Sensory Processes and Perception

Sensory processes and perception 1 Introduction Human beings perceive the world primarily through a system of inputs known as the sensory systems of vision, sound, touch, smell and taste (the five senses). Receptors and Stimulation The first step in sensing the world is to translate energy from the environment into information that can be used by the brain. The first link in the chain, for every sensory system, is a specialized type of cell called a sensory receptor. Sensory Receptor Cells- receive the stimuli from the environment and transfer it to nerve impulses to be processed in the brain. 2 The Character of Sensation In all sensory processes, some form of energy stimulates a receptor cell in one of the sense organs. The receptor cell converts that energy into a neural signal, which is further coded as it travels along sensory nerves. By the time it reaches the brain, the message is quite precise. 3 Sensation Sensation is the process by which our senses gather information and send it to the brain. A large amount of information is being sensed at any one time. With all this information coming into our senses, the majority of our world never gets recognized. We don't notice radio waves, x-rays, or the microscopic parasites crawling on our skin. We don't sense all the odors around us or taste every individual spice in our dinner. We only sense those things we are able to since we don't have the sense of smell like a hound or the sense of sight like a hawk; our sensation thresholds are different from these animals and often even from each other. 4 The absolute threshold The lowest level at which an individual can experience a sensation is called the absolute threshold. The point at which a person can detect a difference between “something” and “nothing” is that person’s absolute threshold for that stimulus 5 Vision Vision is different from all the other senses. It involves receptor neurons that are sensitive to light. Light enters through the pupil and lens of the eye and is projected onto the back surface of the eye called the retina. The retina is composed of, among many other things, receptor neurons called rods and cones 6 CONT. 7 CONT. The rods are sensitive to a broad range of light, i.e. they tell us about “white.” They contain what is called visual purple, a chemical that is sensitive to light. Note that a crucial part of this chemical is derived from vitamin A -- so eat your carrots! The chemical breaks down when exposed to light and releases a protein (opsin) which eventually releases a neurotransmitter to send messages to the brain that “there is light.” Cones are similar, but involve a chemical called iodopsin that is sensitive to more specific wavelengths of light, depending on pigments associated with the chemical. One kind of cone responds to red, one to green, and one to blue. Rods are far more sensitive than cones. This is why you see in “black and white” when there isn’t much light. 8 CONT. The great majority of color-blind people suffer from red-green color blindness. This comes about because they lack either red or green cones, so that red and green are indistinguishable. 9 10 Receptors: rods and cones Rods • • • • 120 million periphery sensitivity night vision Cones •8 million •central •acuity •color vision 11 Hearing (audition) Hearing begins when sound waves strike the eardrum and cause it to vibrate. This vibration, in turn, makes three bones in the middle ear—the hammer, the anvil, and the stirrup—vibrate in sequence. These vibrations are magnified in their passage through the middle ear deep into the inner ear. The oval window, which is attached to the stirrup, and the round window are membranes between the middle and inner ear. In the inner ear, the vibrations cause the fluid inside the cochlea to vibrate, pushing the basilar membrane and the organ of Corti up and down. Inside the organ of Corti are tiny hair cells that act as sensory receptors for hearing. Stimulation of these receptors produces auditory signals that are transmitted to the brain through the auditory nerve. The brain pools the information from thousands of these cells to create the perception of sounds. 12 CONT. 13 Taste There are about 10,000 taste buds on the tongue, clustered in papillae (those bumps all over your tongue). The taste buds are clusters of neuron bodies that line tiny pits in the papillae. Molecules from the food we eat get mixed with saliva and find their way into the little pits and onto the surfaces of the neurons. Like a key fitting into a lock, these molecules open up tiny pores on the cell membranes and begin the process of firing the neuron very much the same way as the neurotransmitters do between neurons. There are only four basic tastes: Bitter – alkaloids, Sweet – sugars, Salt - sodium chlorides and Sour - acids 14 15 Smell (olfaction) Smell works much like taste: It is also a “lock and key” sense. This time, it is a matter of moist air being drawn over a piece of specialized mucous membrane about the size of a dime at the top of your nasal cavity. 16 Touch (tactile) Skin senses involve three basic skin sensations- touch (or pressure), temperature, and pain. 17 Smell and Taste 18 Perception No young child at first sees objects as we see them, or hears sounds as we hear them. This power, the power of perception, is a gradual development. It grows day by day out of the learner's experience in his world of sights and sounds, and whatever other fields his senses respond to. A sense is a physical system that collects information and then translates the information into a meaningful form that the brain can understand. A sensation is message that the brain receives from the senses. There are many pathways by which sensory information can enter the body. Sensory information can come from the external word or from internal world of the body. The sensory system communicates information in an electrochemical form. When the brain receives various sensations, it organizes, interprets and makes sense of the sensory information. The high level processing of information in your brain is perception, which takes up roughly where sensation leaves off. 19 Cont. Perception refers to the cognitive (thinking) process through which we interpret sensory massages. During perception, we synthesize (selectively combine and integrate) and assign meaning to our sensations. We do so by taking in our account our expectations, our experiences, and perhaps even our culture. Consider these marks: 13. If asked what the letter is, you would say "B". If asked what the number is, you would say 13. The actual marks remain the same; their meaning changes in keeping with your expectation to recognize a number or a letter. To a child without appropriate knowledge to perceive either a number or a letter, the marks would probably be meaningless. 20 Cont. Thus, sensations enable our bodies to receive information from the environment and perception enables us to make sense of this information. Sensation and perception are highly interrelated. Sense: A physical system for receiving a particular kind of physical stimulation and translating that stimulation into an electrochemical message. Perception: The mental processes that organize and interpret sensory information that has been translated to the brain. Perception is more than just passive reception of information. Perception is an active process: We should also keep in mind that perception is not something done with the eyes or the ears or any specific sense organ. 21 Sensation and Perception • Sensation – The process through which the senses pick up visual, auditory, and other sensory stimuli and transmit them to the brain; sensory information that has registered in the brain but has not been interpreted • Perception – The process by which sensory information is actively organized and interpreted by the brain 22