Physiology of Vision Flashcards
What does the retina contain?
A sheet of light responsive photoreceptor cells
What does each cell of the retina have?
A distinct receptive field within the visual field
What does light coming from the receptive field generate?
An electrical response in photoreceptor cell = phototransduction
Where is the electrical response to light transmitted to?
Across the retina and visual structures terminating in the cortex
Where does perception of light occur?
The cortex
What is the direct pathway for signal transmission in the retina?
Photoreceptors - bipolar cells - ganglion cells
What are the lateral connections of the retina which influence signal processing?
Horizontal and amacrine cells
What is the role of horizontal cells?
Receive input from photoreceptors and project to other photoreceptors and bipolar cells
What is the role of amacrine cells?
Receive input from bipolar cells and project to ganglion cells, bipolar cells and other amacrine cells
What is the dark current the basis of?
Phototransduction
What happens with the dark current in the dark?
PNa = PK so Vm is therefore between ENa and EK
What happens with the dark current in the light?
PNa is reduced so Vm equals EK and hyperpolarises
What kind of change does the dark current cause?
Local and graded
What does light convert 11-cis-retinal to?
All-trans-retinal (activated form)
What effect does the greater convergence in the rod system have?
Increases sensitivity while decreasing acuity
What kind of light do rods see in?
Dim light
What kind of light do cones see in?
Normal daylight
What is centre surround organisation also known as?
Lateral inhibition
What is the purpose of centre surround organisation?
Emphasises area of contrast = sharpens boundary between objects of different luminance
How is light intensity detected in the visual system?
Detects local differences in light intensity, not the absolute amounts of light
What type of gluR is involved in the on and off pathways?
On = inotropic gluR Off = metabotropic gluR
What effect does lateral inhibition have on ganglion cells?
Modifies the receptive fields of ganglion cells to have centre surround organisation
What cells are primarily involved in centre surround organisation?
Horizontal and amacrine cells
What does centre surround organisation help with?
Aids with localisation = exaggerates difference in stimulus intensity detected by adjacent neurons
How do horizontal cells interact with groups of surround neurons?
Interconnect and sample the total amount of excitation in the surround and responds by releasing GABA
What happens if there is low surround detected by horizontal cells?
Less GABA is released = response of centre on bipolar cell is thus higher
How is retinal output processed?
Parallel processing = simultaneous input from two eyes, info compared in cortex
Where does info about light and dark come from?
On centre and off centre ganglion cells
What are the three different basic types of retinal ganglion cells?
M cells = movement
P cells = form and colour
Nonm-nonP cells = colour
How is a binocular visual field created?
Each eye sees a part of the visual field (monocular visual field) but their visual fields overlap extensively
Where do nerve fibres from the nasal half of each retina cross over?
The optic chiasm
What do the two optic tracts allow?
Right and left visual fields to reach the left and right hemispheres separately
What percentage of nerve fibres cross over?
60% cross over = nasal retina
40% don’t cross over = temporal retina
Where is the entire visual field mapped?
Precisely mapped on to the primary visual cortex = has visuotopic organisation
Is the scaling of the visual field on to the cortex constant?
No = about half of the cortex is dedicated to info from the relatively small fovea
What does the visuotopic organisation of the primary visual cortex allow?
Mapping of lesions in the visual system
Where do inputs to the striate cortex come from?
Magnocellular LGN neurons = layer IVCa
Parvocellular LGN neurons = layer IVCbeta
Konicellular LGN axons = bypass layer IV to synapse in layers II and III
Where are binocular neurons found in the striate cortex?
Mostlyy layer IV
What receptive fields are located in the striate cortex?
Cortical receptive fields
How do the cortical receptive fields display orientation selectivity?
Respond to light/dark bars or edges only if in a particular orientation = peak frequency depends on angle
Where are ocular dominance column located?
Present in VI
Why is ocular dominance important?
Connections from the two eyes compete with each other
Why are retinal synapses in the LGN unaffected by deprivation?
They are monocular
What can monocular deprivation in the cortex during critical stages in development lead to?
Active afferents from one eye and lower activity from the other eye
