Sensation involves the basic elements of experience. Different types of stimuli activate different sense organs. The light stimuli activate the sense of sight, while the sound stimuli activate the sense of hearing. The stimulus energy arrives at our sensory receptors as physical energy, which is then converted to neural impulse. The process by which one form of physical energy is converted into another form such as neural impulses is known as transduction. Each stimulus that is capable of activating a sense organ can also be considered in terms of its strength and intensity.
The issue of how the intensity of a stimulus influences our sensory responses is dealt by a branch of psychology known as psychophysics. Psychophysics is the study of the relationship between the physical nature of the stimuli and the sensory responses they evoke. Psychophysics provides the link between the external physical world and the internal psychological world. Classic research in psychophysics has examined two types sensory thresholds: absolute threshold, and sensory threshold.
Absolute threshold:
A stimulus requires a minimum intensity to activate the receptor cells of a sense organ; otherwise there would be no sensation and perception. That minimum stimulus value which is required to activate the receptor cells of a sense organ is called the absolute threshold or the stimulus threshold for that sense organ. In German, it is called reiz limen (RL). A stimulus below this threshold value does not activate the receptor cells of the sense organ, and hence, the sensation does not pass on to the brain.
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Therefore, in order that a perceptual activity is conducted, the stimulus intensity must reach the absolute threshold. The value of absolute threshold varies from person to person, for different sense organs in the same person, and for varying internal and organic conditions of a person.
For example, suppose a very dim and faint light is presented to you from a distance; you may not be able to see the light. Now, the intensity of the light is increased a bit; you also may not be able to see it. Intensity is increased a bit further; you still do not see the light. Now, if the intensity of the light goes on increasing this way, at one time you will be able to see the light. The light intensity which you can detect 50% of the times it is presented is the absolute threshold for your perception of light.
Thus, the absolute threshold is the least or minimum value of a stimulus which is perceived by a person at least in fifty percent of presentations. This threshold for one person may not be the same for another person as individuals differ from one another in relation to their absolute threshold values for different senses.
Difference threshold:
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Two stimuli of a particular kind may vary from each other in intensity, but may not be perceived as different. For example, the brightness of two lights may be different, but if the difference is very small we do not notice the difference between them. Similarly, when we add a little more salt to our curry, we may not notice a difference in saltiness of the curry.
Thus, in order to actually notice a difference between two stimuli of the same kind, we need a minimum difference in the intensity between the two stimuli. The minimum difference in the intensity of two stimuli, which is required to perceive them as different 50% of the times is called the differential threshold or differential limen.
Thus, the difference in stimulation produced by two similar stimuli should be at least above this threshold to generate any kind of a stable difference in sensation and perception of those stimuli.