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Chapter 4

Chapter 4 PSYA01 TEXTBOOK NOTES.docx

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University of Toronto Scarborough
Steve Joordens

PSYA01 – CHAPTER 4: SENSATION AND PERCEPTION: - Synthesia  the perceptual experience of one sense that is evoked by another sense - for some synthesia, musical notes evoke the visual sensation of color while others see printed letters/numbers in colors (digit 2 in pink or digit 3 in green) - synthesia is neither an isolated curiosity nor the result of faking and the research around synthesia can shed new light on how the brain is organized and how we sense and perceive the world OUR SENSES ENCODE THE INFORMATION OUR BRAINS PERCEIVE: - sensation and perception are two separate activities - sensation  simple stimulation of a sense 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  organization, identification and interpretation of a sensation in order to form a mental representation - ex. when you read, your eyes (the sensory organ) isn’t really seeing words but rather encoding different lines, curves and patterns of ink on a page. Your brain (perceptual organ) is transforming those lines & curves into a coherent mental representation of words and concepts What role does the brain play in what we see and hear? - Despite the variety of our senses, they all depend on the process of transduction  occurs when many sensors in the body convert physical signals from the environment into encoded neural signals sent to the central nervous system - In vision, light reflected from surfaces provides the eyes w/ info about the shape, color and position of objects - In audition, vibrations cause changes in air pressure that propagate through space to a listener’s ears - In touch, pressure ofa surface against the skins ignals its shape, texture and temp. - 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 CNS PSYCHOPHYSICS: - Any type of scientific investigation requires objective measurements - But how do you objectively measure a person`s private, subjective perception - Structuralists Wundt and Titchebner tried using introspection to measure perceptual experiences which failed miserably - Evoked memories and emotions intertwine with what you are hearing, seeing and smelling making your perception an event and therefore you experience of that event is UNIQUE - Gustav Fechner developed an approach to measure sensation and perception called psychophysics --. Methods that measure the strength of a stimulus and the observer`s sensitivity to that stimulus - in a typical psychophysics experiment, the researcher will ask people to make a simple judgement, whether or not they saw a flash o light - the psychophysicist then relates the measured stimulus such as the brightness of the light flash to each observer`s yes or no answer MEASURING THRESHOLDS: - psychophysicists begin the measurement process with a single sensory signal to determine precisely how much physical energy is required to evoke a sensation in the observer ABSOLUTE THRESHOLD: - simplest quantitative measurement in psychophysics is absolute threshold --. The minimal intensity needed to just barely detect a stimulus - threshold is a boundry ex. boundry b/w awareness and unawareness - in finding the absolute threshold for sensation, the two states in question are sensing and not sensing some stimulus (pg.129 table 4.2 for example) - ex. to measure the absolute threshold for detecting a sound, an observer sits in a soundproof room wearing headphones linked to a computer - the experimenter presents a pure toneusing the computer to vary the loudness or the length of time each tone lasts and recording how often the observer reports hearing that tone under each condition - investigators typically define the absolute threshold as the loudness required for the listener to say she or he has heard the tone on 50% of the trials DIFFERENCE THRESHOLDS: - absolute threshold is useful for assessing how sensitive we are to faint stimuli but most everyday perception involves detecting differences among stimuli that are well above the absolute threshold - the human perceptual system excels at detecting changes in stimulation rather than the simple onset or offset of stimulation - as a way of measuring this difference threshold, Fechner proposed the just noticeable difference (JND)  the minimal change in a stimulus that can just barely be detected - it depends on how intense the stimuli being measured are and on the particular sense being measure - ex. measuring JND for light o an observer in a dark room is shown a light of fixed intensity called the standard (S) next to a comparison light that is slightly brighter or dimmer o when S is very dim, observers can see even a very small difference in brightness b/w the two lights (JND IS SMALLER) o when S is very bright, a much larger increment is needed to detect the difference (JND IS LARGER) - weber’s law  states that the just noticeable difference of a stimulus is a constant proportion despite variations in intensity - ex. if you picked up a one-ounce envelope and a two-ounce envelope you probably wouldn’t notice the diff. b/w them - but if you picked up a 20 pound package then a 20 pound 1 ounce package you probably detect no diff. b/w them - when calculating a difference threshold, it is proportion b/w stimuli that is important and the measured size of the difference (brightness, loudness, weight) is irrelevant SIGNAL DETECTION: How accurate and complete are our perceptions of the world? - Sensory signals face alot of competition or noise, which refers to all the other stimuli coming fromt he internal and external world - Memories, moods and motives interwine with what you are seeing, hearing and smelling - This internal noise competes with your ability to detect a stimulus with perfect, focused attention - As a consequence of noise, you may not perceive everything that you sense - During hearing test, you sometimes miss the quiet beeps and even say there is a beep when there really actually wasn’t - Signal detection theory  holds that the response to a stimulus depends both on a person’s sensitivity to the stimulus in the presence of noise and ona persons decision criterion - Observers consider the sensory evidence evoked by the stimulus and compare it to an internal decision criterion - If the sensory evidence exceeds the criterion, the observer responds by saying “yes i detected the stimulus” and if it falls short of the criterion the observer responds by saying “no i didn’t detect the stimulus” - Signal detection theory allows researchers to quantify an observer’s response in the presence of noise - Signal detection theory proposes a way to measure perceptual sensitivity (how effectively the perceptual system represents sensory events) separately from the observers decision making strategy - Signal detection theory offers a practical way to choose among criteria that permit decision makers to take into account the consequences of hits, misses, false alarms and correct rejections SENSORY ADAPTATION: - If you dive into cold water the temp, is shocking at first but after a few minutes you get used to it - This is an example of sensory adaptation the observation that sensitivity to prolonged stimulation tends to decline over time as an organism adapts to current conditions - You are studying and your neighbor blasts the stereo, after awhile you don’t notice the sound but when the music stops you notice - Our perceptual systems emphasize change in responding to sensory events (music stops) - Sensory adaptiation is a useful process for most organisms (imagine if the sting of garbage in your apartment when you first walk in would never dissipate) - Our sensory systems respond more strongly to changes in stimulation than to constant stimulation - A stimulus that doesn’t change usually doesn’t require any action (your car probably makes the same old hum you are used to) but a change in stimulation often signals a need for action (when you car makes different kinds of noises you are more likely to notice them and more likely to do something about it) VISION 1: HOW THE EYES AND THE BRAIN CONVERT LIGHT WAVES TO NEURAL SIGNALS: - 20/20 vision refers to measurement associated with a Snellen chart named after Snellen who developed it as a means of assessing visual acuity  the ability to see fine detail - Our sophisticated visual system has evolved to transducer visual energy in the world into neural signals in the brain - Humans have sensory receptors in their eyes that respond to wavelengths of light energy SENSING LIGHT: - Light is waves of energy that vary in height and wavelengths (distance b/w their peaks) - Three properties of light waves: 1) The length of a light wave determines its hue, or what humans perceive as color 2) The intensity or amplitude of a light wave (how high the peaks are) determines what humans perceive as the brightness of light 3) Purity is the number of distinct wavelengths that make up the light. Purity corresponds to what humans perceives as saturation, or the richness of colors - Length, amplitude and purity are properties of light waves but what humans PERCEIVE from those properties are color, brightness and saturation THE HUMAN EYE: - Eyes have evolved as specialized organs to detect light - Light that reaches the eyes passes first through a clear, smooth outer tissue called the cornea, this bends the light waves and sends it through the pupil (a hole in the colored part of the eye) - The colored part is the iris which is a translucent doughnut shaped muscle that controls the size of the pupil and hence the amount of light that can enter the eye - Behind the iris are muscles inside the eye that control the shape of the lens to bend the light again and focus it onto the retina  light sensitive tissue lining the back of the eyeball - The muscles change 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 - Accommodation  process by which the eye maintains a clear image on the retina - if the eyeball is too long, images are focused in front of the retina leading to nearsightedness (myopia) - if the eyeball is too short, images are focused behind the retina leading to farsightedness (hyperopia) PHOTOTRANSDUCTION IN THE RETINA: - retina is the interface b/w the world of light outside the body and the world of vision inside the CNS - two types of photoreceptor cells in the retina contain light-sensitive pigments that transducer into neural impulses - cones  detect color, operate under normal daylight conditions and allow us to focus on fine detail - rods  become active under low-light conditions for night vision - because all rods contain eh same photopigment they provide no info about color and sense only shades of gray - fovea  an area of the retina where vision is the clearest and there are no rods at all - absence of rods in the fovea decreases the sharpness of vision in reduced light - about 120 million rods are distributed around each retina but each retina only contains only about 6 millionc ones which are densely packed in the fovea and sparsely distributed ovcer the rest of the retina - the distribution of cones directly affects visual acuity and explains why objects off to the side (in your peripheral vision) aren’t so clear - light reflecting from those peripheral objects has a difficult time landing in the fovea making the resulting image less clear - the photoreceptor cells (rods and cones) form the innermost layer of the retina - the middle layer contains bipolar cells (collect neural signals from the rods and cones and transmit them to the outermost layer of the retina where neurons called retinal ganglion cells organize the signals and send them to the brain) - the bundled RGC axons form the optic nerve which leaves the eye through a hole in the retina - b/c the optic nerve contains neither rods nor cones and therefore no mechanism to sense light, the hold in the retina creates a blind spot  location in the visual field that produces no sesnsation on the retina RECEPTIVE FIELDS: - receptive field  region of the sensory surface that when stimulated, causes a change in the firing rate of that neuron - the cells that connect to the touch centers of the brain have receptive fields which are the part of the skin that, when stimulated, causes that cells response to change in some way - most receptive fields have either a central excitatory zone surrounded by a doughnut-shaped inhibitory zone (on-center cell) or a central inhibitory zone surrounded by a excitatory zone (in- center cell) - doughnut shaped regions --> patches of retina - a small spot shining on the central excitatory zone increases the RGC’s firing rate (retinal ganglion cell) - when spot exactly fills the excitatory zone, it elicits the strongest response - light falling on the surrounding inhibitory zone elicits the weakest response or none at all - for response of an off-center cell a small spot shining on the central inhibitory zone elicits a weak response - A spot shining on the surrounding excitatory zone elicits a strong response in the RGC - Retina is organized like this to edetect edges - Edges define the shapes of objects and anything that highlights such boundaries improves our ability to see an objects shape (especially in low-light situations) PERCEIVING COLOR: - Color is nothing but our perception of wavelengths from the spectrum of visible light - All rods contain the same photopigment which makes them ideals for low-light vision but bad at distinguishing colors - Cones contain any one of 3 types of pigment ; each pigment type if sensitive to visible wavelengths that correspond to red, green and blue TRICHROMATIC COLOR REPRESENTATION IN THE CONES: - Light striking the retina causes a specific pattern of response in the 3 cone types - One type responds best o short wavelength (blue) - Second type to medium wavelength (green - Third type to long wavelength of light (red) - S-cones, M-cones and L-cones respectively - Trichromatic color representation  pattern of responding across the three types of cones provides a unique code for each color COLOR-OPPONENT REPRESENTATION INTO THE BRAIN: What happens when the cones in your eyes get fatigues? - Staring too long at one color fatigues the cones that respond to that color, producing a form of sensory adaptation that results in a color afterimage - Color opponent system  pairs of visual neurons work in opposition - Red sensitive cells against green sensitive and blue sensitive against yellow sensitive - Red-green cells are excited (increase firing) in response to wavelengths corresponding to red and inhibited (decrease firing) in response to wavelengths corresponding to green - Color opponent system explains color aftereffects THE VISUAL BRAIN: - Great deal of visual processing takes place within retine but more complex aspects of vision require more powerful processing and enlists the brain - Streams of action potentials containing info encoded by the retina travel to the brain along the optic nerve - Half of the axons in the optic nerve that leave each eye come from retinal ganglion cells that code info in the right visual field whereas the other half code info comes from left visual feild - Visual signal to the halamus and then from there to the area V1  located at the back of the brain ; part of the occipital lobe that contains the primary visual cortex - In this area, the info is mapped into a representation of the visual scene NEURAL SYSTEMS FOR PERCEIVING SHAPE: - Perceiving shape depends on the location and orientation of an objects edges - Area v1 contains populations of neurons ; some neurons fire when an object in a vertical oritentation is perceived; others fire when an object in horizontal orientation is perceived PATHWAYS FOR WHAT, WHERE AND HOW What are the main jobs of the ventral and dorsal streams? - Two functionally distinct pathways/visual streams project from the occipital cortex to visual areas in other parts of the brain - Ventral stream travels across the occipital lobe into the lower levels of the temporal lobes and includes brain areas that represent an objects shape and identity (what it is) - Dorsal stream  travels up from the occipital love to the parietal lobes, connecting with brain areas that identify the
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