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Chapter 5

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Department
Psychology
Course
Psychology 1000
Professor
Dr.Mike
Semester
Fall

Description
Chapter 5 summary Synesthesia: the mixing of the senses - experiences sounds as colours or tastes as touch sensations - there is a sort of cross-wiring in the brain so that activity in one part of the brain evokes responses in another part of the brain dedicated to another sensory modality - may be caused by - the neural connections that occurs in infancy has not been cut - there is a deficit in neural inhibitory processes in the brain that ordinarily keep input from one sensory modality from going into another Sensation is the stimulus-detection process where our sense organs respond to and translate environmental stimuli into nerve impulses that are sent to the brain Perception makes sense of what our senses tells us - actively organizes the stimulus inputs and gives it meaning SENSORY PROCESSES - Every species have different stimuli that they are sensitive to - once they receive that stimuli, they go through transduction (the process whereby the characteristics of a stimulus are converted into nerve impulses) - Study of psychophysics studies relations between the physical characteristics of stimuli and sensory capabilities - concerned with two different kids of sensitivity - the absolute limits of sensitivity - the differences between stimuli STIMULUS DETECTION: THE ABSOLUTE THRESHOLD - How intense must a stimulus be before we can detect its presence? - the absolute threshold is the lowest intensity at which a stimulus can be detected correctly - therefore the lower the absolute threshold, the higher the sensitivity - subliminal stimuli are stimuli that are not consciously perceived - it can influence perceptions and behaviour in subtle ways but not strongly enough to justify concerns about the subconscious control SIGNAL DETECTION THEORY - people’s apparent sensitivity can fluctuate quite a bit therefore there is no such thing as a fixed absolute threshold - there is no single point that separates nondetection from detection - there is a range of uncertainty and people set their own decision criterion (a standard of how certain they must be that a stimulus is present before they will say they detect it) - the decision criterion also changes depending on things like fatigue, expectation, and potential significance of the stimulus - Signal detection theory is concerned with the factors that influence sensory judgments - In a signal detection experiment, participants are given a tone and told to say if they heard something or not - at low stimulus intensities both the participant’s and the situation’s characteristics influence the decision criterion - participants can become bolder or more conservative by manipulating the rewards and costs for giving correct or incorrect responses - eg. doctor less likely to say yes to participate in risky procedure because of problems of consequences - eg. marine more likely to notice a faint blip on screen during war than in peacetime voyage THE DIFFERENCE THRESHOLD - Difference threshold is the smallest difference between two stimuli that people can perceive 50% of the time - Weber discovered there is some degree of lawfulness in our range of sensitivities know as Weber’s Law - Weber’s Law says the difference threshold, or JND is directly proportional to the magnitude of the stimulus with which the comparison is being made and can be expressed as a weber fraction - for weights it is 1/50 meaning that if you lift 50 g you need at least 51 grams to be able to notice the difference SENSORY ADAPTATION - sensory adaptation is when a stimulus diminished its sensitivity towards an unchanging or constant stimuus - eg. feel of watch against skin recedes with time - sensory adaptations occurs in all sensory modalities, including vision - stationary objects would fade from sight fi we stared at them - sensory adaptations frees our sense from the constant to pick up informative changes to the environment THE SENSORY SYSTEMS VISION - usually stimulus for vision is electromagnetic energy - our visual system is only sensitive towards visual spectrum HUMAN EYE - light waves enter the eye through the cornea (protective structure at the front of the eye) - the pupil is behind that and it dilates or constricts to control the amount of light that enters the eye - its muscles are controlled by the muscles in the iris - low lights cause the pupil to dilate - lens is after the pupil and it is an elastic structure that becomes thinner to focus on distant objects and thicker to focus on nearby objects - the images focuses on the retina, a multilayered tissue at the rear of the fluid-filled eyeball - the lens reverses the image from right to left and from top to bottom when it is projected on the retina, but the brain reconstructs it - our ability to see depends on the lens’s ability to focus the image directly onto the retina - myopia (nearsightedness) is when the lens focuses the visual image in front of the retina (eyeball is too long) - hyperopia (farsightedness) when the image is focused behind the retina because the eyeball is too short; gets worse with age PHOTORECEPTORS: THE RODS AND CONES - retina contains specialized sensory neurons - it is actually an extension of the brain - contains two types of light-sensitive receptor cells (rods and cones) because of their shape - 2x more rods than cones - rods - function best in dim light - are primarily black and white brightness receptors - more sensitive to light that cones - so can see in dark but in day cannot see color during the day - cones - colour receptors - function best in bright illumination - animals that are active during the day have only cones in their retinas - animals that are active in both the day and night, have both cones and rods (like humans) - rods are found throughout the retina except in the fovea - a small area in the centre of the retina that contains only cones - cones decrease in concentration as one moves away from the centre of the retina - the periphery of the rod contains mainly rods - rods and cones send their messages to the brain via two additional layers of cells - bipolar cells have synaptic connections with the rods and conds - the bipolar cells then synapse with a layer of ganglion cells, whose axons are collected into a bundle to form the optic nerve - thus input from all the rods and cones is eventually funnelled into only one traffic lane leading out of the retina toward higher visual centres - the rods and cones form the rear layer of the retina - their light-sensitive ends actually point away from the direction of entering light - they only receive a fraction of the light that enters the eye - typically many rods are connected to the same bipolar cell - they can combine or funnel their individual electrical messages to the bipolar cell where the additive effect of the many signals may be enough to fire it - hence we can more easily detect a faint stimulus especially if we look at it from our peripheral vision - cones that lie in the fovea have their own line to a single bipolar cell - our visual acuity (abiility to see fine detail) is greatest when the image projects directly into the fovea - the optic nerve formed by the axons of the ganglion cells exits through the back of the eye produces a blind spot VISUAL TRANSDUCTION: FROM LIGHT TO NERVE IMPULSES - transduction is when nerve impulses are converted into nerve impulses - rods and cones use photopigments (protein molecules) to translate to nerve impulses - when these molecules absorb light, they produce a chemical reaction that changes the rate of neurotransmitter release at the receptor’s synapse with the bipolar cells - the greater change in transmitter release, the stronger the signal to the bipolar cell - therefore stronger signal to ganglion cells - if nerve responses are triggered at the rod/cone, bipolar cell and ganglion cell, the message is instantaneously transferred to the thalamus and to the visual cortex BRIGHTNESS VISION AND DARK ADAPTATION - rods are more sensitive to cones at low light, except for red - cones are more sensitive to low light at the greenish yellow range of the spectrum - Dark adaptation is the progressive improvement in brightness sensitivity that occurs over time under conditions of low illumination - it is because it takes time for the photoreceptor to regenerate its pigment molecules - cones reach their light sensitivity after 10 minutes in the dark - but rods do not reach their max sensitivity until half an hour COLOUR VISION - trichromatic theory - there are three types of colour receptors in the retina - individual cones are most sensitive to wavelengths that correspond to either blue, green or red - these receptors sends messages to the brain based on the extent to which wary were activated - the visual system then combines the signals to recreate the original hue - but people with red-green colour blindness were able to see yellow - so it showed flaw in theory - opponent process theory - proposed that each of the three cone types responds to two different wavelengths - one was red or green, the other blue or yellow, the last black or white - when the neural processes that register the colours become fatigued, opponent reaction occurs as each receptor responds with opposing colours - dual-process theory - combines both theories - the cones contain one of three different protein photopigments that are most sensitive to blue, red and green - different ratios of activity in the cones can produce a pattern of neural activity that corresponds to any hue in the spectrum - the opponent theory was correct except that the opponent reaction does not occur at the level of the cones - certain ganglion cells in the retina as well as some neurons in the visual relay stations and the visual cotrex respond in an opponent process by altering their rate of firing COLOUR-DEFICIENT VISION - people with normal vision are trichromats - people who cannot see red-green or yellow-blue are dichromat - a monochromat is totally colourblind and so is only sensitive to black and white ANALYSIS AND RECONSTRUCTION OF VISUAL SCENES - once the transformation of light energy to nerve impulses occurs, the process of combining the messages from the photoreceptors into the perception of a visual scene begins - from the retina the optic sends nerve impulses to a visual relay system in the thalamus - the input is then routed to various parts of the cortex, especially the primary visual cortex - feature detectors are neurons in the primary visual cortex that are organized to receive and integrate sensory nerve impulses originating in specific regions of the retina - fire to stimuli that have specific characteristics eg. when lines of certain orientation are presented, colours, depth or movement - A could be constructed from response of three different feature detectors - this is known as parallel processing of the information and constructing a unified image - then the information that is analyzed and recombined by the primary visual cortex is routed to the visual association
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