chp 3 continue senses and persception

Introduction to the Eye and Vision

  • Rearrangement of Chapters

    • Some chapters have been rearranged, affecting the order of content in lectures.

    • Apologies for any discrepancies regarding content order, particularly between chapters three and four regarding blind spots.

Anatomy of the Eye

  • Iris

    • The colored part of the eye is called the iris.

  • Eye Color Changes

    • Question: Do eyes change over time?

    • Answer: Possible but usually subtle; significant changes should prompt a visit to an eye doctor.

    • Example: Arcuous senilis - a blue, white, or gray ring that can develop around the iris due to fatty deposits with age.

    • Important Notes:

    • If the change occurs in one eye or someone under 40, see an eye doctor.

    • Change in eye color may indicate cataracts.

Neural Anatomy of the Retina

  • Introduction to Retinal Cells

    • Discussed optic nerve and optic disc from previous lectures.

    • Types of cells in retina: rod cells, cone cells, ganglion cells, and additional cells (bipolar, horizontal, amacrine).

  • Bipolar Cells

    • Function: Receive graded potentials from photoreceptors.

    • Activity: Most active during changes in illumination relative to background light.

    • Response: Affected by levels of glutamate.

  • Horizontal Cells

    • Function: Connect with adjacent photoreceptors.

    • Purpose: Amplify contrast and define edges in the visual field.

  • Amacrine Cells

    • Function: Integrate and modify signals from bipolar cells.

Signal Transmission in the Eye

  • Mode of Communication

    • Neurons communicate through action potentials, which are electrical signals in nature.

    • Bipolar cells primarily receive graded potentials (excitation or inhibition).

  • Retinal Ganglion Cells

    • Only cells in the retina that generate action potentials and leave the eye, forming the optic nerve (cranial nerve II).

    • Generate action potentials in response to specifically localized light stimuli.

    • Convergence: Information from multiple photoreceptors converges to influence ganglion cell activity.

Ganglion Cell Types

  • Midget (P Cells)

    • Small in size with compact dendritic fields.

    • Receive input mainly from the fovea, important for color perception.

  • Parasol (M Cells)

    • Larger cells with a broader dendritic field.

    • Respond to movement and increased light sensitivity.

  • Experimental Study

    • Single-cell recording experiments in cats by Steve Kupper.

    • Procedure: Record activity from a fixed microelectrode in the retina while the cat focuses on a fixation point.

  • Receptive Fields

    • Definition: Area of retina associated with changes in firing of ganglion cells based on light stimulation.

    • Center-surround receptive fields:

    • On-center: Excitation from light in the center; inhibition from the surround.

    • Off-center: Inhibition from light in the center; excitation from the surround.

Visual Processing Pathways

  • Development of Visual Perception

    • Ganglion cells process and transmit visual signals to the brain.

    • Fibers from the optic nerve cross at the optic chiasm:

    • Right visual field info from both eyes travels to the left hemisphere, and vice versa.

  • Lateral Geniculate Nucleus (LGN)

    • 80% of optic nerve fibers project to LGN; 20% to midbrain structures (e.g., superior colliculus).

    • Organization: Retinotopic mapping preserved.

    • Specific layers correspond to inputs from different types of ganglion cells: magnocellular layers for M cells, parvocellular layers for P cells.

Primary Visual Cortex (V1)

  • Structure and Function

    • Comprised of six layers, retains retinotopic organization.

    • Receives input from LGN and transforms information about visual stimuli.

  • Simple and Complex Cells

    • Simple Cells: Respond to certain edges and orientations; organized in excitatory and inhibitory fields.

    • Complex Cells: Respond to moving bars of light; orientation-dependent.

    • Hypercomplex Cells: Respond to higher-order visual features (e.g., corners or angles).

Visual Pathways and Perception

  • Dorsal and Ventral Pathways

    • Dorsal Pathway (Where Pathway): Projects from V1 primarily to the parietal lobe, helping perceive spatial location and actions.

    • Ventral Pathway (What Pathway): Projects primarily to the temporal lobe, crucial for object identification and recognition.

    • Double Dissociation: Damage to one pathway affects one aspect of vision (e.g., recognizing objects visually) without affecting the other (e.g., locating objects in space).

Summary and Implications

  • Information processed by the retina translates into complex perceptions in the brain.

  • Disorders affecting specifics pathways lead to differing effects on perception, like blindsight and other visual anomalies.

  • Critical Periods: Importance of exposure during development for normal visual processing.

Conclusion

  • Advanced understanding of these systems offers insight into how our visual experience is constructed and the implications for disorders affecting perception.

  • Further exploration of these neural pathways can yield better comprehension of visual processing and the necessity of the retinal ganglion cell's activity to vision as a whole.