VISUAL THALAMUS & CORTEX Flashcards
Optic nerve
Axons of ganglion cells in nasal half of retina cross at optic chiasm to opposite tract
– Same half of visual field projects onto temporal hemiretina in one eye, and nasal hemiretina in other eye
– Each optic tract thus has axons representing the contralateral (opposite) half of visual field
Connections from the retina to the lateral geniculate nucleus (LGN)
LGN topographic map:
– Map of visual hemifield in each layer
– Maps for each layer are in register
LGN has 6 layers:
– Individual LGN cells receive input from 1 eye (monocular)
– LGN layers alternate input from each of the 2 eyes
ON-center and OFF-center LGN cells
– Red/green cells in parvocellular (P) layers for form and color info. Projects to sublayer 4Cβ in V1
– Blue/yellow cells in koniocellular (K) layers for color info. Projects to layers 1-3 in V1
– Black/white cells in magnocellular (M) layers for motion and depth info. Projects to sublayer 4Cα in V1
Ocular dominance columns
Left and right eye inputs terminate in alternating patches of layer 4C (IVC) in V1
– Info from each eye is sent to different LGN layers and then to different parts of layer 4C in V1
– A patch of layer 4C in V1 receiving input from a particular eye is called an “ocular dominance column”
– Ocular dominance columns in layer 4C of V1 form zebra stripe-like patterns (left figure)
Info from each eye is first combined in layer 3 (III) of V1
– Layer 3 cells in V1 often receive left and right eye input from different layer 4C cells
V1 cells respond best to oriented lines
Each V1 cell has a preferred stimulus orientation
Orientation selectivity of cells stays the same as you move perpendicular to surface
– Such a radial column of cells with the same orientation selectivity is called an “orientation (mini)column”
Orientation selectivity of cells changes as you move laterally
Direction selectivity
Many V1 cells are direction-selective
– These V1 cells respond to an oriented line moving in one direction but not the opposite direction
– Direction-selective cells in V1 generally receive input from magnocellular cells in the LGN
– Direction-selective cells are thought to contribute to the processing of object motion
Simple cell
Simple cell RF has distinct ON and OFF regions
– E.g., ON-center region flanked on 1 or 2 sides by OFF region
– E.g., OFF-center region flanked on 1 or 2 sides by ON region
– This RF arrangement yields orientation selectivity
Converging LGN input results in simple cell RF
– Multiple LGN cells with slightly offset center-surround RFs
are thought to produce simple cell RF
Complex cell
Complex cell RF has no distinct ON and OFF regions
– Complex cell responds to stimulus at preferred orientation anywhere in the RF
Simple cells with the same orientation selectivity are thought to provide input to a complex cell
Blobs
Enzyme “cytochrome oxidase” differentially distributed across V1
– Found in mitochondria, contributes to cell respiration (energy production)
High amounts occur at regular intervals in V1
– Blobs extend through layers 2 and 3 as well as layers 5 and 6
– If you imagine looking tangentially across layer 3, the blobs would appear like leopard spots
Blobs are thought to contribute to color processing
– Large proportion of cells in blobs are color-sensitive
V1 module
Each module is capable of analyzing every aspect of a portion of visual field:
– Consists of orientation (mini)columns spanning 360º
– Consists of ocular dominance columns from each eye
– Many modules together provide coverage of the visual field
V2, secondary visual cortex
V2 cells respond to angles or the component lines that form preferred angles:
– E.g., a V2 neuron responded best to the circled angle, moderately to gray angles, and poorly to rest
– Such responses from V2 cells could result from summing the responses from two V1 cells
V4, fourth visual area
V4 cells are sensitive to form and color, e.g., they respond to colored shapes
– E.g., Response of two different V4 cells to various curves and colored shapes
– Such responses from V4 cells could result from summing the responses of cells in V2
MT, middle temporal area
V5, fifth visual area
MT cells respond best to stimuli moving in particular directions
– MT cells have a preferred motion direction
– MT cells have preferred stimulus velocities
MT responses correlate with perception of motion
