Our sense organs provide us with first hand information about our external or internal world. Different sense organs deal with different forms of stimuli and serve different purposes. Each sense organ is highly specialised for dealing with a particular kind of information. However, all sense organs function with certain limitations. For example, our eyes cannot see things which are very dim or very bright. Similarly, our ears cannot hear very faint or very loud sounds. As human beings, we function within a limited range of stimulation. A stimulus, to be noticed by a sensory receptor, has to be of an optimal intensity or magnitude. In order to be noticed, a stimulus has to carry a minimum value or weight.

The minimum value of a stimulus required to activate a given sensory system is called absolute limen (AL). Absolute limen is not a fixed point, instead it varies considerably across individuals and situations depending on the people’s organic conditions and their motivational states. Hence, it is assessed on the basis of a number of trials. Further, it is not possible to differentiate between all stimuli. In order to notice two stimuli are different from each other. These has to be some minimum difference between the value of those stimuli. The smallest difference in the value of two stimuli that is necessary to notice them as different is termed as difference limen (DL). Thus, it is not possible to understand sensations without the AL and DL of different types of stimuli.

Besides the stimulus characteristics, sensory processes also depend on other characteristics. Sense organs and the neural pathways connecting them to various brain centres also play a vital role in this process. A sense organ receives the stimulus and encodes it as an electrical impulse. For being noticed this electrical impulse must reach the higher brain centres. Any structural or functional defect or damage in the receptor organs, its neural pathway, or the concerned brain area may lead to a partial or complete loss of sensation.