l8- spatial vision Flashcards
Sine wave gratings
Sine wave gratings
- patterns of alternating light n dark that vary smoothly
- used to study how visual system processess patterns
Sine wave gratings
square wave gratings
- alternating bars of black n white w abrupt edges
- contain many spatial frequencies (sherp edges=more detail)
Sine wave gratings
spatial frequency (SF)
- number of cyckes of light/dark per degree of visual angle
- high sf= fine detail
- low sf= broad shapes
Sine wave gratings
spatial frequency. formula
- f = 1/ wavelength
- frequency is inversly proportional to WL
Sine wave gratings
contrast
- diff in brightness between light n dark areas
- high contrast= big difference
- low contrast= small diff
Sine wave gratings
michelson contrast
- (max luminace-min luminace)/(max+min)
Sine wave gratings
phase of a wave
- the position of the gratings light/dark cycle in space
- cells=phase selective: respond based on alingment
Sine wave gratings
orientation
- angle of grating lines (eg horizontal, vertical)
- V1 neurons r orientation tuned
Sine wave gratings
why sine waves?
- all images can be broken down/built up from sinusoidal components (fourier anlaysis n synthesis)
- visual ssytem may process images this way
- theyre useful n convienent
Spatial frequency in relation to visual resolution (acuity)+sensitivity
visual acuity (resolution limit)
- smallest bar width that can be distinguished
- bar width measured in cycles per degree of visual angle
- measured in arc minutes, normal acuity= 1arc minute
- limited by foveal cone spacing (2 cones per cycle required)
Spatial frequency in relation to visual resolution (acuity)+sensitivity
snellen acuity
- measured by optometrist
- 20/20 or 6/6 vision= acuity of 1 arcmin
- numerator= ur viewing distance
- denominator= avergae vieiwng distance ppl see it
- 20/10 better than average, 20/40=worse
Spatial frequency in relation to visual resolution (acuity)+sensitivity
contrast sensitivty
- how much of the contrast needed to detect a pattern
- contrats sensitivty function: sensitivty=1/contrast threshold
- range:0.1-60 cycles/degree
- peak sensitivty: 5-6cycles/degree
- high frequency cutoff: photreceptor spacing
- low frequency cutoff:lateral inhibition
Spatial frequency in relation to visual resolution (acuity)+sensitivity
contrast sensitivity developement
- low spatial frequencies: mature-like by 33 weeks
- high spatial frequencies: take longer to mature
- contrast sensitity declines w age, esp for high frequency
Spatial frequency in relation to visual resolution (acuity)+sensitivity
michelson contrast
(max luminace-min luminance)/(max+min)
Visual pathway and the lateral geniculate nucleus
Visual pathway and LGN
- visual pathway from retina to cortex
- LGN has 6 layers:
- layers 1-2: magnocellular (M-cells)
- layers 3-6: parvocellular (P-cells)
- monocular input (1 eye per layer)
- concentric centre surrond RFs
- topographic map in register across layers
Visual pathway and the lateral geniculate nucleus
LGN visyal field maping
layers 1,4,6: contralateral eye\
layers 2,3,5: ipsilateral eye
left visual field to right LGN
right visual field to left LGN
Visual cortex: striate and extrastriate
primary visual cortex (V1)
- 6 layers, recieves input from LGN layer 4
- neurons tuned to orientation, spatial frequency, direction
- binocular neurons, ocular dominance columns
- clumnar architecture: orienttaion n eye perfomance
Visual cortex: striate and extrastriate
cortical magnification
- more cortex is devotated to the center of gaze/fovea than periphery
- central 10 deg=50% of V1
- CM in V1: 1 deg at fovea= 20mm of cortex, 1 deg in periphery, 10deg away= 1.5mm of cortex
Visual cortex: striate and extrastriate
receptive fields in V1
- simple cells: clear excitatrory regions, sensisitve to position, respond only if bar is positioned right in RF
- complex cells: less defined, not sensitive to position within RF
- hypercomplex (end-stopped) cells: respond to specific bar lengths
Visual cortex: striate and extrastriate
orientation columns
- neurons w similar orientation tuning grouped in ~0.5mm columns
- ocular dominace columns: respond more to one eye
Visual cortex: striate and extrastriate
extrastriate visual areas
- dorsal pathway: space, where
- ventral pathway: from, what
- includes PPA (places), EBA(body parts), FFA(faces)
- RF=larger n more complex
adaption
adaption
- neural responses decrease after prolonged exposure to stimulus
- causes perceptual aftereffects (eg titl aftereffect)
- indirect way to study neural tuning in humans (‘psychologist electrodes’)
adaption
interocular transfer
- aftereffects remains when test is viewed in opposite eye
- indicates adaption in binocular neurons (visual cortex)
adaption
spatial frequency adaption
- adapting to specific SF (eg 7cpd) causes a dip in CSF at that frequency
- suggests a presence of SF channels
