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
Receptive fields refers to... (x2) For example (x1)
Regions on the sensory organ (retina in vision),
And the features that excite/inhibit the cell
Ganglion cells get input from multiple retinal receptors = concentric circles of inhib/excite regions
Describe centre/surround (concentric) receptive fields (x3) Enabling the (x1)
Cluster of retinal receptors link to particular ganglion/LGN/V1 cell
Cluster of rods and cones connect to particular cell,
Depending on where light strikes will cause vigorous firing or not
Most basic stage of image analysis
On-centre cells of the centre/surround (concentric receptive fields act by…(x2)
Increase firing when a light is shone in the central region,
Decrease in firing when a light is shone on on the surrounding region
Off-centre cells of the centre/surround (concentric receptive fields act by…(x2)
Decrease firing when a light is shone in the central region,
Increas in firing when a light is shone on on the surrounding region
On-centre cells of centre/surround (concentric) receptive fields are sensitive to contrast in that… (x3)
Complete illumination of the centre maximises firing
Complete illumination of the surround minimises firing
Diffuse illumination of entire field also minimises firing
Simple cells of the visual processing system receive signals from… (x1)
Enabling… (x1)
An aggregate of ganglion cells
Second layer of visual image analysis
Simple cells of the visual system respond most vigorously to… (x2)
Static bars of light - to ‘on’ centre of receptive field
Contours of particular orientation that match longer dimension of oblong shape
Five variations of simple cells of the visual processing system are…
Central third as on region: responds to tilted/horizontal/vertical light bar against dark background
Central third as off region: responds to tilted/horizontal/vertical dark bar against light background
Off-set off region (thick and thin on region either side): responds to horizontal dark bar against light background
Two-thirds off region above 1/3 on region: responds to straight border between upper dark region and light lower region
50/50 on/off, tilted: responds to tilted left dark to light right edge (for left off, right on regions)
Complex cells of the visual processing system are…(x1)
Giving… (x1)
An aggregate of simple cells
A larger and more complex recpetive field
The different types of simple cells respond to… (x1)
And combine to give… (x1)
Different orientations, patterns, sizes (spatial scale)
Many kinds of simple cell receptive fields
Spatial code in simple cells of the visual system is analogous to… (x1)
Sound frequency waves - undulations from dark to lights occur more or less often over same unit of space
Low spatial frequency in simple cells of the visual system is… (x1)
And activates… (x1)
Long ‘waves’ - few changes over given unit of space
Simple cells with widely separated subfields (oblong shape at right angle to light bar)
High spatial frequency in simple cells of the visual system is… (x1)
And activates… (x1)
Short ‘waves’ - more changes over given unit of space
Simple cells with less separated subfields (oblong shape parallel to light bar)
Complex cells of the visual processing system are…(x1)
Found in… (x1)
Giving… (x1)
An aggregate of simple cells that are offset
V1
A larger and more complex recepetive field
Third layer of visual processing
The relationship of simple cells is important for the perception of scale because.. (x2)
Need a larger field to respond to low than to high spatial frequencies
The wrong size simple field would perceive diffuse light
Fourier analysis is…(x1)
And is useful in visual processing to show that… (x1)
The breaking down of sinusoidal wave functions into simpler ones
Adding all the sin waves = square wave - the light dark edge
Spatial scale and simple cells of the visual system:
High spatial frequency gives info about… (x1 plus eg), while
Low tells us about… (x1 plus eg)
Edges - add frequencies to sharpen edges in photoshop
Texture - remove frequencies to blur image
Three levels of visual processing
Centre/surround (concentric) receptive fields
Simple cells
Complex cells
Complex cells of the V1 are… (x2)
And will fire.. (x2)
Shaped similar to simple cells – elongated to give pref for particular orientation
If that contour falls anywhere in that field – doesn’t have regions
If that orientation of stimulus moves through the field
Complex cells of the visual system underlie stereoscopic vision, in that… (x4)
Cell will increase firing if inputs arrive from either left or right eye
More vigorous response if inputs arrive from both simultaneously.
Some cells favour/respond more vigorously to one eye over the other – occular dominance.
Some respond well if similar contours fall on nearly same positions in both eyes = a binocular disparity
Columnar organisation of V1 refers to the fact that… (x3)
Functionally similar cells are grouped in columns
Columns alternate in eye dominance
All those in the same have a preference for the same orientation, giving the full complement - hypercolumn
An electrode straight down into the layers of the visual cortex would find cells that… (x3)
While parallel to columns would find… (x3)
Respond to about same position on retina, as well as preferred orientation, and same eye
Cells for different locations, orientations, and preferred eye would switch
Scotoma is… (x2)
Blind spot from visual cortex damage, rather than eye damage
Often not noticed due to brain’s capacity for ‘filling in’
‘Filling in’ is… (x1)
As demonstrated by…(x2)
The neural process of completing blind spots in our visual field (optic chiasm or scotoma)
Low central red dot and high/left grey dot on background of ‘white noise’
Focus on red dot, grey dot disappears after a few seconds
Blindsight is…(x1)
Resulting in… (x5)
Patient’s primary visual cortex has been damaged and a large scotoma is present
= no conscious visual perception within scotoma,
No verbal reporting, but guided action is preserved, can
Judge orientation of lines they report they cannot see
Correctly reach for oriented objects
Correctly intercept objects moving through their scotoma
Two possible explanations for blindsight
Incomplete V1 damage = residual functionality, or
Visual pathways exist that reach secondary visual cortex without passing through V1 - may support visual abilities without conscious awareness
The dorsal visual stream runs… (x1)
And is fed by which pathway?
Governing…(x3)
From V1 tup into parietal lobe Magnocellular Locations Orientations of objects in space Motion
The ventral visual stream runs… (x1)
And is fed by which pathway?
Governing…(x3)
From V1 down into lower temporal lobe Parvocellular Colour Object identity Attributes of stationary objects
The dual dorsal and ventral visual streams were originally thought to comprise the… (x2)
Where (dorsal = visual spatial perception) and
What (ventral = visual pattern recognition) systems
The dual dorsal and ventral visual streams are now thought to comprise the… (x2)
Theory supported by (eg)
Visually guided behaviour (replacing the where system) vs conscious perception/ability to report (replacing the what)
Blindsight
Describe two patients that support modern dual-pathway understandings of visual processing (plus task for testing their abilities)
Ps with dorsal stream damage - issues with visually guided behaviour, could report colour etc
Tasked with posting letter through vertical or horizontal slot, performed at chance rate
Ps with ventral stream damage - manual interactions, but could not verbally report colour, orientation etc
Prosopagnosia is…(x1)
Resulting from… (x1)
Inability to recognise/ID faces (known people, own) but can distinguish from other objects
Damage to ventral stream - impeded conscious awareness but not control of behaviour
