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