22 Cortical vs Subcortical Vision Flashcards
what percentage of the axons are apart of the cortical vision? and what is it involved with?
90%. building blocks of form, colour and motion
Experiment: Single-cell recordings from visual neurons in the superior colliculus
background:
superior colliculus has a layered anatomical organisation,
purpose to mape the receptive field in the superior colliculus.
monekys were tried to fixate on a pot of light that apared in front of them.
each time monkey fixated on light another stimulus was projected onto the screen
at the sight of interest a small lesion was made by passing a current through an eltrode
after mapping the field it became clear that the superfical layers of the superior colliculus provide a retinotopic map.
CONCLUSIONS…
superfical layers contain retinotopic maps
map is distorted with more neurons devoted to analysis of central portion of the visual field
What is the Sprague Effect
The Sprague effect is the phenomenon where homonymous hemianopia, caused by damage to the visual cortex, gets slightly better when the isilateral superior colliculus is destroyed
Experiment on the sprague effect
background: unilateral removal of the visual areas in the cortex of the cat causes the cat to stop orientation towards visual stimulus in the contralateral hemifield.
sprague showed you can restore vision by removing the contralesion superior colliculus or by cutting the fivres that connect the two superior colliculi
important note: subcortical visual pathways were unable to compensate for the damage cortical visual pathways until the ipsilater superior colliculus was released from normal inhibition (thus disinbiting the subcortical visual pathway on the side of the cortical damage).
Experiment with visual cortex vs superior colliculus damage
researchs compared the effects of disrupting cortical vision versus disrupting supcortical vision
rodents were trained too do two tasks
- localisation task (turn head towards a sunflower)
- Discrimination task (run down a two-arm maze and enter the door behind which which a sunflower seed was hidden.
the two lesions yielded a double dissociation
localisation task
- rodents with lesion in the visual cortex performed normally
- rodents with lesions affecting superior colliculus made no attempt to orient towards the seed
discrimination task
- rodents with lesions in visual cortex showed severely impaired performance, had motor capabilities, but they could not discriminate vision.
rodents with lesions affecting the superior colliculus performed normally
Conclusion of rat experiment on the visual cortex vs superior colliculus damage
disruptung subcortical vision impaired their baility to orient toward the position of a stimulus while disrupting cortical vision disrpted object discrimination
results provide compelling evidence for dissociable function of the rodents subcortical visual system and cortical visual system
cannot assume same happens in humans
perhaps evolution of more complex visual system took over the subcortical system
\how can the contribution of subcortical pathways to vision in the absence of cortical pathways be assessed after a stroke involving the primary visual cortex?
if we physically disrupt the visual processing via the retinogeniculistraite pathway, then we can asses any visual abilities that remain and assume that they can be attributed to processing via subcortical pathways.
What does complete damage of V1 within one hemisphere do to humans?
cortical blindness in huamns: perimerty testing
complete damage of V1 within one hemisphere will render the patient hemiantopic
rods and cones still fire but patient is not aware of any stimuli.
can be detected with blind spot. patient can guess that there is a light on when outside his blind spot.
Experiment: residual vision without primary visual cortex
if cortically blind rodents can still locate (orient towards sunflower seeds, then perhaps humans have a residual vision in the absence of V1 as well?
Task when a tone sounds move your eyes to the location of the light
Patient-reported that he could not tell the difference between the experiment trials, for which a sport of light appeared within the hemianopia field, and the control trials for which no light appeared.
results showed that responses on control trails were random but when the spot of light appeared up to 20 into the blind field responses were highly correlated with the position of the light
weiskrantz named this residual vision which occurred in the absences of awareness blindsight
what is blindsight
visual stimuli presented in cortically blind hemifield affect behaviour indicates that the visual stimuli were to some extend processed
processing must of happened in a pathway other than the retinogeniculostraite
given that the superfical layers of the superior collicious contain a retinotopic map, one can hypothesis that when the stimulus appeared within the cortical blidn hemifield it activated the retinotecnal pathway and that activity in this pathway led to the above chance performance.
Experiment: Residual vision without primary visual cortex. Unilateral damage to primary auditory cortex (A1): rafal measured how quickly heinaopic patoemts could look at a stimulus presented in their intact hemifield as a function of whether an irrelevant stimulus appeared in their critically blind hemifield. what was the conclusion?
that blindsight capabilities of patients who are rendered cortically blind after damage to primary visual cortex suggest that the subcortical visual pathway in humans may play an important role in orientation towards visual stimuli.
what happens when there is unilateral damage to A1
a surprising degree of normal auditory function is retained
explained by auditory information is transmitted both ipsilateral and contralateral primary auditory cortex
damage to one side doesn’t result to cortical deafness
