Defining Sensation and Perception
Our sense organs receive messages through sensory receptor cells, which receive outside forms of energy (light, vibration, heat) and translate them into neural impulses that can be transmitted to the brain for interpretation. The process of receiving information from outside world, translating it, and transmitting it to the brain is called sensation. The process of interpreting that information and forming images of the world is called perception. In studying sensation and perception, It is important to understand the term stimulus which refers to any aspect of the outside world that directly influences our behavior or conscious experience.
• Transduction is defined as the translation of energy in the environment into neural impulse.
• Energy is transduced into neural impulses in the sense organs by sensory receptor cells.
• Sensory receptor cells are specialized neurons that are excited by specific kinds of sensory energy and transmit neural impulses along their axons. Some sensory receptor cells respond to sound waves, some to light waves, some to chemicals, and so on.
Limits of Sensation: Absolute Thresholds
Absolute Threshold is define as the minimum intensity of a stimulus that can be detected 50% of the time. This 50% mark is used because the level of the stimulus required for it to just be perceived varies from trial to trial and from person to person during an experiment.
Just Noticeable Difference and Weber’s Law
In addition to establishing absolute thresholds for the senses, psychophysicists have tried to establish the minimum change in the intensity of a stimulus that can be detected 50% of the time. This barely noticeable change in stimulus referred to as the difference threshold or the just noticeable difference (jnd). In the early 1800s, psychophysicist Max Weber discovered an interesting characteristics of the jnd, known as Weber’s law. According to this law, for each of our five senses, the amount of change in the stimulus that is necessary to produce a jnd depends on the intensity at which the stimulus is first presented. For example, if you add one additional teaspoon of salt to a very salty pot of soup, it will probably not be noticeable. But that same teaspoon of salt added to a less salty pot of soup may be very noticeable. Weber’s law helps explain some of the subjectivity we experience in sensation. Under some conditions, one teaspoon of salt won’t make a difference to our enjoyment of recipe. Under other conditions, it might.
The Five Senses
1.1 The Anatomy of the Outer Eye
• Cornea is the clear, slightly bulging surface of the eye that both protect the eye and begins the focusing process. As light waves pass through the material of the cornea, they slow down and bend – just as they do when they pass through a camera lens. This bending of light waves plays an essential role in focusing images on the back of your eye.
• Pupil is located directly behind the cornea. This is a black opening or aperture through which light passes into the center of the eye. Light cannot pass through the white part of the eye, the sclera, therefore, it must pass through the cornea and pupil to enter the eye.
• Iris is the colored part of the eye surrounding the pupil. It is constructed of rings of muscles that control the size of the pupil. In dimly lit conditions, the iris relaxes to dilate the pupil, allowing the maximum amount of light into the eye. In brightly lit conditions, the iris constricts to close the pupil, thus, reducing the amount of light entering the eye so as not to overwhelm the sensitive cells in the eye.
• Lens is clear structures that are attached to the eye with strong ciliary muscles. It is directly behind the iris and the pupil. The lens of the eyes is somewhat soft and flexible. As the Ciliary muscles stretch the lens, it changes shape, or undergoes accommodation, so that the image passing through it is focused properly.
• Retina. Once the light waves have been focused on the back of the eye, conversion of light waves into neural impulses occurs in the retina, The surface that lines the inside of the back of the eyeball. In the retina, specialized cells called rods and cones convert light into neural signals. Without these cells, vision would not be possible. The ganglion cells are on the surface of the retina, followed by successive layers of amacrine, bipolar, and horizontal cells, and finally the light-sensitive rods and cones.
Rods are light-sensitive cells of the retina that pick up any type of light energy and convert it into neural impulse. Rods are long and skinny. If you had only rods in your retina, you’d see everything in black in white.
Cones are shorter and fatter than rods. They are sensitive to specific colors of light and send information to the brain concerning the colors we are seeing.
The rods and cones of the eye are able to convert light into neural impulses because they contain light-sensitive photopigments, chemicals that are activated by light energy. When rods is not receiving light input, its photopigment molecule are stable. However, when light strikes the rod, this incoming light energy splits the photopigments break up, they set off a complex chain of chemical reactions that change the rate at which the neurons of the visual system fire action potentials.
• Optic Nerve. This is the structure that conveys visual information away from the retina to the brain. With no rods and cones in this spot, each of our eyes has a blind spot, which is a point in our visual field that we cannot see.
• Fovea. This is the point of highest visual acuity, cones are concentrated here.
Theory of Color Vision
Three different types of cones, each of which contains
Trichromatic Theory of color vision a slightly different photopigment that makes the cell particularly sensitive to a certain wavelength of light, One type of cone is particularly sensitive to long wavelengths (red), another is very sensitive to medium wavelengths (green), and the third is most sensitive to short wavelengths (blue)
Opponent-Process Theory of Color Vision There are three types of opponent-process cell in our visual system: red/green, yellow /blue, and black/white. The key to opponent-process theory is that these cells can detect the presence of only one color at a time. The colors oppose each other so that the opponent-process cell cannot detect either red or green light at any one time.
1. The Anatomy of the Ear
Outer Ear – the outermost parts of the ear, including the pinna, auditory canal, and the surface of the eardrum.
Pinna – the very outside of the outer ear. The pinna acts as funnel to gather sound waves.
2. Middle Ear – part of the ear behind the eardrum and in front of the oval window, including the hammer, anvil and stirrup.
Auditory Canal – After being gathered by the pinna, sound waves are channeled to the tube connecting the pinna to middle ear - the auditory canal, where sounds are amplified and then strike the membrane at the end of the auditory canal, the eardrum.
Eardrum, or tympanic membrane – it is a very thin membrane that vibrates as the sounds waves strike it, much as the head of a drum vibrates when a drumstick strikes it.
Hammer, anvil, and stirrup – The three bones of the middle ear that are directly behind the eardrum. These very small bones mechanically amplify vibrations coming from the eardrum and transmit them to the inner ear.
3. Inner ear – innermost potion of the ear that includes the cochlea.
Oval window is found on the outer end of the cochlea, one of the major components of the inner ear.
Cochlea is coiled, fluid-filled tube about 1.4inches long that resembles a snail. It is here that sound waves are turned into neutral impulses. If you were to uncoil the cochlea, you would see that it resembles a flexible tube that is closed off at the end. The inside of the tube contains a fluid-filled canal called the cochlear duct.
Round window is a membrane that relieves pressure from the vibrating waves in the cochlear fluid.
Organ of Corti refers to the sensory receptor in the cochlea that transduces sound waves into coded neural impulses.
Basilar membrane. Is the structure in the cochlear duct that contains the hair cells, which convert sound waves into action potentials.
Hair Cells are neurons that grow out of the basilar membrane and convert sound waves into action potential.
The Auditory Pathway of the Brain
Once the hair cells convert sound into neural impulses, these impulses must be sent to the brain for further processing. Attached to the end of the cochlea is the auditory nerve. The bundles neurons of the auditory nerve gather the information from the hair cells to relay It to the brain.
Theories of Pitch Perception
Place theory Different pitches of sound activate specific regions of the basilar membrane more than others. Pitch perception occurs when the brain notices which portions of the basilar membrane are being most excited by incoming sound waves.
Frequency theory The hair cells of the basilar membrane fire action potentials at a rate equal to the frequency of the incoming sound wave. The brain determines pitch by noticing the rate at which the hair cells are firing. This theory explains only perception of pitches up to 1000Hz, the maximum firing rate of hair cell.
Volley Theory Similar to frequency theory, this theory states that groups of hair cells fire as teams to give us the perception of pitches over 1000Hz. For example, three hair cells each firing at 1000hz together yield the perception of 3000Hz tone 0
Duplicity Theory This theory states that a combination of a frequency and place information is used in pitch perception. Exactly how these sources of information are integrated in the brain is still being investigated.
Gustation(Taste) and Olfaction (Smell)
For most of us, the senses of taste and smell are interconnected. These two senses are called chemical senses because they require that certain chemicals come into direct contact with our sense organs. For taste, or gustation, to occur, certain chemicals in foods and other substances must be dissolved in our saliva and come into direct contact with the sense organs commonly known as the tongue. For smell, chemical in the nearby air – from food or other substances – must come into contact with cells in the nasal cavity.
Some Facts about Gustation
There are about 10,000 taste buds on the tongue and each taste bud contains approximately a dozen sensory receptors, called taste cells.
Taste cells are sensitive to chemicals in our food and drink
Papillae are clusters of taste buds on the tongue.
Taste buds respond to thousands of chemicals. There are taste buds that respond primarily to chemicals that give rise to the sensation of sweetness (mostly sugars), sourness (mostly acids), saltiness (mostly salts), and bitterness (in response to a variety of chemicals that have no food value or are toxic).
There is evidence that there is a fifth type of taste bud, which give rise to the sensation of fattiness in response to fats
Some scientists believe that there is another kind of taste bud that give rise to the sensation call umami (the savory taste of meat stock, cheese, and mushrooms), but this has been shown to arise from the same taste buds that give rise to the sensation of sweetness.
We lose taste buds as we age, especially over 45 years of age. Babies have the most taste buds and are very sensitive, whereas older adults are less sensitive to the chemicals that give rise to taste sensation.
Unlike some types of sensory cells, taste buds can regenerates
Some Facts about Olfaction
Olfaction, our sense of smell, has adaptive value. Smells can alert us to danger. The ability to smell smoke enables us to detect a fire long before we see flames.
Chemicals in the air we breathe pass by the olfactory receptors on their way to the lungs. These sheet receptor cells are called olfactory epithelium located at the top of the nasal cavity.
When it comes to discriminating between odors, we can detect roughly 500,000 different scents and we can identify by name about 10,000 different smells, us to capture our attention
Nearly all the chemicals that human can detect as odors are organic compounds, meaning they come from living things. In contrast, we can smell very few inorganic compounds such as rocks and sand.
Some Facts about the Skin
The skin can detect only three kinds of sensory information such as pressure, temperature, and pain.
There are four types of receptors in the skin: free nerve endings, the basket cells, the tactile discs, and the specialized end bulbs.
Free nerve endings are sensory receptor cells in the skin the detect pressure, temperature, and pain. Nocioreceptors in the free nerve endings serve as receptors for stimuli that are experienced as painful.
Pain signals are regulated in three parts of the nervous system: the brain stem, the spinal cord, and in the peripheral pain receptors.
Basket cells at the base of the hair, tactile discs and specialized end bulbs detect pressure.
The Body Senses
Messages about the orientation, balance, and movement of the body come to us from the skin senses (pressure on different parts of our body) and from two kinds of sense organs. A complicated set of sensory structures called the vestibular organs is located in the inner section of the ear, where it provides the cerebral cortex with information about orientation and movement. Individual sensory receptors, called kinesthetic receptors, located in the muscles, joints, and skin provide additional messages about movement, posture, and orientation.
Vestibular Organ is composed of two sets of small sensory structures: the semicircular canals and the linked saccule and utricle are fluid-filled sacs in the inner ear that contain sensory receptors that keep the brain informed about the body’s orientation. Further, the semicircular canal provides the most sensitive message to the brain about orientation.
Kinesthetic Receptors provide detailed information on the orientation of the head and the body, differences in pressure due to gravity and movement on different parts of the body, the movement of each parts, and a host of the other kinds of information.
Perception: Interpreting Sensory Messages
Perception is the interpretation of sensation. It’s an active process in which perceptions are created often go beyond the minimal information provided by the senses. The proceeding discussion of perception focuses on visual perception, rather than all of the perceptual systems because scientist understand better how it works better than they do other systems; and it is representative enough of other systems to tell us something about the process of perception.
Gestalt Principles of Perception
1. Figure ground. When we perceive a visual stimulus, part of what we see is the center of our attention, the figure , and the rest is the indistinct ground.
2. Continuity, We tend to perceive lines or patterns that follow a smooth contour as being part of a single unit.
3. Proximity. Things that are proximal (close together) are usually perceived as belonging together
4. Similarity. Similar. Similar things are perceived as being related.
5. Closure. Missing sensory information is automatically “filled in” the process of perception to create complete and whole perceptions
Type of Perceptual Constancy
Perceptual constancy is the tendency for perceptions of objects to remain relatively unchanged in spite of changes in raw sensation
1. Brightness Constancy
2. Color Constancy
3. Size Constancy
4. Shape Constancy
1. Monocular Cues to Depth Perception can be perceived by one eye. We use this monocular cues in everyday life and artist manipulate them to create images in art that appear to have depth on flat surfaces and to bring computer-animated figures to life.
1.1 Texture Gradient. The texture of an object is larger and more visible up close and smaller when far away. On curved surfaces, the elements of texture are also more slanted when the surface does not squarely face us.
1.2 Linear Perspective. Objects cast smaller images on the retina when they are more distant. As a result, parallel lines, such as railroad tracks, appear to grow closer together the farther away they are from us.
1.3 Superposition. Closer object tend to be partially in front of, or partially cover up, more distant object.
1.4 Shadowing. The shadows cast by objects and highlighted of reflected light suggest their depth.
1.5 Speed movement. Objects farther away appear to move across the field of vision more slowly that do closer objects
1.6 Aerial Perspective. Water vapor and pollution in the air scatter light waves, giving distant objects a bluish, hazy appearance compared with nearby objects.
1.7 Accommodation. The shape of the lens of the eye must change to focus the visual image on the retina from stimuli that are different distances from the eye.
1.8 Vertical Position. When objects are on the ground, the farther they appear to be below the horizon, the loser they appear to be to us. For objects in the air, however, the farther they appear to be above the horizon, the closer they appear to us.
Binocular Cues in depth perception require both eyes to allow us to perceive depth.
The two binocular cues are:
1. Convergence. When both eyes are looking at an object in the center of the visual field, they must angle inward more sharply for a near object than for a distant object.
2. Retinal Disparity. Because our two eyes are a couple of inches apart, they do not see the same view of three-dimensional objects, especially when the object is close. This disparity or difference, between the images on the two retinas is a key factor in depth perception.
Visual Illusions – visual stimuli in which the cues used in visual perception create a false perception.
Example: Muller- Lyer Illusion; Ponzo Illusion
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