Sensory maps

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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.

Contents

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. [1] The central nervous system is attached to this cortex and all other parts of an organism’s body. [2] 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. [3]

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. [4] Each system produces different sensory maps that are connected to analyze an organism’s surroundings more thoroughly. [5] [2] 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. [4] An organism then acts based on the information they receive and already have. [1] 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. [6]

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. [5] Also, for one sensory system there can be multiple different maps working together to analyze different aspects of a stimulus. [4] Some advantages of sensory maps have been elucidated by scientific exploration:

Types

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, [10] as in the tactile sense of touch, or tonotopic, [11] 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. [3]

Phantom limbs activate sensory maps according to scientists. [3] 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. [3]

Examples

  • Wilder Penfield [12] discovered the original topographic map in the form of the internal somatosensory Homunculus. His work on human neural systems showed that the brain areas that processed tactile sensations are mapped in the same fashion that the body is laid out. This sensory map exaggerates certain regions that have many peripheral sense cells like the lips and hands while reduces the relative space for processing areas with few receptors like the back.
  • Hair cells in the auditory system display tonotopic organization. [13] This tonotopic arrangement means that cells are laid out to range from low frequency to high frequency and processed in that same organization within the brain.

Computational maps

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. [5]

Examples

  • The Jeffres Map was a theory of how the brain might compute interaural time differences (ITD), or differences in time of stimulus arrival between the two ears. Jeffres was famous for producing a theoretical mechanism for making a place map out of timing information, this explained how some animals could appear to have a "look-up map" for where a sound came from. The neural system computes this ITD in the Owl Auditory System and the real neural system was found to almost exactly match the Jeffres Map theory. [14] The Jeffres Map shows how ITD signals are used to determine distance and direction in the owl.
  • Feature Detectors in a visual system are another example of computational maps. No part of the physical system in the eyes actually analyzes for features like simple cells in the brain do. This system is well studied in frogs. It is known that frogs detect specific "worm-like" features in their environment and, controlled entirely by the neural system, will lunge at them even if they are a series of white squares in a line imitating a basic worm. [15] Creating illusions within our sensory maps is a way that organisms fill in for unknown information about their surroundings. [3]
  • There is also a Frequency Modulation to Frequency Modulation Comparison in the Bat Auditory System which is used in echolocation. This FM-FM comparison determines flutter of their target and was made famous in work by Suga. [16]
  • When motor and sensory systems were studied by way of fish, scientists found that there could be computational maps made between the two. Fish whose central nervous system was inactivated, for a specific appendage, adjusted their previous natural behavior. Scientists believe that sensory information often precedes the actions and decisions made by organisms. Thus, when there is additional information given by outside stimuli, or lack of it, their behavior changes to adapt to new surroundings. [2]

Abstract maps

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. [6]

Related Research Articles

<span class="mw-page-title-main">Nervous system</span> Part of an animal that coordinates actions and senses

In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events. Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago. In vertebrates it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The PNS consists mainly of nerves, which are enclosed bundles of the long fibers or axons, that connect the CNS to every other part of the body. Nerves that transmit signals from the brain are called motor nerves or efferent nerves, while those nerves that transmit information from the body to the CNS are called sensory nerves or afferent. Spinal nerves are mixed nerves that serve both functions. The PNS is divided into three separate subsystems, the somatic, autonomic, and enteric nervous systems. Somatic nerves mediate voluntary movement. The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal system. Both autonomic and enteric nervous systems function involuntarily. Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves.

<span class="mw-page-title-main">Perception</span> Interpretation of sensory information

Perception is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system, which in turn result from physical or chemical stimulation of the sensory system. Vision involves light striking the retina of the eye; smell is mediated by odor molecules; and hearing involves pressure waves.

<span class="mw-page-title-main">Sensory nervous system</span> Part of the nervous system responsible for processing sensory information

The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons, neural pathways, and parts of the brain involved in sensory perception and interoception. Commonly recognized sensory systems are those for vision, hearing, touch, taste, smell, balance and visceral sensation. Sense organs are transducers that convert data from the outer physical world to the realm of the mind where people interpret the information, creating their perception of the world around them.

<span class="mw-page-title-main">Brodmann area</span> Region of the brain

A Brodmann area is a region of the cerebral cortex, in the human or other primate brain, defined by its cytoarchitecture, or histological structure and organization of cells.

<span class="mw-page-title-main">Afferent nerve fiber</span> Axonal projections that arrive at a particular brain region

Afferent nerve fibers are axons of sensory neurons that carry sensory information from sensory receptors to the central nervous system. Many afferent projections arrive at a particular brain region.

Stimulus modality, also called sensory modality, is one aspect of a stimulus or what is perceived after a stimulus. For example, the temperature modality is registered after heat or cold stimulate a receptor. Some sensory modalities include: light, sound, temperature, taste, pressure, and smell. The type and location of the sensory receptor activated by the stimulus plays the primary role in coding the sensation. All sensory modalities work together to heighten stimuli sensation when necessary.

<span class="mw-page-title-main">Sensory neuron</span> Nerve cell that converts environmental stimuli into corresponding internal stimuli

Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cell bodies of the sensory neurons are located in the dorsal ganglia of the spinal cord.

The receptive field, or sensory space, is a delimited medium where some physiological stimuli can evoke a sensory neuronal response in specific organisms.

Multisensory integration, also known as multimodal integration, is the study of how information from the different sensory modalities may be integrated by the nervous system. A coherent representation of objects combining modalities enables animals to have meaningful perceptual experiences. Indeed, multisensory integration is central to adaptive behavior because it allows animals to perceive a world of coherent perceptual entities. Multisensory integration also deals with how different sensory modalities interact with one another and alter each other's processing.

<span class="mw-page-title-main">Thalamocortical radiations</span> Neural pathways between the thalamus and cerebral cortex

In neuroanatomy, thalamocortical radiations are the fibers between the thalamus and the cerebral cortex.

Neuronal tuning refers to the hypothesized property of brain cells by which they selectively represent a particular type of sensory, association, motor, or cognitive information. Some neuronal responses have been hypothesized to be optimally tuned to specific patterns through experience. Neuronal tuning can be strong and sharp, as observed in primary visual cortex, or weak and broad, as observed in neural ensembles. Single neurons are hypothesized to be simultaneously tuned to several modalities, such as visual, auditory, and olfactory. Neurons hypothesized to be tuned to different signals are often hypothesized to integrate information from the different sources. In computational models called neural networks, such integration is the major principle of operation. The best examples of neuronal tuning can be seen in the visual, auditory, olfactory, somatosensory, and memory systems, although due to the small number of stimuli tested the generality of neuronal tuning claims is still an open question.

Sensory neuroscience is a subfield of neuroscience which explores the anatomy and physiology of neurons that are part of sensory systems such as vision, hearing, and olfaction. Neurons in sensory regions of the brain respond to stimuli by firing one or more nerve impulses following stimulus presentation. How is information about the outside world encoded by the rate, timing, and pattern of action potentials? This so-called neural code is currently poorly understood and sensory neuroscience plays an important role in the attempt to decipher it. Looking at early sensory processing is advantageous since brain regions that are "higher up" contain neurons which encode more abstract representations. However, the hope is that there are unifying principles which govern how the brain encodes and processes information. Studying sensory systems is an important stepping stone in our understanding of brain function in general.

<span class="mw-page-title-main">Lateral inhibition</span> Capacity of an excited neuron to reduce activity of its neighbors

In neurobiology, lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition disables the spreading of action potentials from excited neurons to neighboring neurons in the lateral direction. This creates a contrast in stimulation that allows increased sensory perception. It is also referred to as lateral antagonism and occurs primarily in visual processes, but also in tactile, auditory, and even olfactory processing. Cells that utilize lateral inhibition appear primarily in the cerebral cortex and thalamus and make up lateral inhibitory networks (LINs). Artificial lateral inhibition has been incorporated into artificial sensory systems, such as vision chips, hearing systems, and optical mice. An often under-appreciated point is that although lateral inhibition is visualised in a spatial sense, it is also thought to exist in what is known as "lateral inhibition across abstract dimensions." This refers to lateral inhibition between neurons that are not adjacent in a spatial sense, but in terms of modality of stimulus. This phenomenon is thought to aid in colour discrimination.

Neural coding is a neuroscience field concerned with characterising the hypothetical relationship between the stimulus and the individual or ensemble neuronal responses and the relationship among the electrical activity of the neurons in the ensemble. Based on the theory that sensory and other information is represented in the brain by networks of neurons, it is thought that neurons can encode both digital and analog information.

A topographic map is the ordered projection of a sensory surface, like the retina or the skin, or an effector system, like the musculature, to one or more structures of the central nervous system. Topographic maps can be found in all sensory systems and in many motor systems.

A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of stimuli. Although traditionally five human senses were identified as such, it is now recognized that there are many more. Senses used by non-human organisms are even greater in variety and number. During sensation, sense organs collect various stimuli for transduction, meaning transformation into a form that can be understood by the brain. Sensation and perception are fundamental to nearly every aspect of an organism's cognition, behavior and thought.

Feature detection is a process by which the nervous system sorts or filters complex natural stimuli in order to extract behaviorally relevant cues that have a high probability of being associated with important objects or organisms in their environment, as opposed to irrelevant background or noise.

Sensory-motor coupling is the coupling or integration of the sensory system and motor system. Sensorimotor integration is not a static process. For a given stimulus, there is no one single motor command. "Neural responses at almost every stage of a sensorimotor pathway are modified at short and long timescales by biophysical and synaptic processes, recurrent and feedback connections, and learning, as well as many other internal and external variables".

Neurocomputational speech processing is computer-simulation of speech production and speech perception by referring to the natural neuronal processes of speech production and speech perception, as they occur in the human nervous system. This topic is based on neuroscience and computational neuroscience.

Sensory maps and brain development is a concept in neuroethology that links the development of the brain over an animal’s lifetime with the fact that there is spatial organization and pattern to an animal’s sensory processing. Sensory maps are the representations of sense organs as organized maps in the brain, and it is the fundamental organization of processing. Sensory maps are not always close to an exact topographic projection of the senses. The fact that the brain is organized into sensory maps has wide implications for processing, such as that lateral inhibition and coding for space are byproducts of mapping. The developmental process of an organism guides sensory map formation; the details are yet unknown. The development of sensory maps requires learning, long term potentiation, experience-dependent plasticity, and innate characteristics. There is significant evidence for experience-dependent development and maintenance of sensory maps, and there is growing evidence on the molecular basis, synaptic basis and computational basis of experience-dependent development.

References

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  11. ↑ Kaltenbach J.A., Czaja J.M., Kaplan CR., (1992), Changes in the tonotopic map of the dorsal cochlear nucleus following induction of cochlear lesions by exposure to intense sound. Hearing Research. 59(2):213-23
  12. ↑ Penfield, W., Rasmussen, T., (1950), The cerebral cortex of man: a clinical study of localization of function, Macmillan.
  13. ↑ R.V., Ibrahim, D., and Mount, R.J., (1998), Plasticity of tonotopic maps in auditory midbrain following partial cochlear damage in the developing chinchilla, Experimental Brain Research. Vol 123(4), 1432-1106
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  15. ↑ Carew, T.J. (2000), Behavioral neurobiology: The Cellular Organization of Natural Behavior, Sinauer Associates.
  16. ↑ Suga, N. (1989), Principles of auditory information-processing derived from neuroethology. Journal of Experimental Biology. Vol 146(1): 277-286
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