week 6

what is the Stiles-Crawford Effect of the First Kind

it is where light entering the pupil at different heights is perceived to have different luminous intensities, with peripheral rays appearing dimmer than equal-energy rays entering near the pupil centre

what specifically about the cone photoreceptor causes its effect in SCE-I

cone photoreceptors are oriented toward the pupil centre. its small size and coned shape allows the cone photoreceptor to act as dielectric waveguides. its narrow confinement of light allows incoming light to travel in a single direction and is guided in a predictable, funnel-like manner

does the Stiles-Crawford Effect occur in rods

no because SCE-l is a cone-specific effect as rods don't show directional sensitivity

why does light entering the pupil periphery appear dimmer in SCE-I

light entering near the pupil centre aligns with the orientation of the cones so it enters and travels down the cone efficiently. Oblique peripheral rays enter the cones poorly, so less light reaches the photopigment producing a weaker neural signal and making peripheral light appear dimmer

what is the Stiles-Crawford Effect of the Second Kind

one wavelength entering near the pupil edge is perceived as a different colour than the same wavelength entering the pupil centre, independent of intensity

what may cause SCE-ll

peripheral rays stimulate L, M and S cones in different proportions than central rays due to small differences in cone orientation, angular sensitivity and distribution across the retina. these changes alter the L:M:S activation ratio producing a shift in colour perception

what does the Nyquist limit describe

the minimum physical separation between two image points on a sensor required for them to be resolved as seperate, meaning two stimulated pixels must be separated by one unstimulated pixel

how does the Nyquist limit apply to the retina

two retinal image points are resolved only when two stimulated cones are separated by one unstimulated cone

what is the Nyquist frequency

maximum spatial frequency that can be resolved which is equal to one cycle per two pixels

what is the relationship between pixels and Nyquist limit, as such what is the Nyquist limit for cones with a 2.5 µm diameter

Nyquist limit is double that of pixel number thus 5 um diameter

how does the photoreceptor mosaic determine resolution

regions where two stimulated cones are separated by one unstimulated cone will be resolved clearly whereas if two stimulated photoreceptors are not separated by one unstimulated photoreceptor, they will not be resolved by the photoreceptors

what do black and white dots represent in the photoreceptor mosaic

white dots are the stimulated photoreceptors and black dots are the unstimulated photoreceptors

describe the density and distribution of rods and cones

there are more rods than cones

how do cone sizes vary across the retina

shorter and thicker in the fovea and longer and thinner in the periphery

what is retinal convergence

process where multiple photoreceptors synapse onto one ganglion cell. their signals summate increasing sensitivity to low light but the ganglion cell cannot locate which photoreceptor was stimulated reducing resolution

what is the average convergence ratio of the ganglion cells in the retina

1 ganglion cell receives input from 100 rods and 4-6 cones

what is the convergence pattern at the fovea

1:1 wiring so one cone synapse to one bipolar cell then one ganglion cell

how does convergence differ in the peripheral retina

many rods connect through horizontal and amacrine cells onto a single ganglion cell

what is lateral inhibition

occurs when excited neuron suppresses neighbouring neurons, enhancing contrast edge detection and colour perception

where does lateral inhibition occur in the retina

occurs at both the outer retina and the (photoreceptors + horizontal cells) and inner retina (bipolar, amarine and ganglion cell networks)

how does lateral inhibition work at the photoreceptor level (outer retina)

strong stimulated photoreceptor inhibits neighbouring photoreceptor via horizontal cells. this suppresses weaker signals allowing the dominant photoreceptor's response to stand out, sharpening contrast and edge perception

how does lateral inhibition work at the second synaptic level

a strongly stimulated bipolar cell activates amacrine cells which suppress neighbouring bipolar and ganglion cells, enhancing the dominant signal, sharpening contrast and edge detection

what is vernier acuity

it is a hyperacuity where the visual system detects tiny misalignments between two line segments. it exceeds photoreceptor spacing because cortical processing and lateral inhibition allow precise comparison of edge positions

why can Vernier acuity exceed the optical resolution of the retina

na

what retinal mechanism underlies Vernier acuity

it results from lateral inhibition, which suppresses surrounding cones whose responses are one-eighth or less of the stimulated cone

how fine is Vernier acuity compared to the Nyquist limit

the eyes are able to distinguish displacement as small as 3 sec of arc or 0.05 minutes of arc

what is clinical visual acuity

the visual angle subtended by the smallest resolvable letter where object height and test distance are known

how is Snellen acuity documented what does 6/36 mean

test distance distance a normal observer can read the same letter so 6/36 means the person reads at 6m what a normal eye reads at 36m

what are the common principles of visual acuity charts

charts must be used at specific distances (6m or 3m), letter sizes subtend specific visual angles, and optotypes are built on 5x5 grids

why is Snellen chart design problematic

lines contain different numbers of letters making results inconsistent, letter recognition difficulty is not standardised, letter size progression is irregular, and letter and line spacing are not standardised

what is MAR

the angle subtended by one arm of the smallest resolvable letter measured in minutes of arc

how do you convert Snellen to MAR

invert the Snellen fraction

how do you convert MAR to Snellen

d/(d x MAR)

how is scoring done on a logMAR chart

each line = 0.1 and each letter = 0.02 added to the score

what are the key features of the logMAR chart

five sans-serif Sloan letters per line, letters drawn on 5x4 grids, logarithmic size progression, proportional spacing, every letter contributes to the score