Visual Pathways (2)

Visual Pathways

The Retina: "Inside Out" Organization

• Major Types of Retinal Processing:

  • Feedforward: Sequential processing of visual information.

    • Receptors: Photoreceptors (rods and cones) detect light.

    • Bipolar Cells: Transmit signals from photoreceptors to ganglion cells.

    • Ganglion Cells: Generate action potentials and transmit signals out of the retina.

  • Lateral: Horizontal interactions within the retinal layers.

    • Horizontal Cells: Modulate signals between photoreceptors and bipolar cells.

    • Amacrine Cells: Modulate signals between bipolar cells and ganglion cells, and among ganglion cells.

Visual Acuity

• Definition: The ability to resolve fine details in the visual field.

• Frequency and Detail:

  • Low Frequency: Corresponds to coarse features or large objects.

  • Medium Frequency: Corresponds to moderate detail.

  • High Frequency: Corresponds to fine details.

• Limitation: Visual acuity is directly limited by the spacing of photoreceptors on the retina.

• Peak Receptor Density: Approximately $150,000 - 200,000$ cones/mm$^2$ in the fovea, illustrating the high density needed for fine detail perception.

• Grating Stimulus: Visual acuity is often measured using grating stimuli, where the ability to distinguish lines is dependent on photoreceptor mosaic spacing.

Perceptual Consequence of Convergence

• Convergence: The phenomenon where multiple photoreceptors synapse onto a fewer number of downstream neurons (e.g., bipolar or ganglion cells).

  • Rods: Exhibit more convergence, meaning many rods pool their information onto a single ganglion cell.

    • Consequence: Rods are more sensitive to light (can detect dim light) due to summation of signals, but have lower visual acuity because the pooled signal loses spatial detail. This sensitivity is higher in the periphery than in the fovea.

  • Cones: Exhibit less convergence, meaning fewer cones (sometimes even a 1:1 ratio in the fovea) synapse onto a single ganglion cell.

    • Consequence: Cones have better visual acuity (can see fine details) because each cone's signal is more individually preserved, but are less sensitive to light (require brighter light). This acuity is highest in the fovea, where cone density and a lack of convergence are maximal.

Lateral Inhibition

• Mechanism: Photoreceptors, via lateral connections (horizontal and amacrine cells), inhibit their neighboring photoreceptors or downstream neurons. This process enhances contrast.

• Process: When a photoreceptor is stimulated by light (+$), it excites its direct pathway (+$) but also inhibits its neighbors ($-$) through lateral connections, leading to an amplified difference between stimulated and unstimulated areas.

Receptive Fields (RFs) of Retinal Ganglion Cells

• Definition: The specific pattern of light falling on the retina that elicits the strongest excitatory or inhibitory response from a particular neuron.

• Types of Retinal Ganglion Cell RFs: These are typically concentric.

  • On-Centre RF:

    • Structure: Has an excitatory (+$) center and an inhibitory (-$) surround.</p></li><li><p><strong>Response to Light:</strong></p><ul><li><p>Light falling <em>only</em> on the center elicits a strong excitatory response (increased firing rate).</p></li><li><p>Light falling <em>only</em> on the surround elicits an inhibitory response (decreased firing rate).</p></li><li><p>Light falling on <em>both</em> center and surround elicits a moderate response, as inhibition partially counteracts excitation.</p></li><li><p>Darkness across the entire field results in a baseline firing rate.</p></li></ul></li></ul></li><li><p><strong>Off-Centre RF:</strong></p><ul><li><p><strong>Structure:</strong> Has an inhibitory ($-$) center and an excitatory ($+$) surround.

    • Response to Light:

      • Light falling only on the center elicits an inhibitory response (decreased firing rate).

      • Light falling only on the surround elicits an excitatory response (increased firing rate).

      • Light falling on both center and surround elicits a moderate response, as excitation partially counteracts inhibition.

      • Darkness across the entire field results in a baseline firing rate.

Purpose of This Visual System Setup: Edge Detection

• Advantage: The center-surround organization of receptive fields is fundamentally designed for edge detection and contrast enhancement.

  • A uniform field of light or dark produces less neural response than a stimulus with a light-dark boundary (an edge).

  • Cells respond most vigorously when light falls optimally on either their excitatory center or surround, and darkness on the opposing region across an edge.

• Hermann Grid Illusion:

  • Description: When viewing a grid of black squares on a white background, illusory dark spots appear at the intersections of the white paths, but disappear when directly fixated.

  • Neural Explanation: Retinal ganglion cells with On-Centre receptive fields respond less intensely at intersections compared to the middle of the white paths. This is because at intersections, more of the inhibitory surround falls on the dark squares, leading to relatively less excitation compared to the mid-path where the surround is primarily on the white background.

Two Types of Retinal Ganglion Cells

1. Midget Ganglion Cells (P-ganglion cells):

   • Receptive Fields: Smaller receptive fields.

   • Dendrites: Smaller dendrites, indicating less convergence.

   • Visual Acuity: Contribute to greater acuity due to their small receptive fields and low convergence.

   • Sensitivity: Have less sensitivity to light, requiring higher illumination.

   • Neural Response: Exhibit a sustained neural response to stimuli, providing detailed and prolonged information about stationary objects.

   • Input to LGN: Primarily project to the parvocellular layers of the LGN.

2. Parasol Ganglion Cells (M-ganglion cells):

   • Receptive Fields: Larger receptive fields.

   • Dendrites: Larger dendrites, indicating more convergence.

   • Visual Acuity: Contribute to lower acuity due to their large receptive fields and high convergence.

   • Sensitivity: Possess greater sensitivity to light, good for detecting dim stimuli.

   • Neural Response: Exhibit a transient neural response to stimuli, firing strongly at the onset or offset of a stimulus, important for detecting motion and changes.

   • Input to LGN: Primarily project to the magnocellular layers of the LGN.

The Blind Spot

• Location: The area on the retina where the optic nerve exits the eye.

• Composition: Composed solely of ganglion cell fibers forming the optic nerve; it lacks photoreceptors (rods and cones).

• Perception: Due to the absence of photoreceptors, this area cannot detect light, creating a physiological blind spot in our visual field. However, our brain typically fills in the missing information, making it unnoticeable in everyday vision.

Neural Pathways from the Retina

• Optic Nerve: Neural signals exit the retina via the optic nerve, which is composed of the axons of retinal ganglion cells.

• Retinotopic Organization: The organization of visual information in the retina is preserved as it travels through the optic nerve and subsequent visual pathways. This means adjacent points in the visual field are processed by adjacent areas in the brain.

• Optic Chiasm: At the optic chiasm, fibers from the nasal (medial) halves of both retinas cross over to the contralateral side of the brain, while fibers from the temporal (lateral) halves remain ipsilateral.

  • Result: This crossover ensures that the left visual field (from both eyes) is processed by the right hemisphere, and the right visual field (from both eyes) is processed by the left hemisphere.

• Optic Tract: After the chiasm, the fibers form the optic tracts, which continue towards the brain.

The Lateral Geniculate Nucleus (LGN)

• Location: Most neural signals from the optic tract travel to the Lateral Geniculate Nucleus (LGN), a primary relay station in the thalamus.

• Structure: The LGN is composed of 6 distinct layers.

  • Magnocellular Layers (Layers $1$ & $2$):

    • Cell Size: Contain larger cells.

    • Receptive Fields: Have large concentric receptive fields.

    • Sensitivity & Acuity: Characterized by high sensitivity to light but lower acuity.

    • Input: Receive input primarily from parasol (M-type) retinal ganglion neurons.

    • Function: Specialized for processing gross features, motion, and transient changes.

  • Parvocellular Layers (Layers $3$ through $6$):

    • Cell Size: Contain smaller cells.

    • Receptive Fields: Have small concentric receptive fields.

    • Sensitivity & Acuity: Characterized by high acuity but lower sensitivity to light.

    • Input: Receive input primarily from midget (P-type) retinal ganglion neurons.

    • Function: Specialized for processing fine details, color, and sustained features.

• Eye Input to LGN Layers: The LGN maintains separation of input from the two eyes.

  • Contralateral Eye Input: Layers $1$, $4$, and $6$ receive input from the contralateral eye (the eye on the opposite side of the body).

  • Ipsilateral Eye Input: Layers $2$, $3$, and $5$ receive input from the ipsilateral eye (the eye on the same side of the body).

Optic Radiations and Primary Visual Cortex

• Optic Radiations: After processing in the LGN, neural signals are sent via the optic radiations to the primary visual cortex.

• Primary Visual Cortex (Striate Cortex/V1): Located in the occipital pole, this is the main cortical area for initial visual processing, maintaining the retinotopic organization established earlier in the pathway.

Visual Pathway Lesions

• The diagrams show various potential sites of lesions along the visual pathway (optic nerve, optic chiasm, optic tract, optic radiation, lateral geniculate body).

• Each lesion at a specific point on the pathway results in characteristic and predictable defects or losses in portions of the visual field of one or both eyes (e.g., monocular blindness, bitemporal hemianopia, homonymous hemianopia).