eye as a camera
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
KSJ Fig 27-3
eye as a camera
Carpenter, Fig 26-1
optic disc
optic disc
fovea
demonstration of blind spot
KSJ, Fig 26-1
photoreceptors in the retina
retinal circuitry: laminar organization
KSJ, Fig 26-6
Carpenter, Fig 7-3
dynamic range of light intensity
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
photopic vs scotopic vision
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
KSJ, Fig 29-11
achromatic
colour-opponent
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
contrast = (Lmax - Lmin) / (Lmax + Lmin) x 100%
100 % 50 % 25 % 12.5 %
drifting grating stimuli: contrast
contrast sensitivity = 1 / contrast threshold
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drifting grating stimuli: SF, TF, speed
temporal frequencyspeed = ----------------------------- spatial frequency
cycles/secdeg/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
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)
glaucoma: early detection
Ganglion cell loss in glaucoma
Quigley et al, Fig 11
27 deg superior to fovea
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)
motion coherence: stimulus
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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
see Adler’s, Fig 20-12, 22-11
motion coherence: psychophysical thresholds
% C
orre
ct R
espo
nses
Motion Coherence (%)
motion coherence: loss in glaucoma
Joffe et al (Fig 2)
apparent loss of large cells/fibres might be artifact of cell shrinkage
also find losses of P-cell dependent psychophysics
selective M-cell loss hypothesis: criticisms
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
testing for loss of sparse cell types