Visual System Pathways

• The visual system comprises visual reflexes and higher visual pathways.

Central Projections of Retinal Ganglion Cells

• Retinal ganglion cell axons exit the eye via the optic nerve.
• These axons project to many brain regions.
• Axons reach the optic chiasm, where roughly 60% cross hemispheres contralaterally.
• The remaining axons stay on the ipsilateral side.

Different Visual Pathways

• Conscious vision goes through the lateral geniculate nucleus of the thalamus to the primary visual cortex (V1).
• Subconscious vision uses other retinal pathways.
• These pathways drive reflexes, circadian rhythms, and subconscious visual perception.
• The hypothalamus's suprachiasmatic nucleus is essential for photo-entrainment of the circadian rhythm.
• Retinal ganglion cells that drive circadian rhythms express melanopsin and are intrinsically photosensitive.
• The superior colliculus coordinates head and eye movements to visual targets.

Pupillary Light Reflex

• Certain retinal ganglion cells send axons to the pretectum.
• Pretectal neurons project to the Edinger-Westphal nucleus (EWN) in the midbrain.
• EWN neurons send axons through the oculomotor nerve to the ciliary ganglion.
• Ciliary ganglion cell neurons innervate constrictor muscles of the iris reducing pupil diameter.
• Both pupils respond to monocular visual stimulation.

Optokinetic Reflex

• The optokinetic reflex, or optokinetic nystagmus, is a gaze stabilizing reflex.
• Consists of a fast (saccade) and slow (smooth pursuit) component.
• A subset of directionally selective retinal ganglion cells project to the Nucleus of the Optic Tract.
• This leads to activation of cranial nerves controlling eye muscles.
• Blocking direction selectivity in the retina ablates the optokinetic reflex.
• Even a simple reflex results in wide-scale brain activity.

Primary Visual Pathway

• The primary visual pathway underlies conscious vision.
• Retinal ganglion cells project to the lateral geniculate nucleus of the thalamus (LGN).
• LGN relay neurons then project to the primary visual cortex.

Segregation of Visual Information

• Information from the right visual field projects to the left brain hemisphere.
• Information from the left visual field projects to the right brain hemisphere.
• Temporal-retina-originating signals from each eye join with nasal-retina-originating signals from the other eye at the optic chiasm.

Eye's View of the World

• Each eye has its own visual field.
• Combined, they create a binocular visual field.
• Vision in the periphery of the visual field is monocular.
• Due to lens optics, images entering the eye are flipped vertically and horizontally.

Projections from LGN to V1

• Fibers representing superior vs. inferior visual fields take different paths from LGN to V1.

Parallel Visual Pathways from Retina to V1

• The two ventral LGN layers are magnocellular.
• Magnocellular layers receive input from ganglion cells with larger receptive fields and more transient, motion-sensitive responses.
• Dorsal LGN layers are parvocellular.
• Parvocellular layers receive inputs from retinal ganglion cells with small receptive fields and more sustained responses, also signaling color information.
• Koniocellular layers interdigitate between parvo and magno cellular layers, but the role is unclear.
• Magnocellular cells from the LGN project to layer 4Ca in V1, thought to originate from parasol ganglion cells.
• Parvocellular cells from the LGN project to layer 4Cẞ in V1, thought to originate from midget ganglion cells.
• Koniocellular cells from the LGN project to patches in layer 2/3 of V1, thought to originate from non-midget, non-parasol ganglion cells.

Retinotopic Map

• In the primary visual system, neighboring parts of the visual field are processed by neighboring parts of the brain.
• The foveal region has greater representation in the visual cortex.

Impact of Damage on Visual Pathway

• Damage along the primary visual pathway can lead to differential visual field deficits.

Receptive Field Properties in Visual Cortex

• David Hubel and Torsten Wiesel found small circular spots robustly drove responses in retinal ganglion and LGN relay neurons, but did not strongly drive cells in V1.
• V1 prefer elongated edges oriented at a specific angle (orientation selective cells) to be stimulated.
• When the bar moved orientation cells only responded.
• Cells respond when the bar was moved across the receptive field in one of the angles perpendicular to the long-axis of the bar (i.e. direction selective cells).
• Cells care about spatial frequency and temporal frequency of visual stimuli.

Hierarchical Organization of Visual System

• Center Surround exist in LGN neuron.

Architecture of Primary Visual Cortex

• The cortex is a highly layered neuronal structure.
• It consists of 6 layers which can be grouped or subdivided.
• LGN axons mostly innervate neurons in layer 4C.
• Superficially located pyramidal cells project to other cortical areas.
• Deeper cortical pyramidal cells project to subcortical targets like the LGN and superior colliculus.

Functional Organization of Primary Visual Cortex

• Cells immediately above and below represent the same location and exhibit similar feature selective responses = orientation column.
• Less overlapping receptive fields and different feature selective responses exist beside each other.

Orientation Pinwheels

• Color-coded surface view of the primary visual cortex based on preferred cell orientation.
• Cells with similar orientations cluster together.
• Given a field location, all orientations are present (orientation pinwheel).
• At the next receptive field location, all orientations are again present, which repeats every ~1mm.

Binocular Vision

• Contralateral and ipsilateral retinal axons project to separate layers in the LGN.
• The eye-specific segregation is passed onto the input layer (layer 4) of primary visual cortex, causing ocular dominance.
• Most neurons are binocular, but receive stronger input from one eye.

Hypercolumn

• All the different spatially segregated functional modules overlay in V1.

Stereopsis

• Stereopsis arises from viewing nearby objects with two eyes in slightly different locations.
• Objects in front of or behind the plane of fixation project project to non-corresponding parts.
• Disparity between eyes generates depth perception.
• Visual cortex contains far, near, and tuned zero cells that respond to different disparities.

Strabismus and Amblyopia

• Strabismus, when eyes do not align properly, can be esotropia (cross-eyed) or exotropia (eyes deviate outwards).
• Untreated strabismus in childhood can lead to amblyopia, where the brain stops processing inputs from one eye.

Higher Visual Areas

• There are visual brain areas with complete maps of visual space downstream from V1.
• Higher brain regions are dedicated to specific visual detection tasks.
• The middle temporal area (MT) contains neurons that selectively respond to motion, and V4 contains many color selective cells.

Dorsal and Ventral Streams

• The ventral stream is concerned with object recognition.
• The dorsal stream is concerned with moving objects.

Area MT

• Area MT in the dorsal stream detects motion.

Aperture Problem

• V1's receptive fields are small which can mislead the recognition of movement.

Neurons in Area MT

• Neurons integrate input from neurons with smaller perceptive fields.
• They can detect the overall direction of movement of objects.

Ventral Streams

• The ventral stream is in charge of perception of colour, and object recognition.

Face Cells

• Face cells are regarded as high-odrer, visual object selective cells.