Sensory system: Vision 2 Flashcards
What are the central visual pathways: parallel processing?
optic nerve optic chiasm optic tract hypothalamus: regulation circadian rhythms lateral geniculate nucleus
What is the function of Suprachiasmatic nucleus?
Suprachiasmatic nucleus regulates circadian rhythms (“body-clock”) including sleep, physical activity, alertness, hormone levels, body temperature, immune function, and digestive activity.
What is melanopsin?
Light-sensitive photopigment. Melanopsin is most sensitive to blue light.
Pretectum
reflex control of pupil and lens
Superior colliculus
orienting the movements of head and eyes
4 parallel pathways of retinal projections to the rest of the brain
Parallel processing in hypothalamus (circadian rhythms),
•Superior colliculus (regulation of eye and head movements),
•Pretectum (the pupillary light reflex) and
•LGN (thalamus) for visual processing
How does the lens of the eye project on the retina?
The lens of the eye projects an inverted and left-right reversed image on the retina
Visual field projection and the crossing of fibers
Information from the left visual field is carried in the right hemisphere, and information from the right visual field is processed in the left hemisphere
What are the main features of retinotopic representaitons of the visual field?
- The lens of the eye projects an inverted and left-right reversed image on the retina
- This inverted and left-right reversed representation is maintained in the cortex
- Through the crossing of fibers in optic chiasm, information from the left visual field is carried in the right hemisphere, and information from the right visual field is processed in the left hemisphere
- The upper visual field is projected below calcarine sulcus and lower visual field above
The importance of the Lateral geniculate nucleus
The lateral geniculate nucleus is the principal subcortical site for processing visual information
Striate cortex
Mixing of the pathways from the two eyes occurs only in the striate cortex
How does the LGN look like?
- 6 layers.
- Cells have monocular input (from one eye)
- Layers alternate inputs from each of the two eyes.
- The top four are parvocellular layers, two layers from each eye
- The bottom two are magnocellular layers, one layer from each eye
M-cells, P-cells and K- cells in LGN receive different information from different retinal ganglion cells
P-ganglion cell: color, shape and size
M-ganglion cell: movement
K-ganglion cell: short-wavelength light (blue)
[sumarry] How is visual information conveyed within the primary visual pathway?
From visual field to LGN:
•Information from different eyes arrives in different layers of LGN
•6 layers for 3 streams
•the magnocellular stream conveys information for the detection of motion, the parvocellular stream mediates high acuity vision and color vision together with the koniocellular stream
There are many more cells in the visual cortex than in LGN. Why?
Each LGN neuron shares its information with many cortical simple cells
What can In vivo imaging reveal?
In vivo imaging reveals functional maps of selective responses in rat visual cortex with single-cell resolution
What can optical imaging reveal?
Maps of orientation preference in the visual cortex visualized by optical imaging
Functional columns of cells in the visual cortex analyze a discrete region of the visual field. What does each module contain?
- one complete set of orientation columns
- one set of ocular dominance columns (right and left eye)
- several blobs (color)
Describe the organisation of cortical column. What features of the visual stimulus do different cells represent?
- Visual cortex is organized in cortical columns containing 6 layers
- Information arrives in layer 4
- Each point in visual space is mapped to a certain column. Each column contains a set of cortical neurons specialized for processing different attributes of the visual stimulus (light–dark edges, orientation, movement in a specific direction, color, left or right eye input)
How to perceive motion?
The Parvocellular and Magnocellular Pathways feed into two processing pathways in Extrastriate Cortex (outside V1)
Magnocellular pathway
- Perception of motion begins in the retina (M-cells)
- These cells give rise to magnocellular pathway in LGN)
- After primary visual cortex (striate cortex) this pathway targets MT area
- Finally this pathway joins dorsal (parietal pathway) for spatial information (“where” pathway)
Magnocellular pathway
- Perception of motion begins in the retina (M-cells)
- These cells give rise to magnocellular pathway in LGN)
- After primary visual cortex (striate cortex) this pathway targets MT area
- Finally this pathway joins dorsal (parietal pathway) for spatial information (“where” pathway)
Parvocellular pathway
- Perception of objects (shape and color) begins in the retina (P-cells)
- These cells give rise to parvocellular pathway in LGN)
- After primary visual cortex (striate cortex) this pathway targets V4 area
- Finally this pathway joins ventral (temporal pathway) for object recognition (“what” pathway)
Why do we see 3D images?
Depth perception: monocular cues
- Familiar size
- Occlusion (the nearer object is in front)
- Linear perspective (converging lines)
- Size perspective (the smaller object is assumed more distant)
- Shades and illumination (closer brighter)
- Motion (towards quicker)
Depth perception
Stereopsis and binocular disparity
binocular disparity
binocular disparity cells in V1, V2 and V3 detect differences in the representations of right and left retinas
What is stereopsis?
Stereopsis = depth perception that arises because of binocular disparity
How is stereopsis achieved in the cortex?
Binocular disparity is a difference in the positions of object representations in two retinas;
Which cells are responsible?
It is computed by “far” and “near” neurons in V1 and extrastriate neurons for depth perception
Two streams of information from V1:
Ventral (object recognition) and dorsal (spatial relations between objects)
[Test] Transport: parallel processing
4 central pathways from retina (to hypothalamus, pretectum, superior colliculus and LGN),
2 main streams to V1 (magnocellular and parvocellular), from V1 (dorsal, “where” stream and ventral “what” stream)
[Test] topographical representation
Retinotopic mapping in V1
[Test] cross-over
Visual info from left and right visual field cross at optic chiasm to feed the information to the contralateral hemisphere
[Test] feedback connections
Rich feedback connections from cortex to LGN, but also to V1
[Test] Processing
Neuronal networks in V1 column process visual info from each eye, color, orientation, calculate binocular disparity for depth vision
