Lecture 22- Cortical vs Subcortical Vision

Question 1

Is there more areas invovled in cortical or subcortical vision?

Answer 1

More invovled with cortical vision (retinogeniculostriate
pathway)

Question 2

What is cortical vision?

Answer 2

• We know that the retinogeniculostriate pathway is the target of ~90% of the axons in the optic tract.
  -The retinogeniculostriate pathway goes from the retina to the thalamus to primary visual cortex
• We know that subdivisions of this pathway provide the building blocks (e.g., form, colour, and motion
  perception) that enable us to recognize and identify complex visual scenes.

Question 3

What is subcortical vision?

Answer 3

-If the activity of neurons in our cortical visual pathways account for so much of our visual processing
and experience, then why do we have subcortical visual pathways?
-Subcortical visual pathway is the retinotectal pathway and goes from the retina to the superior colliculus

Question 4

Which system is older: cortical or subcortical vision?

Answer 4

Subcortical visual pathways are phylogenetically older than cortical visual pathways (i.e., the cortical
visual system evolved after the subcortical visual system).

Question 5

What is the big question with regards to the subcortical visual pathway?

Answer 5

Does the subcortical visual system (i.e., the retinotectal pathway) contribute to human experience and
behaviour? Or it is just cortical that is important?

Question 6

What is the superior colliculus (role, arrangement etc.)?

Answer 6

– The superior colliculus has a layered anatomical organization.
– Neurons that receive information from the retinal ganglion cells are located in the superficial layers
of the superior colliculi.

Question 7

What is the purpose of experiments that take Single Cell Recordings from Visual Neurons in the Superior Colliculus? Who are the participants typically?

Answer 7

-To map the receptive fields of neurons in the superficial layers of the superior colliculus.
-Monkeys

Question 8

What is the method of experiments that attempt to take single cell recordings from Visual Neurons in monkey’s Superior Colliculus?

Answer 8

-Before conducting the experiment, the monkeys were trained to fixate a spot of light that appeared on a
screen in front of them.

Procedure for mapping the receptive field:
– Each time the monkey fixated the light, another stimulus was projected onto the screen and used to
determine the receptive field of the cell under study.

Procedure for confirming the location of neurons of interest:
– At the site of each interesting cell, a small lesion was made by passing current through the
electrode. Killing it to allow identification of the specific neuron later on.

Question 9

What was found in the monkey experiment (single cell recording from the superior colliculus)?

Answer 9

After mapping the receptive fields of many cells, it became clear that the superficial layers of the
superior colliculus provide a retinotopic map.

Question 10

What was the training procedure in the monkey experiment (single cell recording from the superior colliculus) and why was it important?

Answer 10

Training procedure:
– Each monkey learned to press a bar to turn on a light.
– The light stayed on for a randomly selected variable period of time between 1 and 3 seconds, and
then the light dimmed for 500 ms before it went out.
– If the monkey released the bar while the light was dim, he received a drop of water.
– The light was small enough that, in order to respond correctly, the monkey had to maintain fixation on the light until the end of the trial.

Important that the monkeys fixate before trying to map the visual field because if their gaze was shifting then the visual field would constantly be changing.

Question 11

What are the general conclusions that can be made about the superior colliculus?

Answer 11

The superficial layers of the superior colliculi contain retinotopic maps of the
visual field.
– The retinotopic map in the left superior colliculus represents the right hemifield. Note that, as with cortical visual pathways, projections from the nasal
hemiretinas to the superior colliculi cross the midline.
– The retinotopic map is distorted, with
more neurons devoted to analysis of the
central portion of the visual field.

Question 12

What does unilateral removal of the visual cortex in a cat cause?

Answer 12

cat stops orienting towards visual stimuli in the contralateral hemifield.

Question 13

What is the Sprague effect?

Answer 13

-Sprague (1966) demonstrated that visual orienting responses can be restored in the cortically blind hemifield by removing the contralesional superior colliculus or by cutting the fibers that connect the
two superior colliculi.

-This restoration of orienting toward the cortically blind hemifield has been termed the “Sprague
effect”

Question 14

What causes the Sprague effect?

Answer 14

-Originally this effect was explained by mutual inhibition between the two superior colliculi: Removal
of the superior colliculus contralateral to the cortical damage disinhibits the superior colliculus
ipsilateral to the cortical damage and therefore restores orienting toward the visual hemifield
contralateral to the cortical damage.
-Subsequent anatomical studies revealed that the Sprague effect actually results from cutting inhibitory
fibers that originate in another nearby structure and project to the superior colliculus on the same side
as the cortical lesion.

Question 15

What are the two important points to do with the Sprague effect?

Answer 15

1. removing visual cortex was devastating for the cat.
2. subcortical visual pathways were unable to compensate for the damaged cortical visual
   pathways until the ipsilateral superior colliculus was released from normal inhibition (thus
   disinhibiting the subcortical visual pathway on the side of the cortical damage).

Question 16

What two tasks in rodent studies have been used to show the difference between visual cortex (cortex) and superior colliculus (subcortical) damage?

Answer 16

– Localization Task: Turn head toward a sunflower seed held in an experimenter’s hand.
– Discrimination Task: Run down a two-arm maze and enter the door behind which a sunflower seed was hidden.

Question 17

In the cortex versus subcortical damage rodent study after training in the two tasks what was done (what lesions were done)?

Answer 17

– Group 1: bilateral removal of visual cortex

– Group 2: bilateral disruption of the retinotectal pathway (transection of the input fibres to both superior colliculi. )

Question 18

In the cortex versus subcortical damage rodent study what result did lesioning have? What type of association is this?

Answer 18

The two lesions yielded a double dissociation…

Localization Task:
– Rodents with lesions in visual cortex performed normally.
– Rodents with lesions affecting the superior colliculus made no attempt to orient towards the seed
(however, whisker contact elicited rapid orienting).

Discrimination Task:
– Rodents with lesions in visual cortex showed severely impaired performance: The animals could run down the maze and had sufficient motor capabilities to enter one of the doors, but they could
not discriminate horizontal from vertical stripes.
– Rodents with lesions affecting the superior colliculus performed normally.

Question 19

What can be inferred from the cortex versus subcortical damage rodent study?

Answer 19

-Disrupting subcortical vision impaired the rodents ability to orient toward the position of a stimulus, while disrupting cortical vision disrupted object discrimination.
-These results provide compelling evidence for dissociable functions of the rodent’s subcortical visual
system and cortical visual system.

Question 20

What is the limitation of the cortical versus subcortical damage rodent study?

Answer 20

-We cannot assume that the same division of functions occurs in humans.
-Perhaps during the evolution of more complex visual systems, cortical areas took over functions that had depended on subcortical areas.

Question 21

How can the role of subcortical visual pathways be assessed in humans?

Answer 21

-In humans, the contribution of subcortical pathways to vision in the absence of cortical pathways can
be assessed after a stroke involving primary visual cortex.

-If we physically disrupt visual processing via the retinogeniculostriate pathway, then we can assess any visual abilities that remain and assume that they can be attributed to processing via subcortical
pathways.

Question 22

How is Perimetry Testing used to assess to cortical blindness in humans?

Answer 22

-The extent of a scotoma (i.e., blind spot) can be mapped by a perimetry test, which involves presenting
a small spot of light at random locations across the visual field while the patient fixates on a central
stimulus. The patient reports whether he/she detected the spot of light.
-When the light falls outside the scotoma, detection is immediate (the patient can see the dot).
-When the light falls within the scotoma, the patient fails to detect the light.

Question 23

What does Complete damage of V1 within one hemisphere cause? and what does this actually mean?

Answer 23

-Contralateral Hemianopia (half of visual field is gone on opposite side)
-The rods and cones still fire, and information is transmitted to the lateral geniculate nucleus of the
thalamus, but the patient is not aware of visual stimuli when they appear in the region normally
represented by the damaged cortex.

Question 24

What experiment is used to demonstrate residual vision without primary visual cortex in humans?

Answer 24

-Weiskrantz (1986) used a task that, unlike perimetry testing, did not require
explicit report but rather tapped into the patient’s implicit knowledge of his hemianopic field.

-Task: When a tone sounds, move your eyes to the location of the light. Measured how far the eyes moved after a tone sounded.

-Experimental trial – a spot of light appeared in the hemianopic field prior to the tone.
-Control trial – the spot of light did not appear, but the tone sounded as on test trials.

-The patient reported that he could not tell the difference between the experimental
trials, for which a spot of light appeared within the hemianopic field (where is blind), and the control trials, for which no light appeared.

-However, results showed that despite no conscious awareness their was a difference in perception between control and experimental trials. that responses on control trials were random, but when the spot of light appeared up to 20 degrees into the
blind field, responses were highly correlated with the position of the light.

-Again this shows blindsight: residual visual that occurs in the absence of awareness.

Question 25

What does the phenomena blind sight suggest?

Answer 25

-The fact that visual stimuli presented in the cortically blind hemifield affected behaviour indicates that
the visual stimuli were, to some extent, processed.

-Although the stimuli that appeared in the cortically blind hemifield were not processed to the point of
conscious awareness, the degree of processing that did occur must have involved a pathway other than
the retinogeniculostriate (assuming that striate cortex was completely dysfunctional).

-Given that the superficial layers of the superior colliculus contain a retinotopic map, one can
hypothesize that, when the stimulus appeared within the cortically blind hemifield, it activated the retinotectal pathway (subcortical) and that activity in this pathway led to the above chance performance.

Question 26

How did Rafal (1990) test the idea of residual vision without the primary visual cortex (blindsight)?

Answer 26

-Measured how quickly
hemianopia patients could look at a stimulus presented in their intact hemifield as a function of whether an irrelevant stimulus (i.e., distractor) appeared in their cortically blind hemifield.

-If no processing was going on for stimuli presented in the ‘blind’ hemifield then would expect equal time to look at presented stimulus regardless of whether distractor appeared or not.

-This was however, not the case. The patient reported that he did not see any stimuli within the cortically blind hemifield. Nonetheless, the latency with which he initiated eye movements toward the spot of light that appeared in the intact hemifield (labelled ‘Target’) was slower when a
distractor appeared in the cortically blind hemifield
compared to when no distractor appeared.
-The increase in reaction time associated with a distractor in
the cortically blind hemifield may be explained by
competing activation by the distractor via the retinotectal
pathway.

-Conclusion: The ‘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 orienting toward visual
stimuli.

Question 27

How does unilateral damage to auditory cortex compare to unilateral damage to the visual cortex?

Answer 27

-Unlike the effects of unilateral damage to primary visual cortex, after unilateral damage to primary
auditory cortex a surprising degree of normal auditory function is retained.

-The relatively minor impact of unilateral damage to primary auditory cortex (in the context of
contralesional stimulation) can be easily explained by the fact that, unlike visual information, auditory
information is transmitted to both ipsilateral and contralateral primary auditory cortex.

-That is, not all auditory information crosses the midline (i.e., each of your ears projects auditory
information to both hemispheres of your brain.).

-Consequently, damage to primary auditory cortex in one hemisphere does not result in cortical
deafness (for input to the contralateral ear). Indeed, the deficit that results from unilateral damage to
primary auditory cortex is minor.