Why does adaptation to stimuli occur

The homunculus is split in half across the brain, with motor representation for each side of the body represented on the the opposite side of the brain. Areas of the body with greater or more complex sensory or motor connections are represented as larger in the homunculus. Those with fewer or less complex connections are represented as smaller. The resulting image is that of a distorted human body with disproportionately huge hands, lips, and face because those regions have huge numbers of nerve endings.

Motor Homunculus : The motor homunculus is a theoretical visualization of the locations in the cortex that correspond to motor and sensory function in the body. A threshold is the minimum level at which a given event can occur. In neuroscience and psychophysics, there are several types of sensory threshold. The recognition threshold is the level at which a stimulus can not only be detected but also recognized; the differential threshold is the level at which a difference in a detected stimulus can be perceived; the terminal threshold is the level beyond which a stimulus is no longer detected.

However, perhaps the most important sensory threshold is the absolute threshold, which is the smallest detectable level of a stimulus. The absolute threshold is defined as the lowest intensity at which a stimulus can be detected.

A classic example of absolute threshold is an odor test, in which a fragrance is released into an environment. The absolute threshold in that scenario would be the least amount of fragrance necessary for a subject to detect that there is an odor. Smell is not the only sense with absolute thresholds. Sound thresholds can be about more than volume; they can also be about frequency. For example, humans cannot hear dog whistles. This is because dog whistles are at a frequency higher than the absolute threshold for frequency for human hearing.

Similarly, the minimum amount of light necessary to see something in the dark is the absolute threshold for vision. Every sense has an absolute threshold.

There are several factors that can influence the level of absolute threshold, including adaptation to the stimulus and individual motivations and expectations.

For example, when you are in a crowded room where a lot of conversations are taking place, you tend to focus your attention on the individual with whom you are speaking.

Because you are focused on one stimulus, the absolute threshold in this case, the minimum volume at which you can hear is lower for that stimulus than it would have been otherwise. Expectations can also affect the absolute threshold.

If you are in a dark hallway searching for the tiny glow of a nightlight, your expectation of spotting it decreases the absolute threshold for which you will actually be able to see it. The minimum amount of change in sensory stimulation needed to recognize that a change has occurred is known as the just-noticeable difference. The just-noticeable difference JND , also known as the difference limen or differential threshold, is the smallest detectable difference between a starting and secondary level of sensory stimulus.

In other words, it is the difference in the level of the stimulus needed for a person to recognize that a change has occurred. Turning Up the Volume : The difference threshold is the amount of stimulus change needed to recognize that a change has occurred. If someone changes the volume of a speaker, the difference threshold is the amount it has to be changed in order for listeners to notice a difference.

The JND is usually a fixed proportion of the reference sensory level. For example, consider holding a five-pound weight the reference level , and then having a one pound weight added. However, if you hold a fifty pound weight the new reference level , you would not be likely to notice a difference if one pound is added.

The absolute threshold is the minimum volume of the radio we would need in order to notice that it was turned on at all. However, determining the just-noticeable difference, the amount of change needed in order to notice that the radio has become louder, depends on how much the volume has changed in comparison to where it started.

This adaptation happens when you enter a dark building after exposure to the sunlight. Your pupils dilate in order for the retina to gain access to additional light.

The cones of your eyes increase in sensitivity as a reaction to the darkness; however, they adapt within approximately five minutes. The rods in your eyes have chemicals that increase with limited light and assist in the adaptation as well.

Individuals adapt to the noise within their environment. For those who live in an area with continuous traffic, their ears adapt to the constant sound until they no longer hear the noise of the traffic. With louder sounds, such as a rock band playing while entering a nightclub, the muscle attached to the inner ear bone contracts, reducing the sound vibration transmission.

This decreases the vibrations to the inner ear, thereby adjusting to the noise level. Those who smoke tobacco do not notice the smell of cigarettes. Nonsmokers can usually smell the cigarette odor intensely and, if in the presence of a smoker, can smell it not only in the smoker's presence but will continue to smell the odor on their clothes, hair and other items long after the two have parted.

This same adaptation happens when wearing perfume or cologne: Within an hour of applying the fragrance, the wearer no longer smells the scent. The feeling of hot and cold is an adaptation to the sensation of touch. Just imagine what it would be like if you didn't experience sensory adaptation. You might find yourself overwhelmed by the pungent smell of onions coming from the kitchen or the blare of the television from the living room.

Since constant exposure to a sensory stimulus reduces our sensitivity to it, we are able to shift our attention to other things in our environment rather than focusing on one overwhelming stimulus.

Here are some more examples of sensory adaptation in different senses. Even hand-eye coordination adjusts when necessary. For instance, if you put on goggles that make everything appear to be a little off and you try to throw a ball at an object, eventually your sensory adaptation will take over and you'll adjust enough to be able to hit it.

If you've heard the term "nose blind," you've heard of sensory adaption; it's the same thing. But it's different from anosmia, or the inability to smell. You also might notice that when you're away from a smell or a sound for a while, such as when you go on vacation and then return to your home, you notice it again. It will probably not take much time for you to adapt to the sensory inputs of your environment and go "blind" to them once again. Ever wonder what your personality type means?

Sign up to find out more in our Healthy Mind newsletter. Coon D, Mitterer JO. Introduction to psychology: gateways to mind and behavior with concept maps.

Wadsworth; Webster MA. Evolving concepts of sensory adaptation. F Biol Rep. Your Privacy Rights.