Lecture 28 Flashcards
Compare and contrast foveal and extrafoveal vision in terms of PR:GC ratio, sensitivity and acuity
in the fovea, there is a ~1.1 ratio between photoreceptors and retinal ganglion cells. This creates small retinal fields with high resolution and low sensitivity. Given that there are a lot of cones in the fovea, it’s good for detecting colour and edges (high acuity)
In extrafoveal areas, retinal ganglion cells are connected to a high number of photoreceptors (~15-45). As there are more rods in this area, it has lower visual acuity and are more specialized for motion detection and background processing
Describe the two types of ganglion cells and their differences
p cells:
- exhibit cone opponency
- small retinal fields
- sustained firing
- colour detection
- form and fine detail detection
- project to the parvocellular layers of the lateral geniculate nucleus of the thalamus
m cells:
- larger retinal fields
- sensitive to luminance, contrast, and movement
project to magnocellular layers of LGN
Describe the pathway visual information pertaining to movement takes from
the retina to the visual cortex and beyond
Scotopic vision
Perceived by rods which hyperpolarize, increasing firing rate of m ganglion cells which project to the magnocellular layers (layers 1 and 2) of the LGN of the thalamus that then project to V1 via the optic radiation pathway. specifically to layer 4Calpha.
From 4Ca, will project to extrastriate regions via the dorsal stream (V1 –>V2 –> V5/6), including the posterior parietal cortex and then to the prefrontal cortex
Describe the pathway visual information pertaining to colour takes from
the retina to the visual cortex and beyond
Phototopic vision
Perceived by cones which hyperpolarize, increasing the firing of p-cells, which project to the parvocellular layers (layers 3-6) of the lateral geniculate nucelus of the thalamus which then project to primary visual cortex (V1) via the optic radiation pathway. Specifically on layer 4Cbeta.
From 4CB, will
project to extrastriate regions via the ventral stream (V1 –> V2–>V4), including the
inferior temporal cortex and then to the prefrontal cortex.
If an image falls on the right monocular segment, which photoreceptors will fire? Describe the path of travel for information
Perceived by photoreceptors in the nasal retina of the right eye. axons leave via the optic disk and enter the optic nerve. axons from the nasal ganglion cells cross at the optic chiasm and the object image will be represented in the contralateral hemisphere
eventually, information will be conveyed to the ipsilateral hemisphere via the corpus collosum
Fill in the blank: If an image falls on the binocular segment of retinas, left half of target is imaged on the ____ ______ retina and _____ _______ retina and vice versa
left nasal
rigt temporal
define: stereopsis
the perception of depth produced by the reception in the brain of visual stimuli from both eyes in combination; binocular vision.
Objects seen with only one eye are perceived in only two dimensions
If an image falls on the temporal retina, explain the path of travel of information
axons leave via the optic disk and enter the optic nerve. axons from the temporal ganglion cells do not cross at the optic chiasm and the object image will be represented in the ipsilateral hemisphere
Explain what is meant by the terms “ocular columns”
and “orientation specificity”
Ocular dominance columns are alternating stripes of neurons in V1.
Each stripe responds preferentially to input from one eye or the other.
The columns from the left and right eyes alternate across the cortex.
These columns help the brain integrate those two inputs into a single, three-dimensional perception. Support binocular vision
orientation specificity: neurons fire in response to stimulus positioned in a particular way (ex: horizontal bar of light)
Explain why some cortical neurons respond best to a
‘bar’ or ‘stripe’ of light presented in a given orientation
Projections from LGN synapse onto layer 4 of V1 where input converges onto simple cells
Simple cells have orientation selectivity, and thus respond best to bars of light presented in certain orientations.
What are layers 5 and 6 for in V1?
output layers
Cortical representation in V1 is flipped left/right and up/down
What does this mean for lesions to V1?
A lesion in the right V1 will cause visual loss in the left visual field of both eyes.
A lesion in the upper part of V1 will cause visual loss in the lower visual field, and vice versa.
Describe complex cell receptive fields
Where can you find complex cells?
Complex cells receive input from multiple simple cells that may or may not have the same orientation specificity
Complex cells exhibit directional specificity (i.e. sensitive to an object or band of light moving in one direction but not the other).
complex cells are in V1,V2,V3
orientation columns
contain neurons with similar
orientation selectivity
Orientations that are in similar angles are next to each other (ice cube model)
Differentiate the responsibilities and location of the dorsal vs ventral stream
Dorsal (where pathway)
- object location representation
- motion
- control of eyes and arms when visual information is used to guide saccades or reaching
- posterior parietal cortex
Ventral (what pathway)
- form recognition
- object representation
- storage of long term memory
- inferior temporal cortex
What is retinotopic organization in V1? Is the retina evenly represented? Why or why not
(functionally and the physiological reason)?
Retinotopic organization means that adjacent areas of the retina are located adjacently in
V1. There is retinotopic cortical magnification, meaning foveal areas take up more space
in the V1. This is because of higher visual acuity, due to a lower convergence ratio.
What are double opponent cells
specialized neurons found in V1 that are tuned to detect color contrast and spatial contrast (edges between colors). They’re different from the color-opponent cells found in the retina and LGN (lateral geniculate nucleus).
integrate color and spatial contrast to help you see color boundaries and support accurate, stable color perception.
Let’s say you’re looking at a red apple on a green table:
A double opponent cell might be excited by the red center of its receptive field (apple), and inhibited by the green surround (table).
That contrast helps you see the edge clearly, even if the red and green are equally bright.
helpful for feature extraction
What do early event related potentials reflect? How can their amplitudes be increased?
How was this discovered?
Early event related potentials (ERPs) such as P1, N1 reflect processing of
information in primary visual cortex (V1)
Attending to visual information, even without moving the eyes, can increase
amplitude of these early ERPs
Attention orienting experiment
- person is directed to move their attention to specific target (covert attention) but without moving their eyes to fixate on specific target (overt attention)
- Example: stare at this plus sign and move your attention to left or right
See graph from lecture
- Stare straight ahead, fixate your eyes on this cross, and then with a cue, move your attention left and right
- Do it many times to average it out and get rid of noise
- When you pay attention to the target you get a larger response from the primary visual cortex. Signal boost!
- When you ignore something, it is suppressed from higher processing (which is why the ERP is smaller in this case as indicated by dotted line)
Differentiate between and describe the receptive
fields of retinal ganglion, LGN, and cortical cells
Retinal ganglion cells
- located in retina
- centre-surround circular shape
- sensitive to luminance contrast
LGN
- located in thalamus
- centre-surround circular shape
- sensitive to luminance contrast (monocular)
cortical cells:
simple cell
- located in V1 (visual cortex)
- oriented bars with ON/OFF regions
- orientation + position
complex cell
- located in V1
- oriented, no fixed ON/OFF zone
- orientation + motion
double opponent
- located in V1
- Center-surround + color opponency
- color contrast + color edges
