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Chapter 1

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McGill University
PSYC 212
Evan Balaban

PSYC 212 – Perception Chapter 1 :Principles of Perceptual Measurement – Notes - In general, ALL visual, auditory, tactile, or chemosensory stimulation that we are able to perceive can be measured. - Perception is a very private experience and since it cannot be exposed to anyone else, it cannot be measured by anyone else either. Most of us are able to correctly identify the color of a stoplight – but are we all experiencing an identical physiological event or could it be slightly different for everyone? We may never know the answer to this. - Many psychologists have asserted that even making the attempt at measuring a perceptual event is fruitless because it is not a measurable thing and therefore can never be verified. - This has generated lots of debate – opposite views held by some experimental psychologists that the perceived intensity of sensations can be reliably estimated by the perceiver. Can also be scientifically validated since information obtained is generally consistent across individuals. A. Scientific Basis of Perceptual Measurement - Advantage of knowing this scientific relationship: might give us an insight about the nature of the brain, the way it processes info and how the biological operations within it lead to sensation and perception Quantitative relationships and their benefits - Provide an estimate of the perceptual quality of a stimulus in numerical terms and allow comparisons with other stimuli - Allow comparison among individuals and species (comparison among the different sensory modalities, aka cross-modal comparison) Must answer this basic question first: Is there a general relationship between physical stimulus and perception? - Early experimentalists were looking for a formula/function that could describe all sensory systems - Increasing function (as physical intensity of the stimuli increases so does our perception of it) - Could be a linear function. For any given increase in physical intensity, there is a certain increment in the perceived intensity. The proportion/slope is constant. - Could be an exponential function. Perceived sensation intensity changes very slowly at the low values of physical intensity, but after a certain point small changes in stimulus intensity produce a dramatic increase in perception. Slope progressively increases with physical intensity. - Could be a logarithmic function. Very large slope at the beginning. Perceived intensities change dramatically with small changes in stimulus intensity. However at higher stimulus intensities, this effect diminishes and the function trails off. Displays a decreasing slope over its entire range. 2 approaches to get precise relationship b/w physical events and perceptual experience: 1. Ask human subjects to rate perceived intensity of a certain stimulus at various physical intensities (eg. Loudness of a sound) - Plot the values and determine which of the 3 functions it best represents 2. Measure the smallest change in stimulus output that still causes a change in sensation To understand how this is important, must examine ideas of experimental psychologists of the 19 century. They understood that a mathematical relationship b/w physical and perceptual qualities needed 2 descriptions of that function – the starting point and the slope. B. Classical Psychophysics - The 3 possible functions do not start at the origin because it is impossible to detect very low levels of stimulus intensity - Even if a stimulus is physically present, the biological elements involved with capturing stimuli and transforming it into a sensory experience cannot function well if the intensity is too low - Intensity must reach a certain minimum level (absolute threshold) before it is registered in the brain as a sensory event. - Stimulus intensities below this point will not produce detectable sensations subthreshold - Stimulus intensities above this point will produce sensation  suprathreshold - The suprathreshold points are the ones forming the slope. One way to obtain this info is by knowing just how small a change in stimulus intensity is required to produce a discriminable change in sensation  difference threshold - Once this is known, the best suitable function can be determined Can be used to - This approach was developed by Gustav Fechner in 1860, calledate how slope changes at psychophysics. Believed that there existed a general relationship between physical and perceptual qualities and that it could behreshold levels determined by knowing the stimulus energy at which the output can be just detected. 1. Psychophysical Methods 3 methods to obtain absolute and difference threshold by Fechner 1. Method of Adjusment = simplest method. Human is told to adjust physical intensity of stimulus until it is barely detectable. Very fast and actively engages subject. 2. Method of Limits = more reliable estimates than method of adjustment. Subject is presented with chosen stimulus whose intensity is ascending or descending. Intensity stimulus is initially set and increases/decreases by a fixed amount until the subject reports that it is perceived/disappeared. Provides a better estimation of threshold if time is not an issue. 3. Method of Constant Stimuli = best method. In contrast to the other 2 methods, this method has randomly chosen intensity values. Neither the experimenter nor the subject knows the intensity of the next stimulus. Subject simply replies whether it was felt or not. Best method since subject has no idea what the next stimulus will be in comparison to the last one (predictability is less accurate) 2. Absolute Threshold - Results should look like a step function – pg. 7 figure 1.2 - HOWEVER, humans are not ideal detectors. Our sensory systems deal with several factors of uncertainty; Stimuli aren’t always perfect every time, our nervous system is inherently noisy, noise interferes with signal detection, and after all that we still have to judge whether it was perceived. - Pg. 8 figure 1.3 shows actual results, which looks like an S-shape (psychometric function/ogive). Conventional approaches to threshold estimation - Which value do we use to represent absolute threshold? Actual results have no well-defined point that can serve as threshold - Must adopt arbitrary response level (most psychophysicists use 50% YES value). The physical intensity at 50% is taken to be the absolute threshold - In reality, there are no all-or-none stimulus detection; our thresholds can fluctuate a little - Over time, some values have been obtained for different sensory systems: -5 Touch – skin dimpling of 10 is sufficient to detected Smell – absorption of 40 molecules by nose detectors produces a smell Hearing – tiny threshold; eardrum movement of 10 -10cm produces a sound Vision – 54-148 photons are necessary to produce a sensation of light 3. Difference Threshold - For the whole function to be determined, Fechner needed to know what the slope of the function was at suprathreshold levels and how that slope changed with increasing intensity - Ernst Weber  How much does a stimulus need to change in order to Constant slope = linear function produce a detectable change in sensatioIncreasing slope = exponential intensity needs to be added or subtractfunctiona target light in order Decreasing slope = logarithm for there to be a just noticeable difference (JND) in sensation? function A difference threshold experiment on the visual system - Experiment: 2 lights, one target light (intensity increases or decreases), one reference light (intensity remains the same). Subject says which one is dimmer or brighter. - Experiment performed using Fechner’s Method of Constant Stimuli. - Just like the other experiments, humans do not behave as ideal detectors when it comes to difference judgments either (gives an s- One of the shape function/ogive) most useful equations - (increment threshold) or (decrement threshold)where b is in the 75% brightness intensity, a is the 50% brightness intensity and perceptual c is the 25% brightness intensity. ∆I is the difference threshold. psychology 4. Weber’s Law Weber  what happens if we change the reference light in the previous experiment to a higher level, will the old difference threshold (∆I) stay the same? Multiple discrimination threshold experiments pg. 11 figure 1.5  the greater the intensity level at which we have to make a JND judgment, the greater the difference threshold (∆I) needed to attain that JND. Weber’s Law: The difference threshold is not constant but increases in a linear fashion with stimulus intensity. where k = proportion (Weber’s fraction) and I = stimulus intensity JND requirement: The incremental amount must be in proportion to the stimulus intensity. How do we find k? Experimentally determined (refer to Table 1.1 pg. 11 for Weber fractions for different sensory systems) Weber fractions are accurate for a broad range of stimulus intensities except for the extremities (eg. very high and very low intensities). At these extremes, Weber’s law no longer applies. 5. Fechner’ Law Fechner’s central goal was to find out the relationship between sensory magnitude and stimulus intensity. As we know, Weber’s law asserts that higher levels of suprathreshold intensity require a correspondingly greater change in intensity (∆I) to produce a change in sensation (∆S) that is just distinguishable (JND). Fechner wanted to know more about ∆S. Fechner’s assumption: All JNDs are produced by equal increments in sensation regardless of the operating level. In other words, exactly the same ∆S value was needed at all sensory magnitudes because JND is standard unit of change (psychological constant). D
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