Lecture 21 - vision continued and hearing Flashcards
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
Astigmatism
Astigmatism caused by aspherical curvature of the cornea and lens
Hypermetropia
Hypermetropia = light is focused beyond the plane of the retina
Majority of refractive power comes from
the cornea
What would annular illumination activate?
Annular illumination would activate an ‘off-centre’ retinal ganglion cell.
What would diffuse illumination activate?
Neither type are activated by diffuse illumination
What is sound?
pressure waves in the atmosphere
Zones of compression
Zones of compression = regions of high pressure due to particles being close together
Zones of rarefactions
Zones of rarefactions = regions of low pressure due to particles being spread further apart
Frequncy =
Frequency = pitch
Measured in hertz
Higher the frequency of the sound, the more waves there are per frequency
The lower the frequency, the less waves there are per second
Amplitude =
Amplitude = loudness Also called intensity Quiet = small amplitude Loud = large amplitude Measured in decibels (dB)
Peaks and troughs in graph represents
Peaks in graph are high pressure and troughs are low pressure
Threshold of hearing
Humans can detect down to about 20 Hz
As you get older, your range of hearing reduces
Human voices range between 700 to 1200 Hz
U shaped graph
Damage to hearing apparatus
Doesnt recover when you damage it so you have permanent hearing damage
