Chapter 3-Spatial Vision Flashcards
what is spatial vision
concerned w/ the ability of the visual system on detecting/discriminating spatially defined features
-changes in space
what does the luminance of a square wave grating change as
as a step function (only min and max luminace values, no intermediate)
what is the luminance of a sine wave grating change as
sinusoidal function
-not only min and max but intermediate also
a luminance grating is defined by its
spatial frequency
phase
orientation
contrast
waht is spatial frequency
the number of cycles per degree of visual angle
-1 cycle is a dark and a bright stripe
high spatial frequency = ____ cycles per degree of visual angle
low spatial freq = _____ cycles per degree of visual angle
more
less
what does phase refer to
refers to the position of the cycle that the luminance grating is in at any given time
what is contrast
contrast is the difference in luminance often expressed as % contrast
- detecting the difference in luminance btwn objects
- difference btwn max and min luminance
if you have a grating w/ a luminance value of 0, how much percent contrast will you have, no matter what the luminance
100%
what is spatial resolution
the smallest spatial detail that can be detected, recognized or resolved
what is target detection
the perception (or not) of the presence of an object
what is target recognition
recognition and naming of the target (snellen)
what is target resolution
the minimum angular size at which 2 objects can be discriminated as 2 diff objects
what is target localization
requires discrimination in the spatial position of objects (vernier acuity)
all the tests for spatial resolution, involving or not cognitive mech, are limited by what
optical and neural factors that determine the lmit of spatial resolution
target localization, detection and resolution give VA higher than the typical VA (landolt C or recognition) indicating that mechanisms involved in making such judgements (recognition) are…
not restricted to the retinal level
what are the limiting factors for the spatial resolution if we assume that the eye’s optical system are aberrations free
optical: diffraction
neuronal: cone density
what is diffraction
pheno when a lightwave encounters an obstacle
-due to diffraction the light wave bends and separates
what is the point spread function
describes the transformation that a point source undergoes as it passes through the optical elements of the eye
in photopic conditions, our spatial resolution limit due to diffraction cant be any better than waht
0.4 arcmins
if there are no aberrations in the eye’s optical system then a point source would be the size of an airy disk and the optical system is limited by….
diffraction
will high or low freq be afftected more by diffraction
high
what is the rayleigh criterion
2 point sources can be resolved as 2 separate point sources only when the centers of their airy disks are no closer than the radius of the airy disks
what does the rayleigh criterion mean for the spatial resolution of the visual system?
in order to resolve 2 point sources, 2 cones must be stimulated w/ light while there is an unstimulated ocne btwn them
what is the cone spacing in the fovea
2.8 micrometers
what is aliasing
the effect that causes diff signals to become indistinguishable
when does aliasing start
for freq higher than the nyquist limit
what is the max freq called
nyquist limit
what is the resolution of smaller pupils limited by
diffraction
what is the resolution of larger pupils limited by
cone density
what are some disadvantages to the snellen chart
the letter size doesnt chnage uniformly throughout the test
- not equal number of letters on each line
- not regular spacing btwn the letters
what does the logMAR chart use
log of the MAR to get a uniform progression of letter size throughout the chart
what are the advantages to logMAR
uniform profession in letter size from line to line
- same number of letters on each line (crowding)
- regular spacing btwn lines
- greater accuracy
for VA charts,
rule 1: the MAR is the ____ of decimal VA
rule 2: the spatial freq (in cycles/degree) is equal to the decimal x ____
reciprocal
30.00
what are the factors affecting spatial resolution
refractive error pupil size illumination contrast retinal area
we need higher contrast for detecting what?
we need lower contrast for detecting what?
high spatial frequency gratings
low spatial freq gratings
what does the contrast sensitivity function describe
the contrast sensitivy of the human visual system for diff spatial freq
what is the highest contrast sensitivity that occurs for spatial freq
~4c/deg
for freq lower and higher than 4c/deg there is a what
low freq drop-off and a high freq cut off
what is CSF often described as
band-pass function
waht does CSF tell us (contrast sensitivity function)
that our visual system is more sensitive for spatial freq around 4cycles/degree
-either side of this freq, our contrast sensitivity drops
what does the peak contrast sensitivity at 4c/deg mean
the majority of the receptive fields respond optimally at this spatial freq
OR
the majority of the receptive fields have such a size so as to respond to 4c/deg
waht is the low freq drop out
the decline in contrast sensitivty at low spatial freq provides info about the strenth of spatial antagonism (or later inhibition)
what is peak sensitivyt
the most effective spatial freq provides info about the overall size of the receptive fields
in low freq drop off…
as the spatial freq increases a single bring (or dark) band occupies both the center and the surround of the recptive field, cuasing what
lateral inhibition
-this means that the center respons to the band but the surround doesnt, resulting in NO REPONSE from this RF
in high freq cut-off..
the decline in contast sensitity at high spatial freq is bc the stimulus is avged by the center and surround of the receptive field, doing what
diminishing the cell’s response
in high freq cut off
as the spatial freq decreases, the avg response of the center and the surround is decreasing, resulting in what
a decrease in contrast sensitivity
waht is our maximum cycles/degree
60 cyc/deg
if the highest spatial freq is 60c/deg then snellen VA will be what, if its 30c/deg what will it be
20/10
20/20
how is snellen VA related to CSF
snellen VA is just a point on the CSF
so CSF provies much more info about the integrity of the visual system than VA
what are the factors affecting contrast sensitivty
- retinal illumination
- refractive error
- media transparency
- reintal and systemic diseases
- sine vs square wave gratings
what is band pass
certain range of visible freq that can be seen in photopic conditions
is the human visual system more sensitive to square or wine wave gratings
square
1.27x more for frequ more than 1c/deg
for sptial freq less than 1c/deg the ratio increases
what happens for spatial freq lower than 1c/deg on the CSF
the contrast sensitivity of a square-wave grating is defined by the contrast sensitivy of a series of sine-wave grating
for spatial freq lower than 1c/deg the contrast sensitivity of a square-wave grating is defined by what
the fundamental and higher order harmonics
if someone adapts (stares for a prolonged time( to a sine wave grating of ex. 4c/deg then there is a reduction in the CSF around….
4c/deg
