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Chapter 5

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Steve Joordens

PSY Chapter 5: - Our senses are the means by which we experience the world; everything we learn is detected by sense organs and transmitted to our brains by sensory nerves - Milner and Goodale: “Vision evolved to provide distal sensory control of the movements that the animal makes in order to survive and reproduce in that world” - Sense systems do that depending on (1) specific modality of the information (2) characteristics of the information and the state of the brain at the time it receives it - Visual system provides stability in the face of rapid shifts in its input. - Sound is not so variable, and is more gradual. Our auditory sense has more time to process signals - When we feel an object, the experience is active, not passive Sensory processing: - Experience is studied by distinguishing between sensation (detection of simple properties of stimuli, such as brightness, colour, warmth and sweetness) and perception (detection of objects both animate and inanimate, their locations, movements and backrounds). - Ex: seeing the colour red is a sensation, but seeing a red apple is a perception Transduction: - Sense organs detect stimuli, provided by light, taste, sound, odor or mechanical contact with the environment. Information about these stimuli is transmitted to the brain through neural impulses – action potentials carried by the axons in sensory nerves - The task of these sense organs is to transmit signals to the brain of the events that have happened in the environment. The task of the brain is to analyze this information and to reconstruct what has occurred - transduction ( leading across): process by which the sense organs convert energy from environmental events (physical stimuli) into neural activity (changes in the activity of receptor cells of sensory organs) - in most senses, specialized neurons called receptor cells ( a neuron that directly responds to a physical stimulus, such as light, vibrations, or smell) release chemical transmitter substances that stimulate other neurons, thus altering the rate of firing their axons Sensory coding - we can tell apart 7.5 million different colours and recognize 10 000 odors - A code is a system of symbols or signals representing information (ex: spoken English, written French, traffic lights…) - if we know the rules of a code, we can convert a message from one medium to another without losing any information - we do not know the precise rules by which the sensory systems transmit information to the brain, but we know that they take two general forms: (1) Anatomical coding and (2) Temporal coding - Firing of a particular set of neurons tells where the body is being touched (anatomical coding), and the rate at which these neurons fire (temporal coding) tells how intense the touch is. 1) since the brain has no direct information about the physical energy impinging on a given sense organ, it uses anatomical coding to interpret the location and type of sensory stimulus according to which incoming nerve fibers are active. Ex: if you rub your eye, you will mechanically stimulate the light-sensitive receptors located there. This stimulation produces action potentials in the axons of the nerves that connect the eyes with the brain (the optic nerves) - Anatomical coding: a means by which the nervous system represents information; different features are coded by the activity of different neurons 2) temporal coding is the coding of information in means of time. Simplest form is rate. Ex: light touch to the skin can be encoded by a low rate of firing. - Temporal coding: a means by which the nervous system represents information; different features are coded by the pattern of activity of neurons. - all sensory systems use rate of firing to encode the intensity of stimulation Psychophysics - a branch of psychology that measures the quantitative relation between physical stimuli and perceptual experience ( sensations they produce) - to study perceptual phenomena, scientists used two methods to measure peoples sensations: (1) the just-noticeable difference and (2) procedures of signal detection theory 1) JND: the smallest difference between two similar stimuli that can be distinguished; also called difference threshold - in germany, Ernst Weber investigated the ability of humans to discriminate between various stimuli. He measured the JND – the smallest change in the magnitude of a stimulus that a person can detect, and discovered that the JND is directly related to the magnitude of the stimulus. - Different senses had different ratios (ex: ratio for detecting differences in the brightness of white light is 1 in 60). These ratios are called Weber Fractions - Gustav Fecher used Weber’s concept of the JND to measure peoples sensations. - His contribution to psychology was to show how a logarithmic function could be derived from Weber’s principle - 100 years after Fechner’s work, S. S. Stevens suggested a power function to relate physical intensity to the magnitude of sensation 2) signal detection theory: a mathematical theory of the detection of stimuli, which involves discriminating a signal from the noise in which its embedded and which takes into account participants’ willingness to report detecting the signal - Psychological methods rely heavily on the concept of threshold (the point in which a stimulus, or a change in the value of a stimulus, can just be detected …. The line between perceiving and not perceiving). The JND can also be called a difference threshold (the minimum detectable difference between two stimuli). An absolute threshold is the minimum value of a stimulus that can be detected --- that is, discriminated from no stimulus at all. - By convention, the threshold is the point at which a participant detects the stimulus 50% of the time - According to the signal detection theory, every stimulus event requires discrimination between signal (stimulus) and noise (consisting of both background stimuli and random activity of the nervous system) - Receiver operating characteristic curve (ROC curve): a graph of hits and false alarms of participants under different motivational conditions; indicates people’s ability to detect a particular stimulus. - Named for its original use in research at the Bell Labratories to measure the intelligibility of speech transmitted through a telephone system Vision - Wavelength: the distance between adjacent waves of radiant energy; in vision, most closely associated with the perceptual dimension of hue - Wavelength of visible light ranges from 380 to 760 nanometers. - Entire range of wavelengths is known as the electromagnetic spectrum - The part we see as light is called the visible spectrum The Eye and its Functions - Cornea: the transparent tissue covering the front of the eye - Sclera: the tough outer layer of the eye; the “white” of the eye - Iris: consists of two bands of muscles (controlled by the brain) that control the amount of light admitted into the eye / the size of the pupil - Aqueous humour: the watery fluid filled in the space space immediately behind the cornea - lens: the transparent organ situated behind the iris of the eye; helps focus an image on the retina. Contains no blood vessels and is functionally dead tissue - cornea and lens help image to be focused on the inner surface of the back of the eye - accommodation: changes in the thickness of the lense of the eye that focus images of near or distant objects on the retina - nearsighted: people who’s eyes are too long (front to back); they need a concave lense to correct the focus - farsighted: people whos eyes are too short. They need a convex lens - retina: the tissue at the back inside surface of the eye that contains the photoreceptors and associated neurons, performs the sensory functions of the eye - photoreceptors: specialized neurons that transducer light into neural activity in the retina - information from the photoreceptors is transmitted to neurons that send axons toward one point at the back of the eye – the optic disc. All axons leave the eye at this point and join the optic nerve, which travels to the brain. - Johannes Kepler is credited with the suggestion that the retina, not the lens, contained the receptive tissue of the eye. - Christoph Scheiner demonstrated in 1625 that the lens is simply a focusing devise. Recorded the functions of the eye from an ox’s eye - Retina has 3 principle layers. Light passes through the ganglion cell layer (front), the bipolar cell layer (middle), and the photoreceptor layer (back) - Photoreceptors are transparent - Photoreceptors respond to light and pass the information on by means of a transmitter substance to the bipolar cells (a neuron in the retina that receives information from photoreceptors and passes it on to the ganglion cells, from which axons proceed through the optic nerves to the brain). Bipolar cells transmit this information to the ganglion cells (neorons whos axons travel across the retuna and through the optic nerves) - Visual information: photoreceptor à bipolar cell à ganglion cell à brain - Human retina contains two general types of photoreceptors: 125 million rods ( function mainly in dim light; are very sensitive to light but cannot detect change in hue. Visual info they provide lacks sharpness) and 6 million cones (function when the level of illumination is bright enough to see things clearly. Responsible for acute daytime vision and for colour perception) - Fovea: a small pit near the centre of the retina containing densely packed cones; responsible for the most acute and detailed vision - Farther away from the fovea, the number of cones decreases and the number of rods increases Transduction of light by photoreceptors - A molecule derived from vitamin A is the central ingredient in the transduction of the energy of light into neural activity - In the absence of light, this molecule is attached to a protein. The two molecules together form a photopigment (a complex molecule found in photoreceptors; when struck by light, it splits and stimulates the membrane of the photoreceptor in which it resides) - Intact photopigments have a characteristic colour. Rhodopsin, the photopigment of rods, is pink - Franz Boll discovered that once the photopigments are split by the action of light, they lose their colour and become bleached. Adaptation to light and dark - Detection of light requires the photons to split molecules of rhodospin or one of the other photopigments. - Dark adaptation: the process by which the eye becomes capable of distinguishing dimly illuminated objects after going from
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