light, eye, retina - first steps in seeing Flashcards
Process through eye
Light hits cornea
Then hits iris and goes through hole in pupil
Hits lens and lens helps eye to focus the light to back of the eye
Retina at the back of the eye is a sheet of sensors to pick up the light
Eye and optics - visual neighbours
Eyes refract light in such a way that the light that comes from one point in visual space is mapped on the same point in the retina
Bent by the cornea
Retina
Earliest sensory unit that interacts with the light that comes from the environment
Has a ridiculously complicated structure
Photoreceptors
Sensory cells that have certain molecules that change their configuration when they are hit by light
Vertical connectivity
Photoreceptors that are connected vertically to interneurons
Interneurons connected to ganglion cells
Ganglion cells take info from eye and feed it into brain
Horizontal cells
Amacrine cells help connect the cells horizontally
Rods
Used when light is dim
Many rods (100 million)
Colourblind
Very sensitive to light and used when very dim
Poor spatial resolution, very blurry
High temporal resolution, used to perceive motion
Cones
Used when light is very bright
Fewer cones (6 million)
S-, M- and L- cones with peak sensitivities at different wavelengths
Less sensitive to light and used in bright light
High spatial resolution, need to use cones when you want to see in high resolution
Poor temporal resolution
Duplex retina
Two systems for seeing
Rod system sensitive to light
Four rod cells feed info into one bipolar cell and the one bipolar cell feeds info to ganglion cell
Only one rod needs to pick up light in order to detect it
Brain doesn’t know where in retina light came from because ganglion cell covers a large cell
Cone system
One cone mapped to one ganglion cell
Brain knows where in visual space and where in retina light comes from due to there only being one
Distribution of photoreceptors
Rods and cones aren’t equally distributed across the whole retina
Fovea
Directly behind iris opening
Lots of cones and no rods
This is why we can’t see stars when we look directly at them because cones are less sensitive to light - so we use our peripheral
Blind spot
No cones or rods
Part of the retina where all the axons of the photo receptors leave our eye
Don’t experience the blind spot due to our other eye filling in the gap
Receptive fields
Specific region of sensory space in which an appropriate stimulus leads to a response in a sensory neuron
Each ganglion cell has a set spatial extent to which it can get info from
Has to be an appropriate stimulus
Receptive fields of ganglion cells
One photoreceptor directly linked to a bipolar cell which feeds directly to a ganglion cell - facilitory
Other photoreceptors surround the central one and link to a HORIZONTAL cell which links to a bipolar cell which then feeds to the ganglion cell - inhibitory
Activity of the horizontal cell inhibits the activation of the bipolar cell
Centre-surround receptive field
Ganglion cell acts like a ballot box - adds up faciliatory and inhibitory votes from photoreceptors
If there are more positive than negative votes then the threshold becomes active
If there is an equal number nothing happens
If it takes in more negative then it will reduce its baseline responding
Receptive field structure
Stimulus has to hit the receptive field of the ganglion cells in order to activate it
Light in centre of receptive field
Ganglion cells will become more active
The photoreceptors in the middle of the receptive field have a facilitatory link to the ganglion cells
Light in edge of receptive field
Ganglion cells become less active
The photoreceptors on the outside of the receptive field have an inhibitory link to the ganglion cells
Light across whole receptive field
Ganglion cells stay at baseline
Not an appropriate stimulus
Local differences in light intensity
Mainly set up to process contrast
Very efficient way to process environment
Edges and receptive fields
With one small part of receptive field in a darker grey area, ganglion cell will respond higher than baseline due to more light hitting the faciliatory centre
Receptive field is exactly in between two greys then the faciliatory and inhibitory cancel each other out
Periphery hits more of the light and the centre is in the dark grey then it will be lower than baseline
Cornsweet effect and receptive fields
When the baseline changes, the ganglion cells tell our brain that there is an edge from light to dark
Our brain fills in the information and assumes that the two different surfaces in the illusion are completely different colours
Efficient way to fill in the information
