Exteroception, together with interoception, forms the totality of human perception. Extroception is the perception of external stimuli by specialized sensory cells called extroceptors. The processing of stimuli occurs in the central nervous system and may be impaired in neurological diseases.
What is exteroception?
Extroception is the perception of external stimuli by specialized sensory cells called extroceptors, such as the sensory cells in the ear. Human perception allows people to form a picture of themselves and their environment. The perception of internal stimuli and the perception of external stimuli make up the overall perceptual ability of humans. Internal stimuli are perceived from within the body and are thus a significant component of self-perception. External stimuli are all external environmental stimuli that allow humans to perceive the outside world. Internal perception is interoception. External perception is analogously called exteroception. It is composed of visual, auditory, gustatory, olfactory and vestibular perception. In addition, sensitivity counts as well. Like internal perception, external perception works with stimulus-specialized sensory cells called receptors. The receptors of external perception are the extroceptors. They are responsible for external stimulus reception, stimulus processing, and transmission of stimulus information in physiologically processable form. Stimulus conduction occurs via afferent pathways and targets the central nervous system, where stimuli from the environment are combined and enter consciousness as an encompassing image.
Function and task
The exteroceptors are the first site that external stimuli pass through on their way into the human body. These receptors are each specialized for specific stimuli. Stimulus molecules bind to the designated sites to excite the receptor, which converts the stimulus into a physiological form of nerve excitation. For example, there are specialized extroceptors for sensing vibration, touch, temperature and many other external stimuli. The opposite of the extroceptors are the interoceptors, which measure internal stimuli. Perceptual structures such as the depth sensitivity of the musculoskeletal system register both external and internal stimuli and thus can be described as interoceptors and exteroceptors at the same time. Exteroceptors include receptors such as the Vater-Pacini corpuscles for the perception of vibrations or the Meissner corpuscles and Ruffini corpuscles for the registration of touch, pressure and pressure differences. The photoreceptors of the eye are receptive to light and the hair cells in the inner ear enable auditory perceptions. All exteroceptors are interconnected via the first neuron to the second neuron. The cell bodies of exteroceptive neurons are located in the spinal ganglion. Their central projections cross the posterior cord pathways without switching or crossing and thus reach the nucleus gracilis or nucleus cuneatus. Only here is the information switched to the second neuron. The fibers originating from there are called fibrae arcuatae internae and extend toward the thalamus. In the decussatio lemnisci medialis they are involved in a crossing. In the nucleus ventralis posterior of the thalamus, the fibers terminate and the information from the exteroceptors is switched to the third neuron. This third neuron runs across the radiatio thalami superior or crus posterior of the capsula interna and from there reaches the primary somatosensitive brain center in the postcentral gyrus. There, Brodmann areas 3,2 and 1 are located. In the brain, besides storage, classification and interpretation of exteroceptive perceptions, an initial stimulus response occurs if necessary. Exteroception is divided by some authors into epicritical sensitivity and protopathic perception. Epicritical sensitivity is defined as perception of subtle touch, vibration perception, and pressure perception and is performed using two-point discrimination. Information collected in this way reaches the brain via the posterior cord pathways fasciculus gracilis and fasciculus cuneatus. By protopathic perception, the authors mean the pain and temperature perceptions that are delivered to the brain via the anterior cord of the tractus spinothalamici anterior et laterales.
Diseases and ailments
Exteroception can be irreversibly damaged by neurological diseases or traumatic lesions of the nerve structures involved and thus be permanently disturbed. Causative diseases in this context include central nervous diseases such as multiple sclerosis or peripheral nerve diseases such as polyneuropathy. However, extroceptive perceptual disorders are not always preceded by actual nerve lesions. In some cases, only the sensory integration of external information is disturbed. This integration occurs in the brain and corresponds to the combination of multiple stimuli into an overall stimulus image. The environmental image is thus a product of the precise interaction of the individual senses. Sensory integration disorders prevent this interaction. Sensory integration disorders are usually related to a person’s attention and correspond to sensory hyposensitivity to certain external stimuli. The brain has to select sensory stimuli in order not to overload itself. Attention to external stimuli is accordingly limited and not always adequately distributed. For example, maintaining posture based on external stimuli requires attention that other activities may lack at the same time. Sensory integration disorders with a postural weakness thus often manifest themselves, for example, in chronic restlessness. Hyposensitivity of the tactile and proprioceptive apparatus manifests itself in insufficient movement planning and clumsiness. Hypersensitivities in this area are modulation disorders and do not allow the nervous system to filter adequately, resulting in tactile defensiveness. Unexpected touch is thus avoided and social anxiety may result. Most commonly, but not exclusively, children are affected by integration disorders. Sometimes sensory-integrative disorders develop from neurological disorders such as stroke. In such a case, the term SI disorder is used. A particular example of a disorder with integrative sensory disorder is autism, which is often also characterized by altered pain perception.
