What are the characteristics of the M ganglion cells?
-magnocellular=large cells -large receptive fields -make up about 10% of ganglion cells -motion detection, flicker and analysis of gross features -can tell if sth is moving or not, not good at fine detail
What are the characteristics of the P ganglion cells?
-parvocellular= small cells -more numerous (about 80% of ganglion cells) -provide fine detail (visual acuity) and colour vision
What is this picture showing?
-this pic shows the AP of a P ganglion cels as reacting to different frequencies of light -one ganglion cell, it responds best to one wavaelength= black it is important for carrying info for colour
What is the output region of the ganglion cells?
-many brain regions -mainly the lateral geniculate nucleus (LGN) in the thalamus
What is the visual pathway?
2. Optic nerve
4. Optical radiations
5. Visual cortex
-starts in the retina= the axons of optic nerve then synapse with the thalamus (LGN), then the LGN neurons have optic radiations (white matter) goes all the way back to the visual cortex V1
Where is the optic chiasm and what happens there?
-lies at the base of the brain, anterior to the pituitary
-the fibres from right and left optic nerves combine to form the optic chasm
What fibres cross at the optic chiasm?
What hemisphere is the right visual field "viewed" by?
What hemisphere is the left visual field "viewed" by?
-the nasal fibres cross over -get information from both eyes in each hemisphere -partial crossing of the visual field
How many layers are there in the LGN?
-6 -contain 2 typesof cells
What layers do the magnocellular cells and the parvocellular cells occupy in the LGN?
-6 layers -Magno=1 and 2 (one layer for info from each eye) -Parvo =3,4,5,6= two of these for info from each eye
What cells in the LGN do the the P and M ganglion cells target?
P ganglion= parvocellular cells M ganglion= magnocellular cells
What is the function of the thalamus?
-functional relay station for sensory information -except for taste!
Is the visual information separated or mixed in the LGN?
-separated, keep M and P streams of information separate, and information from each of the eyes is also kept separate
What are the optic radiations?
-the axons of the magnocellular and parvocellular LGN neurons carrying information to the visual cortex
-white matter, big, enclose the ventricles
Where do the LGN neurons project to?
-the primary visual cortex
Where is the primary visual cortex located?
-occipital lobe, around the calcarine fissure (gap) -very far back in the brain -if you cut corpus callosum medially= then there
What does it mean that V1 is retinotopic?
-whatever the retina sees= the brain will see= retinotopic map= the neurons in V1 will see the same things as the retina sees --Neighbouring cells within the retina project to neighbouring cells in the LGN & Vis Cortex.
Where is each of the visual fields (left/right) represented in the primary visual cortex?
-on the contralateral visual cortex -right field of vision= left V1 -left field of vision= right V1
Does each V1 separate which part of the visual field is presented where?
-central part of vision is on the outermost part of the V1
-more peripheral part of the visual field is closer to the midline
What part of vision is the most vulnerable?
-central -closest to the outside in the V1
How many layers does the visual cortex have?
What layer of the V1 do LGN neurons input to?
4C -P= to layer 4Cbeta -M= to layer 4Calpha
How are cortical (V1) neurons orientation selective?
-neurons respond best to bars moving in particular orientation
What are orientation columns in the V1?
-the cortex is extremely ordered
-in each little area the neurons in all 6 layers will respond to the same direction of the bar of light
-part of the way the information is divided into different bits
What does this picture demonstrate?
-each of the four pics= how different spots in the V1 will see it
-deconstructing what we see
-then brain has to put back together
What are occular dominance columns?
-input from the LGN in the V1 segregated into small regions called occular dominance columns
-infromation from the left eye is thus still separated from the right eye
-it goes: right, left, righ, left...
What is the pathway of the P ganglion cells?
-feed into the parvocellular LGN cells, then via optic radiations (their axons) to the V1 into the 4C beta layer
What is the pathway of the M ganglion cells?
-feed into the magnocellular LGN cells, then via optic radiations (their axons) to the the V1 into the 4C alpha layer
Where does the mixing of visual information occur?
in the layer 4B and 3
What happens to the visual information after V1?
-divided into two large cortical streams
What does the dorsal pathway do?
-the where -where in the visual field something is
What does the ventral pathway do?
-the what -what am I seeing? part of information
What is the MT area?
-part of the dorsal stream -middle temporal (MT) lobe is an area specialised for processing of object motion
Where does the MT receive information from?
-receives retinotopic information from a number of cortical areas (V2 and V3) -receives input from cells in layer 4B of the V1 (so the M type GCs and LGN)
What does the visual system operate on?
-about comparison = what is the colour next to -- red vs green -- blue vs yellow =imprtant comparison -context of the colour
What parts are part of the ventral stream?
V1, V2, V4
Where does the V4 receive input from?
-the blob and interblob regions of the primary visual cortex via V2
What are the receptive fields like in V4 and what is V4 for?
-large -both orientation selective and colour selective -important for perception of shape and colour
What is the area IT?
-major output area of V4 -neurons respond to a wide variety of abstract shapes and colours
-important for visual memory and perception
-important for perception of faces
-when see faces= neurons stimulated -the information from the whole pathway is added up here
What was wrong with David?
-David cannot recognize faces, can tell people only by what their voices are like -think motion perception is off -at the periphery of his vision= can't see -couldn't recognize the prime minister on a photo -lesion affecting the fusiform gyrus (area IT) -also caused a visual field defect, must have been back of the chiasm as the effect is the same in each eye
What was wrong with Giselle?
-cars appear out of nowhere, pouring coffee very difficult= appears frozen until cup overflows -visual acuity and colour vision norma -visual fields= normal -lesion in the MT -she had a tiny stroke in her middle temporal lobe= area spcialised for processing motion
What are the blob and interblob regions?
-areas of 4C beta of V1 (so Parvocellular cells feed into here)
-blob= colour info
-interblob receive the same infor bur react to diretcion of colour