Lecture 3: Visual System Flashcards
What are the 6 steps in the retino-geniculate-striate pathway
- Eyes
- Retina
- Thalamus
- Primary Visual Cortex (Occipital lobe)
- Extrastriate cortex (Occipital lobe)
- Extended Cortex (Temporal and Parietal lobe)
What are the three main phases of seeing something?
- Image formation
- Transduction - image is transducted into signals
- Visual process - understanding the image
What is decussation?
Where the left visual field reaches the right cortex and the right visual field reaches the left cortex
Bilateral visual fields are _____ and unilaterial visual fields are ______
Optic nerves are responsible for bilateral visual fields, optic tracts are responsible for unilateral visual fields
40% of the optic nerves fibres cross at optic chiasm, T/F
False! 50% do
Explain retinotopic
Retinotopic is the idea that adjacent points in the visual field map onto adjacent points on the retina, i.e., mapping of visual information from visual field to retina to brain. This mapping is maintained through visual processing
Explain cortical magnification
More cortex is dedicated to processing the central visual field than the periphery, i.e., fovea disproportionately represented with high acuity, compared to periphery
Explain what is occuring when a neuron fires with light in the centre and dark surround it, and when the same neuron suppresses when light surrounds dark spot
This process is termed receptive fields. The neuron is stimulated (excited) by this image due to its features and where the light lays (region), and is inhibited by the opposite of what stimulate it. This assists with mapping. The neuron describe in the example is a centre-surround cell.
Where does refraction of light occur?
Cornea
What is the role of the lens?
The lens focuses on a light through a process named accomodation reflex, that is, changing its shape to focus
How does the iris and pupil work in conjunction?
The iris controls the size of the pupil, moderating the amount of light the eye receives
Explain the process of how images are formed
An image is formed through light passing through the cornea, which is refracted. The light then passes through the pupil, in which the iris moderates the amount of light it receives by controlling the size of the pupil. Afterwards, the light reaches the lens which focuses the light onto the retina.
Which part of the eye is responsible for transduction?
Retina. Receptors transduce light signals into neural signals
What is the RGC?
Retinal ganglion cells, the final layer of axons to the brain
Fovea is responsible for the small and low acquity central vision, t/f
False! Fovea is responsible for small and HIGH acuity of the central vision
What is the optic disc?
Point of retina where RGC axons leave to become optic nerve
Optic nerve is responsible for…
The neural transmission to the thalamus
How does the fovea solve the backward wiring problem?
The fovea is a solution to the backward wiring problem because due to the fact that light has to pass many layers to reach photoreceptors, the fovea creates an area of high resolution by “pealing through the layers of photoreceptors”
Name the 5 layers of different types of neurons and briefly describe their functions
- Receptors, receives light and transduces
- Horizontal cells, lateral communication
- Bipolar cells, signals pass through and processes
- Amacrine cells, lateral communication
- Retinal ganglion cells (RGC), transmits signal to brain
What is the difference between rods & cones
Cones: Responsible for colour vision and high positional acuity, due to their low convergence. They are low in sensitivity and thus require more well-lit scenes (photopic vision).
Rods: responsible for low position acuity, due to their high convergence (lots of rod cells converging into one area). And as well, due to their high sensitivity, they’re better suited for low light situations
The retina is a ___ _______ ______, convergence is _____ _________, reducing axons to brain. Responsible for ____ detection & ______ detection
low processing centre, early processing. Edge & motion
What is lateral inhibition? How does this occur and how does this create enhancement of edges?
Lateral inhibition is the proess of contract enhancement for edge detection. Each receptor inhibits its neighbours with inhibition being greater when there is increased intensity. If a dim light and an intense light are adjacent, the receptor A receive intense light will inhibit receptor B, and thus enhancing an edge
Optic nerve transmit to….
Thalamus, (lateral geniculate nucleus)
Describe the key processes of V1
The key function of V1 is to identify object boundaries, done by simple cells, and motion information, done by complex cells
How do simple cells work?
Centre-surround cells (from layer 4) send signals to simple cells (in layer 3). They respond to the types of edges e.g., bars of light in dark field, orientation, and location. For example, simple cells can excite/fire when 1. light strikes (2) receptive field of simple cell, in a (3) particular orientation, however, without these, neurons can be inhibited
Explain the process of contour integration
Contour integration is the process by which multiple simple cell signals are combined to detect and represent boundaries within visual map. Since simple cells respond to specific orientation and position of edges/lines, when combined, contours or edges pop out.
Explain lateral facilitation in context of contour integration
Lateral facilitation is the process of neurons enhancing the activity of neighbouring neurons, in which permits an integration of local edge signals into a coherent contour representation.
What is low and high spatial frequency? How is this detected?
Low spatial frequency refers to texture of an object. High spatial frequency refers to edge information. This is detected by simple the position of objects, e.g., wider fields detect texture, narrow fields detect edges
Explain the role of complex cells
Complex cells are sensitive to more complex features such as motion and spatial frequency, that is, they respond to stimuli that move through receptive field in specific directions whilst also integrating information from multiple simple cells
What is the columnar organisation of V1, columnly and across.
Column: - Same receptive field in general area of visual field
- same orientation preference
- same eye (monocular neuron) or same eye dominance (binocular neuron)
Across:
- dominance across shifts
- orientation slowly rotates with column
- receptive field locatiion shifts with column
What is scotoma?
A damage in v1 that produces an area of blindness in contralateral field of both eyes (i.e., damage in left visual cortex, right visual field will be blind). Patients of this damage often have no conscious awareness due to completion
What is blindsight and describe some instances of it at play
Seeing but without any conscious awareness. E.g., respond to visual stimuli in scotoma, see motion, etc.
Describe the extrastriate’s role in processing visuals
Each area within the extrastriate cortex respond retionotopically and respond preferentially to stimuli, i.e., colour, movement, shape. The information is then distributed throughout.
Where does the dorsal stream and ventral stream run to?
- Posterior parietal cortex, 2. Inferior temporal cortex
What is the role of the dorsal stream?
Dorsal stream (where):
- respond to spatial stimuli: object location or direction of motion
- superior longitudunal fasciculus
- large RF, mostly (60%) outside of fovea
- central to spatial attention and selective attention (enhanced processing at some location to select objects) and is highly connected to motor cortex (posterior frontal cortex)
What is the role of the ventral stream?
Ventral stream (what):
- responds to characteristcs of objects; colour, shape, contours, surfaces, fields of motions assembled into objects
- attach semantic significance
2 stages of object recognition in 2 subdivisions:
- posterior: integration of visual features into objects
- anterior: association of object with knowledge of object
- large RF, all (including fovea)
What is the akinetopsia (motion blindness)
Movement becomes snapshots
