eye as a camera
KSJ Fig 27-3
optic disc
fovea
optic disc
Carpenter, Fig 26-1
demonstration of blind spot
photoreceptors in the retina
KSJ, Fig 26-1
retinal circuitry: laminar organization
KSJ, Fig 26-6
dynamic range of light intensity
Carpenter, Fig 7-3
photopic vs scotopic vision
photopic vision
- at high light intensities
- colour vision
- high resolution
- low sensitivity
- best in fovea
- Stiles-Crawford effect
- mediated by cones
scotopic vision
- at low light intensities
- achromatic
- low resolution
- high sensitivity
- foveal scotoma
- no Stiles-Crawford effect
- mediated by rods
operating range: a sliding scale
Carpenter, Fig 7.4
dark adaptation curves
Sekuler and Blake, Fig 3-19
receptive fields of retinal ganglion cells
KSJ, Fig 26-7
retina-LGN-cortex
KSJ, Fig 27-4
LGN laminar organization
KSJ Fig 27-6
LGN (and retinal) receptive fields
achromatic
colour-opponent
KSJ, Fig 29-11
3 kinds of retinal ganglion cells
parasol ("M") - 10 %
- project to magnocellular layers of LGN
- large dendritic fields, large fibres
- large receptive fields -> low spatial frequencies, high velocities
- achromatic
midget ("P") - 80 %
- project to parvocellular layers of LGN
- small dendritic fields, small fibres
- large receptive fields -> high spatial frequencies, low velocities
- colour-opponent (red-green, possibly blue-yellow)
bistratified (“K”) - 2 %
- project to koniocellular layers of LGN
- blue-yellow opponent
drifting grating stimuli: contrast
contrast = (Lmax - Lmin) / (Lmax + Lmin) x 100%
100 %
50 %
25 %
12.5 %
contrast sensitivity = 1 / contrast threshold
drifting grating stimuli: SF, TF, speed
temporal frequency
speed = ----------------------------spatial frequency
QuickTime™ and a
Motion JPEG A decompressor
are needed to see this picture.
Q uic kTim e™ and a
None decom p r essor
ar e needed t o see t his pict u r e.
deg/sec =
cycles/sec
---------------cycles/deg
contrast sensitivity after M-lesions
Merigan et al, Fig 2&3
effects of M vs P lesions: summary
parvo lesion:
- lower acuity
- abolishes colour discrimination
- reduced contrast sensitivity to gratings,
at low temporal / high spatial frequencies (low velocities)
magno lesion:
- no effect on acuity
- no effect on colour discrimination
- reduced contrast sensitivity to gratings,
at high temporal / low spatial frequencies (high velocities)
- does not support idea of magno for motion, parvo for form vision
glaucoma: early detection
central problem: need for early detection
"at risk": ocular hypertension (OHT)
perceptual "filling in" - example is failure to see your "blind spot"
conventional (static) perimetry - detects problem only later
human psychophysics, as approach for early detection:
why you would not expect a deficit on many tasks:
earliest lesions in peripheral vision, but many tasks use foveal vision
-> need to do perimetry (automated) using the task
task may be mediated by unaffected neurons, e.g. color-discrimination (P-cells)
Ganglion cell loss in glaucoma
strategy #1:
earliest effects on larger diameter fibres ( -> M-cells)
theory: intra-ocular pressure block effects greatest on larger diameter fibers
anatomy, in humans: fibre diameters, cell body sizes (Quigley et al)
in animal models: experimentally raise IOP in monkeys (Dandona et al)
27 deg superior to fovea
Quigley et al, Fig 11
motion coherence: stimulus
see Adler’s,
Fig 20-12, 22-11
task:
report direction of motion
QuickTime™ and a
Motion JPEG A decompressor
are needed to see this picture.
noisy random dots:
prevent using change-of-position
Q uic kTim e™ and a
None decom p r essor
ar e needed t o see t his pict u r e.
a demanding task, requiring:
combining responses of multiple neurons
correct timing relations between neurons
vary signal-to-noise (% coherence):
best performance requires all the neurons
% Correct Responses
motion coherence: psychophysical thresholds
Motion Coherence (%)
motion coherence: loss in glaucoma
Joffe et al (Fig 2)
selective M-cell loss hypothesis:
criticisms
apparent loss of large cells/fibres might be artifact of cell shrinkage
also find losses of P-cell dependent psychophysics
testing for loss of sparse cell types
strategy #2:
most sensitive tests for capricious loss are those for sparse cell types:
(explains loss of abilities that depend on M-cells)
-> S-cones, blue/yellow (bistratified ganglion cells)
color: detection of blue spot on yellow background
rationale: blue-yellow ganglion cells (bistratified) are relatively sparse (ca 5%)
results:
Sample et al, Johnson et al: perimetry, longitudinal study