12 - Visual Pathways: Form Serves Function

Microsacades

• mini involuntary eye movements

• constantly doing

• very fast both eyes move at same time

• occurs during fixational gaze

Photoreceptors — Rods

• most sensitive to dim light (scotopic)

• do not convey sense of color

  • do not respond to specific wavelengths

  • lower intensity vision — lower acuity

Photoreceptors — Cones

• work better in bright light (photopic)

• responsible for acute detail

  • respond to both black and white (intensity)

• colour/wavelength specific

• green cones — green sensitive, red cones — red sensitive, blue cones — blue sensitive

Retinal Anatomy

• light enter via pupil — pupil is hole in centre of circular ring of smooth muscle

• info travels from photoreceptors to retinal ganglion cells (RGCs)

• axons of RGCs exit the retina to form the optic nerve — projects to rest of brain

• light must travel through all layers of the retina to reach the receptors

• except in the fovea — other retinal layers pushed aside — light falls directly onto photoreceptors

• decrease in intensity of signal as it goes through all the layers — decreases our abilities — but in fovea only photoreceptors (only cones)

  • gives us max resolution in the fovea

Opthalmoscopic Appearance of the Eye

• at blind spot where RGCs become myelinated

• all the blood vessel come in at blind spot area

Retinal Topography — Rod Density

• outnumber cones overall

• reach peak density abt 7-8mm from fovea at rod ring

• absent from fovea

Retinal Topography — Cone Density

• 5 million cones in avg human retina —- 20% in central 6mm

• cones outnumber rods in central retina

• region of elevated cone density surrounds fovea and extends into nasal retina

Retinal Organisation — Specialisations of Fovea

high acuity, colour vision

• peak density of cone photoreceptors in fovea

• absence of rods

• local absence of retinal blood vessels

• absence of inner retinal layers

• 1:1 r/s of receptors projecting onto RGCs

Retinal Organisation — Specialisations of Peripheral Retinal

low light (scotopic) vision

• low density of cone receptors

• moderate to high density of rods

• retinal blood vessels present

• inner retinal layers

• high convergence of receptors onto RGCs

Sensitivity vs Acuity — Cost of Convergence

photoreceptor → bipolar → RGC (axons form optic nerve/tract) → targets

• single RGC can receive input from

  • single cone

  • multiple cones and rods

  • thousands of rods

• this cluster of cells activating an RGC — defines its receptive field — size of field changes due to degree of convergence and varies across retina

• sensitivity/acuity tradeoff

  • peripheral retina — high convergence of rod input onto RGC — high sensitivity — useful in dim light

    • BUT large receptive fields = low acuity (useful in dim/scotopic conditions)

  • fovea — no convergence — very small RGC receptive fields

    • high acuity BUT low sensitivity (only useful in bright light)

RGC Pathways

each RGC has one axon that exits retina to form optic nerve (and the optic tract)

• each RGC axon projects to one/more structures:

  • lateral geniculate nucleus (LGN) of thalamus

    • projects to primary visual cortex (v1) — CONSCIOUS visual pathway, visual perception

  • superior colliculus — midbrain

    • UNCONSCIOUS — visual reflexes to direct gaze

    • visual, auditory and somatosensory integration

  • pretectal nucleus — midbrain

    • UNCONCIOUS — reflexes for pupil diameter and lens accomadation

    • projects to edinger-westphal nucleus

  • pulvinar of thalamus — thalamus

    • UNCONCIOUS — spatial attention helps stabilise retinal image

    • role in saccades

  • suprachiasmatic nucleus — hypothalamus

    • UNCONCIOUS — circadian rhythm

    • synchronises to day/night cycle

Retinogeniculocortical Pathway — Conscious **

• visual fields of our 2 eyes overlap

• given object will be viewed by temporal retina of one eye and nasal retina of other

• for organised representation of visual world — given object needs to be represented in one location

• optic chiasm sorts this out — axonx from nasal retina CROSS at optic chiasm — partial decussation

• axons then project to LGN — map visuotopically

• LGN projects to V1 via optic radiations — preserves visuotopy

• V1 sits on upper and lower banks of calcarine sulcus

  • also defined by Stria of Gennari

• enables left visual field to be seen by right V1 and right visual field to be seen by left V1

Optic Anatomy

Primary Visual Area Occupies Banks of Calcarine Sulcus

Beyond V1

Pupillary Light Reflex

• unconscious visual circuit

• pupil is hole in centre surrounded by iris — which is 2 layers of smooth muscle

• then ciliary body — contains ciliary muscle

• suspensory ligaments — changes in the tension forces changes of lens — affects focusing for diff types of vision

Autonomic Innervation of the Iris

• inner layer of smooth muscle — pupillary constrictor muscle

  • has smooth muscles fibres that are arranged in circular orientation — constriction decreases pupil diameter

  • mediated by parasympathetic nervous system

• outer layer — pupillary dilator muscle

  • has radial fibres — when constricted increase in diameter

  • flight vs fight situations sympathetic nervous system