It is observed that when the speed of a moving object, the length of a slit behind which the object appears at periodic intervals, and the duration of each exposure of the object at the slit are kept constant, the threshold of detection as measured by the linear size of the object does not vary with changes in distance between the observer and the object over a range of 1 to 5 m. On the other hand, the threshold of detection as measured by the angular size of the object is markedly different for the distances of 1 to 5 m, when the angular velocity of the object, the angular size of the slit, and the duration of exposure of the object are kept constant for the two distances. The results are explained on the basis of a physiological correlate, at the psychovisual cortex, of the phenomenal size and speed of an object. This pattern is assumed to remain constant in spite of the changes produced at the retinal level by variations in distance between the observer and the moving object. The progressive decrease in the visibility of a moving object with increase in its linear velocity is not caused by the changes in the quality of discharge by individual retinal receptors, but is considered to be related to the stimulus pattern at the psychovisual cortex.
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