A stereoscope is a device for viewing a stereoscopic pair of separate images, depicting left-eye and right-eye views of the same scene, as a single three-dimensional image. A typical stereoscope provides each eye with a lens that makes the image seen through it appear larger and more distant and also shifts its apparent horizontal position, so that for a person with normal binocular depth perception the edges of the two images fuse into one "stereo window". In current practice, the images are prepared so that the scene appears to be beyond this virtual window, through which objects are sometimes allowed to protrude, but this was not always the custom. A divider or other view-limiting feature is provided to prevent each eye from being distracted by seeing the image intended for the other eye. Most people can, with practice and some effort, view stereoscopic image pairs in 3D without the aid of a stereoscope, but the physiological depth cues resulting from the unnatural combination of eye convergence and focus required will be unlike those experienced when viewing the scene in reality, making an accurate simulation of the natural viewing experience impossible and tending to cause eye strain and fatigue.
Although more recent devices such as Realist-format 3D slide viewers and the View-Master are stereoscopes, the word is now most associated with viewers designed for the standard-format stereo cards that enjoyed several waves of popularity from the 1850s to the 1930s as a home entertainment medium. Devices such as polarized and shutter glasses which are used to view two superimposed or intermingled images, rather than two physically separate images, are not categorized as stereoscopes; the earliest type of stereoscope was invented by Sir Charles Wheatstone in 1838. It used a pair of mirrors at 45 degree angles to the user's eyes, each reflecting a picture located off to the side, it demonstrated the importance of binocular depth perception by showing that when two pictures simulating left-eye and right-eye views of the same object are presented so that each eye sees only the image designed for it, but in the same location, the brain will fuse the two and accept them as a view of one solid three-dimensional object.
Wheatstone's stereoscope was introduced in the year before the first practical photographic process became available, so drawings were used. This type of stereoscope has the advantage that the two pictures can be large if desired. Contrary to a common assertion, David Brewster did not invent the stereoscope, as he himself was at pains to make clear. A rival of Wheatstone, Brewster credited the invention of the device to a Mr. Elliot, a "Teacher of Mathematics" from Edinburgh, according to Brewster, conceived of the idea as early as 1823 and, in 1839, constructed "a simple stereoscope without lenses or mirrors", consisting of a wooden box 18 inches long, 7 inches wide and 4 inches high, used to view drawn landscape transparencies, since photography had yet to be invented. Brewster's personal contribution was the suggestion to use lenses for uniting the dissimilar pictures in 1849; this allowed a reduction in size, creating hand-held devices, which became known as Brewster Stereoscopes, much admired by Queen Victoria when they were demonstrated at the Great Exhibition of 1851.
Brewster was unable to find in Britain an instrument maker capable of working with his design, so he took it to France, where the stereoscope was improved by Jules Duboscq who made stereoscopes and stereoscopic daguerreotypes, a famous picture of Queen Victoria, displayed at The Great Exhibition. Overnight a 3D industry developed and 250,000 stereoscopes were produced and a great number of stereoviews, stereo cards, stereo pairs or stereographs were sold in a short time. Stereographers were sent throughout the world to capture views for the new medium and feed the demand for 3D. Cards were printed with these views with explanatory text when the cards were looked at through the double-lensed viewer, sometimes called a stereopticon, a common misnomer. In 1861 Oliver Wendell Holmes created and deliberately did not patent a handheld, much more economical viewer than had been available before; the stereoscope, which dates from the 1850s, consisted of two prismatic lenses and a wooden stand to hold the stereo card.
This type of stereoscope remained in production for a century and there are still companies making them in limited production currently. In the mid-20th century the View-Master stereoscope, with its rotating cardboard disks containing image pairs, was popular first for'virtual tourism' and as a toy. In 2010, Hasbro started producing a stereoscope designed to hold an iPhone or iPod Touch, called the My3D. In 2014, Google released. Apps on the mobile phone substitute for stereo cards; the underlying technology is otherwise unchanged from earlier stereoscopes. Several fine arts photographers and graphic artists have and continue to produce original artwork to be viewed using stereoscopes. A simple stereoscope is limited in the size of the image. A more complex stereoscope uses a pair of horizontal periscope-like devices, allowing the use of larger images that can present more detailed information in a wider field of view; the stereoscope is an instrument in which two photographs of the same object, taken from different angles, are presented, one to each eye.
This recreates the way which in natural vis
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted or reflected from a particular area, falls within a given solid angle; the SI unit for luminance is candela per square metre. A non-SI term for the same unit is the nit; the CGS unit of luminance is the stilb, equal to one candela per square centimetre or 10 kcd/m2. Luminance is used to characterize emission or reflection from flat, diffuse surfaces; the luminance indicates how much luminous power will be detected by an eye looking at the surface from a particular angle of view. Luminance is thus an indicator of. In this case, the solid angle of interest is the solid angle subtended by the eye's pupil. Luminance is used in the video industry to characterize the brightness of displays. A typical computer display emits between 50 and 300 cd/m2; the sun has a luminance of about 1.6×109 cd/m2 at noon. Luminance is invariant in geometric optics.
This means that for an ideal optical system, the luminance at the output is the same as the input luminance. For real, optical systems, the output luminance is at most equal to the input; as an example, if one uses a lens to form an image, smaller than the source object, the luminous power is concentrated into a smaller area, meaning that the illuminance is higher at the image. The light at the image plane, fills a larger solid angle so the luminance comes out to be the same assuming there is no loss at the lens; the image can never be "brighter" than the source. The luminance of a specified point of a light source, in a specified direction, is defined by the derivative L v = d 2 Φ v d Σ d Ω Σ cos θ Σ where Lv is the luminance, d2Φv is the luminous flux leaving the area dΣ in any direction contained inside the solid angle dΩΣ, dΣ is an infinitesimal area of the source containing the specified point, dΩΣ is an infinitesimal solid angle containing the specified direction, θΣ is the angle between the normal nΣ to the surface dΣ and the specified direction.
If light travels through a lossless medium, the luminance does not change along a given light ray. As the ray crosses an arbitrary surface S, the luminance is given by L v = d 2 Φ v d S d Ω S cos θ S where dS is the infinitesimal area of S seen from the source inside the solid angle dΩΣ, dΩS is the infinitesimal solid angle subtended by dΣ as seen from dS, θS is the angle between the normal nS to dS and the direction of the light. More the luminance along a light ray can be defined as L v = n 2 d Φ v d G where dG is the etendue of an infinitesimally narrow beam containing the specified ray, dΦv is the luminous flux carried by this beam, n is the index of refraction of the medium; the luminance of a reflecting surface is related to the illuminance it receives: ∫ Ω Σ L v d Ω Σ cos θ Σ = M v = E v R where the integral covers all the directions of emission ΩΣ, Mv is the surface's luminous exitance Ev is the received illuminance, R is the reflectance. In the case of a diffuse reflector, the luminance is isotropic, per Lambert's cosine law.
The relationship is L v = E v R / π A variety of units have been used for luminance, besides the candela per square metre. One candela per square metre is equal to: 10−4 stilbs π apostilbs π×10−4 lamberts 0.292 foot-lamberts Retinal damage can occur when the eye is exposed to high luminance. Damage can occur due to local heating of the retina. Photochemical effects can cause damage at short wavelengths. A luminance meter is a device used in photometry that can measure the luminance in a particular direction and with a particular solid angle; the simplest devices measure the luminance in a single direction while imaging luminance meters measure luminance in a way similar to the way a digital camera records color images. Relative luminance Orders of magnitude Diffuse reflection Etendue Exposure value Lambertian reflectance Lightness, property of a color Luma, the repres
Giambattista della Porta
Giambattista della Porta known as Giovanni Battista Della Porta, was an Italian scholar and playwright who lived in Naples at the time of the Scientific Revolution and Reformation. Giambattista della Porta spent the majority of his life on scientific endeavors, he benefited from an informal education of visits from renowned scholars. His most famous work, first published in 1558, is entitled Magiae Naturalis. In this book he covered a variety of the subjects he had investigated, including occult philosophy, alchemy, mathematics and natural philosophy, he was referred to as "professor of secrets". Giambattista della Porta was born at Vico Equense, near Naples, to the nobleman Nardo Antonio della Porta, he was the third of four sons and the second to survive childhood, having an older brother Gian Vincenzo and a younger brother Gian Ferrante. Della Porta had a privileged childhood including his education, his father had a thirst for a trait he would pass onto all of his children. He surrounded himself with distinguished people and entertained the likes of philosophers, mathematicians and musicians.
The atmosphere of the house resembled an academy for his sons. The members of the learned circle of friends stimulated the boys and mentoring them, under strict guidance of their father, it is possible that his father's interest and influence in providing a well-rounded education helped to turn della Porta into the Renaissance man that he was to become. In addition to having talents for the sciences and mathematics, all the brothers were extremely interested in the arts, music in particular. Despite their interest none of them possessed any sort of talent for it, but they did not allow that to stifle their progress in learning of theory, they were all accepted into the Scuola di Pitagora, a exclusive academy of musicians. The pure impressiveness of their intellect was enough to allow three tone-deaf mathematicians into a school for the musically gifted; the status of the family as a symbol of knowledge and intellectual growth helped in their acceptance as well. More aware of their social position than the idea that his sons could have professions in science, Nardo Antonio was raising the boys more as gentlemen.
Therefore, the boys struggled with singing, as, considered a courtly accomplishment of gentlemen. They were taught to dance, ride, to take part and perform well in tournaments and games, dress well so they could look good doing all these noble activities; the training gave della Porta, at least earlier in his life, a taste for the finer aspects of his privileged living, where he surrounded himself in noble company and lavish things. This kind of lifestyle, the façade and showmanship involved in presenting one's self carried with Giambattista throughout his life. In 1563, della Porta published a work about cryptography. In it he described the first known digraphic substitution cipher. Charles J. Mendelsohn commented: He was, in my opinion, the outstanding cryptographer of the Renaissance; some unknown who worked in a hidden room behind closed doors may have surpassed him in general grasp of the subject, but among those whose work can be studied he towers like a giant. Della Porta invented a method.
During the Spanish Inquisition, some of his friends were imprisoned. At the gate of the prison, everything was checked except for eggs. Della Porta wrote messages on the egg shell using a mixture made of alum; the ink penetrated the egg shell, semi-porous. When the egg shell was dry, he boiled the egg in hot water and the ink on the outside of the egg was washed away; when the recipient in prison peeled off the shell, the message was revealed once again on the egg white. In 1586 della Porta published a work on physiognomy, De humana physiognomonia libri IIII; this influenced the Swiss eighteenth-century pastor Johann Kaspar Lavater as well as the 19th century criminologist Cesare Lombroso. Della Porta wrote extensively on a wide spectrum of subjects throughout his life – for instance, an agricultural encyclopedia entitled "Villa" as well as works on meteorology and astronomy. In 1589, on the eve of the early modern Scientific Revolution, della Porta became the first person to attack in print, on experimental grounds, the ancient assertion that garlic could disempower magnets.
This was an early example of the authority of early authors being replaced by experiment as the backing for a scientific assertion. Della Porta's conclusion was confirmed experimentally among others. In life, della Porta collected rare specimens and grew exotic plants, his work Phytognomonica lists plants according to their geographical location. In Phytognomonica the first observation of fungal spores is recorded, making him a pioneer of mycology, his private museum was visited by travelers and was one of the earliest examples of natural history museums. It inspired the Jesuit Athanasius Kircher to begin a similar more renowned, collection in Rome. Della Porta was the founder of a scientific society called the Academia Secretorum Naturae; this group was more known as the Otiosi. Founded sometime before 1580, the Otiosi were one of the first scientific societies in Europe and their aim was to study the "secrets of nature." Any person applying for membership had to demonstrate they had made a new discovery in the natural sciences.
The Academia Secretorum Naturae was compelled to disband when its members were suspected of dealing with the Occ
Binocular Rivalry Described by Quantum Formalism
Binocular rivalry is a visual phenomenon wherein one experiences alternating perceptions due to the occurrence of different stimuli presented to the corresponding retinal regions of the two eyes and their competition for perceptual dominance. As they compete, alternations between stimuli occur after a few seconds of steady vision. Neuroscientists have used binocular rivalry as a means to test neural responses and have found that the time intervals for these responses correlate with the alternations in perceptual dominance. A famous example of the phenomenon as described by Giambattista della Porta in the sixteenth-century, was to try reading two pages, each from a different book; the reader is to train an eye on each page and attempt to read one without disruption from the other eye. However, the rivalry is not always triggered; the rivalry is triggered by factors such as a difference in colour or brightness between the two stimuli, velocity and low-light settings. Binocular rivalry as a quantum formalism was first proposed by Efstratios Manousakis in his paper Quantum Formalism to Describe Binocular Rivalry in which he theorizes a mathematical description of the increase in dominance duration in binocular rivalry to make quantum predictions in which the observer affects the outcome for the distribution of perceptual alteration in time.
The study of binocular rivalry as a quantum formalism is here based on Neumann’s quantum theory of measurement and conscious observation. According to his theory, conscious events coincide with quantum wave "collapses." This occurs when the event is observed, because it solidifies the result, affects the neural correlates of the brain state, in agreement with calculating the probability distributions of dominance duration of the opposing states in binocular rivalry. The increase in dominance duration in binocular rivalry upon stimuli disruption yields testable predictions for the distribution of perceptual alteration in time, it is argued that the mathematics of quantum theory may describe certain aspects of conscious experience. For the purpose of this study, this idea is applied in order to describe mental experience of the observer undergoing binocular rivalry, rather than how the brain operates during the process; the nature of this study is significant in exploring the quantitative connection of the formalism of quantum theory with consciousness by applying the formation to psycho-physical phenomenon of binocular rivalry.
Studying this connection is important because it can be useful in understanding the role of the central nervous system and its processes in consciousness and qualia. Qualia is the phenomenon of consciousness experience and a subject of much interest in both science and philosophy, which may be able to be explained by quantum theory in this study. In his original paper, Manousakis made the argument linking the realization of conscious events with the collapses of particles represented by the wave function. Supposing that the state of consciousness during binocular rivalry can be modelled by the following equation for a situation of n-indeterminate states, with the vector |ψ> representing the complete potential state in Hilbert space, co-efficients ci for i = 1, …, n numbers in the complex plane related to the probability of each corresponding vector and each vector |i> representing each n-indeterminate state forming an orthogonal basis spanning |i>. An operator is required to act upon the total potential state to actualize one of the n-indeterminate states.
During binocular rivalry, this operator is the consciousness. The paper argues that the consciousness can only perceive change since it is due to the continual influx of light reflecting off an object that causes new neurotransmitters to fire and activate the Neural Correlates of Consciousness to be able to create an image in the brain. So, the operator used when perceiving an object moving in space is equivalent to the Del operator ∇ = and the operator used when perceiving a change in time is equivalent to ∂t. Considering the time evolution of the vector |ψ>, define vectors |i>t to be a basis for all time evolution states of |ψ>. Let ω ^ be a Hermitian operator acting on |i> giving eigenvalues ωi such that: ω ^ | i >= ω i | i >. So expanding |ψ> using the Fourier series yields, And using the definitions of |ψ> and ω ^ reduces the previous equation to, | ψ >= e x p i ω ^ t | ψ >, equivalent to the following time evolution equation: This equation describing the cognitive process resembles the general time-dependent Schrödinger equation but notice that Planck’s constant is not involved with this equation as it is with the one modelling quantum mechanics.
The mathematical model proposed by Manousakis has been correlated with past empirical data pertaining to binocular rivalry. Work describing the observed probability distribution of dominance duration of rival states in binocular rivalry fits well with the proposed formulation when applied to a two state system. Put, binocular rivalry can be regarded as a system in which there is some probability of seeing either state, the relative probabilities of the two
The human eye is an organ which reacts to light and pressure. As a sense organ, the mammalian eye allows vision. Human eyes help to provide a three dimensional, moving image coloured in daylight. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth; the human eye can differentiate between about 10 million colors and is capable of detecting a single photon. Similar to the eyes of other mammals, the human eye's non-image-forming photosensitive ganglion cells in the retina receive light signals which affect adjustment of the size of the pupil and suppression of the hormone melatonin and entrainment of the body clock; the eye is not shaped like a perfect sphere, rather it is a fused two-piece unit, composed of the anterior segment and the posterior segment. The anterior segment is made up of the cornea and lens; the cornea is transparent and more curved, is linked to the larger posterior segment, composed of the vitreous, retina and the outer white shell called the sclera.
The cornea is about 11.5 mm in diameter, 1/2 mm in thickness near its center. The posterior chamber constitutes the remaining five-sixths; the cornea and sclera are connected by an area termed the limbus. The iris is the pigmented circular structure concentrically surrounding the center of the eye, the pupil, which appears to be black; the size of the pupil, which controls the amount of light entering the eye, is adjusted by the iris' dilator and sphincter muscles. Light energy enters the eye through the cornea, through the pupil and through the lens; the lens shape is controlled by the ciliary muscle. Photons of light falling on the light-sensitive cells of the retina are converted into electrical signals that are transmitted to the brain by the optic nerve and interpreted as sight and vision. Dimensions differ among adults by only one or two millimetres, remarkably consistent across different ethnicities; the vertical measure less than the horizontal, is about 24 mm. The transverse size of a human adult eye is 24.2 mm and the sagittal size is 23.7 mm with no significant difference between sexes and age groups.
Strong correlation has been found between the width of the orbit. The typical adult eye has an anterior to posterior diameter of 24 millimetres, a volume of six cubic centimetres, a mass of 7.5 grams.. The eyeball grows increasing from about 16–17 millimetres at birth to 22.5–23 mm by three years of age. By age 12, the eye attains its full size; the eye is made up of layers, enclosing various anatomical structures. The outermost layer, known as the fibrous tunic, is composed of the sclera; the middle layer, known as the vascular tunic or uvea, consists of the choroid, ciliary body, pigmented epithelium and iris. The innermost is the retina, which gets its oxygenation from the blood vessels of the choroid as well as the retinal vessels; the spaces of the eye are filled with the aqueous humour anteriorly, between the cornea and lens, the vitreous body, a jelly-like substance, behind the lens, filling the entire posterior cavity. The aqueous humour is a clear watery fluid, contained in two areas: the anterior chamber between the cornea and the iris, the posterior chamber between the iris and the lens.
The lens is suspended to the ciliary body by the suspensory ligament, made up of hundreds of fine transparent fibers which transmit muscular forces to change the shape of the lens for accommodation. The vitreous body is a clear substance composed of water and proteins, which give it a jelly-like and sticky composition; the approximate field of view of an individual human eye varies by facial anatomy, but is 30° superior, 45° nasal, 70° inferior, 100° temporal. For both eyes combined visual field is 200 ° horizontal, it is 13700 square degrees for binocular vision. When viewed at large angles from the side, the iris and pupil may still be visible by the viewer, indicating the person has peripheral vision possible at that angle. About 15° temporal and 1.5° below the horizontal is the blind spot created by the optic nerve nasally, 7.5° high and 5.5° wide. The retina has a static contrast ratio of around 100:1; as soon as the eye moves to acquire a target, it re-adjusts its exposure by adjusting the iris, which adjusts the size of the pupil.
Initial dark adaptation takes place in four seconds of profound, uninterrupted darkness. The process is nonlinear and multifaceted, so an interruption by light exposure requires restarting the dark adaptation process over again. Full adaptation is dependent on good blood flow; the human eye can detect a luminance range of 1014, or one hundred trillion, from 10−6 cd/m2, or one millionth of a candela per square meter to 108 cd/m2 or one hundred million candelas per square meter. This range does not include looking at the midday lightning discharge. At the low end o
Anaglyph 3D is the name given to the stereoscopic 3D effect achieved by means of encoding each eye's image using filters of different colors red and cyan. Anaglyph 3D images contain two differently filtered colored images, one for each eye; when viewed through the "color-coded" "anaglyph glasses", each of the two images reaches the eye it's intended for, revealing an integrated stereoscopic image. The visual cortex of the brain fuses this into the perception of a three-dimensional scene or composition. Anaglyph images have seen a recent resurgence due to the presentation of images and video on the Web, Blu-ray Discs, CDs, in print. Low cost paper frames or plastic-framed glasses hold accurate color filters that after 2002, make use of all 3 primary colors; the current norm is cyan, with red being used for the left channel. The cheaper filter material used in the monochromatic past dictated red and blue for convenience and cost. There is a material improvement of full color images, with the cyan filter for accurate skin tones.
Video games, theatrical films, DVDs can be shown in the anaglyph 3D process. Practical images, for science or design, where depth perception is useful, include the presentation of full scale and microscopic stereographic images. Examples from NASA include Mars Rover imaging, the solar investigation, called STEREO, which uses two orbital vehicles to obtain the 3D images of the sun. Other applications include geological illustrations by the United States Geological Survey, various online museum objects. A recent application is for stereo imaging of the heart using 3D ultra-sound with plastic red/cyan glasses. Anaglyph images are much easier to view than either crossed-view pairs stereograms. However, these side-by-side types offer bright and accurate color rendering, not achieved with anaglyphs. Cross-view prismatic glasses with adjustable masking have appeared, that offer a wider image on the new HD video and computer monitors; the oldest known description of anaglyph images was written in August 1853 by W. Rollmann in Stargard about his "Farbenstereoscope".
He had the best results viewing a yellow/blue drawing with red/blue glasses. Rollmann found that with a red/blue drawing the red lines were not as distinct as yellow lines through the blue glass. In 1858, in France, Joseph D'Almeida delivered a report to l'Académie des sciences describing how to project three-dimensional magic lantern slide shows using red and green filters to an audience wearing red and green goggles. Subsequently he was chronicled as being responsible for the first realisation of 3D images using anaglyphs. Louis Ducos du Hauron produced the first printed anaglyphs in 1891; this process consisted of printing the two negatives which form a stereoscopic photograph on to the same paper, one in blue, one in red. The viewer would use colored glasses with red and blue or green; the left eye would see the blue image which would appear black, whilst it would not see the red. Thus a three dimensional image would result. William Friese-Green created the first three-dimensional anaglyphic motion pictures in 1889, which had public exhibition in 1893.
3-D films enjoyed something of a boom in the 1920s. The term "3-D" was coined in the 1950s; as late as 1954 films such as The Creature from the Black Lagoon were successful. Shot and exhibited using the Polaroid system, "The Creature from the Black Lagoon" was reissued much in an anaglyph format so it could be shown in cinemas without the need for special equipment. In 1953, the anaglyph had begun appearing in newspapers and comic books; the 3-D comic books were one of the most interesting applications of anaglyph to printing. Over the years, anaglyphic pictures have sporadically appeared in comics and magazine ads. Although not anaglyphic, Jaws 3-D was a box-office success in 1983. At present the excellent quality of computer displays and user-friendly stereo-editing programs offer new and exciting possibilities for experimenting with anaglyph stereo. A stereo pair is a pair of images from different perspectives at the same time. Objects closer to the camera have greater differences in appearance and position within the image frames than objects further from the camera.
Cameras captured two color filtered images from the perspective of the left and right eyes which were projected or printed together as a single image, one side through a red filter and the other side through a contrasting color such as blue or green or mixed cyan. As outlined below, one may now use an image processing computer program to simulate the effect of using color filters, using as a source image a pair of either color or monochrome images; this is called image stitching. In the 1970s filmmaker Stephen Gibson filmed direct anaglyph adult movies, his "Deep Vision" system replaced the original camera lens with two color-filtered lenses focused on the same film frame. In the 1980s, Gibson patented his mechanism. Many computer graphics programs provide the basic tools required to prepare anaglyphs from stereo pairs. In simple practice, the left eye image is filtered to remove green; the right eye image is filtered to remove red. The two images are positioned in the compositing phase in close overlay registration.
Plugins for some of these programs as well as programs dedicated to anaglyph preparation are available which automate the process and require the user to choose only a few basic
Hearing, or auditory perception, is the ability to perceive sounds by detecting vibrations, changes in the pressure of the surrounding medium through time, through an organ such as the ear. The academic field concerned with hearing is auditory science. Sound may be heard through liquid, or gaseous matter, it is one of the traditional five senses. In humans and other vertebrates, hearing is performed by the auditory system: mechanical waves, known as vibrations are detected by the ear and transduced into nerve impulses that are perceived by the brain. Like touch, audition requires sensitivity to the movement of molecules in the world outside the organism. Both hearing and touch are types of mechanosensation. There are three main components of the human ear: the outer ear, the middle ear, the inner ear; the outer ear includes the pinna, the visible part of the ear, as well as the ear canal which terminates at the eardrum called the tympanic membrane. The pinna serves to focus sound waves through the ear canal toward the eardrum.
Because of the asymmetrical character of the outer ear of most mammals, sound is filtered differently on its way into the ear depending on what vertical location it is coming from. This gives these animals the ability to localize sound vertically; the eardrum is an airtight membrane, when sound waves arrive there, they cause it to vibrate following the waveform of the sound. The middle ear consists of a small air-filled chamber, located medial to the eardrum. Within this chamber are the three smallest bones in the body, known collectively as the ossicles which include the malleus and stapes, they aid in the transmission of the vibrations from the eardrum into the cochlea. The purpose of the middle ear ossicles is to overcome the impedance mismatch between air waves and cochlear waves, by providing impedance matching. Located in the middle ear are the stapedius muscle and tensor tympani muscle, which protect the hearing mechanism through a stiffening reflex; the stapes transmits sound waves to the inner ear through the oval window, a flexible membrane separating the air-filled middle ear from the fluid-filled inner ear.
The round window, another flexible membrane, allows for the smooth displacement of the inner ear fluid caused by the entering sound waves. The inner ear consists of the cochlea, a spiral-shaped, fluid-filled tube, it is divided lengthwise by the organ of Corti, the main organ of mechanical to neural transduction. Inside the organ of Corti is the basilar membrane, a structure that vibrates when waves from the middle ear propagate through the cochlear fluid – endolymph; the basilar membrane is tonotopic, so that each frequency has a characteristic place of resonance along it. Characteristic frequencies are high at the basal entrance to the cochlea, low at the apex. Basilar membrane motion causes depolarization of the hair cells, specialized auditory receptors located within the organ of Corti. While the hair cells do not produce action potentials themselves, they release neurotransmitter at synapses with the fibers of the auditory nerve, which does produce action potentials. In this way, the patterns of oscillations on the basilar membrane are converted to spatiotemporal patterns of firings which transmit information about the sound to the brainstem.
The sound information from the cochlea travels via the auditory nerve to the cochlear nucleus in the brainstem. From there, the signals are projected to the inferior colliculus in the midbrain tectum; the inferior colliculus integrates auditory input with limited input from other parts of the brain and is involved in subconscious reflexes such as the auditory startle response. The inferior colliculus in turn projects to the medial geniculate nucleus, a part of the thalamus where sound information is relayed to the primary auditory cortex in the temporal lobe. Sound is believed to first become consciously experienced at the primary auditory cortex. Around the primary auditory cortex lies Wernickes area, a cortical area involved in interpreting sounds, necessary to understand spoken words. Disturbances at any of these levels can cause hearing problems if the disturbance is bilateral. In some instances it can lead to auditory hallucinations or more complex difficulties in perceiving sound. Hearing can be measured by behavioral tests using an audiometer.
Electrophysiological tests of hearing can provide accurate measurements of hearing thresholds in unconscious subjects. Such tests include auditory brainstem evoked potentials, otoacoustic emissions and electrocochleography. Technical advances in these tests have allowed hearing screening for infants to become widespread; the hearing structures of many species have defense mechanisms against injury. For example, the muscles of the middle ear in many mammals contract reflexively in reaction to loud sounds which may otherwise injure the hearing ability of the organism. There are several different types of hearing loss: Conductive hearing loss, sensorineural hearing loss and mixed types. Conductive hearing loss Sensorineural hearing loss Mixed hearing lossThere are defined degrees of hearing loss: Mild hearing loss - People with mild hearing loss have difficulties keeping up with conversations in noisy surroundings; the most quiet sounds that people with mild hearing loss can hear with their better ear are between 25 and 40 dB HL.
Moderate hearing loss - People with moderate hearing loss have difficulty keeping up with conversations when they are not using a hearing aid. On average, the most quiet sounds heard by