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Perception (Cognitive Psychology)
Transcript of Perception (Cognitive Psychology)
Essentially an information processing approach of vision
Focused on computational and algorithmic levels
Ignored implementational level because work mostly involved computer models
Four stages of analysis of retinal image: grey level description, primal sketch, 2.5D sketch, 3D object-centred description
Grey-level description: colourless information based on light-intensity as it reaches the retina
Primal sketch: edge, contour and region of similarity detection (using Gaussian blurring - blur to remove details and focus on important information)
2.5D sketch: size, shape, location, orientation and distance but all in relation to observer
3D object-centred: Gestalt Approach Law of Pragnanz: The whole is greater than the sum of its parts (bottom-up approach)
Explains that perception relies on the application of organizational principles
These are innate, not stored knowledge Similarity - things that are similar are grouped together
Proximity - things that are close are grouped together
Closure - things that look like they approach each other are grouped
Continuity - assume things are continuous 'of several geometrically possible organizations that one will actually occur which possesses the best, simplest and most stable shape' We tend to perceive objects in a way that organizes them most simply Evaluation: Direct/ Ecological Perception J. Gibson (1904-1980) Input Primal sketch 2.5D sketch 3D model representation Can see whole object in mind's eye Some depth but perceived only from viewer's perspective The 'raw' retinal image is not the basis of perception (c.f. Marr) but the multitude of cues present in the environment
The information in our sensory receptors is all we need to perceive anything. No higher cognitive processing is required (no top-down element)
Perception is direct - don't need to think
We perceive objects directly within the context of an environment
Everything is given in the environment (therefore very important)
So experiments have to be ecologically valid!
At the time, perception experiments lacked this, they were very bare
He explained that we don't just look at something but we interact with it Direct/ Ecological Perception Information through the optic array comes in different forms such as optic flow patterns, texture gradients, the horizon ratio and affordances
Optic flow pattern: when going toward a target, things near us move quickly but the further away they are, the slower they move
Horizon ratio: the ratio between an object and the horizon always stays the same unless the object has moved (so we know its the same size, just nearer)
Affordances: meaning is not based on information stored in LTM - meaning is derived from potential uses of objects (affordances) e.g. stairs can be ascended or descended, not eaten(!). Objects can have multiple affordances e.g. an orange can be eaten or thrown. Other bottom-up approaches:
Template theories - have templates of different objects and match others to it;
Prototype theories - have a different prototype of different objects which is the average of all those seen, object match doesn't have to be 100% - it's more flexible;
Feature theories - pick up on important features, build these up until recognise objects
Recognition-by-components theory - So how are optical illusions possible? Useful for understanding how we perceive groups of objects
Lack of realism in terms of 'real world' perception - lack of ecological validity
More descriptive than explanatory Top-down processing must be involves
Prior knowledge of walls Direct/ Ecological Perception Critique Demonstrated value of ecological validity, and the (until then ignored) richness of information available
The human visual system evolved according to our environment
Vague about precise mechanisms Constructive Perception Hermann von Helmholtz (1821-1894) Argued for the importance of experience in perception
1. Analytic stage (sensation)
2. Synthetic stage (sensory elements to perceptions of objects and properties of objects)
Constructive perception is the antithesis of direct perception (it's also known as indirect perception)
Richard Gregory (1923-2010): we have several hypotheses for an image as to how to perceive it/ what it is ('perceptual hypotheses'), and accept the one that best fits.
Irvin Rock (1922-1995): "perception is based on prior perception, implying a perception-perception chain of causality'. He also equated perception with thought. Extreme positions are a bad idea! So, which model? The direct approach is more bottom-up - if it was too extreme we wouldn't 'make sense' of the world
The constructive approach is more top-down - if it was too extreme we'd have hallucinations! A synthesis? (Neisser, 1976) It's a rather vague description of the processes involved but it's probably on the right track! Two visual systems? Through experiments on monkeys with lesioned pathways, Ungerleider and Miskin (1982) found two visual systems.
When information reaches the primary visual cortex in the occipital lobe, it is forwarded through two fasciculi (fibre bundles):
One ascends toward the parietal lobe (the dorsal pathway)
One descends to the temporal lobe (the ventral pathway) Goodale and Millner (1992) examined this in relation their patient DF who had an impaired ventral system.
They argued that ventral is 'allocentric' (objects outside of ourselves) and dorsal is 'egocentric' (allows us to see how to interact with the visual object).
DF had the ability to see objects but could not interact with them. Also called the 'where pathway'
Picks us visual information to allow interaction with the environment (bottom-up)
Responsible for processing location and motion information
Doesn't have much involvement with conscious awareness
Doesn't require cognitive effort for 'interpretation' Also called the 'what pathway'
Dedicated to recognizing objects and events (top-down)
Responsible for processing colour, shape and identity of visual stimuli
Involves more awareness
Requires the use of previously stored information for 'interpretation' Too vague, but therefore safe, not problematic. Makes distinction between sensation and perception - what we perceive is not necessarily what we sense (e.g. optical illusions) 'The set of processes by which we recognize, organize, and make sense of the sensations we receive from environmental stimuli' Physiology of the Eye: Retina Rods - a type of photoreceptor (cell sensitive to light). They are long and thin, and found around the periphery of the retina. They are responsible for night vision and sensitize to light and dark stimuli, but are not very detailed.
Cones - the other type of photoreceptor. They are short and thick, and found near the fovea. They perceive colour and details.
Ganglion cells - its axons contribute the optic nerve.
Photopigments - found within the rods and cones. They react to light and transform physical electromagnetic energy into an electrochemical neural impulse that can be understood by the brain.
The optic disk/blind spot is where all information from the retina goes to the optic nerve so there are no rods/cones here. Subsequently if any light is directed there, it's missed. LIGHT Light passes through the protective covering of the eye, through the pupil, then the lens and the vitreous humor (gel-like substance). It focuses on the retina, where electromagnetic light energy is converted (transduced) into neural electrochemical substances. Vision is most acute in the fovea, so when we look at an object, our eyes rotate so that the image falls directly onto the fovea. Physiology of the Eye: Message The neurochemical messages processed by the photoreceptors travel to the ganglion cells (optic nerve).
The 'lower' ganglion cells nearest to the nose cross through the optic chiasm (where the optic nerves of the two eyes join at the base of the brain) and extend to the opposite hemisphere of the brain. Therefore images from the right visual field travel to the left side of the brain (and vice versa), with the optic chiasm being where they cross over.
All other 'upper' ganglion cells go to the hemisphere on the same side of the body.
The lens of each eye inverts the image as it projects it onto the retina thus the brain receives an upside-down and backward image.
'Lateral geniculate nucleus' = first relay station, combines information from other senses
'Superior colliculus' = allows us to pay attention to other things?
Then it goes to the ~~ cortex, and goes via the dorsal and ventral pathways to the rosterior parietal cortex and inferior temporal cortex respectively Within the retina: Environmental Cues Gibson's Framework for Perceptual Continuum Distal object: object in external world e.g. falling tree
Informational medium: information about this object carried e.g. sound
Proximal stimulation: information coming into contact with the appropriate sensory receptors e.g. inner ear
Perceptual object: the reflection of the properties of the external world, namely perception e.g. falling tree Sensory adaptation No two experiences can be exactly the same (e.g. every apple is shaped differently) but variation is actually necessary for perception
Sensory adaptation: receptor cells adapt to constant stimulation by ceasing to fire until there is a change in stimulation
E.g. visual illusions where after some time of seeing an image, it seems to disappear
This is partly why our eyes are constantly making tiny rapid movements The Ganzfeld Effect: when your eyes are exposed to a uniform of stimulation you will stop perceiving that stimulus after a few minutes and just see a grey field instead. This is because your eyes have adapted to the stimulus. Properties of Dorsal and Ventral Systems Dorsal Pathway: Ventral Pathway: The What/ How Hypothesis: This alternative interpretation of the visual pathways states that instead, what differs between the two pathways is whether the emphasis is on identifying what an object is or on how we can situate ourselves so as to grasp the concept.
There are deficits that can impair people's ability to recognize what they see ('what'), and deficits that impair their ability to reach for what they see ('how) The 'what pathway' Found in the ventral stream
Responsible for the identification of objects The 'how pathway' Found in the dorsal stream
Controls movement in relation to the objects that have been identified through the 'what' pathway Physiology of the Eye Environmental Cues Direct Perception More Bottom-Up Approaches Top-Down Approach & Synthesis Perception = Sensory data + Previous knowledge +Cognitive inferences Perceptual Representations Viewer-centred representation: the individual stores the object the way it looks to them (therefore different angles give different views)
Object-centred representation: the individual stores a representation of the object, independent on its appearance to the viewer (therefore the shape of the object is stable)
The difference between these is whether the person represents the objects and its parts in relation to them (viewer-centred) or in relation to the entirety of the object itself, independent of their own position (object-centred)
Landmark-centred representation: information is characterised by its relation to a well-known or prominent item. Individuals are able to switch between these three strategies. Perception of Objects & Forms Perceptual Constancy Perceptual Constancy occurs when our perception of an object remains the same even when our proximal sensation of the distal object changes Size Constancy: perception that an object maintains the same size despite changes in the size of the proximal stimulus Shape Constancy: perception that an object maintains the same shape despite changes in the shape of the proximal stimulus Depth Perception Monocular Depth Cues These can be represented in just two dimensions and observed with just one eye.
Interposition (whether partially obscured by other objects)
Linear perspective (parallel lines seeming to diverge as they move away from the horizon)
Aerial perspective (clearer/ fuzzier images)
Location in the picture plane (above the horizon, close objects are higher in the picture plane; below the horizon, close objects are lower in the picture plane)
Motion parallax (objects approachng get larger at an ever-increasing speed) Binocular Depth Cues These are based on the receipt of sensory information in three dimensions from both eyes.
Binocular disparity - two eyes send increasingly disparate images to your brain as objects approach you. The degree of disparity indicates distance
Binocular convergence - two eyes increasingly turn inward as objects approach you. Your brain interprets these muscular movements to indicate distance
Depth perception can also be influenced by the effort that would be required to walk to the target. Two Visual Systems Agnosia & Ataxia Difficulty in perceiving sensory information
Often caused by damage to the border of the temporal and occipital lobes, or restricted oxygen flow to areas of the brain.
Sufferers have normal sensations of what is in front of them and can perceive colours, shapes etc. but just cannot recognise them
Visual-object agnosia: cannot recognise objects
Simultagnosia: unable to pay attention to more than one object at a time
Prosopagnosia: impaired ability to recognise faces Agnosia Ataxia Impairment in the ability to use sensory information to guide actions.
Often a processing failure in the posterior parietal corex (where sensorimotor information is processed).
Optic ataxia: can't use visual system to guide movement Colour Perception Dichromacy: only two of the mechanisms functioning (i.e. one isn't). E.g. red-green colour blindness (protanopia), green-blindness (deuteranopia), or blue-green blindness (tritanopia) Red monochromacy/ achromacy: no colour vision at all (true colour blindness) Deficits in Perception