Vision Flashcards
Where are photoreceptors found?
at the back of the retina
What is the fovea?
a small depression within the neurosensory retina where visual acuity is the highest made up of small, densely packed cones
What happens as distance from the fovea increases?
receptor size increases and density decreases
What is the synaptic cascade from cones?
cones synapse on bipolar cells in the retina → bipolar cells synapse on ganglion cells → ganglion cells carry the signal to other areas of the brain
What do the axons of ganglion cells comprise?
the optic nerve
What is transduction?
the conversion of physical energy of the external stimulus to electrical signal by the photoreceptors
What do the inner and outer segments of photoreceptors do respectively?
- inner - carries terminals that synapse with bipolar cells
- outer - receives photon of light
What is the dark current?
a current that brings about depolarisation
What happens in the absence of light?
photoreceptors are more depolarised due to the sodium and calcium influx through open cGMP gated channels
When do cGMP channels close?
in the presence of light due to the breakdown of cGMP
What happens when a photon of light strikes a photoreceptor?
a hyperpolarisation occurs due to the efflux of potassium (higher light intensity = larger depolarisation)
What does the membranous disc of the retina contain?
the photopigment rhodopsin which is made up of 7 transmembrane segments (opsin + 11-cis retinal)
What does transducin do when activated?
activate PDE which breaks down cGMP and closes the gated channels
How can the transducin mechanism be amplified?
activation of 1 rhodopsin molecule closes 200 channels and causes a hyperpolarisation of 1mV
What does hyperpolarisation at the receptor lead to?
excitation of the ganglion cells
What is required to open calcium channels and allow chemical transmission?
a large enough depolarising stimulus at the bipolar synapse
In the dark, what type of transmitter is released at the depolarised receptor?
inhibitory
What is the receptive field of a photoreceptor?
the specific area of the visual field from which that photoreceptor can detect light
Why is the receptive field of an individual photoreceptor very small?
each photoreceptor can only detect light that falls directly on it which corresponds to a tiny point in the visual field
What is the receptive field influenced by?
the spacing and density of photoreceptors in the retina
What part of the retina is responsible for sharp vision and why?
the fovea since the photoreceptors are closely packed and have a small receptive field
What are rod receptive fields involved in?
detecting general light intensity rather than fine detail
What are cone receptive fields involved in?
colour vision and fine detail; each one is tuned to a specific wavelength of light
What are discrete receptive fields characteristic of?
individual photoreceptors or sensory neurons with non-overlapping, specific areas of input that provide high spatial resolution and detailed sensory information
What do convergent receptive fields result from?
multiple sensory neurons converging onto a single higher-order neuron to allow for greater sensitivity and signal integration but reduce spatial resolution, making them more suited for detecting general patterns or stimuli in low-light conditions
When are ON ganglion cells activated (depolarised)?
when light increases in the centre of their receptive field i.e. they fire more APs when the centre of their receptive field is illuminated
What does the receptive field of an ON ganglion cell have?
- a central region where an increase in light intensity leads to excitation (ON)
- a peripheral region where an increase in light intensity can inhibit the cell’s activity (OFF)
What are ON ganglion cells crucial for?
detecting light objects against a dark background to help the visual system respond to brightening events in the visual scene
What do ON ganglion cells receive input from?
ON bipolar cells that depolarise in response to light due to a decreased glutamate release from photoreceptors in the light
When are OFF ganglion cells activated?
when light decreases in the centre of their receptive field i.e. they fire more APs when the centre of their receptive field is darkened
What does the receptive field of an OFF ganglion cell have?
- an OFF centre activated by darkness
- a peripheral ON area where increased light inhibits the cell’s activity
What are OFF ganglion cells important for?
detecting dark objects against a light background to help the visual system respond to darkening events in the visual scene
What do OFF ganglion cells receive input from?
OFF bipolar cells, which depolarise in response to darkness due to increased glutamate release from photoreceptors in the dark
What do ganglion cells respond to?
contrast between the centre and surrounding photoreceptors
What do rods have?
- high sensitivity to light
- large amount of pigment for scotopic (night) vision
What do cones have?
- low sensitivity to light
- smaller amounts of pigment for photopic vision
How does the retina encode for light?
by ON ganglion cells increasing in illumination in the centre of their receptive fields
What does 2-amino-4-phosphonobutyrate do?
block transmission in ON bipolar cells and impair the ability to detect increasing light (e.g. bright spots)
How doe the retina encode for dark?
via OFF ganglion cells
What are the 3 pigments in cones?
blue, green and red
When is yellow perceived?
when red and green cones are excited to 83% of their maximum
What is white and black respectively?
- white - mixture of all wavelengths of light
- black - absence of light
Describe the P (ventral) ascending stream
- starts in primary visual cortex (V1)
- projects to ventral regions of temporal lobe (V2 and V4) through the inferior temporal cortex (IT)
- forms perception, colour and object recognition (‘what’ stream)
Describe the M (dorsal) ascending stream
- starts in primary visual cortex (V1)
- projects to dorsal regions of temporal lobe (V2 and V3) through the middle temporal area (MT)
- ends in posterior parietal cortex responsible for motion, depth and spatial position (‘where’ stream)
What can damage to the middle temporal area cause?
movement agnosia (cerebral akinetopsia) - the inability to distinguish between moving and stationary objects with other perceptual capabilities intact
What can damage to the IT cause in stroke patients?
prosopagnosia - the inability to identify a particular face as belonging to a particular person
