Location and line orientation as distinguishable primitives in spatial vision

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Abstract
In geometry the same parameters define a straight line segment as its two endpoints, yet the human observer's perception of spatial relations can differ depending whether an actual line is shown or merely its terminators. Experiments are reported that demonstrate that the position and orientation senses can be largely decoupled. Because both point localization and line orientation each in its own way has a quality of immediacy, yet manifests high performance in the hyperacuity range, both must be regarded as a primitive each with its own neural processing mechanisms. This can give rise to perceptual dissonances (visual illusions) of which one example is analysed.