Central Visual Pathways and Pupillary Control
Visual Field Concepts
Quadrants of Visual Field
Representation of Images
inverted representation – retinas give inverted view of the world and flipped back in the brain
bilateral representation – a point in space will be represented in one hemiretina in one eye and the other hemiretina in the other eye (ex: nasal hemiretina in left eye, and temporal hemiretina in the other eye)
monocular potions of the visual field are seen by the most medial portion of the nasal retina of each eye (in the most lateral visual field) which has no equivalent temporal visual field on the left eye; everything else is binocular field
- superior and inferior hemiretinas divide field in half vertically; temporal and nasal hemiretinas divide field in half horizontally; these divide field into quadrants (superior temporal quadrant, etc.)
Optic Nerve Organization – the Visual pathway
ganglion cells of retina Þ optic nerve Þ optic chiasm Þ optic tract Þ lateral geniculate nucleus Þ optic radiations Þ visual cortex (Brodmann area 17) in occipital lobe
ganglion cells from the retina synapse to the optic nerve, and from there go on to the optic chiasm
neurons from nasal hemiretina cross the midline at the optic chiasm – ganglion cells come to chiasm, those from nasal hemiretina cross and those from temporal hemiretina stay uncrossed
left visual field hits right half of the retina, therefore entire left visual field is represented in right optic tract and vice versa
Retinotopic organization – a point in space 10 degrees off center for retina is converted in brain to reflect this so you can know where things are in space
Four Principal Optic tract targets
(1) Major path: geniculostriate projection (optic radiations)
(2) Pupil reflex path: pretectum (see later)
(3) Reflexive eye movement (saccades) path: superior colliculus
(4) Circadium rhythm path: suprachiasmic nuclei in hypothalamus are involved with producing these rhythms
Lateral Geniculate Nucleus (LGN)
6 layers (numbered ventral to dorsal), magnocellular layers (1-2), parvocellular layers (3-6)
magnocellular – larger cell body ganglia, conveys information critical for analyzing the movement of objects in space
parvocellular – responsible for high-resolution vision (detailed analysis of the shape, size, and color of objects)
segregation of inputs from two eyes; i.e., take input from both eyes and combine into one batch of information.
center-surround organization (as in of retinal ganglion cells) is maintained in LGN!
small receptive fields (approximately primary)
independent M (for movement in coarse vision) and P (color, fine textures, shapes) channels
- Contralateral eye layers 1,4,6
- Ipsilateral eye layers 2,3,5
- (this was discovered by using tracer studies, injections into neurons of one eye and see where goes in LGN)
Geniculostriate Projections and Primary Visual Cortex (V1)
geniculostriate projections (optic radiations) extend from the LGN to the visual cortex (Brodmann area 17)
upper division contains input from superior retinal quadrants which represents inferior visual field quadrants
lower division (Meyer’s Loop) terminate on lingulate gyrus; contains input from the inferior retinal quadrants, representing superior visual field quadrants
retinotopy in V1 – precise retinotopic map is maintained
there is a proportionally larger map of fovea in visual cortex, therefore visual cortex maps where vision is needed
Laminar Organization of Visual Cortex – six laminar cortex, as in other regions. Striate appearance.
striate target is cortical lamina 4C. Strip in layer 4 Þ LGN recipient zone. Alternating left eye-right eye pattern (happens in layer 4C). Therefore, fidelity of both retinotopy and eye segregation is maintained. Eye segregation leads to ocular dominance- larger areas of left or right representation.
Cortical Visual fields – LGN neurons converge on layer 4C neurons. Alters receptive fields. Two classes of cells in V1: simple and complex. If a bar of light correctly oriented through a sample of neurons will get maximal activation. Vertical bar of light (created by multiple centers of circular fields in eye) will have minimal response. Visual field becomes rectangular in cortex!
Orientation selectivity – there is a group of cells across different layers (other than 4C) that prefer to respond to a bar of light in a certain orientation (i.e., a bar that is parallel for one set of cells won’t be parallel for others).
- (1) V1 simple cells – resemble cells of LGN (defined on and off regions), but with rectangular fields. Best response to bar of light correctly aligned=parallel; all alignments represented for each point in space. Many project to complex cells
- (2) complex cells – larger receptive fields than simple cells; response consistent with summation of several simple cells. Orientation of stimulus still critical, but position is less important (mixed on-off responses).
Columnar Organization of V1 and its significance
orientation preference of the striate cortex demonstrates how the cortex is organized so that neurons with similar response properties are grouped together and span the thickness of the cortex
the same goes for the horizontal groupings of neurons: as one moves across the striate cortex, areas of dominance can be seen and reflect the columnar segregation of inputs from the two eyes in layer IV into ocular dominance columns.
Ocular dominance stripes – if deprive visual cortex from one eye and then stain for a certain enzyme in visual cortex will see alternating lines of where enzyme was (functioning eye) and wasn’t (non-functioning eye)
in newborn there is no clear right and left eye ocular dominance columns in cortex. In kid with only one good eye, only the columns of the good eye will expand and be used. Adults cannot change their ocular dominance because the pathways are already set.
functional organization separates the neurons into columns which penetrate every lamina – therefore, the preferred orientation of the penetrates every lamina
- this is important because columns form AFTER birth. Columns form as inputs from right and left eyes segregate.
- segregation of inputs sharpened by visual experience, so visual mal-experience disrupts columns. (a kid with strabismus will have one eye looking in one direction and other eye in another. Most likely will get suppression of vision from the bad eye and visual cortex won’t form correctly!!! Other option is that the kid will use one eye some of the time and other eye some other time, and will thus lack depth perception)
Higher Order Visual Cortex and Functions
Two major cortical pathways after processing in the occipital lobe:
Parietal lobe – dominated magnocellular stream, spatial aspects of vision such as analysis of motion and understanding the positional relationships between objects in the visual scene
(2) Temporal lobe – dominated by parvocellular stream, high-resolution form vision and form recognition
Anatomical Dissection of Visual Pathway Lesions
if cut optic nerve on right: will be blind in right eye.
if cut in middle of chiasm (pituitary tumor): all crossed axons (nasal hemiretina axons – which sees temporal receptive field) will be damaged – result is that both temporal fields will fail, resulting in bitemporal hemianopia
if cut one tract (right optic tract): will lose one field (left visual field). Left hemianopia (will only see ½ right visual field in each eye)
if cut in Meyer’s loop (connection between LGN and cortex): may get only one quadrant of a field absent (upper homonymous quadrantanopia)
If cut V1 (visual-cortical lesion): will get circular loss of field with macular sparing (smaller round circle of vision) since macula is so largely represented in the cortex
(1) Pupillary light reflex – light shined into one eye causes both to constrict
(2) Acommodation – adjustment of eyes for various distances
- Retinal ganglionic cell Þ optic nerve (bypass LGN) Þ pretectal nucleus Þ Edinger-Westphal nucleus (parasympathetic) Þ CN III Þ ciliary ganglion Þ sphincter muscle of iris
- Note that sensory input from one eye will distribute to bilateral edinger-westphal nuclei so both eyes will respond! If cut optic nerve on left side, will get response in left eye if right eye is activated, but not if left eye is activated
(3) Convergence – occurs as the eyes focus on a near point
- occurs as contraction of the ciliary muscle results in a thickening of the lens and an increase in refractive power
- mediated by the caudal Edinger-Westphal nucleus via CN III
- is mediated by medial rectus innervation via CN III
- Both convergence and accomodation use the same path to invoke changes to the eye:
- Visual Cortex (area 17) Þ Visual Association Cortex (area 19) Þ Pretectal area Þ Perlia’s nucleus of the oculomotor complex, CN III (projects to the rostral and caudal Edinger-Westphal nuclei and the medial rectus subnuclei of CN III)