2101 -week 9 visual system

Key Concepts of the Visual System in Neuroscience

Foundations of Neuroscience

• Foundations covered in the first half of the course will not be heavily tested in detail for the final exam.

Central Visual System Overview

• The visual system can be better understood through:

  • Systems Neuroscience

  • Major components after the optic tract

  • Properties of the visual cortex

  • Functional division into dorsal (where/how) and ventral (what) streams.

Retinofugal Projection

• Definition: The retinofugal projection refers to the pathway by which visual signals leave the retina.

• Components of Retinofugal Pathway:

  • Optic Nerve: Carries signals from the retina.

  • Optic Chiasm: Point where optic nerves cross, allowing visual field information from both eyes to be integrated.

  • Optic Tract: Continues to the thalamus, specifically to the lateral geniculate nucleus (LGN).

  • Optic radiation: LGN to the primary visual cortex (V1)

• Visual Hemifields:

  • Left visual hemifield projects to the right side of the brain and vice versa.

  • Ganglion cell axons from the nasal retina cross at the chiasm while temporal retinal axons remain on the same side.

Visual Deficits from Lesions

• Damage to the retinofugal projection can cause visual field loss.

  • An example includes partial loss of vision if one eye is affected, still preserving parts of the visual field because of contralateral and ipsilateral inputs.

Targets of the Retinofugal Projection

1. Lateral Geniculate Nucleus (LGN): Main target, crucial for processing visual information.

2. Hypothalamus: Influences biological rhythms and sleep-wake cycles.

3. Midbrain Structures (Pretectum and Superior Colliculus): Control pupil response and eye orientation toward stimuli.

Structure of the Lateral Geniculate Nucleus (LGN)

• Each LGN receives input from both eyes, but individual neurons are monocular.

• 6 Major LGN Layers:

  • Layers 1, 4, 6: Input from the contralateral eye.

  • Layers 2, 3, 5: Input from the ipsilateral eye.

• Approximately 0.5 million neurons in the LGN, with significant non-retinal inputs from areas like V1.

Primary Visual Cortex (V1)

• Also known as Area 17 or striate cortex.

• Layers of V1:

  • Composed of six principal layers:

  • Layer IV: Major input layer from the LGN.

  • Layers II, III, IVB: Project to other cortical areas.

  • Layer V: Projects to pontine structures.

  • Layer VI: Projects back to the LGN (feedback connections).

• V1 exhibits retinotopic organization maintaining spatial correspondence.

Major Properties of the Visual Cortex

• Ocular Dominance: Neurons in layer IV respond preferentially to input from one eye, creating stripes.

• Orientation Selectivity: V1 neurons have a strong preference for specific stimulus orientations, associated with their receptive fields.

• Orientation Columns: Neurons within a vertical column share the same orientation preference and receptive field position.

Neuron Types in V1

1. Simple Cells: Orientation-selective and responsive to optimal orientation of light bars at specific positions.

2. Complex Cells: Orientation-selective but responsive to light bars at any position within the receptive field, often requiring motion.

3. Pinwheel and Blob Formation: Orientation columns form pinwheels where multiple orientations converge; color-sensitive blobs exist within these structures, receiving input that is non-selective for orientation but critical for color processing.

Functional Division: Dorsal and Ventral Streams

• Dorsal Stream: Processes visual motion and spatial relations (the "Where/How" pathway).

• Ventral Stream: Processes object recognition (the "What" pathway).

• Specialization in IT: Neurons in the inferior temporal (IT) lobe, particularly sensitive to complex objects and faces, exemplified by studies on face-selective neurons in humans.

Motion-Selectivity and Lesion Studies

• Evidence of motion-selectivity in the dorsal stream, with patient studies revealing the effects of damage in specific areas like MT on motion perception.

Conclusion

• The visual system is highly specialized and organized:

  • The projection pathways from the retina to the cortex involve complex interconnections and functional divisions essential for processing visual information effectively.

  • Understanding these pathways and their functions is crucial for comprehending visual perception and related neural substrates.