Sensory maps


Sensory maps are areas of the brain which respond to sensory stimulation, and are spatially organized according to some feature of the sensory stimulation. In some cases the sensory map is simply a topographic representation of a sensory surface such as the skin, cochlea, or retina. In other cases it represents other stimulus properties resulting from neuronal computation and is generally ordered in a manner that reflects the periphery. An example is the somatosensory map which is a projection of the skin's surface in the brain that arranges the processing of tactile sensation. This type of somatotopic map is the most common, possibly because it allows for physically neighboring areas of the brain to react to physically similar stimuli in the periphery or because it allows for greater motor control.
The somatosensory cortex is adjacent to the primary motor cortex which is similarly mapped. Sensory maps may play an important role in facilitating motor responses. Other examples of sensory map organization may be that adjacent brain regions are related through proximity of the receptors that they process as in the map of the cochlea in the brain, or that similar features are processed as in the map of the feature detectors or the retinotopic map, or that time codes are used in organization as in the maps of an owl's sense of direction via interaural time difference between ears. These examples exist in contrast to non-mapped or randomly distributed patterns of processing. An example of a non-mapped sensory processing system is the olfactory system where unrelated odorants are processed side-by-side in the olfactory bulb. In addition to non-mapped and mapped processing, stimuli may be processed under multiple maps as in the human visual system.

Neurobiology

Sensory maps are created primarily within the somatosensory cortex, also referred to as the sensory cortex. The central nervous system is attached to this cortex and all other parts of an organism’s body. Both the somatosensory cortex and the central nervous system are made up of neurons which create associations with each other to transmit electrical impulses throughout the body.
The central nervous system, when made aware of various stimuli without the body, sends signals to the brain. These signals are sent by different parts of the body e.g. the auditory system, system that uses touch, and visual system. Each system produces different sensory maps that are connected to analyze an organism’s surroundings more thoroughly. For one sensory system there are multiple maps that analyze the stimulus. These maps work together to glean spatial, characteristic, and action information from surroundings. An organism then acts based on the information they receive and already have. Scientists speculate that these nerve connections have grown increasingly over the lifetime of an organism and have also been genetically passed on by earlier generations.

Functions

Mapped sensory processing areas are a complex phenomenon and must therefore serve an adaptive advantage as it is highly unlikely for complex phenomena to appear otherwise. Sensory maps are also very old in evolutionary history as they are nearly ubiquitous in all species of animals and are found for nearly all sensory systems. The dynamic nature of neurons, which collect sensory information to create these maps, allow different stimuli to change maps made by other sensory neurons in the past. Also, for one sensory system there can be multiple different maps working together to analyze different aspects of a stimulus. Some advantages of sensory maps have been elucidated by scientific exploration:

Topographic maps

These maps may be thought of as a mapping of the surface of the body onto the brain structure. Phrased another way, topographic maps are organized in the neural system in a manner that is a projection of the sensory surface within the brain. This means that the organization in the periphery mirrors the order of the information processing in the brain. This organization can be somatotopic, as in the tactile sense of touch, or tonotopic, as in the ear, and the retinotopic map which is laid out in the brain as the cells are arranged on the retina. Neurons on the surface of the body have importance in our day to day life. There are more neurons connected to the parts of the surface of the body when the neuron’s roles are more important than other neurons in relation to our well-being.
Phantom limbs activate sensory maps according to scientists. Because there is no actual connection between the amputee limb and the rest of the body, it is assumed that when the limb was detached from the rest of the body the sensory maps which were created before the amputation, are still active and are being activated without an actual stimulus.

Examples

These maps are organized entirely in the neural system or organized in a manner not present in the periphery. Sensory information for computational maps comes from auditory and visual stimuli. Thus, any auditory or visual information that is constructed by neural computation, which is when the brain relates two or more bits of information in order to obtain some new information from them, can combine to change the already existing sensory map to include the new information. Often these maps involve comparing, as in performing subtraction to get a time delay, two stimuli, like incoming sound information from different ears, in order to produce a valuable new bit of information about those stimuli, as in where they originated. The process just described takes place in the owl's neural system very rapidly.

Examples

Abstract maps are maps that are also created by stimuli outside of an organism, but it has no surface by which it creates a map in the brain. They are ordered like topographical and computational maps, but their features are abstract. These types of maps are associated with seeing color.