lecture 26: vision 2 Flashcards
(15 cards)
what are the different components of colour vision
- photoreceptors contain photopigments
- rods have rhodopsin
- cones have S(blue), M(green) or L (red) photopsin
- light of specific wavelengths is reflected from coloured objects
- perception of colour created by relative activation of 3 cone types
- under optimal conditions, humans can perceive about a million colours
what are the peal wavelengths for the different cones
peak wavelength= more sensitive at this wavelength but still respond to other wavelengths
blue cone = 437nm
green cone = 533nm
red cone = 564nm
mechanisms of colour blindness
- can be inherited (congenital)
- or acquired (due to disease)
- congenital forms affect 8% of males and 0.5% of females
–> genes encoding production of M & L opsins (ie green & red sensitivity) are on the X chromosome - a variety of forms of colour blindness exist
what are the layers of the retina (starting from the front of the eye –> back of the retina)
- optic fibre (photon of light comes through here)
- ganglion cell layer
- inner plexiform layer
- inner nuclear layer
- outer plexiform layer
- outer nuclear layer
- receptor layer
mechanism of retinal processing
- signals from photoreceptors and retinal interneurons are combined so that the electrical responses of ganglion cells depend critically on the spatial and temporal pattern of light stimulation on the retina
- the inputs to each ganglion cell arise from neighbouring photoreceptors in a circumscribed area of the retina, the cells receptive field
ganglion cell receptive fields
- are roughly circular
- are divided into two parts
1. a circular central zone (the “centre”)
2. an annulus around the centre (the “surround”) - ganglion cells respond optimally to differential illumination of the centre and the surround
response in off centre fields
- reduce firing output when there is illumination in the centre of their receptive field
- increase firing when there is illumination in the surround of their receptive field
response in on centre cells
- increase firing output when there is illumination in the centre of their receptor field
- decrease firing when there is illumination in the surround of their receptive field
what makes up our vision
net combo of all the different receptive fields
what is visual acuity
a measure of the sharpness or clarity of vision
visual acuity in low convergence ganglion cells
- small receptive field , high acuity = can pick out details very well
- cones, fovea
visual acuity in high convergence ganglion cells
- large receptive fields, low acuity
- rods and cones periphery
what does the visual pathway involve
- ganglion cells send axons to visual centres in brain
- about 1million ganglion cells per retina, therefore 1 million axons per optic nerve
- axons leave retina at optic disc (optic nerve head)
–> no photoreceptors at this point (blind spot)
nasal retina and temporal retina role in visual pathway
- axons from ganglion cells in nasal retina cross at the optic chiasm to project contralaterally
- axons from ganglion cells in temporal retina do not cross, they project ipsilaterally
- by this means, info concerning images from visual space falling on equivalent parts of the two retinae is brought together for central processing
what are the four main subcortical visual areas that ganglion cell axons project to
- superior colliculus
- concerned with eye movements and orientation to visual stimuli - lateral geniculate nucleus
- concerned with the sensation of vision - pretectum
- control of pupils - suprachiasmatic nucleus
- control of diurnal rhythms
