vision 2 Flashcards
what is lateral inhibition
- object detection
in each eye the axons are connected in a lateral inhibitory neural network.
When light hits the ommatidia they fire at a rate proportional to the intensity of the light.
The more they fire the more they inhibit their neighboours via lateral inhibition
This means that at points of contrasting illumination the difference is exaggerated
occurs only at the edge- amplifies relative difference.
Horizontal cells responsible: They reverse the sign of their input.
The Hermann Gridd
grey blobs at the intersection disappear when they are foveated.
- consequence of lateral inhibition- The intersection is surrounded by more activated receptive fields.
Mach bands
The apparent change in brightness between bands is an illusion caused by lateral inhibition
Mediated by horizontal cells which when activated inhibt the other cells they are in contact with
For example in an on- centre off surround cell the horizontal cells inhibit the outside which causes the difference to be amplified.
Projection to the brainstem accessory optic and pre tectal nuclei
responsible for visual reflexes such as accommodation, vergence, pupillary control
- shining a light in one eye will result in reflex pupil control in both eyes.
Retino- tectal pathway: projections to the superior colliculus
GAZE shift- foveation
- projects contralaterally
- seperate at the optic chiasm.
Properties of the colliculus initiates gaze shifting.
retino- geniculate- striate pathway
scene analysis and object identification
- takes visual information from the retina to the lower layer of the primary visual cortex (V1) via the lateral geniculate nuclei of the thalamus.
- Retinotopic mapping
- The retino- geniculate striate system is retinotopic: each relay is orgorganised according to a spatial map of the retina.
Projections
Temporal hemiretina - Nasal hemiretina
Temporal hemiretina (ipsilaterally) Nasal hemiretina (contralaterally)
binocular overlap
allows for depth perception
the lateral geniculate nucleus : The thalamic relay
P and M layers
P layers: Small cell bodies , responsive to colour, fine detail - scene analysis and object identifcation: CONES
M layers: Large cell bodies, luminance change- on/off, movement. RODS provide input (Similar to superior colliculus)
Receptive fields
The receptive field of a visual neuron is the area in which it is possible to influence the firing of that neuron.
- increase or decrease.
Hubel and Weisel
Recorded the three levels of the retina-geniculate-striate system:
- retinal ganglion cells
- lateral geniculate neurons
- striate neurons of lower layer IV
Four commonalities
- Receptive fields in the foveal area of the retina were smaller than those at the periphery
- All had circular receptive fields
- All neurons were monocular (had a receptive field in one eye but not the other)
- Had receptive fields that comprised an excitatory area and an inhibitory area separated by a circular boundary
Simple cortical cells (v1)
- Simple cells have antagonistic on/ off regions
- borders are straight rather than circular
- respond best to bars of light in specific orientations at a specific location in the visual field
complex cells
more common
do not have static on off regions, they respond best to particular straight edge stimulus of a particular orientation regardless of its position within the receptive field
columnar organisation
flow from on/ off cells to simple cells to complex cells
All simple and complex cells in a column prefer straight line stimuli in the same orientation
Complex receptive field properties required for scene analysis.
horizontal electrode track
if you move an electrode along a horizontal track the spatial location of the receptive fields shift systematically
the preferred orientation of the neurons at the tip shift.
