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Chapter 2

Chapter 2

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Department
Physiology
Course
PSL440Y1
Professor
A
Semester
Winter

Description
What it means to perceive? Sensation and perception provide the raw material for cognition Our perception is not a simple registration of sensory stimuli but sophisticated cognitive processes work on the sensory information almost immediately and produce the brains interpretation, our existing knowledge guides these dynamic processes The sensory stimuli are typically ambiguous and open to multiple interpretations The two problems with sensation and perception: 1. Sensory input does not contain enough information to explain our perception 2. The world presents us with too much sensory input to include into our coherent perceptions at any single moment We do not just register the picture as a whole when we see an image, but we see only relatively fine details in a small region point of fixation. Searching is one of the ways to deal with excess input We engage in selective attention which allows us to choose part of the current sensory input for further processing at the cost of other aspects on input In conclusion, whether too much or too little, cognitive processes are necessary to interpret and understand the material we encounter How it works: The case of visual perception Vision, like hearing, is a distance sense that helps us sense objects without touching them. It tells us what is out there and where is it. Our senses also give us a nudge towards action e.g. where to grab the object from Visual perception takes in information about the properties and locations of objects so we can make sense of it and interact with our surroundings Structure of the visual system: The pattern of light intensity, edges and other features in the visual scene form an image on the retina Light hits the photoreceptors and nerve cells at the back of the eye [job: light gets converted into electrochemical signals] transmitted to the brain via optic nerves (each optic nerve is a bundle of long axons of ganglion cells in the retina optic nerve axons contact with LGC (lateral geniculate nucleus) in thalamus; a structure lying on the surface of the brain Primary visual cortex/striate cortex host of visual areas as well as areas that are not exclusively visual in function. Beyond the primary visual cortex, 2 pathways can be identified: a. Dorsal pathway: reaches up into parietal lobes and is important in processing information about where items are located and how we act on them e.g. touch them 1 b. Ventral pathway: reaches down into parietal lobe and processes information that leads to recognition and identification of objects Top-Down and Bottom-Up processing Most visual areas that send output to another area also receive input from that area i.e. they have reciprocal connections Reciprocal connections lead to the idea that perception is the product of bottom-up and top-down processes Bottom-up processes are driven by sensory input from the physical world Top-down processes seek and extract sensory information and are driven by our knowledge, beliefs, expectations All acts of perception involves both bottom-up and top-down processing e.g. visual search depends on the surrounding objects Perceptions are interpretations of what we see and, and the mental representations are produced by the interaction of bottom-up and top-down processing Learning to see The interpretations of the world around us is determined by the interaction between our biological brain and experience Visual system of an infant typically fixates for about half a second and this visual information starts to accumulate and force lasting mental representations of objects, people and places The early stages of life have critical period during which particular responses must develop, otherwise that ability will fail to properly develop The sensory input of a person competes for representation in the cortex Building from the bottom up: From features to objects Processing Features; the building block of perceptions Visual features includes spots and edges, colors and shapes, movements and textures they are the building blocks of perception In bottom up processing, we look at an object and count the features so that we can figure out the processing that takes place to comprehend the image Spots and Edges This feature is processed by the ganglion cells neurons in the retinas whose axon fibers form the optic nerve Light lands on the receptive field of the photoreceptors; clusters of photoreceptors are linked to ganglion cells of retina that pass on the information to the optic nerve 2 Each ganglion cell responds only to the light that falls on its photoreceptors i.e. each has its own visual field Receptive field: is the area of visual field in which the stimulus affects the activity of the cell Light in some portions of the receptive field excites the cell; makes it more active and white in colour and at the same time it inhibits the surrounding the areas; makes it less active and grey in colour. This is an effective way for a person to focus on the light spot and ignore the surroundings. In a bright field, the total amount of light response is determined by subtracting the signals of inhibitory area from the signals of facilitatory area In a total dark field, the amount of light response is always 0 However, conditions are best where the photoreceptors of the inhibitory are in total dark so the illuminated center is clearly visible. This leads us to conclude that the structure and arrangement of photoreceptors serves to enhance the contrast at the edges Mach bands is a visual illusion where colours of band lie adjacent to each other where it looks like the bands are like a gradient of colours but in reality each band is a solid colour. This illusion is due to the ganglion cells which are well designed to pick our edges f
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