Flashcards
Spatial Vision
Flashcards do conjunto (34)
- 1Contrast sensitivity function ad spatial frequency channels...
- 2How does one specify a sine wave grating?define grating by
* frequency (how many cycles in grating)
* orientation
* amplitude (contrast)
* phase of the grating - 3Fourier Analysis* any 2D visual image can be represented as the sum of a set of sinusoidal grating that differ in spatial frequency, amplitude of contrast modulation, orientation and phase
- 4Spatial contrast sensitivity function of the optical system* at low or high contrast - sensitivity goes down (threshold increases)
* function is made up of a number of channels - 5Square wave luminance profilesharp change in luminance intensity
* made up by a set of sinusoidal waves of same orientation with different frequencies, amplitude and phase. - 6Michelson Contrastchange in luminance / average luminance
- 7High pass filter* remove low frequency - only left with high frequency
* sharp changes in image only - 8Low pass filter* remove high frequencies
* see gradual change in intensity - 9Transparency (how it effects detection of spatial frequency)Decrease in transparency will decrease the detection of higher spatial frequency
- 10Sensitivity of a single channelCertain channels are sensitive to particular aspects (e.g. colour, grating, fine lines, corse line, low spatial freq.)
- 11Adaptation of contrast sensitivityIf observer is exposed to certain levels of spatial frequencies for a period of time, when presented stimulus again - reduced sensitivity to that particular frequency
* suggests that there are specific channels that are sensitive to certain frequencies - 12What is the neural basis of spatial frequency channels?....
- 13Receptive field of retinal ganglion cells* ON centre, OFF surround - light in centre will elicit excitatory response, light on surround will cause inhibitory response
* OFF centre, ON surround - light on centre will cause inhibitory response, light on surround will cause excitatory response - 14Receptive field of visual cortical cellsReceptive field is bar shaped (long)
* Light line detector
* Dark line detector
* Dark to light edge detector
* light to dark edge detector
* respond to specific orientation (orientation selective) - 15Spatial frequency tuningCells responding to particular frequencies depends on their receptive fields
* smaller receptive field can detect high spatial frequencies - 16How does spatial profile relate to spatial frequency tuning?Higher the number and strength of sub-regions, narrower will be the spatial frequency tuning
* more sub-regions - narrower - higher spatial frequency
* fewer sub-regions: wider, detect lower spatial freq.
* centre size determines the peak spatial frequency
* number of sub-regions and strength determines the band width - how narrow the spatial frequency tuning is - 17Different spatial frequency tuning in different cells in visual systemcortical simple cells: narrower
photoreceptors: only response to light (don't have surround- need surround to have spatial frequency tuning)
Retina,LGN: reduce response at lower spatial frequencies. - 18Spatial frequency of catsable to sense lower spatial frequencies gratings - moving large shadows (will be able to detect)
- 19To resolve the finest details of the image, there are two basic requirements:1. detector (receptive field centre) must be small enough (centre size of RGC receptive field determines the high frequency cut-off for that cell)
2. array of detectors must be fine enough (determines whether there would be aliasing or not for those frequencies to which individual cells are sensitive. - 20Adequate samplingNeeds to be adequate to distinguish between two grating
* if not adequate - both can be determine as being the same or aliasing - 21Nyquist frequencyf=N/2
* sampling frequency (N) should be at least twice the highest spatial frequency in the signal to adequately represent the signal - 22Aliasing* results from inadequate sampling
* grating changes to be different in orientation and frequency - 23Density of cones* in central fovea, the L- and M- cone density is high enough to capture all info that survives the blurring by optics of eye
* blurring can prevent aliasing
* periphery: cone density is lower - aliasing is reduced by disarray in sampling mosaic - leads to noisy pattern rather than a plausible object
* S - 10min/arc, M/L- 30sec/arc
* rods - far more numerous than cones (high convergence - improve sensitivity rather than spatial resolution - 24Factors affecting visual acuity* retinal eccentricity: VA decreases with increase in retinal eccentricity
* due to decrease in cone density and increase in RGC receptive field size
* Retinal illuminance: VA increases with retinal illuminance (but too high will cause bleaching - 25Can higher VA at higher illuminance be explained by photoreceptor density or their sensitivity?* cannot be due to receptor density as rods vastly outnumber cones
* cannot be explained by receptor sensitivity as higher illuminance causes more photopigment to be bleached - reduces sensitivity - 26Why does VA improve with increasing illuminance* recruits more functional units
* as each cell has their own threshold, recruitment of more receptive fields will increase the fineness of retinal mosaic - 27How does contrast affect VA?reducing contrast will reduce VA
* larger affect when contrast drops below 50% - 28Other factors affecting VA* duration of stimulus - acuity is dependent on time up to 100ms
* orientation of stimulus - horizontal/vertical gratings are more easily received than oblique orientations
* colour of stimulus - VA is independent of colour (if contrast/luminance is constant)
* target movement - movement reduces VA because of tracking errors (target drifts off fovea)
* Contour interactions - VA reduced when there is crowding contour data
* age: infants have lower VA (vision develops - 3-5 yrs - 29Reasons for infant's poorer VA* not due to optics
* visual cortex has not develop fully (cortical development - 3 years)
* rod-dominated peripheral retina of newborns is normal but all cone fovea contains widely spaced and poorly developed cones - 30Abnormal early visual experience that lead to amblyopia* occlusion amblyopia - when one eye is occluded by congenital cataract
* anisometropic amblyopia - when two eyes have very different refractive errors
* strabismic amblyopia - one eye deviates (eye turn/squint)
* one eye is poorer - observed in contrast sensitivity function - larger difference in higher spatial frequencies. - 31VA changes with age* VA decreases with advancing age - due to loss of ocular transparency and smaller pupils, higher incidence of diseases
- 32refractive error and VArefractive error reduces VA (higher spatial frequency but not at low)
- 33Stiles - Crawford effect* SCE of the first kind: luminous efficiency of a ray of light entering the eye near the periphery of the pupil is less than that of a ray passing through the centre of the pupil
* only in photopic vision
* magnitude of SCE depends on wavelength - 34How is light funnelled down rods and cones to photopigment?* receptors have higher refractive index than their surrounding medium - results in internal reflection
* for cones - light entering at an angle can lose intensity due to leakage - not completely reflected.
* photoreceptors are directed towards the pupil
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- Postgraduate Visual Functions - Optometry (Visual Functions - OD1) FlashCards sobre Spatial Vision, criado por Kim Vu em 24-05-2018.
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- Kim Vu
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