eye as a camera Kandel, Schwartz & Jessel (KSJ), Fig 27-3
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
demonstration of blind spot
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
dynamic range of light intensity rods: lower threshold (higher sensitivity) cones: higher threshold (lower sensitivity): Chaudhuri, Fig 9.9
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
Rod monochromacy rod monochromacy" congenital condition vision provided only by rods, without cone contribution
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
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 stimulus 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 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
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
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 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 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 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 Journal of Ophthalmology 86:823-826. http://bjo.bmjjournals.com/cgi/content/full/86/7/823 British 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.