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Chapter 4

Chapter 4 Textbook Notes

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PSYC 1001
Elaine Waddington Lamont

Chapter 4 Textbook Notes ● Astronauts in space experience reduction in hand-eye coordination ● On Earth gravity provides important cues for peoples judgement concerning the spatial orientation of their bodies ● 3 cues to determine which way is up: visual, gravity, and body direction ○ Use tilting sideways rooms to isolate the 3 cues. Suggests astronauts depend on visual cues ● Sensation - The stimulation of sense organs. ○ Involves absorption of energy, such as light or sound waves, by sensory organs, such as ears and eyes ● Perception - The selection, organization, and interpretation of sensory input ○ Involves organizing and translating sensory input into something meaningful such as your best friend’s face ● Psychophysics: Basic Concepts and Issues ○ Psychophysics - study of how physical stimuli are translated into psychological experience ■ Gustav Fechner important contributor ○ Sensation begins with a stimulus - any detectable input from the environment ○ What counts as detectable depends on who or what is doing the detecting ○ Threshold - a dividing point between energy levels that do and do not have a detectable effect ○ Absolute Threshold - For a specific type of sensory input is the minimum amount of stimulation that an organism can detect ■ Defines the boundaries of an organism’s sensory capabilities ○ As stimulus intensity increases, subjects’ probability of responding to stimuli gradually increases ○ In complete darkness can see a burning candle 50 kilometres away ○ Just Noticeable Difference (JND) - is the smallest difference in the amount of stimulation that a specific sense can detect ■ Absolute threshold is the just noticeable difference from nothing ■ Weber’s Law states that the size of a just noticeable difference is a constant proportion of the size of the initial stimulus ● The constant proportion is called the Weber Fraction ○ JND for lifting weights is 1/30, so you would just be able to notice the difference between a 300 gram weight and a 310 gram weight ■ As stimuli increases in magnitude, the JND becomes larger ○ Psychological Scaling ■ Fechner’s Law states the magnitude of a sensory experience is proportional to the number of JNDs that the stimulus causing the experience is above the absolute threshold ■ Constant increments in stimulus intensity produce smaller and smaller increases in perceived magnitude of sensation ● Ex. Turning 1 light bulb on in dark room causes big difference. Turning a second light bulb on causes a smaller increase in brightness and a third light bulb will cause an even smaller increase ■ Perception can’t be measured on absolute scales, with sensory experiences virtually everything is relative ○ Signal Detection THeory ■ Signal Detection Theory proposes that the detection of stimuli involves decision processes as well as sensory as sensory processes, which are both influenced by a variety of factors besides stimulus intensity ■ 4 Possible Outcomes of Signal Detection: Hits (detecting signals when they are present), Misses (failing to detect signals when they are present), False Alarms (detecting signals when they aren’t present), Correct Rejections (not detecting signals when they are absent) ■ Signal detection theory attempts to account for the influence of decision making processes on stimuli detection. When detecting weak signals your responses will depend in part on the criterion you set for how sure you must feel before you react ● Setting criterion involves higher mental processes rather than sensation and depends on your expectations and on the consequences of missing a signal or of missing a false alarm ■ According to signal detection theory, performance depends on level of “noise”. Noise comes from all the irrelevant stimuli in the environment and neural activity they elicit (evoke/draw out) ○ Perception without Awareness ■ Subliminal Perception - The registration of sensory input without conscious awareness (subliminal means below threshold) ○ Sensory Adaptation ■ Sensory Adaptation - Gradual decline in sensitivity due to prolonged stimulation ● Ex. Longer you are in a room, smells begin to fade ● Sensory adaptation is an automatic, built-in process that keeps people tuned into the changes rather than the constants in their sensory input. Behaviour adaptation sculpted by natural selection ● Sense of Sight ○ The Stimulus: Light ■ Amplitude affects perception of brightness; wavelengths affect perception of colour; purity (how varied the mix is) influences the perception of saturation or richness of colours ■ Vision is a filter that permits people to sense only a fraction of the real world ■ For people to see, incoming visual input must be converted into neural impulses that are sent to the brain ○ The Eye ■ The eye serves 2 main purposes: ● They channel light to the neural tissue that receives it, called the retina ● The eye also houses that tissue ■ Cornea - Transparent window where light enters eye ■ Lens - Transparent eye structure that focuses the light rays falling on the retina ● Accommodation - Curvature of the lens adjusts to alter visual focus (lens gets fatter when focusing on close objects, flattens when focusing on distant objects) ■ Visual Deficiencies from Defects in the Lens: ● Nearsightedness - Close objects are seen clearly but distant objects appear blurry (typical when eyeball is too long) ● Farsightedness - Distant objects are seen clearly but close objects appear blurry (typical when eyeball is too short) ■ Iris - The coloured ring of muscle surrounding pupil ■ Pupil - The black centre/ opening in the centre of the iris that helps regulate the amount of light passing into the rear chamber of the eye ● When pupil constricts, it lets less light into the eye but sharpens the image falling on the retina ● When pupil dilates (opens) more light gets in eye but the image is less sharp ■ Saccades - Tiny eye movements that are essential to good vision. When are eyes scan the environment they make brief fixations on various parts of the stimuli ○ The Retina: Brain’s Representative in the Eye ■ Retina - The neural tissue lining the inside back surface of the eye; it absorbs light, processes images, and sends visual information to the brain ● Part of the CNS ■ Optic Disk - Hole in retina where optic nerve fibres exit the eye. “Blindspot”, can’t see the part of an image that falls on it ■ Visual Receptors: Rods and Cones ● Retina contains millions of receptor cells sensitive to light in the innermost layer of the retina ● Only 10% of light entering cornea reaches these receptors ● Rods and cones get their name from their shape, rods being longer and cones being stubbier ● ~ 125 million rods to 6.4 million cones ● Cones - Specialized visual receptors that play a key role in daylight vision and colour vision ○ Special sensitivities of cones allows them to play a major role in perception of colour ○ Cones do not respond well to dim light, which is why you don’t see colour well in low illumination ○ Cones provide better visual acuity (sharpness and precision to detail) ○ Cones most concentrated in centre of retina, fall off towards periphery ○ Fovea - Tiny spot in the centre of the retina that contains only cones; visual acuity is greatest at this spot ● Rods - Specialized vision receptors that play a key role in night vision and peripheral vision ○ More sensitive than cones to dim light ○ The density of rods is greatest just outside the fovea and decreases towards periphery ■ Dark and Light Adaptation ● Dark Adaptation - Process in which the eyes become more sensitive to light in low illumination ○ Dark adaptation is virtually complete in 30 minutes with considerable adaptation in first 10 minutes ○ Cones adapt quicker than rods in the dark but rods adapt further (detect more light) ● Light Adaptation - Process whereby the eyes become less sensitive to light in high illumination ● Both types of adaptations are due to chemical changes in the rods and cones but neural changes in receptors also contribute ● Information Processing in the Retina: ○ Retina transforms a pattern of light into a very different representation of the visual scene ○ Light striking retina’s receptors (rods and cones) trigger neural signals which send impulses along optic nerve ○ Optic Nerve - Collection of axons that connect the eye to the brain ■ Carry visual information encoded as a stream of neural impulses to the brain ○ Receptive Field - The retinal area of a visual cell that when stimulated, affects the firing of cells ■ Come in a variety of shapes and sizes, commonly circular. In circular fields light falling in the centre has the opposite effect of light falling in the surrounding area ○ Lateral Antagonism - Occurs when neural activity in a cell opposes activity in surrounding cells ■ responsible for the opposite effects that occur when light falls on the inner versus outer portions of centre-surround receptive fields ■ This means visual system can compute the relative amount of light at a point instead of reacting to the absolute levels of light ■ Vision and the Brain ● Light falls on eye but you see with your brain ● Visual input is meaningless until processed by the brain ● Visual Pathways to the Brain ○ Optic Chiasm - The point at which the optic nerves from the inside half of each eye cross over and then project to the opposite half of the brain ■ Ensures signals from both eyes go to both hemispheres of the brain ○ After reaching optic chiasm, the optic nerve fibres diverge along 2 pathways: ■ The Main Pathway projects into the thalamus (brain’s major relay station). 90% of the axons from the retinas, synapse in the lateral geniculate nucleus (LGN) ■ Visual signals are processed in LGN and then distributed to areas in the occipital lobe that make up the primary visual cortex ■ The 2nd visual pathway leaving the optic chiasm branches off to an area in the midbrain called the superior colliculus before going through the thalamus and on to the occipital lobe ■ Function of 2nd pathway appears to be the coordination of visual input with sensory input ○ Main visual pathway is subdivided into 2 more specialized pathways called magnocellular and parvocellular channels ■ Engage in parallel processing ○ Parallel Processing - Involves simultaneously extracting different kinds of information from the same input ○ Ex. parvocellular channel handles the perception of colour, magnocellular channel handles brightness ● Information Processing in Visual Cortex ○ Individual cells in the primary visual cortex don’t respond much to little spots, they are much more sensitive to lines, edges, and other complicated stimuli ○ Hubel and Wiesel identified various types of specialized cells in the primary visual cortex that respond to different stimuli ■ Simple Cells respond best to a line of the correct width, oriented at the correct angle, and located in the correct position in its receptive field ■ Complex Cells also care about width and orientation, but they respond to any in their receptive fields ○ Cells in visual cortex are highly specialized and have been characterized as Feature Detectors - Neurons that respond selectively to very specific features of more complex stimuli ○ After visual input is processed in the primary visual cortex, routed to cortical areas for additional processing ○ Signal Travels through 2 streams: ■ Ventral Stream - Processes details of what objects are out there ■ Dorsal Stream - Processes where the objects are ■ Distinction between vision for perception and vision for action ○ Visual Agnosia - An inability to recognize objects ■ Probably due to damage somewhere along visual pathway that handles object recognition ■ Compensate by acquiring heightened ability to recognize voices ○ Prosopagnosia - Inability to recognize familiar faces ● Multiple Methods in Vision Research ○ Use variety of methods like in chapter 2, from fMRI to microelectrodes ○ McCollough Effect - Afterimage phenomenon that is contingent on colour and pattern/form ■ Viewing the World in Colour ● The Stimulus for Colour ○ Light people see are mixtures of various wavelengths, perceived colour is the dominant wavelength in mixture ○ Long wavelength appears red, short appears blue/violet ■ ***How it appears not physical property of the light ○ Subtractive Colour Mixing - Removes some wavelengths of light, leaving less light than was originally ■ Ex. mixing paint, blue and yellow make green ○ Additive Colour Mixing - Superimposes lights, putting more light in the mixture than exists in any one light by itself ■ Ex. Shining red, blue, green lights on white surface ○ Humans process colour using additive colour mixing ● Trichromatic Theory of Colour Vision - The human eye has 3 types of receptors with differing sensitivities to different light wavelengths ○ Human eye contains specialized receptors sensitive to the specific wavelengths associated with red, green, blue ○ Colour Blindness - Encompasses a variety of deficiencies in the ability to distinguish among colours ■ More common in males ■ Most who are colour blind are dichromats - they make do with only 2 colour channels ● Opponent Process Theory of Colour Vision ○ Complementary Colours - Pairs of colours that produce grey tones when mixed together ○ If you stare at a strong colour then look at a white background, you see an afterimage ○ Afterimage - A visual image that persists after a stimulus is removed ○ The colour of the afterimage will be the complement of the colour you originally starred at ■ Trichromatic theory can’t account for this ○ Opponent Process Theory of Colour Vision - Colour perception depends on receptors that make antagonistic responses to three pairs of colours ○ The 3 pairs of opponent colours: red versus green, yellow versus blue, and black versus white ■ Explains why dichromats have trouble distinguishing red from green or yellow from blue ● Reconciling Theories of Colour Vision ○ Takes both trichromatic colour theory and opponent process theory to explain vision ○ The eye has 3 different types of cones with each type being more sensitive to different bands of wavelengths ○ Found cells in retina, LGN, and visual cortex that respond in opposite ways to red vs green and blue vs yellow ■ Ex. ganglion cells that are excited by green and inhibited by red ● Effects of Colour on Behaviour ○ According to Andrew Elliot colours have an automatic, unconscious effects on behaviour ■ 1. People learn associations based on certain colours being paired repeatedly with certain experiences (ex. red ink marking errors, warns) ■ 2. Through evolution, certain colours had adaptive significance for survival or reproduction (blood and fire both are red, both signal danger) ○ Elliot believed red was associated with danger of failure. Tested by giving IQ test with red, green, white covers. Subjects exposed to red cover scored much lower. ○ Other studies showed red undermines performance by invoking avoidance tendencies that disrupt attention ○ Found red can even affect ratings of attractiveness (sexier) ○ Eye can follow red targets easier than other colours. Red targets look to be travelling faster (red vs grey porsche) ■ Perceiving Forms, Patterns and Objects ● Reversible Figures - A drawing that is compatible with two interpretations that can shift back and forth ● Perceptual Set - Readiness to perceive a stimulus in a particular way ○ Creates a slant in how someone perceives sensory input ● Form perception depends on what people focus their attention on ● Inattentional Blindness - Failure to see fully visible objects or events in a visual display ○ Attributed to a perceptual set that leads people to focus most of their attention on a specific feature in the scene ○ Likelihood of inattentional blindness increases on tasks that require a lot of attention
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