Visual system 2 Flashcards
where does info from the optic nerve travel to?
Optic nerve → Optic chiasm → Optic tract → LGN (Thalamus)
what is the optic chiasm?
The part of the brain where nasal retinal fibres cross over, ensuring visual fields are processed in the contralateral hemisphere.
what happens to nasal retinal fibres at optic chiasm?
Nasal retinal fibres from the right eye cross over to the left optic tract at the optic chiasm
Nasal retinal fibres from the left eye cross over to the right optic tract at the optic chiasm
what are nasal retinal fibres?
They cross over at the optic chiasm and project to the opposite (contralateral) hemisphere of the brain.
This ensures that visual information from the right visual field is processed in the left hemisphere and vice versa.
What happens to temporal retinal fibres at the optic chiasm?
Temporal retinal fibres remain on the same side
Temporal retinal fibres from the right eye travel to the right optic tract
Temporal retinal fibres from the left eye travel to the left optic tract
What brain structure receives visual information after the optic chiasm?
The lateral geniculate nucleus (LGN) in the thalamus.
do retinal ganglion cells and lGN cells respond to motion?
not direction selective and don’t respond to motion
What does the spatiotemporal pattern of light on the retina help detect? and what carries this out
The spatiotemporal pattern of light on the retina helps detect motion. When the same pattern is seen in different places on the retina, milliseconds apart, the brain interprets this as movement
. This process is carried out by neurons in the visual cortex, not by ganglion cells.
What cells send motion information to the LGN?
Magnocellular retinal ganglion cells send their output to layers 1 and 2 of LGN
what info is carried in magnocellular layers of the LGN?
Motion information is carried in the magnocellular layers of the LGN
are magnocellular cells or parvocellular cells bigger?
Magnocellular cells in LGN are much larger than the parvocellular cells
what layers of LGN are magnocellular cells in?
layers 1 and 2 of LGN
what is the difference between magnocellular and parvocellular cells?
Magnocellular cells are fast, large, and good at detecting motion.
Parvocellular cells are smaller, slower, and good at detecting fine details and color
where are magnocellular and parvocellular cells located in the eye
M= peripheral retina
P= central retina
where do parvocellular cells send their output?
layers 3, 4, 5 and 6 of LGN
what information do parvocellular cells carry?
Colour and form information is carried in the parvocellular layers of the LGN
What are the LGN cells that are responsible for colour processing?
Parvocellular cells → Red-green colour processing.
Koniocellular cells → Blue colour processing.
Magnocellular cells → No colour processing, only motion and depth.
where does information for the LGN travel to?
optic radiations
visual cortex
where in the brain does visual info first enter?
occipital cortex in visual brain area V1 (also known as primary visual cortex)
what is area V1 responsible for?
V1 is responsible for processing low level visual information, and passing it on to higher levels of the visual system in the brain for further processing
how many layers does area V1 have?
6
which layer does information from LGN arrive in area V1
layer 4 which is sub divided to 4A, 4B, 4Cα and 4Cβ
What types of processing occur in V1?
There is parallel processing of information in the visual cortex
Different features such as shape and colour are processed separately at the same time
Where do magnocellular and parvocellular cells send their axons in V1?
Magnocellular LGN cells send their output to neurons in sub-layer 4Cα of V1.
Parvocellular LGN neurons send their output to neurons in sub-layer 4Cβ
What do magnocellular neurons process in V1?
Magnocellular neurons process motion and depth in V1.
Where do magnocellular neurons send their information in V1?
Information from magnocellular cells travels from sub-layer 4Cα to layer 4B
From layer VB of V1, information travels to V2
What are the two sub-divisions of the parvocellular pathway (P-pathway)?
Parvocellular blob (PB) pathway – processes color.
Parvocellular inter-blob (PI) pathway – processes form (fine details, contours, and textures)
Where does information from the parvocellular blob (PB) pathway go in V1?
Information from the PB pathway in 4CB is sent to the blobs in layers 2 and 3 of V1.
Where does information from the parvocellular inter-blob (PI) pathway go in V1?
From 4Cβ, PI information is sent to the inter-blob region in layers 2 and 3 of V1
Who discovered simple and complex cells?
Hubel & Wiesel (1950s/60s).
What do simple cells in V1 respond to?
respond best to bars or edges of light with a particular orientation and a particular location in the visual field
What do complex cells in V1 respond to?
Bars/edges of light with a specific orientation and motion direction.
How do receptive fields in V1 simple cells differ from LGN and retinal ganglion cells?
V1 receptive fields are elongated, unlike the circular receptive fields of LGN and retinal ganglion cells.
Simple cells in V1 have antagonistic receptive fields that comprise a central area flanked by surround areas
Simple cells in V1 have receptive fields with separate excitatory and inhibitory sub-regions
how are LGN receptive fields created?
through connections with retinal ganglion cells
How are V1 receptive fields created?
through connections with LGN cells
what do cells in V1 respond to?
simple stimuli: bars/edges of light with a particular orientation
Each cell responds preferentially to a specific orientation. Some cells fire more rapidly in response to a vertical line, some to a horizontal line, some to a bar/edge oriented at 90 degrees, etc
what does no light in raeceptive fields in V1 cells mean?
baseline response
what does light on a excitatory area of a V1 cell mean?
excitation response
what does light on inhibitory area of a V1 cell mean?
inhibitory response
what does light on inhibitory and excitatory areas of V1 mean?
baseline (cancel each other out)
what type of colour opponent receptive fields do retinal ganglion cells have?
single colour
what type of colour opponent receptive fields do V1 neurons have?
double colour opponent receptive fields
do complex cells have fixed excitatory and inhibitory regions in their receptive fields?
no
What do double color-opponent cells in V1 do?
They respond to color contrast by comparing signals in their center and surround. This helps detect color boundaries rather than uniform color.
They allow us to perceive color contrast and edges, helping us distinguish objects based on color differences rather than brightness alone.
They are crucial for detecting color boundaries (e.g., seeing the edge of a red object against a green background).
what do complex cells respond best to?
oriented bars of light that move in a specific direction
maximum excitation to downward motion
no response to horizontal motion
How is V1 organised?
V1 is organised in columns, separating neurons with different orientation preferences.
What are orientation columns in V1?
Columns where all neurons share the same orientation preference, responding to the same stimulus orientation.
How do adjacent orientation columns differ?
Adjacent columns house neurons that are responsive to slightly different orientations, with columns forming an array of 180 degrees
What are ocular dominance columns?
Columns where neurons respond preferentially to input from a specific eye.
How is V1 organised overall?
V1 contains both orientation columns and ocular dominance slabs.
What type of features do V1 cells primarily detect?
V1 cells act as edge detectors but also respond to specific spatial frequencies of contrast.
Define spatial frequency.
Spatial frequency is the number of repetitions of sinusoidal components per unit of distance
This refers to the number of repetitions of pairs of bars in a sine wave grating
Spatial frequency quantifies how rapidly an image changes in terms of brightness or intensity across its spatial dimensions.
What do high spatial frequencies correspond to?
Fine details, sharp edges, and small features in an image.
What do low spatial frequencies correspond to?
Overall shape, coarse structure, and object orientation.
how are square waves created?
by adding sine waves together
squarer the waves, the sharpest the image appears
what is sinewave grating?
light and dark bars follow a sine-wave function
what is square wave grating?
sharp edges between light and dark
How does the visual system process spatial frequencies?
Through spatial frequency channels, where different neurons process different frequencies in parallel.
How does the brain analyse complex visual images?
Through parallel processing, where multiple spatial frequency channels are active at the same time.
What are spatial frequency channels?
Groups of neurons that process specific spatial frequencies.
what sine waves do neurons in V1 respond better to?
sine wave gratings
