Darkthorn’s Blog

- dimensions of color – visual system deals with variations in brightness, hue and saturation, independent of the levels of ambient light

- what you see about an object depends on the context in which it is placed

- reflects the same amount of light, but differs in perceived darkness

- illusory contours

- the visual image focused on the retina is inverted top to bottom and reversed left to right

- myopic- the image is formed before light reaches the eye

- hyperopic – plane of focus is behind the retina

- accommodation – the process by which curvature of the lens changes to allow the eye to focus on a nearby object

- near response – at the same time that accommodation occurs, the eyes converge and pupils constrict to increase depth of field of the focus

- presbyopia – the lens loses elasticity in older age, making accommodation difficult

- fovea – optimized for high acuity vision, the displacement of ganglion cells and the highest density of cone photoreceptors

- adaption to light levels by – adjustment of pupil size and adaptation of sensitivity of receptor cells

- photo receptor à bipolar cell à ganglion cell (modulated by horizontal and amacrine cells)

- cones – 4 million, three classes, low sensitivity, concentrated on fovea

- rods – 100 million, rhodopsin, high sensitivity, outside fovea

transduction in photoreceptors

1. light stimulated rhodopsin molecule activates 500 molecules of G-protein transducin. Cause GTP to bind, not GDP

2. G-protein activates a phosphodiesterase (PDE)

3. Each PDE hydrolyses 2000 molecules of cGMP

4. Reduction of cGMP causes closure of Na+ channels and hyperpolarisation of the receptor

– photoreceptors are somewhat depolarized and release glutamate in the dark

– illumination hyperpolarizes the photoreceptors, reducing glutamate release

– bipolar cells

o preserve the photoreceptor signal, glutamate is excitatory turning off ganglion cell

o invert the photoreceptor signal so that glutamate is inhibitory and ganglion cell on

decussiation of optic neurons

– the right half of the visual field projects to the left half of the brain

– the axons of ganglion cells in the nasal part of the retina cross in the optic chiasm (for information about light colors)

– the axons in the temporal part of the retina do not cross

lateral geniculate nucleus – receives projections for ganglion cells

· forms layers of cells (magnocellular from Y-cells, parvocellular from x cells.

· projections from the LGN to the cortex forms optic radiation (distinct fibre)

o topographically organized