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Ernst Weber (1795-1878) Weber presented an extensive experimental exploration of the sensory phenomenology of tactile experience Coining the phrase, just noticeable difference (JND) to refer to the smallest perceptible difference between two sensations that is detectable by a human being or other animal. Weber provided an existence proof for the possibility of establishing quantitative relationships between variations in physical and mental events Gustav Fechner (1801-1887). A pioneer in experimental psychology and founder of psychophysics, the study of the relationship between stimulus intensity and subjective experience (detection) of the stimulus Psychophysicists usually employ experimental stimuli that can be objectively measured, such as pure tones varying in intensity, or lights varying in luminance. All the senses have been studied: vision, hearing, touch, taste, smell, and the sense of time. The most common use of psychophysics is in producing scales of human experience of various aspects of physical stimuli. For example the physical stimulus of frequency of sound. Frequency of a sound is measured in hertz, cycles per second. But human experience of the frequencies of sound is not the same as the frequencies in hertz. Doubling the frequency of a sound (e.g., from 100 Hz to 200 Hz) does not lead to a doubling of experience. The perceptual experience of the frequency of sound is called pitch Hermann von Helmholtz (1821-1894) He viewed perception as requiring an active, unconscious, automatic, logical process on the part of the perceiver which utilizes the information provided by sensation to infer the properties of external objects and events. Helmholtz anticipated much of later top-down cognitive psychology. Helmholtz had also made another major contribution to physiology. Stimulating nerves at various distances from a muscle and measuring the time it took for muscular contraction, he estimated the rate of travel of the nervous impulse, and in the process incidentally introduced the technique of reaction-time into physiology and psychology
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