Disorders of Perception, Neglect, Blindsight and Prosopagnosia Flashcards
What is the physiology of the eye?
• Light enters the eye via the cornea
• If then passes through the pupil (a hole in the centre of the iris)
- When light is bright the iris causes the pupil to constrict to limit the amount of light entering the eye
• It then passes through the interior lens
• It’s then focussed on the retina (a network of neurones/cells)
• Layers of cells in the retina - ganglion, amacrine, bipolar, horizontal and photoreceptors
• Photoreceptors turn light into neural impulses to send to the brain. Two types:
- RODS (dim light vision, motion)
- CONES (bright light and colour vision)
About perception and the brain…
• Sensation is not the same as perception
• Retina
• Rods and cones
• Optic disk (blind spot)
• Optic chiasma (right field of each eye to right hemisphere and left field of each eye to left hemisphere)
• Lateral geniculate nucleus (LGN)
- Carries different information in the magnocellular and parvocellular layers from rods and cones
We have a blind spot because of the entry point of the optic nerve.
- Visual cortex, V1 (striate cortex) then V2, V3, V4, MT
- The visual system separates the visual features (form, motion, colour etc) of the visual scene but combines them to form a coherent
- Ventral - what pathway
- Dorsal - where pathway - specialised for object recognition or thought of by ohters (Milner and Goodale, 1995 as the how pathway - for visually guided action)
What is the Gestalt approach to perception?
- Assumes that the simplest and most stable interpretations of visual stimulus are favoured (Principle of Pragnanz)
- Gestaltists looked at the configuration of visual scenes. Favour hollistics processing (whole is greater than sum of its parts). It is the relations between parts, and their arrangement that determines perception. Eg spatial rules - bottom up
- Proposed principles that govern how the perceptual input is organised into chunks to provide the ‘best guess’ as to what it is that we are looking at. The perceptual system is considered to have ‘preferences’ for interpreting certain arrangements.
What is form perception?
- Principles of form perception
- Principles are innate so we can’t choose how we see things
- The figure-ground concept - some things stand out (figure)and others fade into background (ground)
- Figure has distinct from and the ground doesn’t
- Figure seen as being in front of the ground
What are some principles of visual perception?
- principle of proximity
- principle of similarity
- principle of good continuity (smoothness)
- principle of closure
What is a summary of Gestalt’s theory of visual perception?
- Proximity - objects close together are perceived as forming a group
- Similarity - group ojects on the basis of similarity
- Continuity - perceive smoothly flowing forms rather than disrupted ones
- Closure - complete objects that are not complete
- BUT - the theory is purely descriptive
- How does this phenomenon occur?
- These principles can be demonstrated very well with 2D designs - but how applicable are they to the real world?
What did Bruner say about visual perception?
Bruner (1957) “all perceptual experience is necessarily the end product of a categorisation process”
What happens to form out visual experience?
the detection of sense ‘information’»_space;> formation of perceptual representation»_space;> interpretation of the perceptual representation
What is perception?
- We experience the world through our sense which receive information from the environment
- Sensory receptors stimulated by physical energy (eg for vision the energy is light reflected off object)
- The sense organs receive these sensory inputs and transmit sensory info to the brain
- How is the physical energy received by the sense organs converted into perceptions?
What is constructivist perception?
Constructivist perception - top-down processing
- Helmholtz (1909); Rock (1983); Gregory (1997) - We construct the perceived stimulus using sensory info as the base but it's our thinking processes that make sense of the info ie we need prior experience to interpret the input
What is direct perception?
Direct perception - bottom-up processing
- Gibson - ecological (1979) and Marr - computational (1982) - The info received by our sensory receptors is all we need in order to perceive (don't need prior knowledge to mediate between sensory input and perception) - We are bon with all we need to make sense of what we see
What are assumptions of constructivist theories?
- Bruner, 1957; Neisser, 1967; and Gregory, 1972, 1980 tend to make the following assumptions:
- Perception is an active and constructive process; ‘something more than the direct registration of sensations…other events intervene between stimulation and experience’. (Gordon, 1989)
- The visual image is ambiguous and needs interpretation
- Perception is not directly given by the stimulus input, but occurs as the end-product of the interactive influences of the presented stimulus and internal hypotheses, expectations and knowledge, as well as motivational and emotional factors (top-down).
- Perception is an active and constructive process; ‘something more than the direct registration of sensations… other events intervene between stimulation and experience (Gordon, 1989)
- Perception is not directly given by the stimulus input, but occurs as the end-product of the interactive influences of the presented stimulus and internal hypotheses, expectations and knowledge, as well as motivational and emotional factors
- Perception is influenced by expectations that are sometimes incorrect, and so it is prone to error. Hence visual illusions.
- Perception is influenced by hypotheses and expectations that are sometimes incorrect, and so it is prone to error
- When information from memory is being used to guide expectations about the information that is being received from the environment, this is referred to as top-down processing
- Gregory talks of ‘perceptual hypotheses’ being generated
What is direct perception?
- Sensations received by the visual system are highly organised and rich in information that we do not need to interpret them to make them meaningful perceptions
- We can perceive them directly
- This means that there are no intermediate stages between light reaching the retina and our response to it
- No need to make use of internal hypotheses or thought processes - perception is automatic. Because of the emphasis on environmental stimuli, Gibson’s theory is also ecological perception
- Characterised best by Gibson’s (1979) ecological approach
- A ‘pattern of light’ reaching the eye is an optic array; this structured light contains all the visual information from the environment striking the eye
- This optic array contains unambiguous information about the layout of objects in space, in the form of texture gradients, optic flow patterns etc
- Perception involves ‘picking up’ the rick information provided by the optic array directly via ‘resonance’, with little or no information processing
- For instance, when thinking of a pilot approaching a landing strip, there is a point in their visual field that appears to remain stationary, with all other points flowing away from it. This gives direct information regarding the point towards which the pilot is currently heading, without any model or internal representation of the environment
- What about meaning? Gibson introduced the notion of direct perception of the affordances of an object, eg a chair “affords” sitting on
- This has proved a useful idea in the context of object design, but is very incomplete as a theory of meaning in general. The idea of ‘resonance’ is also rather difficult to pin down.
What are the successes of Gibson’s perspective?
- More about ‘seeing’ rather than ‘seeing as’
- A proper acknowledgement of the environmental and ecological grounding of perception. What is perception for, if not to allow interaction with the environment?
- The transparency of some illusions with regard to action
- A highlighting of the richness of information present in the visual signal
- An anticipation of theories relating to the so-called ‘dorsal stream’ of brain processing
What are the weaknesses of Gibson’s approach?
- By not specifying exactly what constitutes resonance and invariance detection, Gibson avoids the exact problem that information processing accounts sought to address. He underestimates the amount of processing that must be performed even to detect the invariances that he proposes
- The denial of the role of stored knowledge (memories) in perception is highly questionable eg seeing that a tree has been cut down
- Even the topics Gibson discusses (eg motion towards) are more complex than he assumes
What did Palmer show about visual perception?
Palmer (1975) showed that a preceding picture of a kitchen would facilitate the recognition of a rapidly presented appropriate object(eg a loaf) but would actually slow identification of an inappropriate object (eg a mailbox), both relative to a no-context condition
What are the context effects in object recognition?
- One of the ways in which to look at the interaction of bottom-up and top-down processing is to consider the effects of context
- Experimental data support an influence of context on perception
- Palmer (1975)
- Ps viewed a scene for 2 seconds. After 1.3 seconds delay they were shown a picture of an object for either 20, 40, 60 or 120ms. Their task was to name the object. (line drawings)
- There were 4 types of relationships between the scene and the object:
What did Palmer find in 1975?
- Related context - eg a kitchen followed by a picture of bread
- Misleading context - eg a kitchen followed by a picture of a mailbox (same shape as bread)
- Unrelated context - eg a kitchen followed by a picture of a muscal instrument
- No context (baseline performance measure)
- Measure (DV) was probability of item being correctly identified
- Performance improved with increased viewing exposure (more bottom-up information)
- Performance was best for related condition - better than no context but performance on misleading and unrelated was worse than no context at all (effect on top down processing)
Is visual perception top down or bottom up?
- Top-down or bottom-up
- Most visual experience probably involves combination of bottom-up and top-down processing Neisser (1976)
- Broadbent (1977) and Navon (1981)
- Bottom-up sensory data ‘suggests’ whilst top down information ‘inquires’ or ‘interrogates’ the perceptual representation - guides the final visual experience by selecting that which matches experience/expectations/beliefs
- Under good visual condition bottom-up processing is more prevalent
- Under poor visual conditions, brief exposure top-down processing become more important
How has constructivist approach sought to explain visual illusions?
For example, Gregory’s explanation of the Mueller-Lyer illusion: that the figure on the left looks like the inside corner of the room with the corner therefore far away; that the figure on the right looks like an outside corner, with the corner near. If they look the same size but they are at different distances from the observer then the left one must really be longer than the right one (misapplied size constancy).
What is the Ebbinghaus illusion?
Note: Gregory’s explanation is far from unchallenged. Nevertheless it is a good example of constructivist thinking
Note also: that many perceptual illusions fail to affect actions.
For example, in a three dimensional version of the Ebbinghaus illusion, hand grip aperture was the same for picking up the central object in each case, even thought the objects are perceived as being different sizes
What are some problems with the constructivist approach?
- Illusions are the exception rather than the rule
- Many experimental stimuli are unnatural eg odd designs, brief presentations
- Many illusions remain unaccounted for
About illusions…
- Gregory (1972) - visual information derived form the retinal image is ambiguous
- When perceptual cues conflict, the visual system has to chose the best option based on retinal size, distance cues and expectations
- It formulates a perceptual hypothesis which in the case of illusions is incorrect
- Illusions may be able to help us understand visual perception
- How does the bottom-up approach deal with visual illusions?
- There is no single definitive theory of illusions but they remind us of the different factors which can enter into visual perception eg physiology, depth cues and expectations
What is perceptual constancy?
Even though the retinal image for objects may be different, we still perceive the object in a certain way. There is constancy in what we see even though the retinal image is different, due to different viewpoints or distance. Perceive round table, but retinal image is elliptical.
Perceptual Constancy or Invariance refers to the phenomenon whereby our perception of an object does not change even though the sensory information/image has changed - recall the distinction between perception and sensory experience.
- Size - Colour - Shape
What is size constancy (visual perception)?
• We show size constancy because we take account of the distance when judging the object’s size
• Retinal image changes with movement and distance but the size we perceive does not change
• Imagine someone walking towards you. We perceive them getting closer but we do not perceive someone growing in size
• The retinal image of an object varies with distance of the object from the retina
• Hold finger quite close to eyes then move away to arm’s length - your perception is NOT that is becomes smaller
• When our perception of an object remains the same but our sensation of the object changes
- Eg as you walk towards a person they become larger but you know they’re really the same size - why?
1. We know this from experience which affects our interpretation of the person - ie top-down processing
2. The retinal image and the perceived distance cancel each other out
• Therefore no size constancy when perceived distance of an object is very different form its actual distance
- Eg the Ames room - perceive impossible relative sizes but this is because the shape of the room is distorted
What is colour constancy (visual perception)?
- Colour is a psychological construction
- Hugh, brightness, saturation
- Knowledge of the object’s colour
- Lighting
- Reflective nature of the object
- Surrounding colours
- Shadows
About depth in visual perception…
To know where objects are we need to be able to:
- Segregate objects from each other and from background - Determine distance (depth), and movement. This is done through depth cues
What are depth cues in visual perception?
Monocular (one eye)
- Perspective, things converge in distnace - Texture, gets finer as things get further away - Occluding edges - Motion parallax
Binocular (two eyes)
- Stereopsis - Retinal disparity - Binocular convergence
About monocular cues (visual perception)…
Relative and familiar size
- In a visual scene of different sized objects, smaller ones are usually seen as more distant but we can be fooled
A number of illusions are based on the relationship between size and depth - eg Ponzi illusion
About occlusion and shadows in visual perception…
Near objects will occlude more distant objects. If you cant see all of one object because of another, it is due to the fully visible object being nearer to you than the occluded object.
Other monocular clues to depth such as shadows and shading.
About texture in visual perception…
Another invariant is the texture gradient. The appearance of the surface of objects will change with distance (think about how grass looks close to you and distant from you). As we assume that the texture itself hasn’t changed we take the change in appearance to indicate distance (roads, grass, etc).
What are other monocular cues to depth in visual perception?
• Relative size
- In a visual scene of different sized objects, smaller ones are usually seen as more distant
• Relative brightness
- Brighter objects normally appear closer
• Aerial perspective
- Objects at a greater distance have a different colour (light filtered from atmosphere) blue tint
• Height in the horizontal place
- Distant objects seem higher (closer to the horizon)
What is the motion parallax?
Motion parallax
• Motion also provides important information about the position of items in the world
• Motion parallax describes the phenomenon whereby if the observer is moving - nearby items will appear to move faster than items in the distance (think of looking out of a window of a train or bus)
• Also - more distant objects will appear to ‘move’ more slowly
