Visual Pathway

• Integrator neurons


  Neurons that receive signals from rods, cones, and other integrator neurons and modify those signals before passing them on

  
• Synaptic pathways and signal modification

  1. Rods and cones synapse with integrator neurons

  2. Horizontal cells connect rods, cones, and bipolar cells

  3. Cone > Bipolar cell > Ganglion cell is the simplest and fastest pathway

  4. Bipolar cells receive signals from rods and cones directly or via horizontal cells

  5. Bipolar cell dendrites in outer plexiform layer, axons to inner plexiform layer to synapse with amacrine and ganglion cells

  
• Horizontal cells


  • Always have an inhibitory output
  • Connected laterally to numerous bipolar and rod/cone cells
  • Can suppress signal transmission in some pathways while not affecting others
• Lateral inhibition


  Selective suppression of signal transmission by horizontal cells to increase visual acuity

  
• Light falls on the retina

  Horizontal cells suppress output of less-illuminated areas, allowing only highest-intensity output to get through, improving contrast and visual definition

  
• Bipolar cells


  • Can hyperpolarize or depolarize
  • Can send "positive" or "negative" signals via ganglion cells
  • Enhances lateral inhibition phenomenon

  
• Intra-retinal processing

  1. Rods > Bipolar cell > Amacrine cell > Ganglion cell

  2. Synapse with ganglion cell marks end of intra-retinal processing and start of transmission to visual cortex

  3. Lateral connections between rods, cones, bipolar cells, etc. are possible

  
• Receptive field


  Area in space where presence of appropriate stimulus will modify activity of a neuron

  
• Receptive fields get more complex the higher you go in the processing chain
• Bipolar cell receptive fields


  • Circular, with centre and surrounding area working in opposite ways
• ON-centre bipolar cells


  Light on centre excites, light on surround inhibits

  
• OFF-centre bipolar cells


  Light on centre inhibits, light on surround excites

  
• Ganglion cell receptive fields


  • Concentric with centre-surround antagonism
  • ON-centre and OFF-centre ganglion cells respond by increasing or decreasing action potential frequency, not depolarization/hyperpolarization

  
• Vertical pathway in the retina

  1. Rods and cones project to bipolar cells

  2. Bipolar cells project to ganglion cells

  3. Midget bipolar and ganglion cells provide high spatial resolution

  4. ON and OFF bipolar cells synapse with separate ON and OFF ganglion cells

  
• Light on bipolar cell centre

  Causes hyperpolarization of photoreceptor, depolarization of bipolar cell, excitation of ganglion cell

  
• Centre-surround receptive fields of bipolar cells are preserved and passed on to ganglion cells, enabling contrast discrimination

  
• Parasol/Magnocellular pathway cells


  • Receive inputs from more rods and cones, have fast conduction velocity, respond to low-contrast stimuli
• Midget/Parvocellular pathway cells


  • Receive inputs from fewer rods and cones, have slow conduction velocity, respond only to high-contrast stimuli, involved in form perception
• Koniocellular cells


  • Receive inputs from intermediate numbers of rods and cones, have moderate spatial resolution and conduction velocity, respond to moderate-contrast stimuli, may be involved in colour vision

  
• Post-retinal processing


  1. Half of each optical field is directed to the contralateral portion of the brain via the optic chiasm

  2. Lateral geniculate nucleus is a key "way station" where optic tract fibres synapse

  
• Koniocellular


  Cells as small as dust; their small size made them hard to find
• About 10% of retinal ganglion cells are koniocellular cells

  
• Koniocellular cells


  • Input from intermediate numbers of rods and cones
  • Moderate spatial resolution
  • Moderate conduction velocity
  • Can respond to moderate-contrast stimuli
  • May be involved in colour vision

  
• The brain does not receive signals from each eye unilaterally

  
• Visual signals processing


  1. Half of each optical field is directed to the contralateral portion of the brain

  2. Bundled fibres of the optic nerves meet and cross at the optic chiasm, located on the ventral side of the brain

  3. If the chiasm is split, half of each eye's input to the brain is lost

  
• Lateral geniculate nucleus


  A key intermediate "way station" along the route visual signals follow
• Visual signals processing in lateral geniculate nucleus

  1. Fibres coming via the optic tracts (i.e., the axons of the ganglion cells) synapse with a second set of fibers, the geniculocalcarine tract, which carries the signal into the visual cortex of the cerebrum

  2. Information is interpreted, and true vision resides

  3. Visual tract fibers also run into other regions of the brain, involved in reflex controls of eye movement and behavioral patterns

  
• Dorsal lateral geniculate nucleus

  • Relays the information in exact point-to-point form
  • Faithful spatial representation of the on/off pattern of the visual fibers brought from the retina to the visual cortex
  • Keeps the signals "parallel" and routes the information from each half of each visual field to the appropriate cerebral hemisphere
  • Controls how much of the signal gets to the cortex
  • Has internal inhibitory circuits that can selectively turn individual signals off and regulate exactly which visual information is ultimately passed through to the cortex for processing

  
• Lateral geniculate nucleus


  Has six layers of cell bodies with layers of neuropil in between, in an arrangement something like a layer cake, with cell bodies of LGN neurons as the "cake" and neuropil as the "icing"
• Layers of lateral geniculate nucleus

  • Layers 1 and 2 are large, or magnocellular
  • Layers 3, 4, 5, and 6 are smaller, or parvocellular
  • Between each of the M and P layers lies a zone of very small cells: the interlaminar, or koniocellular (K), layers
• Koniocellular (K) cells


  Functionally and neurochemically distinct from M (where) and P (what) cells and provide a third channel to the visual cortex

  
• Layers receiving input from ipsilateral and contralateral eyes

  • Ipsilateral eye sends information to layers 2, 3 and 5
  • Contralateral eye sends information to layers 1, 4 and 6
• Visual signals processing from lateral geniculate nucleus

  1. Information leaving the LGN travels out on the optic radiations, which form part of the retrolenticular limb of the internal capsule

  2. The axons leaving the LGN go to V1 visual cortex and generally end in layer IV

  3. Axons from layer VI of visual cortex send information back to the LGN

  
• Function of lateral geniculate nucleus

  • Not completely known
  • Introduces coding efficiencies by cancelling out redundant information from the retina
  • Helps the visual system focus its attention on the most important information
  • Spatial attention and saccadic eye movements can modulate activity in the LGN

  
• Retinal ganglion cell pathways and their corresponding LGN layers

  • M pathway: Parasol retinal ganglion cells, most ventral layers 1 and 2, sensitive to higher temporal frequencies (movement)
  • P pathway: Midget retinal ganglion cells, most dorsal layers 3 to 6, sensitive to higher spatial frequencies (detail)
  • K pathway: Mainly bistratified (blue-ON) retinal ganglion cells, within and between principal layers (interlaminar), sensitive to shorter wavelengths (blue-yellow color)

  
• Superior colliculus


  • Located on the dorsal surface of the brain stem
  • Unconscious visual input goes directly from the retina to the colliculus, implicating this neural tissue in navigational processing
  • Output from the superior colliculus goes to motor centres responsible for orienting behaviours
  • Orienting behaviours are immediate, rapid organism responses (reflexes), usually to movement

  
• Layers of superior colliculus

  • Lamina I or SZ, the stratum zonale, is a thin layer consisting of small, myelinated axons together with marginal and horizontal cells
  • Lamina II or SGS, the stratum griseum superficiale ("superficial gray layer"), contains many neurons of various shapes and sizes
  • Lamina III or SO, the stratum opticum ("optic layer"), consists mainly of axons coming from the optic tract

  
• Intrinsically photosensitive (ip) ganglion cells

  • Contain their own photo pigment, melanopsin, meaning they respond directly to light in the eye
  • Cells projecting to the LGN include those making connections with the Edinger-Westphal nucleus (EW) for control of the pupillary light reflex and giant retinal ganglion cells
  • Cells projecting to the SCN go via the retino-hypothalamic tract for setting and maintaining circadian rhythms

  
• Visual cortex


  • Organized into a primary and a secondary region, in each occipital lobe
  • Direct visual signals come into the primary cortex, which is in the occipital region
  • The fovea, the region of the retina with the highest visual acuity, sends signals directly into the primary cortex, and is heavily over-represented there, compared with peripheral retinal regions
  • The secondary visual cortex receives signals secondarily: they are transmitted to these areas for analysis with respect to motion, shape, position, etc. via intra-cortical pathways
  • Different regions of the secondary cortex are responsible for different types of classification and analysis; and depending on the "conclusion" reached, actions can be initiated by motor control areas of the cerebrum

  
• Striate cortex


  • The visual cortex is the part of the cerebral cortex that is responsible for processing visual stimuli
  • It is located at the back of the brain in the occipital lobe
  • It is highly specialized for processing information about static and moving objects and is excellent in pattern recognition
  • Anatomically, the visual cortex occupies the entire occipital lobe, the inferior temporal lobe (IT), posterior parts of the parietal lobe, and a few small regions in the frontal lobe
  • The visual cortex occupies about one third of the surface of the cerebral cortex in humans
  • It is divided into approximately thirty interconnected visual areas
  • The first cortical visual area, the one that receives information directly from the lateral geniculate nucleus, is the Primary Visual Cortex, or V1
  • V1 transmits information to two primary pathways, called the ventral stream and the dorsal stream