In psychology, sensation and perception are stages of processing of the senses in human and animal systems, such as vision, auditory, vestibular, and pain senses. These topics are considered part of psychology, and not anatomy or physiology, because processes in the brain so greatly affect the perception of a stimulus. Included in this topic is the study of illusions such as motion aftereffect, color constancy, auditory illusions, and depth perception. Sensation is the function of the low-level biochemical and neurological events that begin with the impinging of a stimulus upon the receptor cells of a sensory organ. It is the detection of the elementary properties of a stimulus. Perception is the mental process or state that is reflected in statements like "I see a uniformly blue wall", representing awareness or understanding of the real-world cause of the sensory input. The goal of sensation is detection, the goal of perception is to create useful information of the surroundings. In other words, sensations are the first stages in the functioning of senses to represent stimuli from the environment, and perception is a higher brain function about interpreting events and objects in the world. Stimuli from the environment are transformed into neural signals which are then interpreted by the brain through a process called transduction. Transduction can be likened to a bridge connecting sensation to perception. Gestalt theorists believe that with the two together a person experiences a personal reality that is other than the sum of the parts. Senses and receptors While there is debate among neurologists as to the specific number of senses due to differing definitions of what constitutes a sense, Aristotle classified five 'traditional' human senses which have become universally accepted: touch, taste, smell, sight, and hearing. Other senses that have been well-accepted in most mammals, including humans, include nociception, equilibrioception, kinaesthesia, and thermoception. Furthermore, some non-human animals have been shown to possess alternate senses, including magnetoception and electroreception. Receptors The initialization of sensation stems from the response of a specific receptor to a physical stimulus. The receptors which react to the stimulus and initiate the process of sensation are commonly characterized in four distinct categories: chemoreceptors, photoreceptors, mechanoreceptors, and thermoreceptors. All receptors receive distinct physical stimuli and transduce the signal into an electrical action potential. This action potential then travels along afferent neurons to specific brain regions where it is processed and interpreted. Chemoreceptors Chemoreceptors, or chemosensors, detect certain chemical stimuli and transduce that signal into an electrical action potential. There are two primary types of chemoreceptors: Distance chemoreceptors are integral to receiving stimuli in the olfactory system through both olfactory receptor neurons and neurons in the vomeronasal organ. Direct chemoreceptors include the taste buds in the gustatory system as well as receptors in the aortic bodies which detect changes in oxygen concentration. Photoreceptors Photoreceptors are capable of phototransduction, a process which converts light (electromagnetic radiation) into, among other types of energy, a membrane potential. There are three primary types of photoreceptors: Cones are photoreceptors which respond significantly to color. In humans the three different types of cones correspond with a primary response to short wavelength (blue), medium wavelength (green), and long wavelength (yellow/red). Rods are photoreceptors which are very sensitive to the intensity of light, allowing for vision in dim lighting. The concentrations and ratio of rods to cones is strongly correlated with whether an animal is diurnal or nocturnal. In humans rods outnumber cones by approximately 20:1, while in nocturnal animals, such as the tawny owl, the ratio is closer to 1000:1. Ganglion Cells reside in the adrenal medulla and retina where they are involved in the sympathetic response. Of the ~1.3 million ganglion cells present in the retina, 1-2% are believed to be photosensitive. Mechanoreceptors Attribution: Article text available under CC-BY-SA Public domain image source in video

• published: 21 Nov 2013
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