I. Sensory Thresholds
The ability of any organism to perceive and be aware of its environment depends on processes that must detect stimuli and convert those stimuli into a form that can be understood and processed by the nervous system. Often human abilities to do so seem less impressive that those of some other animal species. We can't see as well as a cat in the dark. An eagle flying can see a mouse on the ground from a dizzying height. Bats and dolphins hear so well that they can use echoes to scan their environment as if they we using sonar. Dogs can follow a unique scent trail over miles. But while humans can't do as well as those extremes we actually are pretty good all-around at using our senses. Under ideal conditions our vision can detect a light as dim as a candle flame 30 miles away on a dark clear night, our hearing can detect the soft ticking of a clock when it's quiet, our taste can detect flavors as faint as a teaspoon of sugar dissolved in two gallons of water, our smell can detect the scent of a single drop of perfume dispersed in a volume equal to that of a small house and our sense of touch can detect a contact as delicate as that of a fly's wing dropped on our cheek from a height of one centimeter (about a third of an inch). Other species may be better at detecting fainter stimuli in one or two sensory modalities. But, that's typically because those species are heavily specialized for the use of those particular senses to either catch prey or avoid predators. However, they are often far poorer than humans at other senses. For example, the fine detailed vision of birds is far beyond that of humans but their senses of smell and taste are poorer. Cats have better low-light vision than us but it isn't as good for seeing fine detail and their sense of taste is poorer as well. Humans, while not excelling at any one sensory system, have quite good sensory abilities across the board.

These feats are at the absolute limits of our sensory thresholds. But how are they established? The absolute sensory thresholds for any sensory modality are determined by presenting a series of stimuli, both ascending and descending in intensity from below and above the level of the threshold for that sensory modality. The test subjects are asked if they detected each stimulus after they have been presented. The point of intensity where the likelihood of detecting the stimulus is 50% is called the absolute threshold. Below the threshold the chance of a stimulus being detected is less than a 50-50 random chance. Above the threshold the probability of detecting the stimulus is greater than a 50-50 random chance.

II. Sensory Coding
We've seen that our sensory systems are pretty good at detecting sensory stimuli but how do they do it? All sensory events involve some sort of energy in the environment (light, heat, chemical, etc.,) but our nervous system doesn't detect those events directly. The sensory event must be converted into an electrochemical nerve impulse for it to be processed by the nervous system. This conversion of other forms of energy into an electrochemical nerve impulse is called transduction. This is done for each sensory modality by specialized sensory cells. Sensory cells are not neurons even though they eventually generate the sorts of electrochemical impulses that neurons can interpret. Each sensory modality (vision, hearing, touch, taste (gustation) and smell (olfaction)) has its own sorts of specialized sensory cells. In vision, the cells are photoreceptors and are found in the retina of the eye. In hearing, they are called hair cells and are found in the cochlea of the inner ear. There are several sensory cells involved in touch found in the skin: Pacinian, Ruffini's and Meissner's corpuscles, Merkel's disks and free nerve endings. The three forms of corpuscles and Merkel's provide varying forms of information about pressure, stretch, vibration, and pain. Free nerve endings provide the sense of heat and cold. However it may not always be that simple. The cornea of the eye can feel pressure, stretch, vibration, pain, heat and cold but only has free nerve endings. In gustation the sensory cells are chemoreceptors found in the taste buds of the tongue. In olfaction different chemoreceptors are found on nerve endings descending from the brain's olfactory bulb into the olfactory mucosa of the sinuses.

In all cases if a sensory event doesn't activate the sensory cells, there is no sensation or perception of that event. For example radio waves are electromagnetic radiation, just like light but they are unable to activate the photoreceptors of the retina. If they could, we could see radio or television transmissions as some sort of pattern of colors floating in the air. But they don't, so we can't. Just because we can't detect a sensory stimulus doesnÕt necessarily mean it's not there. It just means that for whatever reason, our sensory cells weren't activated.

III. Sensory Modalities
A. Vision
1. The Nature of Light
Light is electromagnetic radiation that is visible to the eye. For humans that means light with wavelengths from 400 to 700 nanometers (nm) with maximal daytime sensitivity at 555 nm. The color violet is from 400-450 nm and the color red from 610-700 nm. Our daytime maximal sensitivity is in the middle of the range for the color green. Ultraviolet light has shorter wavelengths than violet and Infrared has longer wavelengths than red. While we can't see in the ultraviolet range, some animal species like insects can see in the ultraviolet range. We also can't see infrared, but some animals with thermal receptors like some snakes are detecting infrared radiation, though with different specialized sensory cells, not with their eyes.

When we see an object we are seeing light coming from it. If it is a light source like the sun, a light bulb or a flame, it is the light emitted by that source. But we see most objects because of the light that they reflect back to our eyes. These objects absorb the rest of the light from the light source. White light is light that is a combination of all the frequencies from 400-700 nm. If we see a red light bulb shining it is red because it is emitting only red light. If we see a red ball outside in the sunlight it is absorbing all other colors in the white light produced by the sun and is reflecting back only the red wavelengths to our eyes.

2. The Eye

a. General Structure

Our eyes are our visual organs. The white, tough outer wall of the eye is called the sclera. Along with the pressure of the eyes' internal fluid (the vitreous humor) , it keeps the eyes' shape and protects its internal delicate structures. The cornea is a clear, rounded surface made of a transparent protein sheet that covers the front of the eye. It is the first and most powerful lens in the eye's optical system. To ensure its transparency the cornea is without blood vessels. It is nourished by the tears over it and fluid called the aqueous humor in a chamber behind it.. The cornea can be damaged by accidents, infections, and genetic defects and in these cases the person can receive a corneal transplant. The iris is the colored part of the eye. It is a ring of muscle fibers located behind the cornea and in front of the lens. It contracts and expands, opening and closing the pupil, in response to the brightness of surrounding light. The pupil is the (apparently) black hole in the center of the iris that light passes through. The lens in the eye is kind of like the adjustable lens in a camera. However, it is not made of glass. Like the cornea it is also a transparent protein structure. Positioned just behind the cornea, it is responsible for keeping images in focus on the retina. It is adjustable for distance and close work. Those lens adjustments are done by the ciliary muscles. They relax to flatten the lens for distance vision and contract, rounding out the lens, for near vision. As we age, the ciliary muscles and the lens lose their elasticity and as such the ciliary isn't as good at adjusting the lens for near vision. This is why most people need reading glasses by their 40's. The aqueous humor is a water-like fluid that fills the front of the eye between the lens and cornea and provides the cornea and lens with oxygen and nutrients. The vitreous humor is a jelly-like liquid that fills of the eye, from the lens to the retina As we age it changes from a gel to a liquid and gradually shrinks separating from the retina. This is when most people start seeing floaters, dark specks floating in their in their field of vision. However, people of all ages can see them. They are usually nothing to worry about. They are just occasional cells that slough off from the inner surface of the eye. However in the cases of some eye injuries or degenerative diseases the incidence of floaters will increase. The retina is the layered sheet-like structure lining the back of the eye. It converts light into electrochemical nerve impulses and sends them to the brain through the optic nerve. The lateral sides of the retina are responsible for our peripheral vision. The center area, called the fovea is used for our fine central detailed vision and color vision. At the very back of the retina are the photoreceptors which are the sensory cells which do the transduction of light into electrochemical nerve impulses. There are two types of photoreceptor cells. When light falls on one of these cells, it causes a chemical reaction in photopigments (light-sensitive pigments) that generates the electrochemical pulses that nervous system can interpret. Cone cells give us our fine detail color daytime vision. There are 6 million of them in each human eye. Most of them are located in the fovea area. There are three types of cone cells: one sensitive to red light, another to green light, and the third sensitive to blue light. They require bright light to activate their frequency specific photopigments. Rod cells are about 500 times more sensitive to light then cone cells. They give us our dim light or night vision (largely black and white/grayscale vision with a tinge of purplish-blue response) and they are also more sensitive to motion then cone cells. There are 120 million rod cells in the human eye, most located in our peripheral or side vision. During bright light all their especially sensitive photopigments are essentially bleached out and are unable to function. If we enter into a darkened room (like a movie theater) from bright light we will find it difficult to see because of this "bleaching" out. But after a period of time called dark adaptation, the rod cell pigments are regenerated and dim-light vision returns. Sitting in front of the photoreceptor layer are the bipolar cells receive the initial input from the photoreceptors and begin the process of processing the visual signal and pass the information to the first true axon-possessing nerve cells in the retina the ganglion cells. Horizontal cells link adjacent bipolar cells close to where the bipolars make contact with the photoreceptors, and amacrine cells, in turn, link adjacent bipolar cells where the bipolars and retinal ganglion cells make contact. There are about 1 million ganglion cells in the eye. Ganglion cells are the front-most cell in the retina. They compare signals from many different photoreceptors. His comparison across many other photoreceptors is possible because of the horizontal and amacrine cells. But even though they may compare the input from many photoreceptors, in the fovea area the ratio of photoreceptor cell to bipolar cell to ganglion cell is 1:1:1. In the sides of the retina, each ganglion cell will be responsible for the inputs from many photoreceptors. The axons of the ganglion cells come together to form the optic nerve. The optic nerve has about 1.2 million nerve fibers, connecting the eye to the brain. The optic disk (also called the blind spot) is the spot on the retina where the optic nerve leaves the eye. There are no sensory cells here, hence creating a blind spot. The brain fills in the missing information by a process of approximation and by comparing what sort of stimuli surrounded the blind spot and "filling in" the missing information.

3. The Visual Brain

The optic nerve proceeds toward the brain entering the brain just in front of the pituitary gland. The optic chiasm is where the optic nerve enters the brain. Each eye takes a slightly different picture of the world. At the optic chiasm each picture is divided in half. The outer left and right halves continue back toward the visual cortex. The inner left and right halves cross over to the other side of the brain then continue back toward the visual cortex. In this way, the left side of the external visual world is processed entirely by the right side of the brain and the right side of the external visual world is processed entirely by the left side of the brain. As it enters the brain, the optic nerve is now known as the optic tract which connects the optic chiasm to the lateral geniculate nucleus (LGN) of the thalamus. The LGN acts as a relay station doing only preliminary decoding visual information from the optic tract before sending it to the visual cortex for final processing. However, some inputs are sent to the optic tectum (in particular the superior colliculus) to help in guiding eye movements and tracking moving objects with the eyes. The visual cortex takes up most of the occipital lobe of the brain and processes and combines visual information and converts it into sight. There are several levels of visual cortex each which different jobs. Primary visual cortex (V1) does the most basic and general processing of the visual world, each column of cells responsible for a small "pixel"-like element of the visual scene. Damage to the primary visual cortex results in a condition called cortical blindness. Other areas of visual cortex are V2, V3, V4 and V5. V2 is responsible for complex patterns generated from the simpler patterns processed by V1. V3 does some more detailed processing of color than V1 and also processes movement. V4 also process color and form such as geometric shapes (think of "form" as even more complex patterns than V2). V5 is involved in the processing of form and motion.

4. Perception

The summed activity of Vs 1-5 are further processed in the dorsal stream which extends from the occipital lobe to the parietal and allows us to understand where objects are in space or where they are in relation to where our bodies in space. In comparison the activity of visual cortex is also sent to the ventral stream which extends from the occipital lobe along the lower (i.e., inferior) parts of the temporal lobe. The ventral stream is involved in determining what an object is. Here specific faces, hands and specific objects are represented.

B. Audition
1. The Ear

The sense of hearing (audition) has two parts: the behavior of the mechanical apparatus and the neurological processing of the information acquired. The mechanics of the outer, middle and inner ears are straightforward and well understood, but the processes the brain uses to interpret sounds is still a matter of dispute among researchers. The outer ear consists of the external ear, called the pinna or auricle, and the ear canal, structures which serve to protect the more delicate parts inside. However, the shape of the auricle does help concentrate and funnel sound into the ear canal which provides some small amplification of sound. The outer boundary of the middle ear (which essentially a hollow chamber) is the eardrum, a thin membrane which vibrates because of sound vibrations entering the ear canal. The eardrum's vibrations are transferred across the middle ear via three small bones named the hammer, anvil, and stirrup (also called the malleus, incus and stapes, respectively). These bones are supported by small muscles which normally allow free movement but can tense up and reduce their movement when sounds are extremely loud. The ear drum's vibrations are very efficiently transmitted. The Eustachian tube connects from the chamber of the middle ear to the back of the pharynx, (throat). The boundary of the inner ear is the oval window, a thin membrane structure of the coiled structure called the cochlea, which is contact with the stirrup. The snail shell-like cochlea also contains several tubes which wind in various ways within the skull. These tubes, called the semicircular canals, contribute to our senses of balance and motion in space. They contain fluid that spins when we are in motion. The fluid displaces small stone like crystals called otoliths. When the stirrup transfers the sound vibrations to the oval window, the vibrations are then transferred to the fluid which fills the cochlea which is divided in two the long way by the basilar membrane. The basilar membrane is supported by the sides of the cochlea but is not tightly stretched. Sound vibrations introduced into the cochlear fluid flex the basilar membrane and sets up traveling waves along its length. The basilar membrane is tapered in such a way the vibrations in the fluid set up traveling waves in the membrane that are not of even height along the entire distance of the membrane, but grow in height to a certain point and then quickly fade out. The point of maximum height depends on the frequency of the sound wave and the tapered thickness on the membrane. The basilar membrane is covered with tiny structures called hair cells (the sensory cell for the sense of hearing), which look like little bulb-like structures with bristly hair-like projections coming from them. The base of the hair cells connected to a bundle of nerves. Motion of the basilar membrane bends the hairs which in turn excite the associated nerve fibers. These fibers will come together to form the auditory nerve which will carry the sound information to the brain. The location of the hair cells along the basilar membrane is associated is highly correlated with the frequency (pitch) of the sound. A complex sound will produce a series of active areas along the basilar membrane that accurately matches the complex frequencies of the sound. The auditory nerve enters the spinal cord and makes contact with its first relay called the cochlear nucleus. From there some projections go to the inferior colliculus while other projections go to other brainstem nuclei as well as to the medial geniculate nucleus (MGN) of the thalamus. From there the auditory system projects to the primary auditory cortex in the lateral temporal lobe.

C. Gustation

Humans can detect different types of chemical substances through the sense of gustation (taste). Scientists generally refer to four basic classifications for taste: salty, sweet, bitter and sour. However, researchers in Japan have identified a fifth taste classification called umami, which translates as "savory" or "meaty" and is sensitive to the amino acids found in proteins, especially the amino acid glutamate.

The sensory cells are found on the tongue in the papillae, which are small projections containing the taste buds (which in turn contain the MOST of sensory cells of taste). However, some taste receptor cells are also found scattered in the throat, pharynx and epiglottis. The taste receptor cells, which sense the presence of chemicals in the mouth connect to axons which relay the information to the brain via the cranial nerves. Some taste receptors also found directly activate ion channels, while others are similar to G protein-coupled receptors in structure. They respond to chemicals in the environment by depolarizing the membrane potential, which leads to release of neurotransmitters via Ca⁺⁺-influx and activation of the sensory axons that relay the signal to the brain. These enter the brain via cranial nerves (facial nerve, anterior 2/3 of the tongue; glossopharyngeal nerve, posterior 1/3 of the tongue; and the vagus nerve, throat, epiglottis and pharynx). These nerves synapse in the gustatory nucleus in the dorsal medulla. Neurons from the gustatory nucleus synapse upon neurons in the ventral posterior medial nucleus (VPM) of the thalamus. From there, the VPM sends sensory information to the primary gustatory cortex in the ventral parietal lobe.

D. Olfaction
Organisms detect odors through the olfactory (smell) system. Olfactory cues seem to be especially important for learning and memory, which may be a reflection of the olfactory system's closeness, literally and figuratively to the hippocampus. There are direct projections from the primary olfactory cortex to the hippocampus.

The olfactory system originates in the nasal cavity's olfactory epithelium which contains three main cell types: the olfactory receptor cells, supporting cells and basal cells. Olfactory receptor cells are neurons, and send axons directly (not via the thalamus) to the olfactory bulb in the central nervous system. The supporting cells do not participate directly in sensory transduction, but do produce mucus. Basal cells are the source of new olfactory receptor cells, which grow, die and regenerate over a period of 4 - 8 weeks. Olfactory receptor cells send cilia-like dendrites up to the mucus layer. Chemicals in the air (odorants) become dissolved in the mucus lining the nasal cavity where these chemicals bind to the surface of the dendrites and activate the olfactory receptor cells. The chemical stimulation of the olfactory cells occurs via G protein-coupled receptors. Olfactory receptor cells send unmyelinated axons via the olfactory nerve into the olfactory bulb. From the olfactory bulb, the olfactory tract projects directly to the primary olfactory cortex. However, some olfactory bulb neurons project via the olfactory tract into the olfactory tubercle which in turn projects to the medial dorsal nucleus (MDN) of the thalamus which then projects to the parts of the prefrontal cortex.

E. Touch
The sense of touch originates in the bottom layer of your skin, called the dermis, which is filled with the various sensory cells we talked about earlier (Pacinian corpuscles and the others) which activate nerve fibers which carry the information to the spinal cord, which sends messages to the brain where the sensation is registered. Some areas of the body are more sensitive than others because they have more nerve endings per area of skin than other areas. When these nerve endings are activated by their sensory cells, they activate their cell bodies which are in the dorsal root ganglions of the nerves exiting the spinal cord. These dorsal root ganglion cells project into the CNS contacting neurons in either the grey matter of the spinal cord or brainstem. These neurons project in turn to the ventro-posterior nucleus (VPN) of the thalamus which in turn projects to the primary somatosensory cortex. The size of the area of the primary somatosensory cortex devoted to each body part is proportional to the density of the nerve endings coming from that area. Therefore, sensitive areas with high nerve ending density such as the fingertips and lips have a much greater area devoted to them than areas with lower nerve ending densities like the middle of the back, even though the back is physically much larger.

IV. A parting word about PAIN.
Pain is part of the sense of touch but it is special in its own right. Its technical name is nociception which means the perception of damage or injury. There are specialized fibers to carry only pain information. They are called Ad fibers and C fibers. They differ in size and speed of transmission. The Ad fibers are myelinated (fast conduction velocity) and carry sharp intense pain like a burn or a needle poke. The C fibers are unmyelinated (slow conduction velocity) and carry duller, throbbing pain. There are also Aa and Ab fibers which carry other somatosensory modalities but when they fire intensely enough, their signals can also be interpreted as painful. When tissue is damaged the injured cells release chemicals which produce inflammation such as histamine and prostaglandin. These chemicals sensitize nerve ending and lower their threshold to fire. This is why an injury is often especially sensitive to even the slightest touch or sensory disturbance. These chemicals have made it far easier for stimuli to cause pain fibers to fire when they normally would not have. Many of the over-the-counter pain relievers like aspirin, ibuprofen, acetaminophen and naproxen sodium are inhibitors of the production of prostaglandin, which is how they are able to reduce the sensation of pain associated with an injury. As pain fibers ascend towards the thalamus the synapse on an area called the periaqueductal grey (PAG) (which is the primary source of pain-relieving opioid endorphin peptides). These ascending pain fibers also can activate the raphe nucleus (the main source of serotonin, 5HT). These two areas send descending projections to the spinal cord and can inhibit the activity of the pain fibers at the level of the spinal cord.