Visual System

Question 1

where does the pathways subserving visual perception originate?

Answer 1

retina

Question 2

where does the pathway of vision go from retina?

Answer 2

retina –> dorsal nucleus of lateral geniculate body of thalamus –> primary visual cortex (striate cortex, area 17, V1)

then to occipital, temporal and parietal association areas

Question 3

where do the retinal ganglion cell axons pass via?

Answer 3

optic nerves, optic chiasma and optic tract

Question 4

how many axons does the optic nerves have?

Answer 4

1 million myelinated axons

Question 5

describe the optic chiasm

Answer 5

–Partial crossing of optic nerve fibres
-ganglion cell axons from nasal retina cross here
–Essential for binocular vision

Question 6

describe the optic tract

Answer 6

–Curves around the rostral midbrain and ends in the lateral geniculate body of the thalamus

Question 7

what is binocular and monocular crescents field?

Answer 7

binocular –>visual field viewed by both eyes
monocular crescents –> visual field viewed by only one eye

Question 8

where does the left hemifield project to in the brain? and vice versa

Answer 8

left hemifield to right side of the brain –> the right LGN and right visual cortex (right hemifield to left side of brain

Question 9

describe the upper half of the visual field (which fall on the bottom parts of the retina)

Answer 9

– processed by lateral LGN and visual cortex below the calcarine sulcus

Question 10

describe the lower half of the visual field (which fall on the top parts of the retina)

Answer 10

– processed by medial LGN and visual cortex above the calcarine sulcus

Question 11

describe the macula (colour, information of high acuity)

Answer 11

– processed by posterior LGN and
posterior 1/3 of visual cortex

Question 12

describe the lateral geniculate body

Answer 12

-The lateral geniculate body is a small swelling beneath the posterior projection of the pulvinar of the thalamus
- Fibres of the optic tract terminate in the dorsal nucleus
-Receptive fields of LGN neurons: almost identical to the ganglion cells that feed them
-At the level of the lateral geniculate body the information from the two eye are kept separate

Question 13

how many layers does the LGN have and describe the magnocellular LGN neurons, parvocellular LGN cells and nonM-nonp cells

Answer 13

-6 distinct layers
-Magnocellular LGN neurons: large centre-surround receptive fields with transient response –> layers 3-6 and these layers innervated by P-type retina cells
-Parvocellular LGN cells: small centre-surround receptive fields with sustained response –> layers 1, 2 and these layers innervated by M-type retina cells
-nonM-nonP retina cells project to K1-K6 (areas between layers) to the Koniocellular LGN layer

Question 14

what layers for crossed and uncorssed fibres end in the LGN

Answer 14

-Crossed fibres end in layers 1, 4, &6
- Uncrossed fibres end in layers 2, 3, &5

Question 15

what does the visual cortex (aka striate cortex) occupy?

Answer 15

-primary visual cortex occupies the upper and lower lips of the calcarine sulcus of the medial occipital lobe (area 17)

Question 16

describe the geniculocalcarine tract

Answer 16

from the LGN to the V1 via this tract

Question 17

how many layers of the striate cortex

Answer 17

-6, I - VI
-IV –> broken up into IVA, IVB, IVC (alpha and beta)

Question 18

what is contained in layer IVC in V1?

Answer 18

spiny stellate cells (spine-covered dendrites)

Question 19

What layers are the pyramidal cells (spines and thick apical dendrites) contained in V1?

Answer 19

layers III, IVB, V, VI

Question 20

What layers are the inhibitory neurons (lack spines) in the V1?

Answer 20

all layers, form local connections

Question 21

Where do magnocellular LGN neurons project to in the V1?

Answer 21

layer IVC-alpha

Question 22

Where do parvocellular LGN neurons project to in the V1?

Answer 22

layer IVC-beta

Question 23

Where do Koniocellular LGN neurons project to in the V1?

Answer 23

-Koniocellular LGN axons make synapses primarily in layers I and III
-direct projections from koniocellular layers of LGN to blobs in layer II/III

Question 24

What are the patterns of intracortical connections in the V1

Answer 24

-Connections within the cortex are vertically organised

-preserves retinotopic mapping of layer IV through different cortical layers - hence cortical “columns”.
thought to be basis of information processing in cortex
- there are also lateral connections in layers III and V that may be involved in processing across the cortex e.g. for “perceptual binding

Question 25

describe ocular dominance columns and cytochrome oxidase blobs

Answer 25

-ocular dominance columns in layer IV of primary visual cortex revealed by the radioactive amino acid tracer technique

-Cytochrome oxidase:
mitochondrial enzyme used for cell metabolism

-Blobs: cytochrome oxidase-stained pillars in striate cortex

-Each blob centered on an ocular dominance column in layer IV.
-Receive koniocellular inputs from LGN

-neurons in blob regions tend to have monocular, circular RFs with colour opponency

Question 26

Describe the efferents of the striate cortex

Answer 26

-Layer II, III, and IVB cells project to other cortical areas.

-Layer V cells project to the superior colliculus and pons.

-Layer VI cells project back to the LGN

Question 27

Describe the dorsal stream of visual information

Answer 27

–Analysis of visual motion and the visual control of action (“where”)
-V1, V2, V3, MT, MST, other dorsal areas

-Area MT (temporal lobe):
– Most cells are direction-selective, respond more to the motion of objects than their shape.

-Beyond area MT—three roles proposed for cells in area MST (parietal lobe):
–Navigation
–Directing eye movements
–Motion perception

Question 28

Describe the ventral stream of visual information

Answer 28

–Perception of the visual world and the recognition of objects (“what”)
-V1, V2, V3, V4, IT, other ventral areas

-Area V4—shape and colour perception
– Achromatopsia: clinical syndrome caused by damage to area V4—partial or complete loss of colour vision

-Area IT:
– Major output of V4
– Receptive fields respond to a wide variety of colours and abstract shapes.
– May be important for both visual perception and visual memory (such as faces)

Question 29

What are the nonthalamic targets of the optic tract

Answer 29

-Participate in visual reflexes

-Pretectal area: control size of the pupil, certain types of eye movement

-Superior colliculus: orients the eyes in response to new stimuli—move fovea to objects of interest

Question 30

Describe the role of pretectal area in vision

Answer 30

-Group of small nuclei rostral to the superior colliculus

–Axons from the optic tract bypass the lateral geniculate body and enter the superior brachium

-Innervates the olivary pretectal area

–These neurons innervate the Edinger-Westphal nucleus of the
occulomotor complex

–These in turn innervate the ciliary ganglion which controls the sphincter pupillae muscle of the iris

-Participates in the pupillary light reflex

Question 31

describe the role of superior colliculus in vision

Answer 31

-Axons from the optic tract bypass the lateral geniculate body and enter the superior brachium

-Collaborates with inputs from the parietal and occipital cortex, frontal eye field, pallidum, and spinal cord

-Indicating the considerable integrative activity in the superior colliculus

-Efferents innervate the oculomotor nuclei, pontine reticular formation, pretectal area, and spinal cord

-Contributes to both voluntary and involuntary movement of the eyes and head required for fixation of gaze

Question 32

describe the role of suprachiasmatic nucleus in vision

Answer 32

–Synchronises the intrinsic circadian rhythm in response to ambient illumination

Question 33

What innervates the cerebellum with optics connections and what does this do?

Answer 33

-Accessory optic tract innervates the cerebellum via the tegmentum and inferior olivary nuclei

–Coordination of movement of the eyes and head