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Home , Monochromacy, Night vision, Receptive field, Scotopic vision

eye as a camera

Kandel, Schwartz & Jessel (KSJ), Fig 27-3 specialization

• fovea: highest density of photoreceptors, aimed at “where you are looking” -> highest acuity

• optic disk: -free area, where retinal nerve fibres exit the eyeball -> blind spot

KSJ, Fig 26-1 demonstration of blind spot photoreceptors in the

Two types of photoreceptor cells: • rods – abscent at fovea, more in periphery - mediate • cones – highest density at fovea - mediate day vision

Chaudhuri, Fig 9.1, 9.2 dynamic range of intensity

rods: lower threshold (higher sensitivity)

cones: higher threshold (lower sensitivity):

Chaudhuri, Fig 9.9 photopic vs scotopic vision - at high light intensities - at low light intensities - colour vision - achromatic - high resolution - low resolution - low sensitivity - high sensitivity - best in fovea - foveal - Stiles-Crawford effect - no Stiles-Crawford effect - mediated by cones - mediated by rods Rod monochromacy rod monochromacy congenital condition vision provided only by rods, without cone contribution Neural circuitry in the retina three layers of retinal neurons: – photoreceptors – bipolar and amacrine cells

Chaudhuri, Fig 9.11 Electrophysiology of retinal neurons

Chaudhuri, Fig 9.12, 9.13 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 affects a ganglion cell’s response 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 Receptive fields of retinal ganglion cells

Retinal ganglion cells are optimized for detecting : • 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 retina-LGN-cortex

KSJ, Fig 27-4 LGN (lateral geniculate nucleus)

KSJ Fig 27-6 LGN 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 - 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 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 Sinewave gratings: spatial frequency

spatial frequency: cycles per degree of visual angle

Chaudhuri, Fig 9.26 Sinewave gratings: contrast

contrast = (Lmax - Lmin) / (Lmax + Lmin) x 100%

100 % 50 % 25 % 12.5 %

contrast sensitivity = 1 / contrast threshold 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 sinewave gratings that move

temporal frequency! speed = ------! spatial frequency! ! ! cycles/sec! deg/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 : early detection central problem: need for early detection

"at risk": (OHT)

perceptual "filling in" - example is failure to see your "blind spot"

conventional (static) perimetry - detects problem only later

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 : 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

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 motion coherence: psychophysical thresholds % Correct Responses % Correct

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

References general textbooks: Carpenter RHS (2003) Neurophysiology, (4th Ed) London: Arnold. Chaudhuri A (2011) Sensory . Oxford: Oxford Press. Kaufman PL, Alm A (Ed) (2003) Adler's Physiology of the , 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 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 motion perception depend on the magnocellular pathway ?" J. Neuroscience 11: 3422-4329.

Quigley HA, Dunkelberger GR, Green WR (1989) " atrophy correlated with automated perimetry in human 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- color visual fields in glaucoma suspects at risk", Am. J. Ophthal. 115: 225-233 (1993).

Shapley R, Perry VH (1986) " and retinal ganglion cells and their visual functional roles", Trends in Neurosciences 9:229-235.