Lecture 5: Spatial Vision Flashcards
state what is spatial vision
visual system’s ability to detect and analyse changes in brightness across space
explain detection acuity task
perception of the absence or presence of an aspect of the stimuli
eg. landolt C, tumbling E
explain recognition acuity task
requires naming of a target
eg. snellen letter chart, near reading chart
explain resolution acuity task
ability to detect a gap/separation
eg. grating acuity
explain localisation acuity task
ability to discriminate differences in spatial position (stimulus aligned or not) of a test object
eg. vernier acuity
explain hyperacuity
when some aspects of vision are so sensitive that they cannot be explained by dense packing of cones. hyperacuity arises from cortical processing
eg. vernier acuity, stereoacuity (3D), sharpness/roughness of an edge
explain diffraction
diffraction is the spreading of light (in airy pattern) after it passes through an aperture (pupil)
explain rayleigh’s criterion and equation (MAR)
rayleigh’s criterion states that 2 objects will just be resolved if the diffraction minima of 1 light source corresponds with the maxima of another
angular resolution = 1.22(lamda) divide by pupil diameter
explain scoring in LOGMAR charts
tested at 4m, snellen notation as eg. 4/10
score for each decreases by 0.1 starting from 1.0
wrong answer plus 0.02. bigger the score, the worse the VA
explain spatial modulation transfer and contrast sensitivity function
SMTF describes how an optical system transfers information. CSF presents how humans ‘transfer and process’ visual information
explain how to obtain CSF
(1) present a grating of a specific spatial frequency
(2) slowly increase the contrast of grating from NS to S (sensitivity = 1/threshold)
(3) repeat for a range of spatial frequencies
explain shape of CSF (receptive fields)
*anything under graph is SEEN
a ganglion cell’s receptive field has an ON centre and OFF surrounding
ON - cell is excited, OFF - cell is inhibited
at middle spatial frequencies, there is HIGHER EXCITATION as the light part of the sine grating falls on the ON centre while the dark parts falls on the OFF surround.
at low spatial frequencies, there is LESS RESPONSE as light parts fall on ON centre & OFF surround simultaneously. OFF surround cancels some excitation response in the centre hence lateral inhibition
the cut off at high spatial frequencies (30cpd/60cpd), due to the limitation of optical system to resolve details
explain relation between CSF & resolution acuity
CSF tests the eyes ability to resolve details for various contrasts. typical snellen acuity tests only for high contrast
- 30 cpd = 6/6
- 60 cpd = 6/3
explain CSF in refractive error vs cataract
uncorrected refractive error results in loss at HIGH spatial frequencies
cataract results in loss at ALL spatial frequencies
explain effect of luminance + location of ret on CSF
CSF declines with luminance
CSF varies with retinal eccentricity (reduced in periphery & highest in central)