Lecture 7 The first steps in vision: From light to neural signals Flashcards
Steps 1-3 of how the eye works
- Light passes through the cornea, some of this light enters the eye through the pupil
- Then through the aqueous humor (nourishment)
- Next the lens to focus the image on the retina
Steps 4-7 of how the eye works
- Vitreous humour,
- Retina, light is converted into electrical signals
- Fovea,
- Optic disc, information transmitted onwards to the brain
Key info about the fovea
maximum resolution focus, responsible for high acuity vision it is densely saturated with cone receptors to help us see fine detail.
Emmetropia
- Normal vision
Light rays hit at the back of the eye with no blur
Myopia
Short sighted
if the eyeball is too long or the cornea is too curved
raised light converge before they hit the retina so the image appears blurry
how glasses correct myopia
diverges light rays entering the eye so they focus a little further back. landing directly on the retina instead of infront of it.
Hyperopia
long sighted
can’t see objects close as light converges too far back of the eye
Accomodation
the eyes ability to adjust focus to see objects clearly at different distances
Key facts about accomodation
Happens in a quarter of a second
15 dioptre accommodation range at birth
How eyes accommodate to see near objects
lens becomes thicker, this allows the light rays to refract more strongly
How eyes accommodate to see objects far away
lens is pulled thin, allowing the light rays to refract slightly
How the ability to accomodate is affected by age
Presbyopia - ability declines with age
1D drop every five years until 30
Less than 2.5D range by age 40-50 yrs
Astigmatism definition
Blur due to irregular curvature of cornea
Image not focusing on the eye due to lens or the shape of the cornea
Retina: vertical connections
photoreceptors
bipolar cells
ganglion cells
Retina: Horizontal connections
Horizontal cells
Amacrine cells
Role of photoceptor’s
converts light into electrical signals that the brain interprets as sight
Key facts about cones
See in colours and fine details in light
densely packed in the fovea
Key facts about rods
responsible for vision in low light
no rods in the fovea
responsible for our peripheral vision
Three cone types
Short, medium, long
No S cones at the fovea
Difference between photoreceptor peaks at different wavelengths
Rod sensitivity peaks at a lower wavelength than cone sensitivity (20 deg)
What is the blind spot
Where the optic nerve connects to the retina, lacks photoreceptors
Rods maximum density
between 0-10 mm on either side of the fovea
Fundus definition
interior lining of the eye, including retina, vascular tree, macula, fovea, optic disc
Bipolar cells: Midget
Fovea, cones, low convergence (1:1), good resolution
Bipolar cells: diffuse
periphery, rods + cones, high convergence (many: 1) good sensitivity
Bipolar cells: ON
Depolarize when light increases
depolarize definition
membrane potential more positive
Bipolar cells: OFF
hyperpolarize when light increases
hyperpolarize definition
membrane potential becomes more negative
Low convergence definition
1:1 connection, a single cone connects to a single bipolar cells, high resolution, low sensitivity
High convergence definition
Mnay:1 connection; many rods connect to a single bipolar cell
Low acuity
high sensitivity
Ganglion P cells
70% midget bipolar, parvocellular pathway, small dendritic fields and receptive fields
Ganglion cells M cells
10% diffuse bipolar, magnocellular pathway
Large dendritic fields and receptive fields
Light transduction definition
The pattern of light falling on the retina is encoded as graded potentials and spikes
Photoreceptors: Outer segment
photopigment molecules stored near pigment epithelium
Photoreceptors: Inner segment
photopigment molecules made
Pigment = protein + chromophore
G-protein coupled receptors
Photoreceptors: synaptic terminal
signal sent onward to the next (bipolar)
Light transduction at the photoreceptor steps 1-4
- Photon captured by chromophore in outer segment
- Photon energy transferred to chromophore (photoactivation)
- Chromophore changes configuration (isomerization) 11-cis-retinal to all-trans-retinal, breaks free of opsin
- Hyperpolarization: membrane channels close, Na+ reduced, cell becomes negatively charged
Light transduction at the photoreceptor steps 5-7
- Calcium channels close, calcium decreases, glutamate decreases
- Reduction in glutamate is a signal to bipolar cell that the rd/cone has captured a photon the signal cascades through to ganglion cells and onwards to the brain through the optic nerve
- Photopigment is regenerated
Receptive field
the region in visual space to which a neuron responds
ON-center OFF-surround receptive fields
cell response increases when light falls on its center, decreases when light falls on its surround
OFF-center ON-surround receptive fields
Cell response decreases when light falls on its center, increased when light falls on it’s surround
Effect of spot size on ON-center cell response
cell responds most when spot size is perfectly matched to the center
Lateral inhibition: reduction in cell firing rate due to stimulation outside the center of the receptive field enabled by horizontal cells
Lateral inhibition in ganglion cells
Antagonistic neural interaction between adjacent retinal cells
Produces center-surround organization of ganglion cell receptive fields
Acts as a filter
Signals information about change in illumination across a scene
Enhances the edges in a present scene
Lateral inhibition in ganglion cells: filter
Large response for differences in light intensity between center and surround small response for average or ambient illumination
Edge enhancement by ganglion cell responses: center-surround receptive fields
emphasizes edges
What an ON-center OFF-surround cell wants
darkness in the surround part of the receptive field
Hermann grid classical explanation 1.
Grey sports seen at crossings and not streets
The cell on the street has more darkness in it’s surround at the crossing has less darkness in it’s surround
Hermann grid classical explanation ON-centre receptive fields
fires more when dark in surround and less when light in surround
therefore, fires less at crossings and more on the streets so grey spots seen at crossings
Hermann gris grey sports seen in periphery and not fovea
receptive fields large in periphery, cover both light and dark regions
receptive fields small in fovea, fit within the crossing, no differential response
