break down real-world complex stimuli into sine wave components, through the mathematical process called fourier analysis. many neurons have a strong preference for certain sine frequency components in contrast to others. the way that simpler sounds and images are encoded during sensation can provide insight into how perception of real-world objects happens. sensory neuroscience and the biology of perception main article: sensory neuroscience perception occurs when nerves that lead from the sensory organs (e.g. eye) to the brain are stimulated, even if that stimulation is unrelated to the target signal of the sensory organ. for example, in the case of the eye, it does not matter whether light or something else stimulates the optic nerve, that stimulation will results in visual perception, even if there was no visual stimulus to begin with. (to prove this point to yourself (and if you are a human), close your eyes (preferably in a dark room) and press gently on the outside corner of one eye through the eyelid. you will see a visual spot toward the inside of your visual field, near your nose.) sensory nervous system main article: sensory nervous system all stimuli received by the receptors are transduced to an action potential, which is carried along one or more afferent neurons towards a specific area (cortex) of the brain. just as different nerves are dedicated to sensory and motors tasks, different areas of the brain (cortices) are similarly dedicated to different sensory and perceptual tasks. more complex processing is accomplished across primary cortical regions that spread beyond the primary cortices. every nerve, sensory or motor, has its own signal transmission speed. for example, nerves in the frog's legs have a 90 ft/s (99 km/h) signal transmission speed, while sensory nerves in humans, transmit sensory information at speeds between 165 ft/s (181 km/h) and 330 ft/s (362 km/h). the human sensory and perceptual system physical stimulus sensory organ sensory system cranial nerve(s) cerebral cortex primary associated perception(s)) name light eyes visual system optic (ii) visual cortex visual perception sight (vision) sound ears auditory system vestibulocochlear (viii) auditory cortex auditory perception hearing (audition) gravity and acceleration inner ear vestibular system vestibulocochlear (viii) vestibular cortex equilibrioception balance (equilibrium) chemical substance nose olfactory system olfactory (i) olfactory cortex olfactory perception, gustatory perception (taste or flavor) smell (olfaction) chemical substance mouth gustatory system facial (vii), glossopharyngeal (ix) gustatory cortex gustatory perception (taste or flavor) taste (gustation) position, motion, temperature skin somatosensory system trigeminal (v), glossopharyngeal (ix) + spinal nerves somatosensory cortex tactile perception (mechanoreception, thermoception) touch (tactition) multimodal perception main article: multimodal integration perceptual experience is often multimodal. multimodality integrates different senses into one unified perceptual experience. information from one sense has the potential to influence how information from another is perceived. multimodal perception is qualitatively different from unimodal perception. there has been a growing body of evidence since the mid-1990s on the neural correlates of multimodal perception. philosophy the philosophy of perception is concerned with the nature of perceptual experience and the status of perceptual data, in particular how they relate to beliefs about, or knowledge of, the world. historical inquiries into the underlying mechanisms of sensation and perception have led early researchers to subscribe to various philosophical interpretations of perception and the mind, including panpsychism, dualism, and materialism. the majority of modern scientists who study sensation and perception take on a materialistic view of the mind. human sensation absolute threshold some examples of human absolute thresholds for the nine to 21 external senses. sense absolute threshold (obsolete system of signal detection used) vision stars at night; candlelight 48 km (30 mi) away on a dark and clear night hearing ticking of a watch 6 m (20 ft) away, in an otherwise silent environment vestibular tilt of less than 30 seconds (3 degrees) of a clock's minute hand touch a wing of a fly falling on the cheek from a height of 7.6 cm (3 inches) taste a teaspoon of sugar in 7.5 liters (2 gallons) of water smell a drop of perfume in a volume of the size of three rooms multimodal perception humans respond more strongly to multimodal stimuli compared to the sum of each single modality together, an effect called the superadditive effect of multisensory integration. neurons that respond to both visual and auditory stimuli have been identified in the superior temporal sulcus. additionally, multimodal "what" and "where" pathways have been proposed for auditory and tactile stimuli. external external receptors that respond to stimuli from outside the body are called exteroceptors. human external sensation is based on the sensory organs of the eyes, ears, skin, vestibular system, nose, and mouth, which contribute, respectively, to the sensory perceptions of vision, hearing, touch, spatial orientation, smell, and taste. smell and taste are both responsible for identifying molecules and thus both are types of chemoreceptors. both olfaction (smell) and gustation (taste) require the transduction of chemical stimuli into electrical potentials. visual system (vision) main article: visual system the visual system, or sense of sight, is based on the transduction of light stimuli received through the eyes and contributes to visual perception. the visual system detects light on photoreceptors in the retina of each eye that generates electrical nerve impulses for the perception of varying colors and brightness. there are two types of photoreceptors: rods and cones. rods are very sensitive to light but do not distinguish colors. cones distinguish colors but are less sensitive to dim light. at the molecular level, visual stimuli cause changes in the photopigment molecule that lead to changes in membrane potential of the photoreceptor cell. a single unit of light is called a photon, which is described in physics as a packet of energy with properties of both a particle and a wave. the energy of a photon is represented by its wavelength, with each wavelength of visible light corresponding to a particular color. visible light is electromagnetic radiation with a wavelength between 380 and 720 nm. wavelengths of electromagnetic radiation longer than 720 nm fall into the infrared range, whereas wavelengths shorter than 380 nm fall into the ultraviolet range. light with a wavelength of 380 nm is blue whereas light with a wavelength of 720 nm is dark red. all other colors fall between red and blue at various points along the wavelength scale. the three types of cone opsins, being sensitive to different wavelengths of light, provide us with color vision. by comparing the activity of the three different cones, the brain can extract color information from visual stimuli. for example, a bright blue light that has a wavelength of approximately 450 nm would activate the "red" cones minimally, the "green" cones marginally, and the "blue" cones predominantly. the relative activation of the three different cones is calculated by the brain, which perceives the color as blue. however, cones cannot react to low-intensity light, and rods do not sense the color of light. therefore, our low-light vision is—in essence—in grayscale. in other words, in a dark room, everything appears as a shade of gray. if you think that you can see colors in the dark, it is most likely because your brain knows what color something is and is relying on that memory. there is some disagreement as to whether the visual system consists of one, two, or three submodalities. neuroanatomists generally regard it as two submodalities, given that different receptors are responsible for the perception of color and brightness. some argue that stereopsis, the perception of depth using both eyes, also constitutes a sense, but it is generally regarded as a cognitive (that is, post-sensory) function of the visual cortex of the brain where patterns and objects in images are recognized and interpreted based on previously learned information. this is called visual memory. the inability to see is called blindness. blindness may result from damage to the eyeball, especially to the retina, damage to the optic nerve that connects each eye to the brain, and/or from stroke (infarcts in the brain). temporary or permanent blindness can be caused by poisons or medications. people who are blind from degradation or damage to the visual cortex, but still have functional eyes, are actually capable of some level of vision and reaction to visual stimuli but not a conscious perception; this is known as blindsight. people with blindsight are usually not aware that they are reacting to visual sources, and instead just unconsciously adapt their behavior to the stimulus. on february 14, 2013, researchers developed a neural implant that gives rats the ability to sense infrared light which for the first time provides living creatures with new abilities, instead of simply replacing or augmenting existing abilities. visual perception in psychology according to gestalt psychology, people perceive the whole of something even if it is not there. the gestalt's law of organization states that people have seven factors that help to group what is seen into patterns or groups: common fate, similarity, proximity, closure, symmetry, continuity, and past experience. the law of common fate says that objects are led along the smoothest path. people follow the trend of motion as the lines/dots flow. the law of similarity refers to the grouping of images or objects that are similar to each other in some aspect. this could be due to shade, colour, size, shape, or other qualities you could distinguish. the law of proximity states that our minds like to group based on how close objects are to each other. we may see 42 objects in a group, but we can also perceive three groups of two lines with seven objects in each line. the law of closure is the idea that we as humans still see a full picture even if there are gaps within that picture. there could be gaps or parts missing from a section of a shape, but we would still perceive the shape as whole. the law of symmetry refers to a person's preference to see symmetry around a central point. an example would be when we use parentheses in writing. we tend to perceive all of the words in the parentheses as one section instead of individual words within the parentheses. the law of continuity tells us that objects are grouped together by their elements and then perceived as a whole. this usually happens when we see overlapping objects. we will see the overlapping objects with no interruptions. the law of past experience refers to the tendency humans have to categorize objects according to past experiences under certain circumstances. if two objects are usually perceived together or within close proximity of each other the law of past experience is usually seen. auditory system (hearing) main article: auditory system hearing, or audition, is the transduction of sound waves into a neural signal that is made possible by the structures of the ear. the large, fleshy structure on the lateral aspect of the head is known as the auricle. at the end of the auditory canal is the tympanic membrane, or ear drum, which vibrates after it is struck by sound waves. the auricle, ear canal, and tympanic membrane are often referred to as the external ear. the middle ear consists of a space spanned by three small bones called the ossicles. the three ossicles are the malleus, incus, and stapes, which are latin names that roughly translate to hammer, anvil, and stirrup. the malleus is attached to the tympanic membrane and articulates with the incus. the incus, in turn, articulates with the stapes. the stapes is then attached to the inner ear, where the sound waves will be transduced into a neural signal. the middle ear is connected to the pharynx through the eustachian tube, which helps equilibrate air pressure across the tympanic membrane. the tube is normally closed but will pop open when the muscles of the pharynx contract during swallowing or yawning. mechanoreceptors turn motion into electrical nerve pulses, which are located in the inner ear. since sound is vibration, propagating through a medium such as air, the detection of these vibrations, that is the sense of the hearing, is a mechanical sense because these vibrations are mechanically conducted from the eardrum through a series of tiny bones to hair-like fibers in the inner ear, which detect mechanical motion of the fibers within a range of about 20 to 20,000 hertz, with