Explain the activities of the sensory system and predict sensory behavior
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The ability to sense stimuli is calledReceptivity。The size of receptivity is measured by the size of the sensory threshold.Each feeling has two receptivities and sensory thresholds: absolute;And absolute threshold, differential sensitivity and differential threshold.The purpose of these laws is to explain the activities of the sensory system and predict sensory behavior.
Not all stimuli of any intensity can cause our feelings.Too weak stimulation, such as dust falling on the skin, is invisible to us.Only when the stimulus reaches a certain intensity can we be aware of it.The minimum stimulus that can be just perceived is called absolute threshold.Absolute receptivityIt refers to the ability to detect the minimum stimulus just now.
Absolute threshold is not a single intensity value, but a statistical concept.When measuring the sensory threshold, as the amount of stimulus gradually increases, the subjects will be able to detect the stimulus from imperceptible, sometimes perceptible, sometimes imperceptible, to completely perceptible.As shown in Figure 7-2, with the increase of stimulus amount, the percentage of times that subjects reported perceiving stimulus increased.【This curve is called psychophysical function, which indicates that psychological quantity (sensory experience) andphysical quantity(physical intensity of stimulus).】According to the convention, psychologists set the absolute threshold value of 50% of the times the stimulus value is perceived.Therefore, it is obvious that there is no special value between what we feel and what we do not feel.The threshold is a gradually transitional intensity range.
Absolute receptivityIt is inversely proportional to the absolute threshold in quantity.If E represents absolute receptivity R represents absolute sensory threshold, the relationship between them can be expressed by the following formula:
E=1/R
The absolute thresholds of various senses are different.Under appropriate conditions, the threshold of human sensation is very low.For example, under the condition that the air is completely transparent, people can see the candle light one thousandth of a kilometer away.If one gram of water is heated to 1 ℃ with this energy, it will take 60 million years.People can smell a milligram of artificial musk in a liter of air.Of course, the absolute thresholds of different individuals are quite different, and even the same individual will change due to the body condition and motivation level.
Although we cannot detect the stimulus below the absolute sensory threshold, it can cause certain physiological effects.For example, sound stimuli below the auditory threshold can cause changes in brain waves and pupil dilation.Therefore, the conscious sensory threshold is not identical to the physiological stimulation threshold.Generally speaking, the threshold of physiological stimulation is lower than the threshold of conscious sensation.Because a certain physiological process has already occurred before a person said "I feel it".
difference sensitivity and difference threshold
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On the basis that stimuli can cause sensation, if the stimulus quantity is changed, not all changes in quantity can be detected by us.For example, if we add one candle to the original 200 candles, we will not be able to detect the change of light intensity;It is necessary to add 2 candles or more to detect the difference in intensity between the two lights.In order to cause a differential sensation, the stimulus must be increased or reduced to a certain amount.The minimum difference that can detect two stimuli is calledDifferential sensory thresholdOr just notifiable difference (jnd).The ability to sense this minimum difference is called differential sensitivity.
In 1834, German physiologist E.H. Weber (1795-1878) found when studying the difference threshold of sensation that if R (abbreviation of Reiz in German) was used to represent the initial stimulus intensity, and R+Δ R was used to represent the stimulus intensity that had just been perceived to have changed, then within a certain range, the difference threshold of each sensation is a relative constant, which can be expressed by mathematical formula as:
ΔR/R=K
This formula is calledWeber's law(or Weber fraction, Weber ratio, Weber constant), that is, when the size of R is different, the size of Δ R (the physical quantity of the minimum deviation) is also different, but West R is a constant.For example, if the weight of 50g (R) was originally lifted, the difference threshold was 2g, then at least 51g weight was perceived to be slightly heavier than the original weight;If the weight is 100g, at least 102g will be perceived as slightly heavier than it;If it is 150 grams, at least 153 grams will be perceived as slightly heavier than it.It can be seen that here, the difference threshold value is the same fraction of the stimulus weight: l/50=2/100=3/150=0.02.This Weber fraction shows that it must be increased by 2% on the basis of the original weight to detect that it is slightly heavier than the original weight.
The Weber scores of different sensory systems vary greatly.Table 7-1 shows the Weber scores of different sensory systems under the condition of moderate intensity stimulation.The smaller Weber's score, the more sensitive he feels.
Weber scores of various sensory systems are correct in the range of moderate intensity stimuli, but they are not correct in the condition of extreme stimuli (process or too weak).Fichner (C.T. Fechner, 1861-1887) determined the change of Weber's fraction when approaching the absolute threshold, further assumed that a minimum sense error is a sensory unit, and deduced the following formula on the basis of Weber's law:
S=KlogR+C
Here, S is the sensory intensity, R is the stimulus intensity, and K and C are constants, that is, the stimulus intensity increases geometrically, while the sensory intensity increases only arithmetically.This is it.Fechner's law。Later studies have shown that Fischner's law is not universal, and it has only approximate meaning, that is, it is only applicable to the range of moderate stimulus intensity, although this range is quite large.
The difference threshold is relative, not absolute.According to the convention, the threshold of difference is the smallest difference of stimuli recognized in 50% of the number of experiments.There is also an inverse relationship between differential sensitivity and differential threshold.
