Week 9 Parallel & Cortical Pathways Flashcards
Pathways vs. processes?
Pathways = dorsal vs. ventral
Processes = RG to V1, V1-V5
What is the superior colliculus and what is its role in primates?
- Receptive fields have ill-defined on and off regions - they respond to anything!
- Multi-sensory, they can receive auditory input, especially when it is corresponds to its visual receptive field
- Attending to peripheral vision, attention - from evolutionary perspective, you need to disengage and move eye movements reflexively
AND
4. It orients the visual system to the point of peripheral attention so that other systems can do more detailed processing
Where are m and p cell located and what do they receive input from?
- LGN - 6 distinct layers of cells
two layers for the M-cells, four layers for the P-cells - stay separated until V1 and beyond - Retinal ganglion cells fed into distinct M and P cells
What is contrast sensitivity and which cell is faster and steeper?
Luminance contrast sensitivity is how many activation spikes per second as a function of contrast in luminance
- M-cells are super sensitive to small increases in low contrast, but saturate with higher contrast
- P-cells are less sensitive to contrast sensitivity but give you more linear response to increases in contrast sensitivity
A. Why might M and P cells have the same discrimination ability?
B. Why don’t they fire more?
A. Because over time the slopes of the curves of contrast levels between M and P cells are the same - cannot distinguish contrast sensitivity at higher levels of contrast
B. If cells fire at higher rates, it’s a huge metabolic cost/not necessary!
How does the nature of the firing response differ in m and p cells?
P-cells = Sustained response
- maintained for the duration of the stimulus
- Initially goes up and decreases a little but maintains firing
-except in Troxler fading
M-cells = transient response
- Increase in firing response in the brief presence of a stimulus and goes back to baseline activity quickly
How does the transient firing response of m-cells help explain their high temporal sensitivity?
Only get the M-cell to respond when it comes on or goes off, not sustained in the middle - so more firing occurs as stimulus changes from point A to B in a visual scene across time, when an object is moving or flickering or changing in luminance
How does Bar’s transmission model relate to transmission speed in m-cells?
M-cells have a faster axon transmission speed to process large global features of a stimuli to V1 and prefrontal cortex quicker than P-cells
Processes information is global features and outline of stimulus and low-spatial frequencies: broad outline/course info of the stimulus
A. How could you design a stimuli to activate M or P cells?
B. Is this representative of everyday life processing stimuli?
A1. Low-spatial frequencies + rapid temporal frequency = activates M cells
A2. High-spatial frequencies + slow temporal frequency = activates P cells
B There is no simple segmentation in the functions of M and P cells, and the formation of visual percepts in everyday life require the activation of both
Does dyslexia a result of M or P system dysfunction?
While it may seem that dyslexia would be a result of p-system dysfunction, as p-cells are involved in distinguishing fine details like letters, dyslexia only impairs reading, not other visual tasks
Reading requires
a. constant saccadic movements
b. shifts in temporal attention
c. sensitive to changes in visual stimuli,
d. fine manipulation of attention which are skills enabled by the M-system function.
If the M-system was impaired, it would be difficult to move eyes to new location to process new words easily
How do we test M-cell impairment in dyslexia?
Remove the need for person to make a saccade by flashing words sequentially / moving sentences instead of reading a static sentence
Ifa dsylexic person can more easily read the flashing words in temporal order instead of a spatial order, it indicates that saccadic suppression and M-system involves at fault in dyslexia
How does saccadic suppression relate to m-cells and dyslexia?
Saccadic suppression is the brain’s ability to discriminate eye movement from moving images.
As M-cells are responsible for regulating saccadic suppression, an impairment in M-cells may cause the appearance of letters to look blurry in motion, even when the image is non-moving, which is a common perception of reading with dyslexia. This is why M-cell impairment is thought to be related to dyslexia.
What cells and pathways are involved in dorsal vs. ventral streams?
Ventral stream V1 (bottom of cortex)
Goes to inferotemporal cortex, FFA, PHG, to V4 (colour processing) + parvocellular cells
Dorsal stream
Goes to posterior-parietal cortex to V5/MT with many magnocellular cells
= motion and spatial localisation
What studies are needed for evidence in dorsal and ventral streams of visual processing?
- Lesion studies: Studying people with localised damage to V4/V5 to see patterns of impairment
- Clinical case studies
- Non-human primates = single cell recording
What type of neuroimaging and cognitive task could be used as evidence for dorsal and ventral demarkation streams?
1.Neuroimaging - fMRI and MEG
- Spatial vs.face-matching tasks to see differences in neural processing for object location vs object recognition
What tasks are used in lesion studies to show regions of dorsal and ventral streams in primates?
After microstimulation to brain area!
Object discrimination task: showing objects and new object, train to pick the new object (failure should imply damage to ventral pathway)
Landmark task: pick food near landmark (failure should imply damage to dorsal pathway)
Review of cells in visual processing roles
- Cones/rods = uniform luminance
- Retinal ganglion cells = changes in luminance dots of lights
- LGN = bars and edges/spatial and temporal acuity (M-cells can see topographic mapping)
- V1 = motion/texture/colour
What features needed to be processed already in initial stages before ventral/dorsal pathways?
- Topographic mapping
- Feedforward and feedback connection occurs within the cortex
- Population responses from cell pulling information across neighbouring cells to combine small receptive field sizes in V1 cells
- Integration of information is a transformative process in ventral and dorsal pathways
Motion is not 100% to fit neatly into dorsal or ventral streams. What in neuroimaging and behavioural studies to determine where motion processing is localised?
- Functional imaging with adequate spatial resolution or single cell recording to measure where motion processing is localised
- Behavioural methods
a) the motion-aftereffect/waterfall illusion appear to distort the appearance of stimuli - where the brief exposure of motion stimuli shown before a non-moving stimuli makes it appear moving and in the opposite direction - shows we have a motion adapting system
How does the tilt aftereffect show we have motion cells?
This behavioural test shows that there are cells turned to different directions of motion - and when motion cells become adapted to a particular orientation of motion, the connection between cells are tuned to opposite direction of motion are inhibited, causing an imbalance of neural response which appear to have an aftereffect
Using motion-after effect, how to determine whether motion cells are in the retina or cortex?
EYES!
To distinguish monocular (retinal systems) vs. binocular (V1 systems),
- we close one eye and look at a moving stimulus
- Switch eyes, close the open eye, and open the other eye that is not adapted to the moving stimulus.
- We get interocular transfer of moving after effect, which shows motion units are in the V1 cortex using binocular processing and not retinal level using monocular processing
