Lecture 21 - vision continued and hearing Flashcards
(37 cards)
Retina is responsible for
The retina is responsible for converting light energy into patterned changes of membrane potential
Information from what is combined for the retina …
nfo from photoreceptors and interneurons is combined, so that the output from ganglion cells depend critically on the spatial and temporal pattern of light stimulation on the retina.
Receptive fields
Receptive fields – in the fovea very small, in the periphery of the retina much bigger (therefore the fovea has the best vision
Refractions is done by
Lens and cornea
Complete the pathway … Photoreceptors receive light
Photoreceptors receive light - goes through complex network of interneurons - then get an action potential firing from the retinal ganglion cells - axons of the retinal ganglion cells project to optic disk ( blind spot, called this because there are no photoreceptors here)
Visual pathway
Information is sent from eye to visual centres in brain in such a way as to preserve spatial relationships between visual stimuli.
So, visual info from objects in left visual space is processed in right brain, and vice versa. For this to happen, some info must cross to opposite side of brain, some must remain on same side.
Nasal retina
Nasal retina – contralateral (cross)
Temporal retina
Temporal retina – Ipsilateral (don’t cross)
Information from our nasal retina crosses over to the other side of our brain contralaterally, information from our temporal retina stays on the same side of the brain (projects ipsilaterally)
The optic chasm is where the nasal retina retinal ganglion cells cross
Axons of the retinal ganglion cells project down the optic tract
There is one lateral geniculate nucleus on each side, here the info synapses and is sent to the visual cortex in the occipital lobe
Retinal ganglion cells send
Retinal ganglion cells send axons to visual centres in the brain
How many retinal ganglion cells per retina
There are approximately 1 million ganglion cells per retina (1 million nerve cells per optic nerve)
Axons leave retina at
Axons leave retina at the optic disc
No photoreceptors at this point (blind spot)
Axons from ganglion cells in nasal retina
Axons from ganglion cells in nasal retina cross at the optic chiasm to project contralaterally
Axons from ganglion cells in temporal retina
Axons from ganglion cells in temporal retina do not cross, they project ipsilaterally
Information from the two retinae is …
By this means, information concerning images from visual space falling on equivalent parts of the two retinae is brought together for central processing
Ganglion cell axons project to 4 main subcortical visual areas
1) Superior colliculus
Concerned with eye movements and orientation to visual stimuli
2) Lateral Geniculate Nucleus
Concerned with the sensation of vision
these cells send axons and information to the visual cortex
3) Pretectum
Control of pupils
Near response = accommodation, convergence, constriction - constriction is controlled by the pretectum
4) Suprachiasmatic nucleus
Control of diurnal rhythms
Responsible for adapting our eyes to day and night vision
Superior colliculus
Concerned with eye movements and orientation to visual stimuli
Lateral Geniculate Nucleus
Concerned with the sensation of vision
these cells send axons and information to the visual cortex
Pretectum
Control of pupils
Near response = accommodation, convergence, constriction - constriction is controlled by the pretectum
Suprachiasmatic nucleus
Control of diurnal rhythms
Responsible for adapting our eyes to day and night vision
After processing by visual cortex, information can be sent to…
Temporal lobe- for identification of objects in visual space (the “what”) ( what they are in our visual space)
Parietal lobe- for processing of information regarding the location of objects in space (the “where”) (whereabouts the object is)
(Once we sensed vision, you can see object but might require an action such as moving out of the way) information passes to frontal cortex (from the temporal and parietal lobe), where it is used to help direct actions
Function in the visual cortex
The place where objects in and out of their visual context are analysed in detail.
For analysis of form, there are cells responding to edges and corners
For analysis of movement, there are cells responding to direction of movement and direction of movement in relation to background
For analysis of colour, there are cells responding to wavelengths coming from an object in relation to wavelengths coming from objects nearby, and an average of wavelengths from objects in other parts of the field of view.
All of this comes together to form our image/visual perception of the world
This information is used to discriminate between objects and backgrounds, to tell us what objects are, where they are, and where they are moving.
Retinal is a
Retinal is a chromophore that sits within the membrane spanning protein called opsin
In the dark =
Dark = intracellular concentration of cGMP level to be high
Action potentials and neurotransmitter release in retina processing
All the photoreceptors and interneurons release neurotransmitter and excite each other locally and once it gets to the retinal ganglion cell, if they have been excited enough then they will fire action potentials to other parts of the brain
