Visual System: Pathway, Decussation & Organization

• The University of Queensland (UQ) acknowledges the Traditional Owners and their custodianship of the lands on which they meet.
• Respect is paid to their Ancestors and descendants, who maintain cultural and spiritual connections to Country, and their contributions to Australian and global society are recognised.

Visual System Overview

• Visual Pathway Overview involves image formation, transduction, and visual processing.
• Key concepts include decussation, retinotopic organization, cortical magnification, and receptive fields.

Decussation

• Partial decussation explains how the left visual field projects to the right cortex and vice versa.
• Approximately 50% of the optic nerve fibers cross at the optic chiasm.
• Optic Nerves represent bilateral visual fields.
• Optic Tracts represent unilateral visual fields.

Retinotopic Organization

• Adjacent points in the visual field map onto adjacent points on the retina.
• This mapping is maintained throughout the processing hierarchy.

Cortical Magnification

• More cortex is dedicated to processing the central visual field (fovea) than the periphery.

Receptive Fields

• Particular neurons respond based on how the retina is stimulated.
• Receptive fields (RFs) refer to regions on the retina that excite or inhibit a cell.
• The nature of a cell's RF provides clues about its function.
• RFs can be small (high spatial resolution) or large (low spatial resolution).
• RFs typically have both excitatory and inhibitory regions.

The Eye

• The eye forms an image, generates a neural signal (transduction), performs early neural processing of the signal, and transmits the visual signal to brain.

Anatomy of the Eye

• Key structures include the cornea, iris, lens, pupil, retina, sclera, choroid, macula lutea, fovea centralis, vitreous body, optic disc, optic nerve.

Image Formation

• The cornea is a transparent outer layer where most light bending (refraction) occurs.
• The lens fine-tunes image formation, is adjustable, and its reflex stiffens with age.
• The iris and pupil regulate the size of the opening by contractile tissue and vary focal length, and engage in reflex actions.

Transduction/Processing

• Retina contains receptors that transduce light signals into neural signals
• Layers of neurons in the retina facilitate early processing of the signal.
• Retinal Ganglion Cells (RGCs) form the final layer with axons extending to the brain.
• The fovea is a small, specialized area responsible for high acuity central vision, solving the "backward wiring" problem.

Transmitting to the Brain

• The optic disc is the point on the retina where RGC axons leave to become the optic nerve.
• There is a blind spot at the optic disc because there are no receptors, resulting in no vision at that specific point
• Neural transmission occurs from the optic nerve to the thalamus, involving partial decussation at the optic chiasm, and continuing via the optic tract beyond the optic chiasm.

The Blind Spot

• Each eye has a blind spot, but there is no black hole in vision.
• Vision is constructed with completion.
• Receptors around the blind spot provide information to fill in the gaps.
• Edges are continued and surfaces are interpolated to create a complete image.
• Best guess at what is in the blind spot is based on what is around it.

The Retina

• The retina consists of 5 layers of different types of neurons (numerous subtypes).
• Cells within the retina include receptors, horizontal cells, bipolar cells, amacrine cells, and retinal ganglion cells.
• Light stimulates the receptors and the signal travels through bipolar cells and reaches the RGCs en route to the brain.
• Horizontal and amacrine cells are responsible for lateral communication.

Transduction and Receptors

• 3 types of Cones which are short (S), medium (M), and long (L) wavelength exist
• Cones facilitate photopic vision in well lit conditions and colour perception, totaling 6 to 7 million per retina
• Cones have lower sensitivity and high positional acuity because of low convergence.
• Rods facilitate Scotopic vision in low light and account for 120 million per retina.
• Rods have high sensitivity and low positional acuity due to high convergence.

The Fovea

• RGC clearance and very high acuity result in a solution to backward retina

Early Processing

• The retina is more than a sensory organ; it is a processing center.
• Convergence is a simple early processing step that reduces axons to the brain.
• 130 million photoreceptors per retina pair with approximately 1 million axons in each optic nerve.
• Edge and Motion detection are more features of early processing

Lateral Inhibition

• Lateral inhibition is used for contrast enhancement and edge detection.

Transmission to Brain

• Retinal Ganglion Cell (RGC) axons form the optic nerve
• Lateral Geniculate Nucleus forms the first synapse at the thalamus
• The Central Nervous System not the Peripheral Nervous System sends 10% to other areas (esp SC) and consists of meninges
• The optic chiasm has 50% decussation in humans and first synapse at thalamus
• CNS not PNS. ODCs not Schwann cells.

Optic Chiasm

• 50% decussation in humans, but more complete decussation in prey animals (less binocular vision).
• 75% decussation in rodents, 85% in horses is apparent.
• Birds have almost complete decussation, but owls have good stereopsis.
• Albinism results in disruption of melanin synthesis, abnormal projection to the thalamus, and larger/faster responses in the contralateral hemisphere to eye stimulation.

Receptive fields (retinal Ganglion Cells)

• Centre-surround RFs is a form of receptive field used by retinal ganglion cells
• 'ON' or 'OFF refers to centre of the RF regarding if cells fire to light or dark
• Used for small image elements, and contrast rather than light detection

Visual Thalamus

• The relay station for all sensory, except smell, signals through thalamus includes projections that are cortical
• Integration of bottom-up inputs and top-down is the role of this area of the brain
• The Lateral Geniculate Nucleus is the first synapse after the optic nerve leaves the eye
• Other nuclei include the Medial Geniculate Nucleus for auditory relay and the Ventro Posterior Nucleus for sensorimotor relay

LGN

• The Lateral Geniculate Nucleus consists of 6 layers for visual streams separated for Left and right eyes that transmit P channel and M channel information
• Also contains same center surround RFs as RGCs

Primary Visual Cortex - V1

• Relays retino-geniculate-striate pathway
• Axons from the lateral geniculate nucleus (LGN) project to lower layer 4
• There is lots of pre-cortical processing to identify object boundries
• It starts integrating basic contrast and integrates motion Information.
• Has First line segments and spatial scale
• V1 cells are 'simple' or 'complex'

Simple Cortical Cells

• Centre-surround cells in layer 4 project to simple cells in layer 3
• Are about detecting line segments and are monocular with the LGN / RGCs
• Shows a preference for type of edge (orientation / location) with an appropriate response for entering / leaving an OFF region

Simple Cortical Cells and Integration

• Edges give outlines of the object - good initial step
• Also relates to the Gestalt principle of continuity
• Small steps, with local and global direction close together

Contour Integration

• Connection between directionally similar and retinotopically adjacent

Simple Cells and Spatial Scale

• Spatial Frequency describes: contrast that changes different aspects in spatial frequencies
• Has high edge and low texture info
• Involves high SF and narrow subfields and low SF activates cells with wide subfields.

Complex Cortical Cells

• Multiple overlapping simple cells project to complex cells
• No distinct ON/OFF regions exist
• Cells respond to any simple cell inputs that respond or to straight edge simulus anywhere
• Continuously respond as an edge traverses through RF perpendicular to orientation

Complex Cells and Depth Perception

• Receive input from binocular cells, that increases in firing with left or right eye and even better with both eyes
• Some cells favour an eye depending on ocular dominance regarding similar contours / disparity

Organization of V1

• Functionally similar cells live in RF near a general area of general visual fied that share orientation/eye preference in binocular neurons
• Cells in dominance alternate with columns

Damage to V1

• Can produce scotoma with conscious completion
• Perimetry test can be used to determine
• Can cause blindsight with motion and intact V1

Extrastriate Cortex

• The extrastriate cortex Contains visual areas beyond V1 in occipital lobe
• There is extensive non sequential interconnections and retinotopic preference that is also not a heirarchy
• Visual stimulus distributed amongst visual space

What V4 and V5 do

• zeki (1993) showed static / moving effects with V5 and colour rectangles with V4

Visual System - Processing

• Can occur through extrastriate cortex

Dorsal and Ventral Streams

• 2 visual pathways exists:
  • Through extrastriate cortex that lead to extended cortical areas such as the Posterior Pariatal / Temporal Cortexes
• The Dorasal is for object size where the ventral is for seeing the size

Dorasal stream - (A.T.)

• Recognizes obj / finger sizes with impaired motor skill function but fixed object size recall

Ventral Stream (D.F)

Has bilateral damage and incorrect size estimate with an accuracy for obj size / gripping due to ventral damage

Comparison

• The dorsal stream responds to objects near a spatial location while the ventral responds to characteristics of a object
• Superior / Inferior long. fasciculus
• Also: Large RF most but mostly outside fovea and large rf all include fovea (d v)

Theories on Dorsal vs Ventral

• 82 vs where-spatial vision -visual perception
• 92 Dorsal specializes in visually guided behaviour and ventral specialise in visual behaviour and conscious visual perception

Role of the Dorsal Steam

• Responsible for enviromental awareness and motor skill fixation through Posterior areas of the brain

Dorsal Stream Dysfunction

• Akinetopsia – Motion Blindness
• Defect can cause inability to determine motion when something ceases
• Also can can be caused be drug side-effects