Vision: from retina to cortex

what is the ‘problem’ with vision?

there is an inherent ambiguity in the visual signals our eyes receive

what is the pupil?

where light enters the eye

what is the iris?

adjustable aperture, constricts in bright light to make pupil smaller

what does the cornea and lens do?

focuses light on retina

what is accommodation?

when ciliary muscles change shape of lens to bring objects into focus at different distances

what are photoreceptors?

cells with light sensitive photopigments in outer segments

what are rods?

• contain rhodopsin

• respond in dim light

• none in the fovea

what are cones?

• three types with photopigments sensitive to different wavebands (long, medium, short)

• daytime vision

what are the large parasol ganglion cells?

• connects to the magnocellular system

what are the small midget ganglion cells?

• connects to the parvocellular system

what are receptive fields?

the part of the retina from which the ganglion cells receive input

how does lateral inhibition affect the action potential of retinal ganglion cells?

• centre-surround relationship (on-centre, off-surround)

• light in the centre of the cell increases activity

• light surrounding the cell decreases activity

features of retinal ganglion processing

• poor at spotting change

• good at picking out sharp edges

• filters the input for useful info

what is the lateral geniculate nucleus?

• contains pathways and cells that are responsive to vision??

• in the thalamus

what do (parasol) magnocellular cells respond to?

movement and flicker

what do (midget) parvocellular cells respond to?

colour and detail

where are the koniocellular cells located?

in between the layers of the lateral geniculate nucleus

what do the koniocellular cells respond to?

blue-yellow colour??

what percentage of the cortex is dedicated to vision?

greater than 50%

other names for the primary visual cortex

• V1

• striate cortex

what are the cells in V1 sensitive to? (Hubel and Wiesel)

lines and specific orientations of lines

what does v1 contain?

a retinotopic map (not all parts of the visual system has this)

what are the streams of processing?

• dorsal

• ventral

what does the dorsal stream do?

• ‘where’ pathway

• focuses on where things are in space

• movement

• runs along the top of the brain

what does the ventral stream do?

• ‘what’ pathway

• focuses on object identification and recognition

• runs along the bottom of the brain

what is a retinotopic map?

a representation of each point in the visual field arranged so that neighbouring parts of the retina are represented by neighbouring parts of V1

how do centre-surround receptive fields and v1 simple cells relate to each other?

if you take the outputs from the receptive fields and organise them in a specific way you can get a rectangular receptive field which is responsive to a certain orientation of line but not others