Wk 6 - Vision Flashcards
Name and describe anatomical components of the eye (x7)
Cornea - first point of contact, first refraction of light
Lens - gets thinner, thicker in order to focus - ageing thickens/freezes it
Vitreal chamber - fluid filled chamber
Receptors - across whole retina on sensory membrane
Optic disc is blind spot – where all axons of ganglion cells in retina exit the eye
Optic nerve goes back to occipital cortex
Fovea – very small relative to rest of retina…
In reference to the eye, distal and proximal mean (x2)
Distal – means outside the eye – an object.
Proximal – means image on the retina.
The six cell types of the retina, from front to back of eyeball
Retinal ganglion cells Amacrine cells Bipolar cells Horizontal cells Cone and rod receptors
The fovea is the solution to the eyes… (x1)
Achieved by… (x2)
Inside out design - cell layers all in front of photoreceptors
Cell bodies thinning out
= direct path of light for that 2% portion of retina
Rods, describe (x5)
Far more numerous than cones None in fovea High convergence – hundreds of rods to single ganglion cell; so Has poor spatial certainty, but Is sensitive to dim light
Cones, describe (x5)
Do colour vision, three types – for different colours
High concentration in fovea
Direct one to one wiring to neural unit in visual cortex
= very high positional accuity, but
Takes strong light to activate it (less sensitive to dim light)
Antsis 1974 demonstrated the high accuity of the fovea in study involving…
Ps focus on the dot, kept sliding letters in from the left or right until they could be identified
Smaller ones had be closer to focal point to be reliably identified
Satisfy the viewing conditions of study = all letters in the diagram (that get increasingly larger in concentric rings) equally identifiable
An example of the use of degrees to measure retinal image size (x2)
Your finger at about half an arm’s length is 1 degree wide on your retina.
Precisely: a 1cm wide object viewed at a distance of 57cm will subtend 1 degree of visual angle
Visual transduction is… (x1)
The change of physical energy into neural signals
Mechanisms of visual transduction (x4)
Photo pigment in receptors (rhodopsin) respond to stimulation by light (not to NTs)
In the dark, it’s slightly dedpolarised = steadily releasing glutamate (inhibitory), = reduction in neural impulses
When light absorbed, rhodopsin hyperpolarises receptor = reduction of inhibitory NT = increased firing
So shining light = more firing due to reduction of inhibitory action
Amacrine and horizontal cells of the retina have… (x1)
Facilitating (x3)
Lateral connections
Horizontal communication across many receptors,
Generally inhibitory connections = beneficial for accenting edges of objects
Lateral inhibition amacrine and horizontal cells of the retina is demonstrated by… (x1)
Which occurs because (x4)
The Mach Bands illusion
The cell on the edge of intense lights gets an inhibitory signal from bright side, but less so from dark side
= more vigorous firing than others in the bright section
Cell on edge of dimmer side gets opposite
Combine to accentuate the edge
Omatidia are…(x1)
And are useful models for explaining…(x1)
Receptors in the coarse mosaic in horseshoe crab retinas
Lateral inhibition
The visual neural pathway runs…(x4)
From retina
Through optic chiasm - hemidecuscation occurs
To lateral geniculate nuclei - organisation rather than processing
Finishing in primary visual cortex
Two points to remember about visual cortex organisation
Adjacent regions on retina connect to adjacent regions in cortex - retinotopic organsiation
Fovea is over-represented - 2% of retina, but 25% of cortex
The cortical magnification factor is… (x2)
The overrepresentation of the fovea in cortex,
That means neural representation of retina favours fovea
Retinotopic organisation refers to…
That of the ganglion cell - lateral geniculate nucleus - V1,
= adjacent regions on retina stimulated, adjacent nerves in LGN and V1 are too
The six layers of the lateral geniculate cortex use which two neural pathways?
Parvocellular
Magnocellular
Parvocellular neurons comprise the (x1)
Describe shape and functions (x5)
Top four layers of the lateral geniculate nucleus
Small cell bodies (think cones)
Small receptive fields
Do detail, colour and stationary objects
Magnocellular neurons comprise the (x1)
Describe shape and function
Bottom 2 layers of the lateral geniculate nucleus Large cell bodies (think rods) Large receptive fields Achromatic Larger details, b/w and moving objects
The retinal geniculate striate pathway runs… (x3)
With four features common to the three stages…
Retina - LGN - V1 Fovea receptive fields smaller than the periphery All receptive fields are circular All neurons are monocular Concentric excitatory/inhibitory regions
