Sample Q 2018a Flashcards
What are the major differences between parvocellular and magnocellular
divisions of the primate LGN?
Magnocellular Pathway: • Large receptive fields • High contrast sensitivity • Poor isoluminant response • Low spatial resolution • Good motion sensitivity (high temporal resolution) • Fast conduction velocity • X-like (linear) or Y-like (non-linear) • Transient response • Input from Parasol cells • Projects to layers 4Cα & lower 6 in area V1
Parvocellular Pathway: • Small receptive fields • Lower contrast sensitivity • Chromatic selectivity • High spatial resolution • Poor motion sensitivity • Slow conduction velocity • X-like (linear) • Sustained response • Inputs from Midget cells • Projects to layers 4Cβ, 4A & upper 6 in V1
Which cortical layer/s do the parvocellular cells project to?
- Mainly goes to layer 4Cβ
- Some to layer 6 and 4A
Which cortical layer/s do the magnocellular cells project to?
- Mainly goes to layer 4Cα
- Some to layer 6
Which cortical layer/s do the koniocellular cells project to?
- Mainly goes to supergranular layers 2 & 3 (within blob areas)
What is the defining feature of a simple cell in the primary visual cortex?
Simple cells:
- found in layer 4, 6
- responds to bars of light in a certain orientation
- still respond to simple stimuli, but not very well
- distinct ON and OFF subregions
- small receptive field
- linear spatial summation (when you have a spot of light, the central surround gets added up and you don’t get a response)
What is the defining feature of a complex cell in the primary visual cortex?
Complex cell:
- found outside layer 4
- receive input from multiple simple cells of the same orientation
- overlapping ON and OFF subregions
- large receptive field
- non-linear properties (also won’t respond to diffuse light, adding up central surround will get a response in some situations)
What is the defining feature of a hypercomplex cell in the primary visual
cortex?
Hypercomplex cell:
- respond well for short bars, but have end inhibiting areas
- poor responses for bars that aren’t the optical length
- receive input from multiple complex cells
What is the basic model that Hubel and Wiesel proposed to explain cortical
simple cells’ orientation selectivity?
Simple cells get orientation selectivity:
- convergent excitatory connections from a number of lateral geniculate cells
- circular receptive fields are arranged in a row in visual space
- when there’s a long bar aligned across the cells, stimulating all the receptive fields, then it will stimulate all the geniculate cells at the same time= simple cells get vigorous input
- if the light bar had an orientation perpendicular to the orientation that it moves across, this will only simulate 1 or 2 cells at a time, which isn’t sufficient for the simple cells to fire
What are some of the criticisms of their model (Hubel and Wiesel orientation selectivity model)?
ORIENTATION SELECTIVITY- may also arise before the level of the striate cortex
- removal of inhibition on a striate cell abolished the orientation selectivity, this wouldn’t happen if it was a result of excitatory convergence
HIERARCHY SCHEME- only partially true
- complex cells can respond to stimuli that simple cells don’t respond to
- both complex and hypercomplex cells can be directly excited from the LGN without going through simple cells
What are orientation and ocular dominance columns?
ORIENTATION COLUMNS:
- cells with similar orientation selectivity were arranged perpendicular to the surface of the brain
- cells measured obliquely across the cortex tend to differ more
OCULAR DOMINANCE COLUMNS:
- cells arranged into alternating left/ right eye input dominant columns perpendicular to the surface
What are some of the methods used to demonstrate them (orientation and ocular dominance columns)?
RADIOACTIVE AMINO ACIDS (ocular dominance columns):
- inject into the eye, where they travel to the cortex
- image the ocular dominance columns in V1 (layer 4) and the lateral geniculate nucleus (LGN) a week after injection
2-DEOXYGLYCOSE (2-DG) (orientation columns):
- glucose analog that is taken up into active regions of the visual cortex, but isn’t metabolised
- images of orientation columns in the visual cortex are found in infragranular layers 5 & 6
CYTOCHROME OXIDASE (orientation columns):
- mitochondrial enzyme that correlates with cellular activity
- cytochrome oxidase staining reveals ‘blobs’ in V1 and stripes (both thick and thin) in V2
- blobs are typically at the centres of ocular dominance columns, rarely at boarders
What kind of visual stimulation would you do to show up ocular dominance
columns with optical imaging?
- Radioactive amino acid
What kind of visual stimulation would you do to show up orientation columns
with optical imaging?
- 2- deoxyglucose (2-DG)
- Cytochrome oxidase
What are pin-wheel centres in optical imaging maps?
- point where all the different orientation domains converge
- tend to fall in the centre of ocular dominance columns where the iso-orientation lines (lines of cells with the same orientation selectivity) converge
What are the functions of the two major cortical streams?
DORSAL STREAM- where
- gated by area MT
- informs goal directed actions by outputting to the posterior parietal cortex
VENTRAL STREAM- what
- gated by area V4
- conscious visual perception that we use to identify objects by outputting to the inferior-temporal cortex (area IT)
Which cortical regions do the two streams project to?
DORSAL STREAM:
Magnocellular > 4Cα in V1 > thick stripes in V2 > V3 > MT > parietal
VENTRAL STREAM:
Parvocellular > 4Cβ, 4A in V1 > thin stripes in V2 > V4 > inferotemporal
Which cortical region is specialised for detection of motion?
AREA MT
- dorsal stream
- posterior parietal cortex
- WHERE=paRietal
What are the areas involved in processing objects?
AREA V4
- ventral stream
- inferior-temporal cortex (Area IT)
- WHAT= Temporal
What is visual agnosia?
- can’t identify objects
- patients with lesion in the temporal lobe (ventral stream)
- will be able to identify objects using their other senses eg. touch
What is prosopagnosia?
- face blindness
- damage to the fusiform face area
- impaired ability to recognise faces
- cognital, which goes unnoticed because of compensatory mechanisms (eg. voice,) or acquired
Where in the brain do the signals from the two eyes come together for the first
time?
V1- convergence between parallel pathways in the striate cortex and information from both eyes
What is the neurophysiological basis for stereopsis?
STEREOPSIS- the perception of depth
- visual info deriving from 2 eyes by individuals with normally developed binocular vision
STEREOSCOPIC VISION
- depth perception using binocular cues
- if the eyes are strabismic (have abnormal alignment) during the critical period, then binocular cells will not develop and this will result in a lack of stereoscopic vision in adulthood
What are the different cues for depth, besides stereopsis?
Binocular cues to depth perception:
- animals with front facing eyes like primates have 2 visual fields that overlap significantly
- when we are fixing on an object, any light on the arc corresponding with the object’s depth will fall on equivalent points in each eye, called the horopter
- RETINAL DISPARITY- light originating further away from, or closer to the horopter will fall on non-equivalent points in each retina
Monocular cues for depth perception:
- RELATIVE SIZE OF OBJECTS- typically decrease when they are further away. Size contrast is also used ie. big things will seem bigger when surrounded by smaller things
- LINEAR PERSPECTIVE gives us the sense that parallel lines getting smaller are moving away from us
- OCCLUSION- the object that occludes another is always the one that is closer to us
- TEXTURE tends to be more detailed and larger when it’s closer to us
- SHADING & SHADOWS gives us information about how light is hitting an object eg. the further away the shadow is from an object, the higher that object is from the ground, etc
- ATMOSPHERIC/ AERIAL PERSPECTIVE CUES- light coming from distant objects will have more attenuated colours ie. when you’re on top of a mountain, the grass closer to you will be greener than the pale grass below
What type of motion signal processing is the most complex? – motion
detection, speed discrimination, optic flow, etc.
- SIMPLE SMOOTH MOTION- an object moves across our visual field eg. ball thrown
- APPARENT MOTION- when the light falling on the retina gives the impression that something is moving across our visual field when nothing is actually moving eg. sequence of lights in a neon sign
- OPTIC FLOW- movement of the world across our retina as we move through it
- COMPLEX MOTION- various combinations of the above movement types eg. circular, radial motion
What is binocular rivalry?
- when different images are presented to equivalent locations in each eye then these stimuli will compete for perception
- this results in us seeing alternating moments of what each eye sees but never a combination of both (either one or the other)
- experiments show that both images are encoded in the visual cortex but something (not yet understood) is choosing which one reaches perception
What are cytochrome oxidase blobs?
CYTOCHROME OXIDASE
- a mitochondrial enzyme that correlates with cellular activity
- V1 blobs
- V2 stripes
- Blobs- located within the centres of ocular dominance columns, rarely at boarders
What is blindsight?
- phenomenon of subconscious abilities of sight in individuals with blindness arising from a non-functioning primary visual cortex
- capable of guessing where an object is more better than chance
- achieved by direct dorsal stream projections from the retina to the posterior parietal cortex (via the superior colliculus and pulvinar)
- the dorsal stream also gets subconscious input straight fro the retina and koniocellular cells in the LGN
What is visual neglect? Where is the site of brain damage in neglect?
- neuropsychological disorder that prevents patients from being aware of stimuli on one side of their visual field
- posterior parietal lesion
- mot frequently associated with lesions in the right hemisphere
How do serial and parallel searches differ?
PARALLEL SEARCHES:
- pops-out
- has unique features that are easily seen (eg. orientation and colour)
SERIAL SEARCHES:
- has a combination of features (eg. red and horizontal)
- requires time to find as there is no unique features
- reaction time rises linearly with an increase in the number of items in the visual field
What is inattentional blindness?
- focusing on one aspect of a visual scene and this results in us not perceiving another part of the scene
(eg. walking bear experiment)
What is attentional blink?
- the second of 2 targets cannot be identified when it appears close in time to the first
- momentary blindness of attention revealed when the subject tries to spot 2 numbers within 500ms of one another, it can only attend to one of them, not both
What do you understand by “binding problem” in perception? What cortical
area is critical to help binding?
BINDING PROBLEM:
- combining all the features of an object, which are distributed across the cortex in one integrated image
- the parietal cortex and our attention spotlight is vital, since it chooses the visual field location that will be focused on for further processing
What is possibly the role of synchronised neuronal oscillations?
- a stimulus shown at a particular location would want to be more attentive in that location for the same stimulus again (increase attention to that area)
- so the stimulus of the same orientations and features would be detected much quicker and easier
- synchronised neuronal oscillations between the mid-temporal area (MT) and lateral-intraparietal area (LIP)
What was Hebb’s idea regarding memory and learning?
HEBBIAN MODIFICATION:
- the strengthening/ weakening of synapses based on their activity
LONG TERM POTENTIATION (LTP)- strengthened
- synapse with high pre and post synaptic activity
- requires the firing of multiple synapses at once
LONG TERM DEPRESSION (LTD)- weakening
- synapse with poor or no presynaptic activity
- if the postsynaptic neuron fires before the input from one of the synapses
What is the most recent neural explanation for the “phantom limb” sensation some amputees get?
- following the amputation of limbs, the cortex that loses its afferent input gets input now from adjacent cortical areas
What is ocular dominance shift?
MONOCULAR DEPRIVATION
- involves covering one of the eyes
- some diffuse light reaches the retina, but not detailed like formed vision
OCULAR DOMINANCE SHIFT follows monocular deprivation
- the deprived eye won’t be able to drive any of the cells in the visual cortex
- the non-deprived eye has taken over most of the cortical space
What do you understand about critical period in development?
- period where there needs to be input or proper development will not occur
- input during this time helps to fine tune and calibrate cortical cells, and adapt them to the environment
What are the different types of amblyopia?
OCCLUSION AMBLYOPIA
- when one eye is occluded for some reason during the critical period
eg. occluded by a cataract, lid ptosis (drooping)
ANISOMETROPIC AMBLYOPIA
- when 2 eyes have different refractive errors
STRABISMIC AMBLYOPIA
- when one eye is deviated (eye turn, squint) and this affects its development
What is the basis of stereoscopic vision?
STEREOSCOPIC VISION
- depth perception using binocular cues
STRABISMUS
- if the eyes are strabismus during the critical period, binocular cells won’t develop and this will result in a lack of stereoscopic vision in adulthood (need to fix before 3yo)
What is saccadic suppression? What neural correlate do you find in area MST?
SACCADIC SUPPRESSION
- mechanism of how our brain removes the fast motion of saccades from our perception
- when we do a saccade, our contrast sensitivity dips quite significantly, both slightly before the saccade and during and after the saccade
NEURAL CORRELATION IN AREA MST
- saccades interact with the dorsal pathway- ‘where’ things are
- when a flash is presented, neurons in the medial superior temporal (MST- global motion processing brain region) will respond
- if he flash occurs 65ms before a saccade, then the neurons won’t respond (saccadic supression)
How do MST neurones respond to visual stimuli just after you make a
saccade?
POST SACCADIC ENHANCEMENT:
- when a flash is shown 85ms after the saccade, the MST neurons respond more strongly than normal, have more spontaneous activity, and a lower latency (time between flash and response)
What are time compression and temporal inversion during saccades?
TIME COMPRESSION DURING SACCADES:
- on average, we tend to lose 30-70ms of perceived time every time we make a saccade
- results in us perceiving events in close proximity to the saccade as shorter
TEMPERAL INVERSION
- two stimuli occurring within close proximity of one another can be reversed in time
eg. perceive 2 stimuli within a saccade in the opposite order to the one they actually occur in
