Visual System Pathway

Question 1

Visual field

Answer 1

. What is visible to an individual when eyes are fixed on an object in direct line of vision
. Each eye has its own
. Supraorbital ridges and cheek bones limit sup. And inf. Portions of field
. Fields for each eye overlap but not completely due to nose
. Allows for predominantly binocular vision w/ monocular segment at extreme peripheral portion of field

Question 2

Visual field divisions

Answer 2

. Fixation point: center of visual field w/ the image transmitted to fovea of both retinas
. Imaginary line drawn vertically through fixation point deivides the field into nasal and temporal halves
. Imaginary line drawn horizontally through fixation point divides field into upper and lower quadrants

Question 3

Visual field versus retinal fields

Answer 3

. Each retina is also divided into nasal and temporal halves and upper and lower quadrants
. There is complete inverse relationship btw visual field and retinal field
. Visual field is inverted and reversed onto retina

Question 4

Light path through retina

Answer 4

. Light passes through transparent cornea, ant. Compartment, through lens and vitreous body and through all retinal layers
. Excess light scatter absorbed by RPE melanin and light is redirected back toward photoreceptor outersegments
. Light energy converted into electrochemical gradients via visual transduction
. Synaptic connections send signal from photoreceptors to bipolar cells to ganglion cells
. Axons of retinal ganglion cells exit the eye at optic disc

Question 5

Optic n. And optic chiasm

Answer 5

. Retinal ganglion cell axons form optic n.
. Fibers in optic n. Are organized by site of origin w/in retina
. After entering cranial cavity, optic nn. Unite to form chiasm and undergo partial decussation
. Fibers from temporal visual fields (nasal retinas) cross to opposite side while fibers from nasal visual fields do not decussate
. Net result is that entire left field for each eye projects to right brain and entire right field projects to left brain
. Fibers then sweep lat. and caudally to synapse in LGN of thalamus

Question 6

Lateral geniculate nucleus (LGN)

Answer 6

. 6 layered structure
. Fibers maintain topographic order w/in LGN (80-90% fibers go here)
. Synaptic connections from both eyes line up w/ one another in precise order
. Uncrossed fibers from ipsilat. Eye synapse in layers 2,3,5
. Decussate fibers from contralat. Eye synapse 1,4,6
. Magnocellular division: 1,2: light and motion detection
. Parvocellular division: 3-6: spatial discriminationa dn color
. Projections rom divisions synapse in different parts of visual cortex

Question 7

Where do other 10% fibers form optic chiasm go?

Answer 7

. Fibers enter brachium of sup. Colliculus to terminate in other diencephalic or mesencephalic nuclei

Question 8

Geniculocalcarine tract

Answer 8

. Optic radiations

. LGN leave to form geniculocalcarine tract

Question 9

Temporal (meyer’s) loop

Answer 9

. Optic radiation Fibers carrying info from upper visual fields pass ant., loop downward and forward into temporal lobe
. Pass sup. And lat. to inf. Horn of the lat. ventricle before running post. As the ventral portion of optic radiations

Question 10

Parietal loop

Answer 10

. optic radiation Fibers carrying info from lower visual field follow a more direct anteroposterior course through parietal lobe
. Occupy the dorsal portion of the optic radiations

Question 11

What occupies the intermediate portion of optic radiations?

Answer 11

Fibers carrying info from macula (central vision)

Question 12

Striate cortex

Answer 12

. Primary visual cortex (area 17)
. Incoming LGN axons terminate almost exclusively in layer IV of the 6-layered striate cortex
. Forms white line that is visible in gross brain sections as line of Gennari

Question 13

Topographic order of LGN projections w/in striate cortex

Answer 13

. Upper visual field project inf. To calcarine sulcus
. Lower visual field project sup. To calcarine sulcus
. Macular vision projects to caudal region of striate cortex while fibers from successively peripheral regions of visual fields terminate in successively more ant. Regions along the calcarine sulcus
. Amt of cortical area devoted to unit area of sensory surface is not uniform but reflects the density of receptors w/in that area
. Cortical representation of fovea is large, peripheral retina is small

Question 14

Components fo striate cortex

Answer 14

. Hypercolumn: processes input from 1 small fragment of visual image ( 2 dominance + 2 blobs)
. 2 ocular dominance columns (R/L eye): process form and motion for 1 eye, visual input from 1 eye is adjacent to other eye input
. Blobs: cylindrical structure interspersed along layers 2,3 of ocular dominance columns: process color info
. Orientation columns: w/in ocular dominance columns, responds to stimulus in form of bar of light w/ specific orientation
. All axes of orientation encompass 360 degrees are represented by adjacent orientation columns

Question 15

T/F retinal ganglion cells will converge on LGN neurons that also have a circular receptive field

Answer 15

T

Question 16

How is circular receptive field turned into rectangular receptive field?

Answer 16

. Orientation column stimulated when 3 adjacent on center LGN neurons line up in rectangular receptive field which the orientation column is set to recognize

Question 17

Most projections from LGN synapse in layer ___

Answer 17

IV
. Projections from magnocellular division and most of the projections rom parvocellular divisions do this
. Some fibers from parvocellular division synapse w/in blobs in layers 2 and 3 for color processing

Question 18

What occurs when orientation column is stimulated?

Answer 18

. Pyramidal cells in layer IV form horizontal connections w/ adjacent orientation columns of the same axis orientation

Question 19

How does processing each individual aspect of visual input occur?

Answer 19

. Separate neuronal circuits functioning simultaneously and w/ feed forward and feedback control btw circuits
. When fragments of visual input from all functional columns are together, the primary visual cortex is ale to analyze the topographic nature of input in order to make an image

Question 20

Visual association cortex

Answer 20

. Contain magnocellular and parvocellular projections

. Give visual cortex meaning and significance to image

Question 21

Magnocellular projections from LGN in visual association cortex

Answer 21

. Synapse on layer IV mostly
. Some fibers synapse on layer 2 and 3 where a dorsal pathway projects to parietal lobe (area 7) for spatial aspects of vision (analysis of motion and positional relationships)
. Some fibers cross splenium of corpus callosum for integration in contralat. Visual cortex
. Some project to layer 5 for projections to subcortical areas (pons, pulvinar, sup. Colliculus)
. Some project to layer 6 for projections back to LGN and claustrum

Question 22

Parvocellular projections w/ visual association cortex

Answer 22

. Synapse on blobs in layer 4
. Some project from 4 to layers 2 and 3 where ventral pathway projects to temporal lobe (area 20,21,37) for high resolution analysis of form, color, and face recognition (neurons in blobs also do this)
. Some fibers cross splenium to contralat. Visual cortex
. Some fibers from layer 4 project to layer 5 for projections to subcortical areas (pons, pulvinar, sup. Colliculus)
. Some fibers form from layer 4 project to layer 6 for projections back to LGN and claustrum

Question 23

Prospagnosia

Answer 23

. Neurons in inf. Temporal cortex (20,21) function in face recognition
. Some neurons are set to recognize a frontal face view, some set to recognize side face view and others are set to recognize facial expressions
. Lesion in this region may mean that person can see and describe facial components but cannot understand that it is a face or whose face it is

Question 24

Achromatopsia

Answer 24

. Area 18 and 37 function in color recognition
. Lesion means person is unable to distinguish hues and sees only shades of fray
. Problem is inability to interpret color input even though everything in retina is functioning fine

Question 25

Movement agnosia

Answer 25

. Neurons at junction of occipital and temporal cortices
. Function in discerning the movement of objects
. Lesions causes person to not distinguish btw stationary and moving objects

Question 26

Pretectal area

Answer 26

. Area of midbrain involved in pupillary light reflex and possible accommodation reflex

Question 27

Superior colliculus

Answer 27

. Area of midbrain involved in coordination of head and eye movements in response to visual stimuli
. Receives input from visual cortex, spinal cord, and inf. Colliculus
. Sends projections back to cervical spinal cord via tectospinal tract and inf. Colliculus
. In charge of visuomotor reflexes

Question 28

Suprachiasmatic nucleus

Answer 28

. Area of hypothalamus located sup. To optic chiasm

. Involved in influencing visceral functions entrained to day/night cycle