Lecture 20- The Visual System

Question 1

What happens to the layers of bipolar and retinal ganglion cells at the fovea?

Answer 1

The bipolar cells and ganglion cells are displaced laterally to allow light to strike the foveal
photoreceptors directly. This means have best vision for cones here (in center of visual field)

Question 2

What is the visual field for one eye?

Answer 2

• The visual field is the total amount of space that can be viewed by the retina.
• Note that images are inverted on the retina.

Question 3

Where do the blood vessels of the eye originate?

Answer 3

The blood vessels originate from the optic disk, which is also where the optic nerve fibres (i.e., the
axons of retinal ganglion cells) exit the retina and then form the optic nerve. There is a lack of blood
vessels at the region of the macula.

Question 4

What are the terms used to spilt the retina?

Answer 4

-The half of the retina that is closer to the nose is called the nasal hemiretina.
-The other half of the retina is called the temporal hemiretina.

Question 5

Which part of the retina is effected by the blind spot?

Answer 5

-Since your optic disk is located in your nasal hemiretina, the blind spot affects your temporal
hemifield.
-Your blind spot is about 15 degrees eccentric in your temporal hemifield.

Question 6

Which ganglion/ which part of the retina is responsible for vision in each of part of the visual field? (draw a diagram)

Answer 6

• The axons of the ganglion cells
located in each nasal hemi-retina
cross the midline via the optic
chiasm.
• The axons of the ganglion cells
located in each temporal hemi-retina
do NOT cross the midline.
• Consequently, ganglion cells in both
retinas that respond to visual stimuli
in the right hemifield project axons
into the left optic tract.

Question 7

What happens to vision if the left optic nerve is cut?

Answer 7

vision in
the left eye will be lost completely,
resulting in a loss of left peripheral
vision.

Question 8

What happens to vision if the optic chiasm is transected?

Answer 8

peripheral vision will be lost
bilaterally.

Question 9

What happens to vision if left optic tract is cut?

Answer 9

vision of the right hemifield will be lost completely.

Question 10

Which terminology do we use to describe the retinal ganglion cell axons before and after the optic chiasm?

Answer 10

The axons of ganglion cells are called the optic nerve before they cross at the optic chiasm.
• The axons of ganglion cells are called the optic tract after they cross at the optic chiasm.

Question 11

Where do the axons of the optic tract project to?

Answer 11

• The axons of the optic tract project to:
– Superior Colliculus
– Thalamus (lateral geniculate nucleus)

Question 12

What are the names of the two visual pathways?

Answer 12

Subcortical Vision: Retinotectal Pathway
– Retina
– Superior Colliculus

• Cortical Vision: Retinogeniculostriate Pathway
– Retina
– Thalamus (LGN)
– Primary Visual Cortex

Question 13

Describe the Retinotectal Pathway….

Answer 13

About 10% of the ganglion cells in the retina project to a part of the midbrain called the superior
colliculus.
• In non-mammalian vertebrates, the superior colliculus is called the optic tectum. tectum = colliculus (explains why this is pathway is called this)
• There is a retinotopic map of visual space in the superior colliculus.
• The retinotopic map is distorted, with more neurons devoted to analysis of the central visual field.

Question 14

Describe the Retinogeniculostriate Pathway…

Answer 14

Flow of visual information:
– Retina (light energy is converted into a neural signal)
• Photoreceptors (rods and cones)
• Bipolar cells
• Ganglion cells
(optic nerve → optic chiasm → optic tract)
– Lateral geniculate nucleus of the thalamus
– Primary visual cortex in the occipital lobe
– Extrastriate cortex in the occipital, temporal, and parietal lobes

Question 15

In cortical vision/ the Retinogeniculostriate pathway what is the LGN?

Answer 15

The left and right LGN, located in the thalamus, are the major targets of the two optic tracts (i.e.,
most retinal ganglion cells synapse on LGN neurons).
• The right LGN receives information about the left half of the visual field, and vice versa.
• Like the superior colliculi, the LGN contain retinotopic maps.
• Most neurons in the LGN project their axon to primary visual cortex.

Question 16

In the cortical vision/ the Retinogeniculostriate pathway what is the role of the primary visual cortex?

Answer 16

Receives visual inputs (relayed through the thalamus)
• Primary visual cortex is the first region of cortex to process visual information via the
retinogeniculostriate pathway.
• Because the projections are orderly, striate cortex contains a complete neural map of the retina.

Question 17

Where is primary visual cortex located?

Answer 17

Located primarily in the medial part of the occipital lobe and buried within the calcarine fissure

Question 18

What are alternative names for primary visual cortex?

Answer 18

Primary visual cortex = striate cortex = V1 = Brodmann’s area 17

Question 19

What did monkey experiments show about the retinotopic visual map of primary visual cortex?

Answer 19

While viewing the stimulus on the left, the monkey was injected with a radioactive agent.
• Metabolically active cells in striate cortex absorbed the agent, revealing how the topography of the
retina is preserved across striate cortex (see image on right).
• Thus, in primary visual cortex, the receptive fields of the cells define a retinotopic map.

Question 20

What does Transcranial Magnetic Stimulation (TMS) Over Primary Visual Cortex show?

Answer 20

TMS is a non-invasive method that causes a transient disruption of brain activity by emitting a brief
magnetic pulse. The disruption can be excitatory or inhibitory.
• What if the TMS coil is placed over primary visual cortex?
• When placed over the occipital cortex, TMS can elicit light sensations (phosphenes) in the absence
of any visual stimuli.
• The minimum TMS intensity that evokes phosphenes is referred to as the “phosphene threshold”.
• A reduction in the minimum TMS intensity that evokes phosphenes reflects increased visual cortex
excitability, whereas an increase in the phosphene threshold reflects reduced visual cortex
excitability.

Question 21

What relationship has been shown between ecstasy use and V1 excitability?

Answer 21

Question 1: Might heavy/chronic ecstasy use impact the excitability (i.e., responsiveness) of visual
cortex?
• Question 2: How might the responsiveness of visual cortex relate to the experience of visual
hallucinations?
• Test group: Ecstasy users
• Method: Transcranial magnetic stimulation (TMS)
• The goal was to measure the excitability of primary visual cortex and make comparisons between:
– Ecstasy group vs. control group
– Within the ecstasy group, hallucinators vs. non-hallucinators
• The participants in the ecstasy group were not on ecstasy during the experiment, but had taken it
recently.
• Results 1: Participants in the ecstasy group had a significantly lower phosphene threshold than
participants in the control group. Moreover, frequency of ecstasy use correlated negatively with
phosphene threshold.
• Results 2: Within the ecstasy group, the phosphene threshold of participants who had hallucinations
was significantly lower than the phosphene threshold of participants who did not hallucinate. Do you
think that the ecstasy caused some participants to develop a more excitable primary visual cortex, or
that the participants that hallucinated already had more excitable primary visual cortex prior to
taking ecstasy?

Question 22

What is sensory integration? Why is it so important?

Answer 22

Although information about vision and audition is initially transmitted to unimodal areas of cortex,
those primary sensory areas transmit the information to heteromodal regions of cortex so that
information from the different senses can be integrated and influence your perception.
• Integration of sensory information also occurs in some subcortical structures.
• For example, the inferior colliculus sends axons not only to the medial geniculate nucleus in the
thalamus but also to the superior colliculus, which is a target for visual information.
• Thus, visual and auditory information is integrated in the superior colliculus.

Question 23

What illusion shows sensory integration at work?

Answer 23

The ventriloquist illusion provides an example of how visual and auditory information are
integrated.
• Ventriloquist Illusion – speech seems to be coming from the puppet’s mouth rather than the
puppeteer’s mouth.
• The ventriloquist illusion occurs due to the sound source being mislocalized towards a synchronous
but spatially discrepant visual event (in this case, the puppets mouth moving).

Question 24

What are some examples of how the sensory systems overlap?

Answer 24

-Most sensory information is relayed through the thalamus en route to cortex.
– Spontaneous activity can lead to perceived sensation (e.g., hallucinations, tinnitus).