Vision: From Eye to Brain - Detailed Notes

Vision: From Eye to Brain

Introduction

• The lecture covers the journey of vision from the eye to the brain, detailing the functions of various components and potential disorders.

Blindsight Explained

• G.Y., a patient with damage to the occipital cortex, was clinically blind.

• Despite claiming to see nothing, G.Y. could accurately point to a light source 99% of the time when asked to guess.

• This phenomenon is known as blindsight, where individuals can react to visual stimuli without conscious awareness.

Anatomy of the Eye and Retina

• Key structures include the cornea, sclera, pupil, iris, lens, ciliary muscle, choroid, and retina.

• The retina contains photoreceptors (rods and cones), horizontal cells, bipolar cells, amacrine cells, and ganglion cells.

Photoreceptors: Rods and Cones

• Rods:

  • Contain rhodopsin, a photopigment extremely sensitive to light.

  • Function well in dim light but not in bright light.

  • Do not distinguish color.

  • Located in the peripheral retina.

• Cones:

  • Contain iodopsin, which requires bright light to function.

  • Work well in daylight but not in dim light.

  • Three types of iodopsins respond to different wavelengths (red, blue, green).

  • Located in the central retina (fovea).

Lateral Processing Cells

• Amacrine cells: Contact bipolar and ganglion cells.

• Horizontal cells: Contact photoreceptors and bipolar cells.

Neural Signals

• All cell types except ganglion cells generate graded potentials.

• Ganglion cells fire action potentials.

Receptive Fields

• Receptive field: The area of the visual world that a receptor can "see."

• Cones:

  • Small receptive fields.

  • Fewer cones per ganglion cell.

  • In the fovea, each cone has its own ganglion cell.

  • High visual acuity (ability to see details).

• Rods:

  • Large receptive fields.

  • Many rods share a ganglion cell.

  • Enhances light sensitivity but reduces acuity.

Phototransduction

• Rhodopsin Activation:

  • Rhodopsin consists of opsin and retinal.

  • When light strikes rhodopsin, retinal is activated.

• Transduction Process:

  • In the dark, Na^+ channels are open, allowing sodium ions to enter the cell.

  • Light activates rhodopsin, which activates transducin (G-protein).

  • Transducin activates phosphodiesterase, which hydrolyzes cGMP.

  • Reduced cGMP closes Na^+ channels, causing hyperpolarization and turning rods off.

Genetic Vision Disorders

• Leber’s Congenital Optic Degeneration:

  • Caused by a defective RPE65 gene, leading to photoreceptor degeneration.

  • Gene therapy can be used as a treatment.

Mach Bands and Lateral Inhibition

• Mach Bands: Optical illusion where changes in light intensity are exaggerated due to lateral inhibition.

• Lateral Inhibition:

  • Inhibiting one's neighbors produces contrast.

  • Enhances the perception of edges and boundaries.

  • Receptor cells and ganglion cells are involved.

  • 

Visual Pathways

• The visual pathway consists of:

  1. Retina

  2. Optic chiasm

  3. Lateral geniculate nucleus (LGN) in the thalamus

  4. Visual cortex (occipital lobe)

     

Hemianopia

• Hemianopia: Loss of vision in half of the visual field.

• A case study discussed a 29-year-old man with worsening vision in his left visual field due to a temporal lobe tumor.

• Resection of the tumor, followed by chemotherapy and radiation, initially showed a good response, but the patient developed left homonymous hemianopia.

• 

Receptive Fields in the Retina: On-Center and Off-Center Cells

• On-Center/Off-Surround Cells (Type 1):

  • Excited by light in the center of their receptive field and inhibited by light in the surround.

  • 

• Off-Center/On-Surround Cells (Type 2):

  • Inhibited by light in the center and excited by light in the surround (reverse of Type 1).

  • 

Lateral Geniculate Nucleus (LGN)

• The LGN in the thalamus has three cell types:

  • Parvocellular: Small cells, small receptive fields.

  • Magnocellular: Large cells, large receptive fields.

  • Koniocellular: Very small cells, located between the main layers.

Primary Visual Cortex (V1)

• Most visual information first arrives at the primary visual cortex (V1) in the occipital lobe.

• Brain maps of visual space are mostly devoted to the fovea.

Cortical Cells

• Simple Cortical Cells: Respond to an edge or bar of a particular width, orientation, and location.

• Complex Cortical Cells: Respond to a bar of a particular width and orientation but can be anywhere in the visual field.

Visual Cortex Processing

• V1 is needed to form all visual images.

• V2, V4, and the inferior temporal lobe perceive complex forms.

• V5 is specialized for motion perception.

V1 Function

• V1 breaks down the visual image into components: color, shape, and location, processing these inputs simultaneously.

V2 Function

• V2 'fills in the gaps,' extrapolating and predicting from what is actually seen.

V4 Function

• V4 cells respond to concentric and radial stimuli.

• V4 is involved in color perception.

Motion Detection

• Motion blindness (akinetopsia) occurs when the brain cannot process motion.

• The brain determines if an object is moving before identifying it.

• The retinal periphery (rods) is sensitive to motion and projects to V5.

Inferotemporal Cortex (IT)

• Cells in the inferotemporal cortex respond to complex forms, including forms learned through recognition.

• Receptive fields in IT cortex develop through experience and learning.

• IT cortex is the final stage of visual processing.

Visual Processing Hierarchy

• The visual processing hierarchy moves from simple edges and borders (V1) to complex forms (IT).

Three Channels from Retina to Higher Visual Cortex

1. M Channel (Magnocellular Pathway):

   • Orientation selective, directional sensitive for movement.

   • No color sensitivity.

   • Analysis of object motion.

2. P-IB Channel (Parvocellular Interblob Pathway):

   • High orientation sensitivity, no color sensitivity.

   • Small receptive fields.

   • Analysis of object shape.

3. Blob Channel (Parvocellular Blob and Koniocellular Pathway):

   • No orientation sensitivity, color sensitivity.

   • Analysis of object color.

Cortical Processing Disorders

• Synesthesia: A condition where stimulation of one sense triggers experiences in another sense.

  • Example: A person might see colors when hearing sounds.

• Alice in Wonderland Syndrome:

  • Characterized by micropsia (objects appear smaller than they are) and macropsia (objects appear larger than they are).

  • Occurs in migraine, EBV infection, and is common in children.

• Integrative Agnosia:

  • Inability to attend to more than a small area of the visual field despite normal visual acuity.

  • Patients can see individual objects but cannot perceive a visual scene as a whole.

• Palinopsia:

  • Patients see afterimages, both as a reduced amount of time required to form an afterimage and an increased duration of the afterimage.

• Visual Agnosia:

  • Sensation intact and speech normal, but patients are unable to name objects, although they know how to use them.

  • Example: Patient GS.

Stroop Test

• A test demonstrating interference in the reaction time of a task; for example, naming the color of a word when it differs from the word's meaning.

Blindsight Revisited

• Patients with damage to the primary visual cortex can tell where an object is, although they claim they cannot see it.

• In patient G.Y., blindsight is thought to be due to minor pathways into the extrastriate cortex that bypass V1.