Chapter 4 Notes-Sensation and Perception
Chapter 4 Notes-Sensation and Perception
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Date Created: 04/13/14
October 19 2013 PSYCH 101 Tori Timmons Chapter 4 Notes Sensation and Perception o These unusual perceptual events are varieties of synesthesia the perceptual experience of one sense that is evoked by another sense For some synesthetes musical notes evoke the visual sensation of colour o Other people with synesthesia see printed netters or numbers in specific consistent colours Still others experience specific tastes when certain sounds are heard o Whatever the ultimate explanations research on synesthesia can shed new light on how the brain is organized and how we sense and perceive the world o Our Senses Encode the Information Our Brains Perceive Sensation is simple stimulation of a sensory organ It is the basic registration of light sound pressure odor or taste as parts of your body interact with the physical world After a sensation registers in your central nervous system perception takes place at the level of your brain Sensation and perception are related but separate events We all know that we have five senses vision hearing touch taste and smell Despite the variety of our senses they all depend on the process of transduction which occurs when many sensors in the body convert physical signals from the environment into encoded neural signals sent to the central nervous system October 19 2013 PSYCH 101 Tori Timmons In vision light reflected from surfaces provides the eyes with information about the shape colour and position of objects In audition vibrations cause changes in air pressure that move through space to a listener39s ears In touch the pressure of a surface against the skin signals its shape texture and temperature In taste and smell molecules dispersed in the air or dissolved in saliva reveal the identity of substances that we may or may not want to eat In each case physical energy from the world is converted to neural energy inside the central nervous system Psychophysics Measuring the physical energy of a stimulus such as the wavelength of a light is easy enough but how do you quantify a person s private subjective perception of that light In the mid 1800s German scientist and philosopher Gustav Fechner developed an approach to measuring sensation and perception called psychophysics methods that measure the strength of a stimulus and the observer39s sensitivity to that stimulus In a typical psychophysics experiment researchers ask people to make a simple judgment whether or not they saw a flash of light for example The psychophysicist then relates the measured stimulus such as the brightness of the light flash to each observer s yes or no response October 19 2013 PSYCH 101 Tori Timmons Measuring Thresholds 39 Psychophysicists being the measurement process with a single sensory signal to determine precisely how much physical energy is required to evoke a sensation in an observer 39 The simplest quantitative measurement in psychophysics is the absolute threshold A threshold is a boundary In finding the absolute threshold for sensation the two states in question are sensing and not sensing some stimulus 39 Investigators typically define the absolute threshold as the loudness required for the listener to report hearing the tone on 50 of the trials The absolute threshold is useful for assessing how sensitive we are to faint stimuli but most everyday perception involves absolute threshold As a way of measuring this difference threshold Fechner proposed the just noticeable difference or JND 39 The JND is not a fixed quantity rather it is roughly proportional to the magnitude of the stimulus 39 This relationship was first noticed in 1834 by German physiologist Ernst Weber now called Weber39s Law it states that the just noticeable difference of a stimulus is a constant proportion despite variations in its intensity When calculating a difference threshold it is the proportion between stimuli that is important the measured size of the October 19 2013 PSYCH 101 Tori Timmons difference whether in brightness loudness or weight is irrelevant Signal Detection Measuring absolute and difference thresholds requires a critical assumption that a threshold exist But much of what scientists know about biology suggests that such a discrete all or nothing change in the brain is unlikely Humans don t suddenly and rapidly switch between perceiving and not perceiving in fact the transition from not sensing to sensing is gradual An absolute threshold is operationalized as perceiving the stimulus 50 of the time which means the other 50 of the time it might go undetected Our accurate perception of a sensory stimulus then can be somewhat haphazard Sensory signals face a lot of competition or noise which refers to all the other stimuli coming from the internal and external environment This internal noise competes with your ability to detect a stimulus with perfect focused attention As a consequence you may not perceive everything that you sense and you may even perceive things that you haven t sensed October 19 2013 PSYCH 101 Tori Timmons An approach to psychophysics called signal detection theory holds that the response to a stimulus depends both on a person39s sensitivity to the stimulus in the presence of noise and on a person s decision criterion That is if a stimulus exceeds the criterion it is detected if it falls short of the criterion it is not Signal detection theory is a more sophisticated approach than was used in the early days of establishing the absolute thresholds because it explicitly takes into account observers response tendencies such as liberally saying yes when there is any hint of a stimulus or conservatively reserving identifications only for obvious instance of the stimulus Signal detection theory offers a practical way to choose among criteria that permit decisions makers to take into account the consequences of hits misses false alarms and correct rejections Sensory Adaptation Sensory adaptation is the observation that sensitivity to prolonged stimulation tends to decline over time as an organism adapts to current conditions Sensory adaptation is a useful process for most organisms Our sensory systems respond more strongly to changes in stimulation than to constant stimulation o Vision I How the Eyes and the Brain Convert Light Waves to Neural Signals October 19 2013 PSYCH 101 Tori Timmons Humans have sensory receptors in their eyes that respond to wavelengths of light energy When we look at people places and things patterns of light and colour give us information about where one surface stops and another begins The array of light reflected from those surfaces preserves their shapes and enables us to forma a mental representation of a scene Sensing Light Visible light is simply the portion of the electromagnetic spectrum that we can see and it is an extremely small slice Think about light as waves of energy Light waves vary in height and in the distance between their peaks or wavelengths There are 3 properties of light waves each of which has a physical dimension that produces a corresponding psychological dimension 39 The length of a light wave determines its hue or what we perceive as colour The intensity or amplitude of a light wave how high the peaks are determines what we perceive as the brightness of light The purity is the number of distinct wavelengths that make up the light it determines what we perceive as saturation or richness of colours The Human Eye Eyes have evolved as specialized organs to detect light October 19 2013 PSYCH 101 Tori Timmons Light that reaches the eyes passes first through a clear smooth outer tissue called the cornea and then through the pupil a hole in the coloured part of the eye This coloured part is the iris a translucent doughnut shaped muscle that controls the size of the pupil and hence the amount of light that can enter the eye Immediately behind the iris muscles inside the eye control the shape of the lens to bend the light again and focus it onto the retina The muscles changes the shape of the lens to focus objects at different distances making the lens flatter for objects that are far away or rounder for nearby objects This is call accommodation KEY TERMS Sensation Simple stimulation of a sensory organ Perception The organization identification and interpretation of a sensation in order to form a mental representation Transduction What takes place when many sensors in the body convert physical signals from the environment into encoded neural signals sent to the central nervous system Psychophysics Methods that measure the strength of a stimulus and the observers sensitivity to the stimulus Absolute Threshold The minimal intensity needed to just barely detect a stimulus detect it 50 of the time October 19 2013 PSYCH 101 Tori Timmons Just Noticeable Difference JND The minimal change in a stimulus that can just barely be detected Weber39s Law The just noticeable difference of a stimulus is a constant proportion despite variations in intensity Signal Detection Theory An observation that the response to a stimulus depends both on a person s sensitivity to the stimulus in the presence of noise and on a person s response criterion Sensory Adaptation Sensitivity to prolonged stimulation tends to decline over time as an organism adapts to current conditions Retina Light sensitive tissue lining the back of the eyeball Accommodation The process by which the eye maintains and clear image on the retina Cones Photoreceptors that detect colour operate under normal daylight conditions and allow us to focus on fine detail Rods Photoreceptors that become active under low light conditions for night vision Fovea An area of the retina where vision is the clearest and there are no rods at all Blind Spot A location in the visual field that produces no sensation on the retina because the corresponding area of the retina contains neither cones nor rods and therefore has no mechanism to sense light Receptive Field The region of the sensory surface that when stimulated causes a change in the firing rate of that neuron Area V1 The part of the occipital lobe that contains the primary visual cortex Visual Form Agnosia The inability to recognize objects by sight October 19 2013 PSYCH 101 Tori Timmons Binding Problem A phenomenon that concerns how features are linked together so that we see unified objects in our visual world rather than free floating or miscombined features Feature Integration Theory The idea that focused attention is not required to detect the individual features that comprise a stimulus but is required to bind those individual features together Perceptual Constancy A perceptual principle stating that even as aspects of sensory signals change perception remains consistent Monocular Depth Cues Aspects of a scene that yield information about depth when viewed with only one eye Binocular Disparity The difference in the retinal images of the two eyes that provides information about depth Apparent Motion The perception of movement as a result of alternating signals appearing in rapid succession in different locations Change Blindness A phenomenon that occurs when people fail to detect changes to the visual details of a scene Intentional Blindness A failure to perceive objects that are not the focus of attention Pitch How high or low a sound is Loudness A sound s intensity Timbre A listener s experience of sound quality or resonance Cochlea A fluid filled tube that is the organ of auditory transduction Basilar Membrane A structure in the inner ear that undulates when vibrations from the ossicles reach the cochlear fluid Hair Cells Specialized auditory receptor neurons embedded in the basilar membrane October 19 2013 PSYCH 101 Tori Timmons Area A1 A portion of the temporal love that contains the primary auditory cortex Place Code The mechanism by which the cochlea encodes different frequencies at different locations along the basilar membrane Temporal Code The mechanism by which the cochlea registers low frequencies via the firing rate of action potentials entering the auditory nerve Haptic Perception The active exploration of the environment by touching and grasping objects with our hands Referred Pain Feeling of pain when sensory information from internal and external areas converges on the same nerve cells in the spinal cord Gate Control Theory A theory of pain perception based on the idea that signals arriving from pain receptors in the body can be stopped or gated by interneurons in the spinal cord via feedback from two directions Vestibular System The three fluid filled semicircular canals and adjacent organs located next to the cochlea in each inner ear Olfactory receptor neurons Receptor cells that initiate the sense of smell ORNs Olfactory Bulb A brain structure located above the nasal cavity beneath the frontal lobes Pheromones Biochemical odorants emitted by other members of its species that can affect an animal s behavior or physiology Taste Buds The organ of taste transduction
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