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PSYC236 Semester Study Notes
Introduction
Definitions
- Sensation = when your sensory organs transform physical properties (e.g. light
energy) from the environment into electrical signals which are sent to the brain
- Perception = turning the sensory information into meaningful
experience/information
• PERCEPTION IS CONSCIOUS
- Cognition = processes by which we elaborate/store/organise/recall sensory
information
• Involves things like memory, recall, storing, relating
• Face perception = perception, Face recognition = cognition
- Pre-conscious and conscious visual processing involved
Senses
- Our senses turn physical properties in the world into psychological information
(perceptions)
Visual perception
- Light reflected off objects into eye onto back of retina
- Rod + cone receptor cells in retina stimulated > convert light into electrical/neural
energy > sent to visual cortex in brain (V1) > V2, V3…
Visual perception and parallel processing
- Parallel processing = the brain has separate systems that process information
independently/separately before integration
• Different areas specialised for specific tasks
• Info about an image > each element (motion, colour, size) individually
extracted by parallel/separate pathways
Visual perception
- Visual perception is very automatic and usually very accurate
- Visual perception OVERCOMES these problems so we get complete image
• We have 2 eyes but see 1 world/complete image
• Retina is 2D but we correctly see the 3D world
• When shake head the world doesn’t “shake”
• When get partial view of object can recognise the objects is still whole
, 2
Our perception can get things wrong – shown by optical illusions
- Muller-Lyer Illusion
- The Poggendorff Illusion
- The Thatcher Illusion = shows facial configuration is crucial for facial recognition but
only when the face is upright
- Illusion of spinning dancer = “accommodation/adaption” to one direction causes it to
change
- Brain interprets things but can be wrong (e.g. subjective contours)
Why visual illusions are important
- They show us our systems aren’t always accurate
- They show us perception is subjective (reality is what we perceive)
- They show us we are always “guessing” based on visual cues
- “Change blindness” good example that we pay attention to what we attend to
• “The grand illusion” = thinking that we perceive everything around us
Two important questions
- What visual info is available in the environment?
- What mechanisms does the brain have for utilizing this info?
Ways we study visual perception
- Physiological approaches = directly observe which areas of the brain are “lighting up”
in response to a stimulus
• PET, fMRI, lesion studies, single cell recordings, etc
- Cons of physiological approaches:
• A lot more is happening in brain not just responding to the selected stimulus
• Lesions are coarse not fine
• Restricted to studying simple visual stimuli
- Psychophysical approaches = examine the relationship between the visual stimulus
and perception
- Examples of psychophysical approaches:
• Thresholds = minimum amount of stimulus energy to detect a stimulus (e.g.
dimmest light can see)
• Sensitivity = 1/threshold
Useful measures of visual perception
- Visual acuity = clarity of image
• Measure of central vision only
• The minimum resolvable separation between two lines
• Example of visual acuity test is an eye test
• 20/20 vision = normal visual acuity
, 3
The Visual System
Main areas of the visual system
- The eye (retina)
- The Lateral Geniculate Nucleus (LGN) in the thalamus
- The visual/striate cortex in the occipital lobe
- The extra striate cortex (areas in the temporal/parietal/frontal lobes)
Top-down and bottom-up processing
- Bottom up = begin with simple sensory information first > then perception
- Top down = perception driven by cognition first
Vision
- Light is reflected off 3d objects in the environment and create 2d image on our
retina
- First stages of the visual system (bottom up) break up image into small components
(lines, colours, etc)
Two main visual pathways for parallel processing
- The “what” pathway = responsible for shape, motion, colour, brightness, depth
(VENTRAL)
- The “where” pathway = responsible for perceiving location of object in space
(DORSAL)
Accommodation
- Focusing vision by changing shape of the lens
- Use accommodation to get a SINGLE CLEAR IMAGE
- 70% of the focusing is done by the Cornea - the lens only does fine tuning
- Closer focus = fat lens, further focus = narrow lens
The retina
- Where the photoreceptors are
- Light onto eye > onto retina (where image falls)
- Focus light to put a clear image on the back of the retina
- The retina is a network of neurons which cover the back of the eye
- Light must pass a lot of neurons before it reaches the receptors (rods & cones)
Rod and cone receptor cells
- Neurons that turn the light on the retina into electrical energy
- This is achieved with light sensitive chemicals called visual pigments
, 4
- When these pigments absorb light they trigger a chemical reaction, which generates
an electrical signal in the receptors
- They have differences in:
• Shape
• Retinal distribution
• Number
• Dark adaptation
• Spectral sensitivity
• Spatial summation
Rod/cones retinal distribution
- The fovea is a small region on the centre of the retina which has the highest visual acuity
- The fovea has only cones
- Peripheral regions of the retina have both rods and cones
- No rod or cone cells in blind spot
• Blind spot = optic nerve
Rod/cones total number
- More rods than cones = 20:1
- 120 million rods and 6 million cones
Rod/cones dark adaption
- Our sensitivity (dimmest light can detect) increases when illumination decreases
- Rod vision has much higher sensitivity than cone vision (referred to as the rod-cone
break)
• At rod cone break the cones reach max sensitivity at 10 mins in dark but rods
continue to maximise sensitivity longer
• Rods increase to maximise sensitivity for up to 20 minutes
• Rod cone break = point where only rods continue to become more sensitive
Rod/cones spectral sensitivity
- Rods more sensitive to short wavelengths of light (BLUE)
- Cones responsible for “colour vision”
- Different colours for wavelengths:
• Short = blue
• Medium = yellow/green
• Long = red
Rod/cones spatial summation
- Rods more sensitive than cones
- Rods high summation
PSYC236 Semester Study Notes
Introduction
Definitions
- Sensation = when your sensory organs transform physical properties (e.g. light
energy) from the environment into electrical signals which are sent to the brain
- Perception = turning the sensory information into meaningful
experience/information
• PERCEPTION IS CONSCIOUS
- Cognition = processes by which we elaborate/store/organise/recall sensory
information
• Involves things like memory, recall, storing, relating
• Face perception = perception, Face recognition = cognition
- Pre-conscious and conscious visual processing involved
Senses
- Our senses turn physical properties in the world into psychological information
(perceptions)
Visual perception
- Light reflected off objects into eye onto back of retina
- Rod + cone receptor cells in retina stimulated > convert light into electrical/neural
energy > sent to visual cortex in brain (V1) > V2, V3…
Visual perception and parallel processing
- Parallel processing = the brain has separate systems that process information
independently/separately before integration
• Different areas specialised for specific tasks
• Info about an image > each element (motion, colour, size) individually
extracted by parallel/separate pathways
Visual perception
- Visual perception is very automatic and usually very accurate
- Visual perception OVERCOMES these problems so we get complete image
• We have 2 eyes but see 1 world/complete image
• Retina is 2D but we correctly see the 3D world
• When shake head the world doesn’t “shake”
• When get partial view of object can recognise the objects is still whole
, 2
Our perception can get things wrong – shown by optical illusions
- Muller-Lyer Illusion
- The Poggendorff Illusion
- The Thatcher Illusion = shows facial configuration is crucial for facial recognition but
only when the face is upright
- Illusion of spinning dancer = “accommodation/adaption” to one direction causes it to
change
- Brain interprets things but can be wrong (e.g. subjective contours)
Why visual illusions are important
- They show us our systems aren’t always accurate
- They show us perception is subjective (reality is what we perceive)
- They show us we are always “guessing” based on visual cues
- “Change blindness” good example that we pay attention to what we attend to
• “The grand illusion” = thinking that we perceive everything around us
Two important questions
- What visual info is available in the environment?
- What mechanisms does the brain have for utilizing this info?
Ways we study visual perception
- Physiological approaches = directly observe which areas of the brain are “lighting up”
in response to a stimulus
• PET, fMRI, lesion studies, single cell recordings, etc
- Cons of physiological approaches:
• A lot more is happening in brain not just responding to the selected stimulus
• Lesions are coarse not fine
• Restricted to studying simple visual stimuli
- Psychophysical approaches = examine the relationship between the visual stimulus
and perception
- Examples of psychophysical approaches:
• Thresholds = minimum amount of stimulus energy to detect a stimulus (e.g.
dimmest light can see)
• Sensitivity = 1/threshold
Useful measures of visual perception
- Visual acuity = clarity of image
• Measure of central vision only
• The minimum resolvable separation between two lines
• Example of visual acuity test is an eye test
• 20/20 vision = normal visual acuity
, 3
The Visual System
Main areas of the visual system
- The eye (retina)
- The Lateral Geniculate Nucleus (LGN) in the thalamus
- The visual/striate cortex in the occipital lobe
- The extra striate cortex (areas in the temporal/parietal/frontal lobes)
Top-down and bottom-up processing
- Bottom up = begin with simple sensory information first > then perception
- Top down = perception driven by cognition first
Vision
- Light is reflected off 3d objects in the environment and create 2d image on our
retina
- First stages of the visual system (bottom up) break up image into small components
(lines, colours, etc)
Two main visual pathways for parallel processing
- The “what” pathway = responsible for shape, motion, colour, brightness, depth
(VENTRAL)
- The “where” pathway = responsible for perceiving location of object in space
(DORSAL)
Accommodation
- Focusing vision by changing shape of the lens
- Use accommodation to get a SINGLE CLEAR IMAGE
- 70% of the focusing is done by the Cornea - the lens only does fine tuning
- Closer focus = fat lens, further focus = narrow lens
The retina
- Where the photoreceptors are
- Light onto eye > onto retina (where image falls)
- Focus light to put a clear image on the back of the retina
- The retina is a network of neurons which cover the back of the eye
- Light must pass a lot of neurons before it reaches the receptors (rods & cones)
Rod and cone receptor cells
- Neurons that turn the light on the retina into electrical energy
- This is achieved with light sensitive chemicals called visual pigments
, 4
- When these pigments absorb light they trigger a chemical reaction, which generates
an electrical signal in the receptors
- They have differences in:
• Shape
• Retinal distribution
• Number
• Dark adaptation
• Spectral sensitivity
• Spatial summation
Rod/cones retinal distribution
- The fovea is a small region on the centre of the retina which has the highest visual acuity
- The fovea has only cones
- Peripheral regions of the retina have both rods and cones
- No rod or cone cells in blind spot
• Blind spot = optic nerve
Rod/cones total number
- More rods than cones = 20:1
- 120 million rods and 6 million cones
Rod/cones dark adaption
- Our sensitivity (dimmest light can detect) increases when illumination decreases
- Rod vision has much higher sensitivity than cone vision (referred to as the rod-cone
break)
• At rod cone break the cones reach max sensitivity at 10 mins in dark but rods
continue to maximise sensitivity longer
• Rods increase to maximise sensitivity for up to 20 minutes
• Rod cone break = point where only rods continue to become more sensitive
Rod/cones spectral sensitivity
- Rods more sensitive to short wavelengths of light (BLUE)
- Cones responsible for “colour vision”
- Different colours for wavelengths:
• Short = blue
• Medium = yellow/green
• Long = red
Rod/cones spatial summation
- Rods more sensitive than cones
- Rods high summation
