Visual perception
Visual illusions (e.g. Kanisza triangle and neon colour Steven’s power law is more refined than Binocular cues:
spreading) - brain constructs things that aren’t there Weber’s - p(l) = k(l)a - Binocular disparity creates a felt compelling sense of depth
P = subjective experience of intensity - Binocular disparity only works over a few 10s of meters
Light projects an image onto retina, and visual brain l = objective intensity - Diff in relative +ve on left and right eyes increases as we
‘interprets’ retinal image and creates representation - K = proportionality constant
visual experience is the interpretation a = exponent
Diff stimuli have diff values Other cues:
Our visual and sensory experiences can be described - Accommodation - we can focus on objects near or
as ‘percepts’ = visual representations Relationship between physical stimulus far by changing the shape of the lens - shape of lens
and sensation often non-linear - explains is proportional to distance
Sensations are more basic elements which are Weber-Fechner scaling - Vergence - how turned in the eyes need to be to
elaborate into percepts by visual parts of brain focus - cross-eyedness is proportional to distance
A ‘sensory modality’ is a dedicated part of the Gestalt psychology:
nervous system that responds to specific physical - Percepts are ‘wholes’ made of an arrangement ‘Vision’ by Marr (1982):
inputs (e.g. visual system responds to light) of parts, but can’t be reduced to the sum of - Showed we could think of representations being
their parts, their arrangement matters constructed by applying operations to the image
Exteroception provides the brain with signals about - Interested in grouping principles and - To understand any system, we should
energies in the outside world perceptual organisation understand it at computational, algorithmic, and
- Interested in the way we segment the scene implementational levels
Interception provides the brain with signals about into figure and ground - figure owns the - Final representation produced by vision is an
what’s going on inside the body (e.g. contraction of borders, ground is shapeless object represented as aggravates of connected
muscles, heart rate, breathing etc.) - Some bi-stable objects allow us to feel brain and distorted cyclinders
‘pinging’ between alternative figure-ground
Transduction = conversion of physical energy in interpretations
action potentials Biederman’s Geons:
- Proposed that object representations in brain
Psychophysics - plots lawful relationships between Monocular cues: are made of an alphabet of 36 primitive
the physical world and subjective experience, it then - Occlusion - near things generally occlude far shapes (= geons)
tries to explain why the laws are like they are in terms things - Even most complex objects can be coded as
of optics and neuroscience - Linear perspective - parallel lines converge on a an aggregate of geons
vanishing point
Thresholds - e.g. some sounds are too low or high - Texture gradients - texture becomes finer further
pitched to hear away Face perception:
- Height in visual field - far things are higher in - Fusiform face area (FFA) responds strongly to
Psychophysicists like to determine just-noticeable- visual field faces, although some people argue to
differences (JNDs) - 75% correct, 50% is just chance, - Motion parallax - near things move across retina responds strongly to all hyper familiar classes
100% is too easy) when we move our heads of objects
- Familiar size - some things have a standard size - Phenomena of pareidolia suggests FFA is
JND increases with stimulus intensity - Weber’s law - so we can simply assume that is their size and over-excitable and gets activated by any face-
e.g. no light vs light from candle - diff in brightness is changes in retinal size are attributable to like configuration
obvious, light from sun vs light from sun + candle - distance - FFA damage causes prosopagnosia (face-
diff is invisible blindness)
Visual illusions (e.g. Kanisza triangle and neon colour Steven’s power law is more refined than Binocular cues:
spreading) - brain constructs things that aren’t there Weber’s - p(l) = k(l)a - Binocular disparity creates a felt compelling sense of depth
P = subjective experience of intensity - Binocular disparity only works over a few 10s of meters
Light projects an image onto retina, and visual brain l = objective intensity - Diff in relative +ve on left and right eyes increases as we
‘interprets’ retinal image and creates representation - K = proportionality constant
visual experience is the interpretation a = exponent
Diff stimuli have diff values Other cues:
Our visual and sensory experiences can be described - Accommodation - we can focus on objects near or
as ‘percepts’ = visual representations Relationship between physical stimulus far by changing the shape of the lens - shape of lens
and sensation often non-linear - explains is proportional to distance
Sensations are more basic elements which are Weber-Fechner scaling - Vergence - how turned in the eyes need to be to
elaborate into percepts by visual parts of brain focus - cross-eyedness is proportional to distance
A ‘sensory modality’ is a dedicated part of the Gestalt psychology:
nervous system that responds to specific physical - Percepts are ‘wholes’ made of an arrangement ‘Vision’ by Marr (1982):
inputs (e.g. visual system responds to light) of parts, but can’t be reduced to the sum of - Showed we could think of representations being
their parts, their arrangement matters constructed by applying operations to the image
Exteroception provides the brain with signals about - Interested in grouping principles and - To understand any system, we should
energies in the outside world perceptual organisation understand it at computational, algorithmic, and
- Interested in the way we segment the scene implementational levels
Interception provides the brain with signals about into figure and ground - figure owns the - Final representation produced by vision is an
what’s going on inside the body (e.g. contraction of borders, ground is shapeless object represented as aggravates of connected
muscles, heart rate, breathing etc.) - Some bi-stable objects allow us to feel brain and distorted cyclinders
‘pinging’ between alternative figure-ground
Transduction = conversion of physical energy in interpretations
action potentials Biederman’s Geons:
- Proposed that object representations in brain
Psychophysics - plots lawful relationships between Monocular cues: are made of an alphabet of 36 primitive
the physical world and subjective experience, it then - Occlusion - near things generally occlude far shapes (= geons)
tries to explain why the laws are like they are in terms things - Even most complex objects can be coded as
of optics and neuroscience - Linear perspective - parallel lines converge on a an aggregate of geons
vanishing point
Thresholds - e.g. some sounds are too low or high - Texture gradients - texture becomes finer further
pitched to hear away Face perception:
- Height in visual field - far things are higher in - Fusiform face area (FFA) responds strongly to
Psychophysicists like to determine just-noticeable- visual field faces, although some people argue to
differences (JNDs) - 75% correct, 50% is just chance, - Motion parallax - near things move across retina responds strongly to all hyper familiar classes
100% is too easy) when we move our heads of objects
- Familiar size - some things have a standard size - Phenomena of pareidolia suggests FFA is
JND increases with stimulus intensity - Weber’s law - so we can simply assume that is their size and over-excitable and gets activated by any face-
e.g. no light vs light from candle - diff in brightness is changes in retinal size are attributable to like configuration
obvious, light from sun vs light from sun + candle - distance - FFA damage causes prosopagnosia (face-
diff is invisible blindness)
