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Psychophysics is the discipline that studies the relations between physical external stimuli and subjective human perceptions. Despite the impression of vagueness that its name can evoke (due to its contiguity with the nondeterministic world of psychology), it is a rigorous scientific discipline aimed at deriving quantitative connections between a subjective response and the stimulus that originated it. The underlying assumption is that the human perceptual system can be seen as a measuring instrument that can yield results (responses and/or judgements) that can be systematically analyzed. Visual psychophysics characterizes human visual perception by decomposing its complex global phenomenology into a number of elementary tasks afforded by the human eye, and then finding quantitative descriptions for each of them. These include color perception, motion detection, spatial resolution, pattern recognition, 3D perception, temporal response, etc. The issue of vision sharpness, that is, the individual ability to resolve fine details, was the first topic to be investigated, both for the intrinsic ease of its characterization and for the purpose of studying and correcting subnormal visual performance. This led to the development of concepts such as minimum angle of resolution and visual acuity (mid-19th century), as well as to suitable tools (letter charts) to grade performance level. In the 20th century, it was realized that visual acuity is just one aspect of a multifactorial problem, where resolvable size is not the only variable, others are involved as well (for example, contrast and luminosity). In the effort to go beyond the domain of visual resolution, the question of measuring the visibility of a spatial pattern was solved through Fourier decomposition of the pattern itself. This in turn determined the development of concepts, such as spatial frequency, threshold contrast, and CSF, again accompanied by reliable means (interferometric fringe technique, sinusoidal and square bar charts) to quantify the psychophysical response.
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