VIS Lecture 2 - Basics of vision II: The brain

• The lecture covers the basics of vision, focusing on the brain.

Rods vs. Cones: Number

• There are approximately 6 million cones per eye.
• There are approximately 120 million rods per eye.

Rods vs. Cones: Distribution

• Cones are concentrated in the fovea and periphery.
• Rods are located in the retinal periphery.
• The fovea contains cones only.
• The retinal periphery contains more rods than cones.
• Receptor density for both decreases with increasing distance from the fovea.

Rods vs. Cones: Dark Adaptation

• Cones are light-adapted and operate at high luminance.
• Rods are dark-adapted and operate at low luminance.
• Complete dark adaptation, is achieved by the rods due to their higher sensitivity to light.

Rods vs. Cones: Acuity

• Cones operate in light-adapted conditions, have sensitivity to low light, and enable high visual acuity.
• Rods operate in dark-adapted conditions, have sensitivity to high light, and enable low visual acuity.
• Neural convergence affects both

Neural Convergence

• Many neurons synapse onto fewer neurons.
• On average one ganglion cell receives input from 120 rods and 6 cones.
• More convergence occurs for rods than cones.

Rods vs. Cones: Light Sensitivity

• Cones are light-adapted, operate at high luminance, and have low light sensitivity.
• Rods are dark-adapted, operate at low luminance, and have high light sensitivity.
• Neural convergence increases the likelihood to detect a light stimulus, and thus light sensitivity.

Convergence and Acuity

• Separate ganglion cells receive input from separate cones, meaning detailed information about different light sources is available.
• The same ganglion cell receives input from multiple rods, meaning information about different light sources are combined.
• Cones are responsible or increased acuity (detailed vision)

Rods vs. Cones: Acuity

• Cones are in the fovea & periphery, are light-adapted and operate at high luminance, and have low light sensitivity and high visual acuity.
• Rods are in the retinal periphery, dark-adapted and operate at low luminance, and have high light sensitivity and low visual acuity.
• Neural convergence decreases the ability to locate a light stimulus & vision becomes less detailed.

Rods vs. Cones: Spectral Sensitivity

• Cones are sensitive to long light waves.
• Rods are sensitive to short light waves.
• Rods are more sensitive to shorter wavelengths, with a maximum at 500 nm.
• Cones are more sensitive to longer wavelengths, with a maximum at 560 nm.
• Purkinje shift is a shift in spectral sensitivity from long to short waves in the light versus dark-adapted eye.

Rods vs Cones: Color Vision

• Cones in the fovea & periphery are light-adapted, operate at high luminance, have low light sensitivity and high visual acuity, are sensitive to long light waves and facilitate colour vision.
• Rods in the retinal periphery are light-adapted, operate at low luminance, have high light sensitivity and low visual acuity, are sensitive to short light waves and do not facilitate colour vision.
• Rod Vision = Dark-adapted vision and facilitates no colour sensation.
• Cone Vision = light-adapted vision composed of:
• S cones respond to short wavelengths (blue).
• M cones respond to medium wavelengths (green).
• L cones respond to long wavelengths (red).

Photopic vs Scotopic Vision

• Photopic vision is associated with cones, the fovea & periphery, is light-adapted and operates at high luminance, with low light sensitivity, high visual acuity, sensitivity to long light waves and facilitates colour vision.
• Scotopic vision is associated with rods, the retinal periphery, is dark-adapted, operates at low luminance, with high light sensitivity, low visual acuity, sensitivity to short light waves and no colour sensation.
• Mesopic vision = rod and cone vision together

Visual Pathway Transformations

• The first transformation involves the environmental stimulus.
• The second involves light transformation.
• Receptor processing completes the third.
• Neural processing encompasses the 4th transformation.

Ganglion Cells

• Each ganglion cell receives input from ~126 photoreceptors, with variance.
• There's a 1:1 ratio in the fovea and a 1:100x ratio in the periphery.
• Input either excites or inhibits a ganglion cell

Receptive Fields

The receptive field of a neuron is the part of the visual field in which a stimulus can modify the neuron’s firing rate.

On-Off Center Surround Receptive Fields

• Ganglion cells have on(+)/off(-) centre/surround receptive fields.
• An excitatory area (+) 'on' area increases the firing rate of a neuron when stimulated.
• inhibitory area (-) 'off' area decreases the firing rate of a neuron when stimulated.
• Uniform illumination does not activate on/off cells.
• Luminance discontinuity, for example, an edge of an object, causes activation.

Types of Ganglion Cells

• Magnocellular (M) cells receive mostly input from rods and aren't color specific.
• Parvocellular (P) cells receive input from single M or L cones and are color specific (green or red on/off).
• Koniocellular (K) cells receive excitatory input from S cones and inhibitory input from M and L cones (blue on).
• Axons of M, P, and K cells = optic nerve

Retino-Geniculo-Striate Pathway

• This demonstrates a Lateralized pathway.
• The retina -> Lateral geniculate nucleus -> Striate visual cortex.
• Nasal axons cross over to the other side of the brain
• Temporal axons stay on the same side
• Visual fields are now represented in contralateral hemispheres, i.e., the left visual field in the right hemisphere, the right visual field in the left hemisphere.

Lateral Geniculate Nucleus

• The Lateral geniculate nucleus has 6 Layers with axons from:
• Parvocellular (P) neurons
• Magnocellular (M) neurons
• Koniocellular (K) neurons interlayered.

Visual Cortex

• The visual pathway from the Primary Visual Cortex includes Extrastriate Visual Areas

Primary Visual Cortex

• Primary visual cortex = V1 (visual area one), also known as the striate visual cortex
• As visual information travels through the cortex, and on to the Extrastriate visual areas such as V2, V3 and V4, neural convergence is ongoing.

V1 Layers

• The V1 or Striate cortex (stripy appearance) has Magno and parvo layers from the LGN and retina preserved in V1 input layers, then they merge subsequently.

V1 Neurons

• Some V1 neurons are orientation selective.
• V1 Neurons respond when a stimulus in their receptive field matches their preferred orientation.
• Other V1 neurons are selective for colour and brightness.

V2

• V2 receptive fields are about twice as large as V1 receptive fields.
• As V1, V2 neurons respond to basic stimulus features such as orientation, motion direction, brightness,...
• V2 neurons also respond to more complex features such as length, angles, arcs, shapes, components of natural scenes, texture.

Visual Streams

• There are two visual processing streams of cortical processing in the extrastriate areas. They involve increasing receptive fields and increasing complexity of visual information
• The Temporal/ventral stream (V1/2/4/IT) or "what" stream facilitates object identification and has neurons that respond to object-defining features.
• The Parietal/dorsal stream (V1/2/3/MT) or "where" stream that facilitates object localization and has neurons that respond to motion.
• The Parietal/dorsal stream processes object locations.
• The Temporal/ventral stream processes object identities.
• Receptive fields are five times larger in V3 and eight times larger in MT than in V1.

Controversy

• The dorsal and ventral streams interact heavily.
• For example, reaching and grasping is more precise towards familiar objects & directional movements, controlled by the dorsal stream, are influenced by object recognition in the ventral stream.