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Chapter 1

# Chapter 1 - Chapter 1 Principles of Perceptual Measurement...

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Chapter 1- Principles of Perceptual Measurement - Three possible functions that may relate the physical intensity of a stimulus to the perceived intensity of sensation: o Linear (stable slope) o Exponential (increasing slope) o Logarithmic (opposite of exponential: decreasing slope) suggests that a sensory system would no longer be additionally responsive to further increments in stimulus intensity past a certain point. Classical Psychophysics : (study of quantitative relationships btw physical events + psychological responses) - 19 th c. experimentalists understood that a mathematical relationship btw physical and perceptual qualities could be established by obtaining two basic characteristics: starting point and slope. - Functions start a bit to the right of zero, as we are unable to detect very low-intensity stimuli. - The minimum threshold is called the absolute threshold . Below this, stimulus intensities are called subthreshold and will not produce detectable sensation. (Above the AT= suprathreshold). - How small of a change in stimulus intensity is required to produce a discriminable change in sensation? The answer= called the difference threshold - used to estimate how the slope changes. - ^This general scientific approach was formulated by German physicist Gustav Fechner in 1860. Psychophysical methods: - 1. Method of Adjustment : human subject is told to adjust the intensity of a stimulus until it is barely detectable (to obtain the absolute threshold). - 2. Method of Limits : subject is presented with a stimulus whose intensity is chosen from an ascending or descending series; if ascending is used, the stimulus is initially set at subthreshold value and increased until it is perceived (opposite is done for descending values). - 3. Method of Constant Stimuli : intensity values are randomly chosen from a preset range; neither experimenter nor subject knows the value of the coming stimulus intensity= preferable. o The subject is presumed to behave like an ideal detector in such a scenario. o The expected response profile is a step function. Absolute threshold: - However, humans are not ideal detectors- too much variability: our nervous systems are “noisy” and a judgement of perceptual intensity (etc) can be interfered with by misperceptions and physical, cognitive, and emotional factors. - The function from an actual experiment would be more S-shaped ( ogive )= psychometric function. - Psychophysicists use the 50% detection response level as the absolute threshold. - Examples of sensory system’s absolute thresholds: o Touch: a dimpling of the skin by as little as 10^-5 cm is sufficient to be detected. o Smell: absorption of only 40 molecules by nose detectors produces a detectable smell. o Hearing: a movement of the eardrum by only 10^-10 cm is necessary to hear a sound.

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