Visual cortex and perception Flashcards
What happens at the optic chiasm
Fibres from the nasal portion of both the left and right retina cross over- this means all info from the left visual field is directed to the right side of the brain, and vv
Where is info from each hemifield processed
All info from the left visual hemifield is processed by the right hemisphere, while all info from the right visual hemifield is processed by the left hemisphere
Summarised pathway of visual info through the brain
Retina -> Optic chiasm -> Optic tract -> LGN -> Optic radiation -> V1-> Other subcortical visual nuclei
How is the LGN divided
Split into 6 layers, magnocellular/parvocellular/ koniocellular info goes to different layers
For P and M cells, each layer gets a precise retinotopic input from one eye (ipsi vs contra)
Where does M vs P vs nonMnonP info project to in the LGN
M- layers 1-2
P- layers 3-6
nonMnonP- ventral to each principal layer, called layers K1-6
How are the electrophysiological properties of LGN cells similar to the ganglion cells supplying them
ON and OFF cells remain independent, P cells retain colour opponancy, circular receptive fields with surround antagonism
eg magnocellular LGN neurons have similar properties to M cells
Is there retinotopic mapping in the LGN?
Yes, precise retinotopic map in each layer
Experimental lesion monkey study, effect of damage to magnocelllar layers of LGN
Merrigan et al (1991)- sharp reduction in ability to perceive rapidly changing stimuli, no effect on visual acuity or colour vision
Experimental lesion monkey study, effect of damage to parvocelllar layers of LGN
Merrigan et al (1991)- no effect on motion perception, severely impaired visual acuity and colour perception
Experimental lesion monkey study LGN- conclusion about role of parvocellular stream
Merrigan et al (1991)-Important for high spatial resolution- size, shape and colour of objects
Experimental lesion monkey study LGN- conclusion about role of magnocellular stream
Merrigan et al (1991)-Critical for tasks that require high temporal resolution- location, speed and direction of a rapidly moving object
Why is the striate cortex considered to be the primary visual cortex (V1)
Most LGN axons terminate in V1, all V1 neurons respond exclusively to visual stimuli, ablating V1 causes blindness, electrical stimulation of v1 elicits visual sensations
What is the result of a stroke destroying neurons in the right V1
Loss of left (contralateral) visual field, as V1 is needed for initial cortical processing of visual info necessary for visual perception
Study showing effect of shrapnel wounds on visual field
Holmes- studied different visual field losses caused by different shrapnel wounds causing different V1 lesions in WW1 soldiers.. could map out the whole of the visual field in V1 in these individuals
What is a scotoma
An area of blindness in the visual field- the region of the scotoma is characteristic of the area of the lesion, showing the retinotopic organisation of the visual pathways
What distortion is there in the cortical retinotopic map
Cortical magnification of area receiving and analysing foveal input aka it is disproportionately large
Peripheral areas have much smaller areas for cortical processing
What is the retinotopic map in the cortex based on
No of receptors present in fovea vs periphery, and no of ganglion cells that output that info
What important research did Hubel and Wiesel provide us- development of the visual system?
Critical periods and descriptions of ocular dominance columns
What important research did Hubel and Wiesel provide us- bases for understanding visual physiology
How the brain interprets visual info to generate edge detectors, motion detectors, stereoscopic depth detectors and colour detectors
How is V1 divided
9 layers with distinct cytoarchitecture- I (ventral-most), II, III, IVA/B/Ca/Cbeta, V, VI
Different pathways dock in different layers
What do stellate cells do in v1
Receive info streams from the LGN and pass it to the dendrites of pyrAmidal cells
What carries info from the LGN to V1
Optic radiations
Where does info from the magnocellular pathway terminate in v1
Layer IVCa
Where does info from the parvocellular pathway terminate in V1
Principally IVCbeta