eye as a camera - McGill University · 2017. 11. 21. · effects of M vs P lesions: summary! parvo...
Transcript of eye as a camera - McGill University · 2017. 11. 21. · effects of M vs P lesions: summary! parvo...
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eye as a camera
Kandel, Schwartz & Jessel (KSJ), Fig 27-3
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retinal specialization • fovea: highest density of photoreceptors, aimed at “where you
are looking” -> highest acuity
• optic disk: cell-free area, where retinal nerve fibres exit the eyeball -> blind spot
KSJ, Fig 26-1
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demonstration of blind spot
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photoreceptors in the retina
Two types of photoreceptor cells: • rods – abscent at fovea, more in periphery - mediate night vision • cones – highest density at fovea - mediate day vision
Chaudhuri, Fig 9.1, 9.2
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dynamic range of light intensity
rods: lower threshold (higher sensitivity) cones: higher threshold (lower sensitivity):
Chaudhuri, Fig 9.9
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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
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rod monochromacy" congenital condition vision provided only by rods, without cone contribution
Rod monochromacy
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Neural circuitry in the retina three layers of retinal neurons:
outer nuclear layer – photoreceptors inner nuclear layer – bipolar and amacrine cells ganglion cell layer
Chaudhuri, Fig 9.11
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Electrophysiology of retinal neurons
receptive field: – A small, circular region of the retina that affects response of a ganglion cell – Equivalently, a small circular region of the visual field, within which a
light stimulus affects a ganglion cell’s response
Chaudhuri, Fig 9.12, 9.13
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Receptive fields of retinal ganglion cells Two kinds:
• ON-center/OFF-surround cell: – Centre circular region of receptive field is excited by light, surrounding
zone is inhibited by light. • OFF-center/ON-surround cell:
– Centre circular region of receptive field is inhibited, surrounding zone is excited by light.
Chaudhuri, Fig 9.13
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Receptive fields of retinal ganglion cells Retinal ganglion cells are optimized for detecting contrast:
• Centre-surround antagonism: – results from the concentric
spatial arrangement of the ON and OFF subregions
• Consequence is that retinal output sent to the brain by ganglion cells is driven by light contrast, i.e. differences in luminance
Chaudhuri, Fig 9.14
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retina-LGN-cortex
KSJ, Fig 27-4
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LGN (lateral geniculate nucleus)
KSJ Fig 27-6
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LGN receptive fields
KSJ, Fig 29-11
achromatic
colour-opponent
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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 - small 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
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Visual angle • Resolution:
– Often express acuity in terms of visual angle – Visual angle = angle subtended by image on the retina – An object at a greater distance subtends a smaller visual angle
http://en.wikipedia.org/wiki/Visual_angle
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Sinewave gratings: spatial frequency spatial frequency: cycles per degree of visual angle
Chaudhuri, Fig 9.26
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contrast = (Lmax - Lmin) / (Lmax + Lmin) x 100%
100 % 50 % 25 % 12.5 %
Sinewave gratings: contrast
contrast sensitivity = 1 / contrast threshold
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Contrast sensitivity function
Measure minimum contrast to make a grating of a particular spatial frequency just visible. Plot threshold data in terms of sensitivity = 1 / threshold.
Chaudhuri, Fig 9.27
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sinewave gratings that move
temporal frequency!speed = -----------------------------! spatial frequency!! ! cycles/sec!deg/sec = ----------------! cycles/deg!
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contrast sensitivity after M-lesions
Merigan et al, Fig 2&3
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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
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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
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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)
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motion coherence: stimulus
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
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motion coherence: psychophysical thresholds
% C
orre
ct R
espo
nses
Motion Coherence (%)
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motion coherence: loss in glaucoma
Joffe et al (Fig 2)
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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
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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
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general textbooks: Carpenter RHS (2003) Neurophysiology, (4th Ed) London: Arnold. Chaudhuri A (2011) Sensory Perception. Oxford: Oxford Press. Kaufman PL, Alm A (Ed) (2003) Adler's Physiology of the Eye, 10th ed. St.Louis: Mosby. Kandel, Schwartz, and Jessell , Principles of Neural Science (4th Ed.) journal articles: Ansari EA, Morgan JE, Snowden RJ (2002) “Glaucoma: squaring the psychophysics and neurobiology” British Journal of Ophthalmology 86:823-826. http://bjo.bmjjournals.com/cgi/content/full/86/7/823 Joffe KM, Raymond JE, Chrichton A (1997) "Motion coherence perimetry in glaucoma and suspected glaucoma" Vision Research 37:955-964. Johnson CA, Adams AJ, Casson EJ (1993) "Blue-on-yellow perimetry can predict the development of glaucomatous visual field loss" Arch. Ophthalmol. 111: 645-650. Maddess, T., Goldberg, I., Dobinson, J., Wine, S., Welsh, A.H., and James, A.C., “Testing for glaucoma with the spatial frequency doubling illusion”, Vision Research 39: 4258-4273 (1999). Merigan WH, Byrne CE, Maunsell HR (1991) "Does primate motion perception depend on the magnocellular pathway ?" J. Neuroscience 11: 3422-4329. Quigley HA, Dunkelberger GR, Green WR (1989) "Retinal ganglion cell atrophy correlated with automated perimetry in human eyes with glaucoma", Am. J. Ophthal. 107: 453-464. Sample, P.A., Taylor, J.D.N., Martinez, G.A., Lusky, M., and Weinreb, R.N., "Short-wavelength color visual fields in glaucoma suspects at risk", Am. J. Ophthal. 115: 225-233 (1993). Shapley R, Perry VH (1986) "Cat and monkey retinal ganglion cells and their visual functional roles", Trends in Neurosciences 9:229-235.
References