Lecture4 Flashcards
In what order does light enter the eye?
Passes the cornea, then pupil, then lens, then reaches the retina which converts light into neural signals & conducts them toward the CNS
Fovea subserves the finest:
Visual acuity
Where are light sensitive cells situated?
In the sensory membrane (retina)
What is the retina responsible for?
Transduction of light into neural signals
Explain the cross section of the retina
Retinal ganglion cells, amacrine, bipolar, horizontal cells, cone/rod receptors
What is the fovea a solution for?; How?
The inside out design of the retina; The layer of cell bodies & axons thin out around this area of the retina and reduce the distortion of incoming light
There’s high acuity in the central but not in the….
Peripheral
Light is sometimes referred to as waves of electromagnetic energy that are between…
380 & 760 nanometres in length
When illumination is high the pupils are…; so the image is….; When illumination is low the pupils…; which sacrifices….
Constricted; Sharper with depth of focus; Dilate to let in more light; Acuity & depth of focus
What are the amacrine cells & horizontal cells specialised for?
Lateral communication (communication across the major channels of sensory input
In what 2 ways do retinal neurons communicate?
Both chemically via synapses & electrically via gap junctions
What occurs at 17 degrees at the centre of the fovea?
there is a gap in the receptor layer called the blind spot; all axons from the ganglion cells leave the eye
What’s the difference between rods & cones?
Rods are achromatic, none are in the fovea, high convergence (many rods share a single ganglion cell), higher sensitivity to dim light & lower positional acuity; There are 3 classes of cones, they’re sensitive to colour, exclusive to the fovea, have low convergence (each cone in fovea gets a direct circuit back to visual cortex), less sensitive to dim light, high positional acuity, less ambiguity about location of stimulus
What’s the difference between Photopic vision and Scotopic vision?
Photopic is cone-mediated - predominates in good lighting, provides high acuity coloured perceptions; Scotopic is rod-mediated - predominates in dim light; lacks fine detail & colour
In the centre of the fovea at what point do the rods reach a maximum density?
20 degrees
Why do we measure size & eccentricity in degrees of visual angle?
Because the physical size of an object or the lateral separation of 2 objects does not tell us how big or separated they’ll be on the retina; all the information our brain has about the visual scene is contained in the retinal image
Your finger at about half an arms length is…; A 1cm wide object viewed at a distance of 57cm will subtend…
1 degree wide on your retina; 1 degree of visual angle
What is visual transduction?; How does this occur?
The conversion of light into neural signals; Photoreceptors contain a pigment (rhodopsin) that responds electrochemically to stimulation by light
Explain the process of visual transduction in darkness
Rhodopsin molecules are inactive, sodium channels are kept open so sodium ions flow into the rods partially depolarising them (inhibitory signals flow freely from receptor) & rods continuously release glutamate;
Explain the process of visual transduction in light
Light bleaches rhodopsin molecules, as a result sodium channels close (do not respond to neurotransmitters), sodium ions cannot enter rods so receptor cell hyperpolarises & reduces the release of glutamate, inhibition is reduced so the bipolar cell increases firing
Hyperpolarising is…; Depolarising is…
More negative - reduces neurotransmitter release; More positive - increases neurotransmitter release
With an increase of light absorption, photoreceptors…; ON Bipolar cells…; OFF Bipolar cells…
Hyperpolarise; Depolarise; Hyperpolarise
With a decrease of light absorption, photoreceptors…; ON Bipolar cells…; OFF Bipolar cells…
Depolarise; Hyperpolarise; Depolarise
What does the Mach bands demonstration show?
Lateral inhibition; Our perception of scalloped edges between borders of brightness between colours
What do Ommatidia receptors do?; How are they connected?
Absorb light; By a lateral neural network
Explain the horseshoe crab theory in regards to the middle ommatidia receptors labelled D & E
Adjacent signals are inhibitory; D is intense light, E is dim; E receives a weak inhibitory signal from F and a strong inhibitory signal from D so D’s firing is accentuated
Describe the process of transduction from the visual field of the left eye
Projection on left retina; crosses to right hemiretina, half to nasal, half to temporal side; axons travel down the optic nerve; at the optic chiasm, temporal side axons travel ipsilaterally & nasal side axons decussate to contralateral side; on each side they pass through optic tracts, lateral geniculate nuclei to primary visual cortex
How do signals from the left visual field reach the right primary visual cortex?
Either ipsilaterally from the temporal hemiretina of the right eye or contralaterally (via the optic chiasm) from the nasal hemiretina of the left eye
What proportion of the primary visual cortex is dedicated to the analysis of the fovea’s input?
about 25% (2 degrees of entire visual field)
Explain retinotopic organisation
Each level of the system is organised like a map of the retina; two stimuli presented to adjacent areas of the retina excite adjacent neurons at all levels of the system
How many layers is the Lateral Geniculate Nucleus composed of?
Six
What are P channels?; What size cell bodies do they have?; Do cones or rods provide the majority of input?
Parvocellular neurons: top 4 layers of the lateral geniculate nucleus; they have small receptive fields, are particularly responsive to colour, fine pattern details & to stationary objects; Small; Cones
What are M channels?; What size cell bodies do they have?; Do cones or rods provide the majority of input?
Magnocellular neurons: bottom 2 layers of the LGN; they have large receptive fields, are achromatic & responsive to motion; Large; Rods
From the LGN, the M & P channels project to different sites in the lower part of….
Layer 4 in the Primary Visual Cortex
What are 4 common features of the 3 stages of the retina-geniculate-striate system (i.e. from retina to LGN to Primary Visual Cortex)?
Fovea receptive fields are smaller than the periphery; all receptive fields are circular; all neurons are monocular (i.e. activated by 1 eye only); concentric excitatory/inhibitory regions
What do Receptive Fields refer to?
Regions on the sensory organ (i.e. the retina in vision) & the features that excite or inhibit the cell. The nature of the receptive field of a cell gives clues about the cell’s function
Receptors converge onto a particular ganglion cell or LGN. What are the responses of an On-centre cell?
There’s an ON response when a spot of light is shone anywhere in the centre of the field (complete illumination maximises firing) & an OFF response when shone in the periphery field (illumination of the surround minimises firing)
What are the responses of an Off-centre cell?
There’s an OFF response when a spot of light is shone anywhere in the centre of the field & an ON response when shone in the periphery field
What is the most effective way of maximising the firing of an on-centre or off-centre cell?; What does diffuse illumination of the entire receptive field do?
To completely illuminate either the ON area or the OFF area of its receptive field; Minimise firing (if both areas are illuminated together there is little reaction from the cell
What are the main differences between Simple Cells & lower layer 4 neurons?;
The borders between the ON & OFF regions of the cortical receptive fields are straight lines/rectangular rather than circles & they respond best to bars of light in a dark field or single straight edges between light & dark areas
When does a simple cell respond maximally or most vigorously?
When static bars with an appropriate orientation falls onto the ON subfield of the receptive field; they’re selectively responsive to specific patterns
What do Simple VI cells code?; Low spatial frequency activates simple cells with…; High spatial frequency activates simple cells with…
Spatial Scale; Widely separated subfields; Less separated subfields
Gradual undulations of simple cells are like…; The more undulations the….
Sine waves for the eye (number of times undulated from dark to light per unit of space); Higher the Spatial Frequency
What does signalling changes from light to dark at a different spatial scale allow simple cells to do?; If we take out high spatial frequency what do we get?
Code information about different features: edges & texture; A blurred image - borders are less defined & texture is more prominent
Where do complex cells reside?; When will a complex cell fire?
VI, layers 1-3 & 5-6; If it gets any input from its contributing simple cells
Many complex cells receive inputs from both eyes, what is this called?; If inputs arrive from either the left or right eye what will happen?; If inputs arrive from both eyes simultaneously what happens?; What is occular dominance?
Binocular; The cell will increase firing; There’s a more vigorous response; Some cells favour one eye over the other & respond more vigorously to one eye
How do some complex cells respond if similar contours fall on nearly the same positions in the 2 eyes?; What is this called?; Therefore what do complex cells underlie?
Well; Binocular disparity; Stereotopic depth perception
Explain the columnar organisation of the primary visual cortex
Signals flow from simple to complex; functionally similar cells are grouped in VI; columns alternate in eye dominance
In columnar organisation a block of tissue is assumed to analyse visual signals from one area to the visual field where half is dominated by…; & half by…; What is each slice of the block of tissue presumed to specialise in?
Right eye input; Left eye input; The analysis of straight lines in a particular orientation
When proceeding vertically within cortical columns of VI, all neurons in a column have receptive fields in the…; What do all the simple & complex neurons in a column prefer?; In a given column, all monocular & binocular neurons are dominated by…
Same general area of the visual field; Straight line stimuli in the same orientation; The same eye
When neurons proceed across a horizontal electrode track of VI cortical columns, the position of the receptive fields of the neurons…; & the preferred orientation of the neurons at the tip…; As the electrode advances, how does the tip move?
Shifts systematically at the tip; Also shifts systematically; Alternately through columns of right & left-eye dominance
What does the Howard Dolman test teach us about comparisons between Monocular & Binocular view when looking at targets near & far?
It’s hard to tell the difference through one eye (monocular); with binocular we can compare horizontal differences & see the difference in separation; adjustments with 2 eyes are very precise compared to monocular vision
Significant cortical mechanisms of vision & conscious awareness occur in ventral & dorsal streams of…
Inferotemporal Cortex & Prestriate Cortex
What occurs if you have a left VI cortical scotoma?
You’ll have a blind spot in the right side of visual space
What is Blind Sight?; Although a patient will report no conscious visual perception within the scotoma, how do they perform on visual tasks when tested?
A condition where the primary visual cortex is damaged & a large scotoma is present (usually covers half of visual field); Above chance on the following: judging orientation of lines they report they can’t see, correctly reaching for oriented objects, & correctly intercepting objects moving through their scotoma
Name two possible explanations for visual task performance despite Blindsight
Damage to VI is not complete & some residual functionality remains; visual pathways exist that ascend to secondary visual cortex without passing through VI. These pathways may support visual abilities without conscious awareness
Explain the “where vs. what” theory; What does this theory predict?
Dorsal stream specialises in visual spatial perception & Ventral stream specialises in visual pattern recognition; That damage in either area will disrupt the corresponding ability
Explain the “control of behaviour vs. conscious perception” theory; What does this theory predict?
Dorsal stream specialises in visually guided behaviour & Ventral stream specialises in conscious visual perception; That damage in either area will disrupt the corresponding ability without affecting the other ability
What deficit tests the Visual Pathway theory?; What is it?; They can distinguish between objects & faces but not…; What does this suggest?
Prosopagnosia; Where a patient can’t recognise faces (sometimes even their own); Between faces; There appears to be neural structures specific to faces
What does Prosopagnosia result from?; Therefore theoretically what should be impeded in this condition?; What evidence supported this theory in an experiment recognising familiar & unfamiliar faces?
Damage to the Ventral stream; Conscious awareness but not control of behaviour; None of the faces were recognised but familiar faces yielded heightened galvanic skin responses (GSR is subserved by the dorsal stream, ie. control of behaviour)
