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PSYA01H3 (849)

chapter 4

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University of Toronto Scarborough
Malcolm Mac Kinnon

Chapter 4 continuation: Signal Detection  An absolute threshold is operationalized as perceiving the stimulus 50% of the time  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 observers response in the presence of noise  False alarm: when stimulus is not present but the observer says it is  Signal detection theory: proposes a way to measure perceptional sensitivity-how effectively the perceptual system represents sensory events. Sensory adaptation  Sensory adaption: sensitivity to prolonged stimulation tends to decline over time as an organism adapts to current conditions  Our perceptual systems emphasized change in responding to sensory event  Our sensory systems respond more strongly to change in stimulation that to constant stimulation Vision1:  Hernmann Snellen developed visual acuity: the ability to see fine detail Sensing Light:  Visible light is the portion of the electromagnetic spectrum that we can see  There are three properties of light waves:  The length of the light wave determines its hue (colour)  The intensity/amplitude of the wave determines how bright the light is  Purity is the number of distinct wavelengths that make up the light (Saturation, richness of the colour) Human Eye:  Light reaches the eye passes the first through a smooth outer tissue called: cornea  This bends the light and sends the light through the pupil  The coloured part of the eye is the iris. It controls the size of the pupil and the amount of light that can enter the eye  Len bends the light and focuses it onto the retina-light sensitive tissue and the back of the eyeball  The muscle changes shape of the lens to focus the objects  Accommodation, the process by which the eye maintains a clear image on the retina  If the eyeballs are too long/short the lens cannot focus images properly on the retina Photo transduction  Photoreceptors cells in the retina contain light-sensitive pigments that transduce light into neural impulses  Cones detect colour, operate under normal daylight conditions and allow us to focus on fine detail  Rods 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. Absence of rods will decreases the sharpness of light  Retina contains cones, and the distribution of the ones affect visual acuity and explains why objects off to the side of your peripheral vision aren’t clear  The retina is thick with cells. The middle layer contains bipolar cells: collect neural signals from rods and cones and transmits it to the retina  Then the retinal ganglion cells organizes the signals and sends them to the brain  The optic nerve leaves the eye through a hole in the retina (contains neither rods nor cones).  This hole is referred to as a blind spot: the location in the visual field that produces no sensation of the retina Receptive Fields  RCGs respond to input not from a single retinal cone or rod but from a patch of photoreceptors  RCG with respond to light falling anywhere within the receptive field: the region of the sensory surface that, when simulated, causes a change in the firing rate of that neuron  RCG responds to a spot of light projected anywhere within the retina. Most receptive fields contain an inhibitory zone (on-centre cell) and excitatory zone (off-centred 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 creates a week spot  The retina is organized in this way to detect edges and abrupt transitions from light to dark. Perceiving Colour:  Colour is nothing about our perception of wavelengths  Rods contain the same photo pigment which makes them ideal for low-light vision  Cones contain any of the three types of pigments (green, red, blue)  Colour perception results from different combinations o three elements in the retina that respond to the wavelengths corresponding to the three primary colours of light  Additive colour mixing: increasing light to create colour  Subtractive colour mixing: works by removing light from the mix Trichromatic Colour Representation:  Trichromatic colour representation means that pattern of responding across the three types of cones provides a unique code for each other  Colour deficiency occurs when one of the cones are missing resulting into colour blindness Colour-Opponent Representation into the Brain  Staring too long at one colour fatigues the cones that respond to the colour, producing a dorm of sensory adaptation that result in a colour afterimage.  Colour opponent systems: where pairs of visual neurons work in opposition: red-sensitive cells against green sensitive cells etc.  The colour-opponent systems explains colour aftereffects The Visual Brain:  Streams of action potentials containing information encoded by the retina level to the brain along the optic nerve  The two nerve bundles link to the left and right hemispheres of the brain  The optic nerve travels from each eye to the lateral geniculate nucleus in the thalamus  From there the visual signal travels back to the brain to a location called area V1-the part of the occipital lobe that contains the primary visual cortex Neural Systems for Perceiving Shape:  Vision involves perceiving the shapes of objects  Neurons in the visual cortex selectively respond to bars and edges in specific orientation in space.  Area V1 contains population of neurons that respond to edges oriented at each position in the visual field Pathways for What, Where and How:  The brain system identifies people and things and another tracks their movements  The ventral stream travels across the occipital lobe into the lower levels of the temporal
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