Vision: Central visual pathways

Lecture 5 – Vision: Central visual pathways

optic nerve

Ganglion cell axons exit the retina through the optic disk and form the ___ ___

Optic Chiasm

In the ___ ___ ~60% of fibers cross over to the contralateral side

Optic Tract

The ___ ___ contains information from both eyes

Dorsal lateral geniculate nucleus of thalamus

primary visual (striate) cortex

A major target for projections of retinal ganglion cells

Suprachiasmatic nucleus of hypothalamus

Regulation of circadian rhythms

Another for projections of retinal ganglion cells

Pretectum

Reflex control of pupil and lens

Another for projections of retinal ganglion cells

Superior colliculus (midbrain)

Orienting the movements of head and eyes

Another for projections of retinal ganglion cells

Pupillary light reflex

Ganglion cells (bilaterally) → Pretectum → Edinger-Westphal nucleus (both) → Oculomotor nerve (III) → Ciliary ganglion

should be identical for both eyes → diagnostic tool

Ciliary ganglion neurons regulate constriction of the iris (decrease diameter of pupil when activated)

Automatic adjustment of the pupil’s size in response to light

Edinger-Westphal nucleus

A man is brought to the emergency room after a car crash. A doctor shines a light in his right eye and only the right pupil constricts. Which of the following regions is most likely damaged?

visual field

Each eye sees a part of visual space that defines its ___ ___

Nasal, temporal

Vertical line divides retina into ___ and ___ divisions

superior, inferior

Horizontal line divides retina into ___ and ___ divisions

B

A binocular portion of the left visual field

→ nasal retina of the left eye and the temporal retina of the right eye

C

A binocular portion of the right visual field

→ nasal retina of the right eye and the temporal retina of the left eye

A and D

Monocular portions of the left and right visual fields (what each eye sees)

→ left (_) and right (_) nasal retinas

left, right

The (left/right?) half of the visual world (coming from either the left or right eye) is represented in the (left/right?) half of the brain

Nasal diviision

Ganglion cells in the ___ ___ of each retina cross in the optic chiasm.

temporal division

Projections of cells that lie in the ___ ___ stay on the same side

opposite

The optic tract carries information from the (same/opposite?) side visual field

fovea

The ___ is represented disproportionally large in the posterior striate cortex, while peripheral stimuli are represented further anterior

upper, below, lower, above

The (upper/lower?) visual field is represented (above/below?) the calcarine sulcus, the (upper/lower?) visual field is represented (above/below?) the calcarine sulcus.

Meyer’s loop

(temporal cortex) → carries info about contralateral superior visual field (inferior retinal quadrants)

Course of the optic radiation to the striate cortex

Baum’s loop

(parietal cortex) → carries info about contralateral inferior visual field (superior retinal quadrants)

Course of the optic radiation to the striate cortex

Scotomas

small visual field deficit

Anopsias

large visual field deficits

Optic Radiation

Paths of fibers and axons that represent the superior and inferior visual fields

Bitemporal (heteronomous) hemianopsia

• Often the result of pituitary tumors

• Vision loss confined to the temporal visual field of each eye

• The parts of the visual field that are lost in each eye do not overlap

• Peripheral vision is lost

lesion in chiasm → nasal component

Left homonymous hemianopsia

• Loss of sight in the left visual field

• Blindness in the temporal visual field of the left eye & the nasal visual field of the right eye

lesion after chiasm → left (right) visual field

Visual field deficits

Spatial relationships in the retinas are maintained in the central projections → helps to pinpoint the site of neurological damage

Left superior quadrantanopsia

lesion along Meyer’s loop

Loss of visual information from the superior visual field

Left optic tract

A researcher interested in studying how the brain adapts to complete loss of the right visual field should lesion which region to create an animal model of this anopsia?

Magnocellular layers

(layers 1-2, large neurons)

Target in primary visual cortex → layer 4Cα

Parvocellular layers

(layers 3-6, small neurons)

Target in primary visual cortex → layer 4Cβ

Koniocellular

layers in between

Target in primary visual cortex → patchy 2/3

Parvocelluar Pathway

Important for spatial resolution

Detailed analysis of shape, size and color of object

P ganglion cells

Have small receptive fields and are slow → sustained responses

Magnocellular pathway

Critical for high temporal resolution

→ Evaluate location, speed, and direction of a fast-moving object

M ganglion cells

Have large receptive fields and are fast → respond only transiently (short time)

Koniocellular pathway

Also transmits some color information

primary visual cortex

Cells in this area respond selectively to oriented bars/edges → The “preferred” orientation is the orientation to which a cell is most responsive

Layer 4C

Axons from LGN terminate primarily on spiny stellate cells of ___ ___

→ axons convey LGN activity to other cortical layers

Layers 2/3

Pyramidal cells in these layers project to higher order (visual) cortices (red in E)

Layers 5/6

Pyramidal cells in these layers project to subcortical areas, including LGN and superior colliculus

Columns

feature-selective (e.g. orientation)

Microelectrode penetrations perpendicular to the cortical surface encounter ___ of neurons that have similar receptive field properties

• Receptive fields are centered on the same region of visual space (upper right)

• Exhibit similar orientation preferences

Combining inputs from two eyes

→ In the LGN the inputs from the two eyes are separated in different layers.

→ Distinction is maintained in visual cortex in ocular dominance columns (layer 4 inputs).

→ Neurons outside layer 4 integrate inputs from both eyes

far cells

discharge to retinal disparities beyond fixation point (point C)

Type of binocular neuron in the primary visual cortex

near cells

retinal disparities that arise from points in front of the plane of fixation (point A)

Type of binocular neuron in the primary visual cortex

tuned zero

respond selectively to points that lie on the plane of fixation (point B)

Type of binocular neuron in the primary visual cortex

Binocular Neurons

Relative activity in these classes of neurons mediates the sensation of stereoscopic depth

MT (middle temporal area)

Cells in this area respond selectively to direction of a moving edge

V4

Cells in this area respond selectively to color without regard to direction of movement

Cerebral akinetopsia

• Unable to appreciate the motion of objects

• Ex: difficulty pouring tea into a cup because the fluid seemed to be “frozen”

• Problems crossing street – can’t judge movement of approaching cars

Cerebral achromatopsia

• Lose the ability to see the world in color

• Shades of gray

• Normal cone functioning

Dorsal Pathway

Spatial awareness and guidance of actions (e.g., reaching) → Selectivity for direction and speed of movement.

Spatial vision “where” pathway

Located in the parietal lobe

Ventral Pathway

Object recognition and form representation → Selectivity for shape, color, and texture → Preferential response to faces and objects.

Does not merely provide a description of the elements in the visual world → it also plays a crucial role in judging the significance of these elements.

Objection recognition “what” pathway

Located in the temporal lobe