eye%20as%20a%20camera

1
eye as a camera
KSJ Fig 27-3
2
optic disc
fovea
optic disc
Carpenter, Fig 26-1
3
demonstration of blind spot
4
photoreceptors in the retina
KSJ, Fig 26-1
5
retinal circuitry laminar organization
KSJ, Fig 26-6
6
dynamic range of light intensity
Carpenter, Fig 7-3
7
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
8
operating range a sliding scale
Carpenter, Fig 7.4
9
dark adaptation curves
Sekuler and Blake, Fig 3-19
10
receptive fields of retinal ganglion cells
KSJ, Fig 26-7
11
retina-LGN-cortex
KSJ, Fig 27-4
12
LGN laminar organization
KSJ Fig 27-6
13
LGN (and retinal) receptive fields
achromatic
colour-opponent
KSJ, Fig 29-11
14
3 kinds of retinal ganglion cells
parasol ("M") - 10 - project to magnocellular
layers of LGN - large dendritic fields, large
fibres - large receptive fields -gt low spatial
frequencies, high velocities -
achromatic midget ("P") - 80 - project to
parvocellular layers of LGN - small dendritic
fields, small fibres - large receptive fields
-gt high spatial frequencies, low velocities -
colour-opponent (red-green, possibly
blue-yellow) bistratified (K) - 2 - project
to koniocellular layers of LGN - blue-yellow
opponent
15
drifting grating stimuli contrast
contrast (Lmax - Lmin) / (Lmax Lmin) x 100
100
50
25
12.5
contrast sensitivity 1 / contrast threshold
16
drifting grating stimuli SF, TF, speed
temporal frequency speed
-----------------------------
spatial frequency
cycles/sec deg/sec ----------------
cycles/deg
17
contrast sensitivity after M-lesions
Merigan et al, Fig 23
18
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
19
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 -gt need to do perimetry (automated)
using the task task may be mediated by
unaffected neurons, e.g. color-discrimination
(P-cells)
20
Ganglion cell loss in glaucoma
strategy 1 earliest effects on larger
diameter fibres ( -gt 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
21
motion coherence stimulus
see Adlers, Fig 20-12, 22-11
task report direction of motion noisy
random dots prevent using
change-of-position 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
22
motion coherence psychophysical thresholds
Motion Coherence ()
23
motion coherence loss in glaucoma
Joffe et al (Fig 2)
24
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
25
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) -gt 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