SENSATION AND PERCEPTION

SENSATION AND PERCEPTION

1. The Visual System

   • Light
     • Is a form of electromagnetic radiation that travels as a wave.
     • Amplitude
       • Is the height of the light wave and allows us to perceive brightness.
       • Wavelength
         • Is the distance between peaks and allows us to perceive colour. The lights humans see are usually mixtures of several wavelengths (colours). Therefore, the purity can vary with we experience as saturation of a colour.
   • The Cornea
     • The transparent layer forming the front of the eye. This is where light enters the eye. 
     • The Pupil
       • The opening of the iris. The pupil constricts (smaller) in bright light or dilates (larger).in dim light.  
     • The Lens
       • A transparent structure with muscles attached to it to focus the light.  It is called accommodation when the lens curvature adjusts to let us focus on a closer or more distant object. 
     • The Retina
       • Is the neural tissue lining the inside back surface of the eye; it absorbs light, processes images and sends visual information to the brain. It is actually a part of the central nervous system. It is as thin as a piece of paper but has three layers.  The axons that go from the retina to the brain come together at a place known as the optic disk, a hole in the retina where the neural fibres exit the eye. This actually leaves a blind spot but each eye compensates for the blind spot in the other.     
   • Nearsighted
     • unable to see things clearly unless they are relatively close to the eyes, owing to the focusing of rays of light by the eye at a point in front of the retina; myopic.
     • Farsighted
       • unable to see things clearly, especially if they are relatively close to the eyes, owing to the focusing of rays of light by the eye at a point behind the retina; hyperopic.
   • Visual Receptors
     • Rods and Cones
       • The innermost layer of the retina has the visual receptor cells called rods and cones. There are 125 million rods and 6.4 million cones in one retina.  They are shaped as their name states. Cones are specialized visual receptors that play a key role in daylight vision and colour vision.  Rods are specialized visual receptors that play a key role in night vision and peripheral vision and are sensitive to dim light. Rods are more concentrated in the periphery of the retina. Our sharpest vision for detail and colour is in the centre of the retina called the fovea which only has cones.      
     • Dark Adaptation
       • is the process in which the eyes become more sensitive to light in low illumination, e.g. going into a dark theatre. Cones adapt faster but not as much. Rods adapt slower but adapt more than cones.    
       • Light Adaptation
         • Light adaptation is the process whereby the eyes become less sensitive to light in high illumination, e.g. turning on the lights after the show.        
   • Information Processing in the Brain
     • The retina has 130 million rods and cones that need to be funnelled into only 1 million axons in the optic nerve. Signals move from rod and cone receptor layer to bipolar cell layer to the ganglion cell layer of the retina and then on to the optic nerve.
     • Optic Nerve
       • Is a collection of axons that connect the eye with the brain.    
     • Receptive Field
       • The receptive field of a visual cell is the retinal area that when stimulated affects the firing of that cell. For example, a bipolar cell’s receptive field would be the rods and cones connected to that bipolar cell. A ganglion cell’s field includes the bipolar cells, rods, and cones attached to it. A common type of receptive field has a centre-surround arrangement with the centre being excitatory and the surround inhibitory or vice versa.  This allows us to see edges clearly.      
   • Trichromatic Theory of Colour Vision
     • (Tri = three; Chroma = colour) Hermann von Helmholtz (1852) stated that the human eye has three types of receptors with differing sensitivities to different light wavelengths, specifically, red, blue, and green.  The eye mixes these three colours to make any colour we can see.  Televisions work this way.       
   • Colour Blindness
     • Colour blindness encompasses a variety of deficiencies in the ability to distinguish among colours. This occurs more frequently in males. Most colour blind people are dichromats meaning they see with only two channels. They lack either the red, green or blue channel though it is rare not to have the blue colour working.  Colour blindness supports trichromatic theory.     
   • Gestalt Principles
     • Form perception due to top down processing is shown in the principles of Gestalt psychology. These psychologists showed that “The whole is greater than the sum of the parts". Max Wertheimer (1912) Gestalt psychologist who observed the phi phenomenon – illusion of movement created by presenting visual stimuli in rapid succession (movies, neon lights).  Movement is not in any one of the parts. Gestalt Principles showing how the visual system organizes a scene: Figure and ground:  Figure is the object being looked at and the ground is the background.     
   • Perceiving Depth and Distance
     • Depth Perception
       • Interpretation of visual cues that indicate how near or far away objects are.
       • Binocular Depth Cues
         • Are clues about distance based on the differing views of the two eyes. Our eyes are set apart from each other so that they each have a slightly different view of an object.  1. The most important binocular depth cue is retinal disparity – objects within 25 feet of your eyes project images to slightly different locations on each retina. The closer an object is, the more difference there is in what each eye sees.  2.  Convergence – involves sensing the eyes converging toward each other as they focus on closer objects of up to 10 feet away.  The two eyes turn slightly inward when focusing on an object.      
         • Monocular Depth Cues
           • Cues about distance based on the image in either eye alone.             1. Motion parallax – movement of images of objects at different distances move across the retina at different rates.  If you are driving, the fence posts close to your vehicle appear to move faster than the trees in the distance.  2.  Pictorial depth cues – clues about distance in a flat picture.  These include linear perspective, texture gradients, interposition, relative size, height in plane, light and shadow.  Perceptual constancy is a tendency to experience a stable perception in spite of continually changing sensory input.  E.g. size constancy – you know that though a man seems smaller as he walks away from you, he is still the same size.Optical illusions – E.g. Muller Lyer illusion and Ponzo illusion operate through size constancy and misperception of depth.  Ames room – room built without any right angles so one side of the room is shorter than the other.  Because there is no such thing in the real world the children walking in it appear of different heights though they are not.     
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2. The Auditory System-Hearing

   • The Vestibular System
     • People are equipped with a vestibular system which responds to gravity and keeps us informed of our body’s location in space.  It provides you with a sense of balance or equilibrium, compensating for changes in the body’s position. If you are running down a steep hill, this system would help you know when to slow down so you don’t take a tumble.  If you were running with a video camera on, your eyes would not perceive the world as moving up and down as you raise and lower your legs because you have a vestibular system. However, the movie picture would look like the world was bouncing. Receptors for the vestibular system are in the semicircular canals in the inner ear. When the head rotates, the fluid in this area changes and is detected by hair cells.  Sensory integration puts information from all senses together in the brain.     
     • The Kinesthetic System
       • We actually have more than five senses. The kinesthetic system monitors the positions of the various parts of the body. Some receptors are in the joints and let us know how much we are bending. Others, in the muscles, let us know if the muscle is extended or not. The kinaesthetic system uses the same pathway as pressure/tactile stimulation.  If you raise your hand in class you are using your kinesthetic system    
   • Sensing Pain
     •  Although we dislike pain, it is very important in helping us change our behaviour so we do not hurt ourselves. Pain messages are sent to the brain through 2 different pathways and through 2 different parts of the thalamus. The fast pathway uses myelinated neurons and registers localized pain in a fraction of a second, e.g. sharp pain.  The slow pathway uses unmyelinated neurons and lets us know about less localized, longer lasting aches and burning pain that comes later. People feel pain differently from one another depending on their personality, mood, expectation, culture, etc.  One may feel less pain after being distracted or receiving a placebo and more pain by focusing on surgery for the problem.  This shows that cognitive and emotional processes in the higher brain centres must be able to block signals from the skin receptors.     
     • Sensing Touch
       • We experience the sense of touch due to nerve endings in our skin. We can sense pressure, pain, and temperature. Sensing pressure (tactile stimulation): Amazingly accurate so we can even sense where a feather touched us. Some parts of the skin have receptive fields with a centre surround arrangement like the eye has with an excitatory centre and inhibitory surround or vice versa. Nerve fibres that carry incoming messages from the outside world go to the spinal cord and then the brain stem where they cross to the opposite side of the brain. Tactile signals then pass through the thalamus to the somatosensory cortex in the parietal lobe. Some cells in the brain are feature detectors that respond to specific features of touch such as movement in a particular direction.
     • Olfactory System: Sense of Smell
       • Physical stimuli are chemical senses dissolved in the mucous in the nose. Receptors for smell are called olfactory cilia, hairlike structures in the upper portion of the nasal passages. The receptors have axons that synapse with the cells in the olfactory bulb at the base of the brain (prefrontal cortex) rather than the thalamus. There about 10,000 different odors and they have never been categorized. Pheremones are chemical messages that can be sent by one member of a species to another and usually linked to sexual attraction
     • The Gustatory System-Taste
       • Taste and smell are chemical senses. The physical stimuli for taste are chemicals that can be dissolved in water. The gustatory receptors are taste buds that line the trenches around tiny bumps on the tongue called papillae. When the chemicals are dissolved in saliva, they trigger neural impulses that travel through the thalamus to the cortex. Four primary types of taste: sweet, sour, bitter, and salty. Although taste cells respond to more than one taste, they are most sensitive to a particular one. Babies naturally like sweet tastes and dislike sour, bitter, and salty so this is considered innate (hereditary). However, most taste preferences are learned. People are born with a certain density of taste buds on the tongue. Supertasters have 4 times as many taste buds as nontasters. These are two extremes on a continuum. Women are more likely to be supertasters than men. Supertasters are often less fond of sweets and high fat foods, and dislike vegetables, alcohol and smoking. Odor contributes to our sense of taste/flavour. If a person lacks a sense of smell, taste will be affected (like when you have a cold).
   • Frequency Theory
     • This theory of how we hear sounds states that there are pulses that travel up the auditory nerve, carrying the information about sound to the brain for processing, and that the rate of this pulse matched the frequency of whatever tone you are hearing exactly. We thus hear the tone because the pulse traveling up the auditory nerve matches the actual tone. Essentially, we are getting a copy of the real sound. 
     • Place Theory
       • Place theory is a term that refers to how sound is received and perceived by the human ear. It refers to how sound waves affect different areas of the tympanic membrane, or eardrum, to create the perception of different types of sounds. A similar effect can be seen by hitting a tamborine in different spots; hitting near the side gives a flatter sound that hitting it in the center. 
   • Auditory Localization
     • Where is the sound coming from? Our ears are set apart so one ear hears 1) louder (greater intensity) and 2) sooner than the other. The head also casts a “shadow”, or partly blocks out the sound to the ear that is not on the side nears the sound’s location.
   • Sensory Processing in the Ear
     • Outer Ear
       • sound is conducted by vibration of air molecules. The main part is called the Pinna – sound collecting cone which funnels sound waves into the auditory canal and then to the eardrum – a taut membrane that vibrates in response to sound waves
     • Middle Ear
       • Sound is conducted by vibration of movable bones (smallest bones in body) called the hammer, anvil, and stirrup, known together as the ossicles. They operate as a lever system and amplify tiny changes in air pressure   
     • Inner Ear
       • Sound is conducted by waves in a fluid which are then converted into neural signals and sent to the brain. It includes the cochlea snail like structure with neural tissue (like retina in eye). The basilar membrane runs the length of the cochlea and holds the auditory receptors called hair cells which are stimulated by waves of fluid (like rods and cones in eye) Neural signals are routed through the thalamus to the auditory cortex in the temporal lobe which has feature detectors of sound. 
   • Wavelength or Frequency
     • Pitch (high or low musical note or voice)  Is measured in Hertz (Hz). Humans can only hear from 20 – 20,000 Hz, but usually 2000-4000 Hz.
     • Wave Amplitude
       • Corresponds to the psychological quality of loudness. It is measured in decibels (dB) After 120 dB it is painful.
     • Wave Purity or mixture
       • Timbre or complexity of the sound wave.  What is it the sound of? (car running)  The purest sound has only one frequency of vibration (e.g. tuning fork).  It is the difference between playing a note of the same loudness and pitch on piano and the guitar.   
   • The Gate-Control Theory
     • (Melzack and Wall, 1965) states that incoming pain sensations pass through a process in the spinal cord that functions as a gate which can be closed by skin receptor signals or brain receptor signals. However, no actual neural circuitry has been found. Instead endorphins may be released by placebos or acupuncture.  It has recently been discovered that certain types of glial cells may decrease pain or may cause chronic pain. Some may be activated by immune system responses to infection or by signals from neurons in pain pathways. Once activated, they may affect continuation of pain.  
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