Lecture 12 - notes summary

Announcements

• Lecture material beyond this point will not be found in the book.

• Importance of keeping up with notes, lecture material, and assigned readings.

Today's Lecture: The Visual System

I. Mechanisms for the Formation of an Antagonistic Surround

• A. Key Mechanism: Horizontal Cell

  • Horizontal cells play a critical role in forming the antagonistic surround in visual processing.

• B. Light Interaction Diagram

  • Diagram demonstrates the function of a horizontal cell (HC) when light shines on a photoreceptor (PR) within the bipolar cell (BP) and retinal ganglion cell (RGC) surround:

    • Responses when light hits the center of ON center receptive field.

    • Responses when light falls on the surround of the ON center RF.

II. Effects of Light on Bipolar Cells and RGCs

• C. Antagonistic Effects of Light

  • Light in the surround produces an opposite effect on the membrane potential of a photoreceptor compared to light in the center.

  • This is facilitated by the horizontal cell's synaptic connections between photoreceptors in the center and surround of bipolar cells and RGCs.

• D. Bipolar Cell Functionality

  • Reference Figure 9.22 in Bear et al. to see synaptic inputs from photoreceptors in a bipolar cell’s receptive field.

III. Outputs from the Retina

• A. Diagram Overview of Processing

  • Represents general features of serial and parallel processing of retinal outputs.

  • Important to note the massive scale of parallel and serial processing within the visual system.

Different Functions of Visual System Components

1. Superior Colliculus

   • Acts as a tracking system for orienting the eyes to visual, auditory, and somatosensory stimuli.

2. Suprachiasmatic Nucleus (SCn)

   • Functions as the brain's clock controlling sleep/wake cycles.

   • Light input from the retina re-establishes the SCn clock with sunrise and sunset events.

IV. Visual Mapping and Projections

• B. Visual Information Mapping

  • The visual system maintains orderly point-to-point mapping characterized as:

    • Visuotopic Map: Mapped according to visual field coordinates.

    • Retinotopic Map: Mapped according to retinal coordinates.

• C. Mechanisms Behind Orderly Representations

  • The orderly location of RGCs in the retina ensures organized point-to-point neuronal projections throughout visual brain areas.

  • Diagrams illustrating right and left hemispheric projections from nasal and temporal hemiretinas:

    • Nasal projections to contralateral brain side.

    • Temporal projections to ipsilateral brain side.

V. Visuotopic Mapping Mechanism

• Diagram Analysis

  • Bird’s Eye View: Explains how extreme peripheral light is processed only by the ipsilateral eye (blocked by the nose).

  • The superior colliculus on either side contains complete visual representations of the contralateral visual field.

  • Mechanism: orderly anatomical connections maintain visuotopic map in the brain.

  • RGCs from each eye converge on the same neurons in the superior colliculus.

VI. Integration of Information Across Sensory Modalities

A. Multimodal Integration in the Superior Colliculus

1. Distinct Layers in the Superior Colliculus

   • Each layer responds to different sensory modalities:

     • Visual, auditory, and somatosensory stimuli are aligned in the colliculus.

   • Example: Visual stimulus at location B leads to firing in corresponding auditory neurons at that location.

2. Motor Layer Functionality

   • Stimulation in this layer can result in eye and head movement towards the processed stimuli.

VII. Conclusion

• Integration of different sensory modalities occurs effectively in the brain using mechanisms within the superior colliculus, highlighting the connectivity and functional specialization in neural processing.

The visual system processes information through a series of steps that involve specialized cells and mechanisms in the retina and brain. Here’s a simplified explanation:

1. Horizontal Cells and Antagonistic Surround: In the retina, horizontal cells help create an antagonistic surround. This means that when light hits the center of a photoreceptor, it activates a response, while light hitting the surrounding area causes an opposite response. This contrast helps in detecting edges and differences in light intensity.

2. Bipolar Cells and Retinal Ganglion Cells (RGCs): Bipolar cells relay information from photoreceptors to RGCs. The response difference between light in the center versus the surround is critical for how these cells process visual information. So, light in the center excites the bipolar cell while light in the surround inhibits it, which helps in refining the visual signal.

3. Output Processing: The outputs from the retina are processed in both serial and parallel ways. This means that while some information is processed step-by-step, other information can be processed simultaneously, creating efficiency in how we perceive the visual world.

4. Visual Mapping: The brain organizes visual information in maps. There are visuotopic maps that represent how we see the world based on visual coordinates and retinotopic maps that represent it based on retinal coordinates. This organization helps the brain to quickly interpret the visual input it receives.

5. Integration of Sensory Information: The superior colliculus integrates information from different senses (like sight and sound). It has distinct layers that respond to visual, auditory, and physical sensations, allowing for a coordinated response to stimuli. For example, if you see something at a specific location, your brain can quickly align auditory information with that visual input, directing your attention and movement towards it.

Overall, the visual system relies on intricate connections and mechanisms to help us interpret and respond to what we see.