L22 - subcortical vision

What is subcortical?

deeper structures in the brain beneath the cortex

What is the subcortical visual pathway?

retina → superior colliculus

What did a single cell recording the neurons in the SC find?

the SC has a layered organisation and cells in the superficial layer of the SC receive visual information from the retina

How did an experiment test that the superficial cells of the SC receive info from the retina?

cells in the superficial SC respond to light within their respective visual fields and produced a retinotopic map

What is the sprague effect?

a unilateral lesion of the visual cortex causes hemianopia on the opposite side, but contralesional removal of the SC resolves the deficit

How does the sprague effect work?

by removing the contralesional SC, it stops the inhibitory fibres between both SC

What did a study about the sprague effect in cats find?

when right visual cortex damaged cat acted as if blind to the left hemifield, but when damage to the left superior colliculus the cat could orient again

What did a study about rodents and visual cortex vs superior colliculus damage find?

found a double dissociation: localisation/orienting task - rodents with visual cortex lesions performed normally but rodents with SC lesions couldn’t orient towards the sunflower seed, discrimination task between different striped doors - rodents with visual cortex lesion couldn’t distinguish vertical vs horizontal striped doors to find the sunflower seed, but rodents with SC lesions performed normally

What did studies find the unilateral damage to visual cortex caused?

contralesional hemianopia

What is a perimetry test and what can it be used to find?

presenting a small spot of light at random locations across the visual field while patient fixates on a central stimulus, can be used to detect the extent of a scotoma (blindspot) as patient can report lights outside the scotoma but not within

Do humans have residual vision without V1 like rodents?

patients reported that the couldn’t tell the difference between the experimental trials (light presented into hemianopic field) and control trials (no light) but oriented their eyes towards where the last light had appeared

How did a study test for the responsibility of subcortical vision in orienting towards objects in humans using light and distractors?

participants were told to look at light when it appeared in their intact hemifield, in some trials a distractor was simultaneously presented in their hemianopic field, latency was slower when a distractor was presented suggesting the retinotectal/subcortical pathway is involved in some degree of unconscious processing

How does unilateral A1 damage affect hearing?

only has minor effects as auditory information is transmitted bilaterally