theta, T = theta/theta, where T is the time to contact, theta is the instantaneous angular size and theta is the rate of increase of angular size. We describe a rationale and an experimental procedure for demonstrating(More)
1. Visual sensitivity to movement in depth was measured as a function of the relative distances through which the left and right retinal images moved. This relative distance (left:right ratio) provides a sensitive cue to the direction along which a target moves in three-dimensional space.2. Gazing at a target which moved along a fixed direction in space(More)
The direction of motion in depth of a monocularly-viewed rigid sphere can be quantified in terms of the distance by which the sphere's centre will miss the centre of the pupil of the observing eye. If we express this distance as ns (where s is the sphere's radius and n is a scaling factor), then n approximates the ratio (d phi/dt)/(d theta/dt) between the(More)
After adapting to changing size by viewing a square whose dimensions increased with a ramp waveform. a subsequently-viewed test square appeared to move continuously away in depth. Adapting to decreasing sire produced the opposite aftereffect. This depth movement aftereffect could be measured by cancelling it by some unique rate of change of size. The(More)