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Chapter 4

PSYA01 Chap. 4.docx

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University of Toronto Scarborough
Michael Krashinsky

 Synesthesia: the perceptual experience of one sense that is evoked by another sense  Example: Seeing a poster of JB makes me throw up (no offence to JB fans) 4.1 OUR SENSESENCODE THE NFORMATION O URBRAINSP ERCEIVE  Sensation is a simple stimulation of a sense organ  Basic regulation of light, sound, pressure, odor, or taste as parts of your body interact with the physical world  Perception takes place at the level of your brain: It is the organization, identification, and interpretation of a sensation in order to form a mental representation  As you are reading this message your eyeballs are registering different patterns of light reflecting off the page  The brain is integrating and processing that light information into the meaningful perception of words  Our senses depend on the process of transduction, which occurs when many sensors in the body convert physical signals from the environment into encoding neural signals sent to the central nervous system 4.2 PSYCHOPHYSICS  Psychophysics: methods that measure the strength of a stimulus and the observer‟s sensitivity to that stimulus 4.3 M EASURINGTHRESHOLDS Absolute Threshold  The minimal intensity needed to just barely detect a stimulus  It is a boundary  Example: If you baked cupcakes, I can probably still smell it from a room that is 20 meters away. If you were outside, that is a different story. You better not hide those mouth watering cupcakes  Difference Thresholds  The human perceptual system excels at detecting changes in stimulation rather than the simple onset or offset of stimulation  Difference threshold: The minimal change in a stimulus that can just be detected (Can you tell the taste difference between Nutella and a regular chocolate bar?)  Weber’s Law: states that the just noticeable difference of a stimulus is a constant proportion despite variations in intensity  Example: you can probably tell the difference between a 10-pound cake and a 15-pound cake, compared to a 10-pound cake and 10.1-pound cake. 4.4 IGNAL DETECTION  Transition from not sensing to sensing is gradual  Signal detection theory: 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 decision criterion  This theory allows researchers to quantify an observer‟s response in the presence of noise  Example: when you are getting your peripheral vision checked, there will be a setup with flashing lights with different intensity. Now and then you have to respond with a “yes” if you see a flash of light. If you say “yes” when there isn‟t a flash, it results in a false alarm. If you say “no” when there was no light, this results in a correct rejection (good job you passed the test).  The theory proposes a way to measure perceptual sensitivity – how effectively the perceptual system represents sensory events – separately from the observer‟s decision-making strategy 4.5 ENSORY A DAPTATION  Sensory adaptation: sensitivity to prolonging stimulation tends to decline over time as an organism adapts to current conditions 4.7 VISION: OW THE EYES AND THB RAINC ONVERTLIGHTW AVES TO NEURALS IGNALS  Visual acuity, the ability to see fine detail  Example: the Snellen chart 4.8 ENSING LIGHT  Visible light is the portion of the electromagnetic spectrum that we can see  The length of a light wave determines its hue or colour  The intensity or amplitude of a light wave – how high the peaks are – determines what we perceive as the brightness of light  Purity is the number of distinctive wavelengths that make up the light. Purity corresponds to what humans perceive as saturation, or the richness of colours The Human Eye  Light reaches through the cornea, which bends the light wave and sends it through the pupil, a ole in the colored part of the eye (iris)  Behind the iris, muscles inside the eye 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, the process by which the eye maintains a clear image on the retina  Nearsightedness (myopia), images are focused in front of the retina  Farsightedness (hyperopia), images are focused behind the retina Phototransduction in the Retina  The retina is the interface between the world of light outside the body and the world of vision inside the central nervous system  Two type of photoreceptor cells in the retina contain light-sensitive pigments that transduce light 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 of night vision  Rods are more sensitive photoreceptors than cones  They contain the same photopigment, they provide no information about colour and sense only shades of gray  Fovea, an area of the retina where vision is the clearest and there are no rods at all  The absence of rods in the fovea decreases the sharpness of vision in reduced light  Each retina contains only about 6 million cones, which are densely packed in the fovea  The distribution of cones directly affects visual acuity and explains why objects off to the side, in your peripheral vision, aren‟t so clear  The more fine detail encodes and represented in the visual system, the clearer the perceived image  The photoreceptor cells form the innermost layer of the retina  The middle layer contains bipolar cells, which collect neural signals from the rods and cones the 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 retinal ganglion cells (RGC) axons form the optic nerve, which leaves the eye through a hole in the retina  This creates a blind spot, a location in the visual field that produces no sensation on the retina Receptive Fields  The region of the sensory surface that, when stimulated, causes a change in the firing rate of that neuron  Most receptive fields contain either a central excitatory zone surrounded by a doughnut-shaped inhibitory zone (on- center cell), or a central inhibitory zone surrounded by an excitatory zone (off- center cell)  A small spot shining on the central excitatory zone increases the RGC‟s firing rate  A small spot shining on the central inhibitory zone elicits a weak response, and a spot shining on the surrounding excitatory zone elicits a strong response in the RGC 4.9 PERCEIVINGC OLOUR Seeing Colour  Rods are bad at distinguishing colour (life must suck seeing in only shades of gray .. Except that new moving coming out, 50 Shades of Grey)  Cones can contain any of the 3 types of pigments  Each cone absorbs light over a range of wavelengths, but its pigment type is especially sensitive to visible wavelengths that correspond to red (long-wavelength), green (medium-wavelength), or blue (short wave-length) light Trichromatic Colour Representation in the Cones  The pattern of responding across the three types of cones provides a unique code for each color  Color deficiency (color blindness) is a sex-linked trait affecting more men than women (If you are a biology student then you should know why )  Color blindness = missing cone(s)  Color-opponent system: where pairs of visual neurons work in opposition; red against green, blue against yellow  Red-green cells are excited (they increase their firing rates) in response to wavelengths corresponding to red and inhibited (they decrease their firing rates) in response to wavelengths corresponding to green 4.10 HE VISUALBRAIN  Half of the axons in the optic nerve that leave each eye come from retinal ganglion cells that code information in the right visual field, whereas the other half code information in the left visual field  The optic nerve travels from each eye to the lateral geniculate nucleus (LGN), located in the thalamus  The thalamus receives inputs from all of the senses except smell  The visual signal travels to the back of the brain, to V1, the part of the occipital lobe that contains the primary visual cortex  30-50 brain areas specialized for vision, located mainly in the occipital lobe at the back of the brain and in the temporal lobes on the sides Neural Systems for Perceiving Shape o I can only hope you can tell the difference between a cookie and a stalk of celery, otherwise your meals will become a traumatic experience o Perceiving shapes depends on the location and orientation of an object‟s edges o V1 is specialized for encoding edge orientation  Visual cortex selectively respond to bars and edges in specific orientations in space Pathways for What, Where, and How  Two functionally distinct pathways, or visual streams, project from the occipital cortex to visual areas in other parts of the brain  The ventral (below) stream travels across the occipital lobe into the lower levels of the temporal lobes and includes brain areas that represent an object‟s shape and identity – identify what it is  The dorsal (above) stream travels up from the occipital lobe to the parietal lobes (including some of the middle and upper levels of the temporal lobes), connecting with brain areas that identify the location and motion of an object – identify where it is  Betty suffered from visual-form agnosia, the inability to recognize objects by sight  Patients with brain damage to the parietal section of the dorsal stream have difficulty using vision to guide their reaching and grasping movements, a condition termed optic ataxia 4.13 THE„G LU‟ THATBINDS NDIVIDUALFEATURES INTO AW HOLE  Binding problem in perception, which concerns how features are linked together so that we see unified objects in our visual world rather than free-floating or miscombined features Illusory Conjunctions: Perceptual Mistakes  Illusory conjunction, a perceptual mistake where features form multiple objects are incorrectly combined  Feature integration theory: focused attention is not required to detect the individual features that comprise a stimulus, such as the color, sha
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