From Retina to Cortex

1.    Physiology of the Eye:

  • Pupil: Entry point of light.

  • Iris: Adjusts the pupil size based on light conditions.

  • Cornea & Lens: Focuses light onto the retina.

  • Accommodation: Ciliary muscles adjust the lens shape to focus on objects at varying distances.

  • Photoreceptors:

    • Rods: Sensitive to dim light, absent in the fovea.

    • Cones: Responsible for daytime vision; three types for different wavelengths (long, medium, short).

2.    Retina and Retinal Ganglion Cells:

  • Retina: Contains photoreceptors and retinal ganglion cells (RGCs) responsible for visual processing.

  • Receptive Fields (RFs): Each ganglion cell receives input from a part of the retina.

    • On-Centre, Off-Surround RFs: Excited by light in the centre, inhibited by light in the surround.

  • Retinal Ganglion Cell Types:

    • Parasol Ganglion Cells: Sensitive to movement and flicker.

    • Midget Ganglion Cells: Sensitive to colour and detail.

  • Lateral Inhibition: Enhances contrast, crucial for edge detection.

3.    Lateral Geniculate Nucleus (LGN):

  • Part of the thalamus, responsible for processing visual signals from the retina before they reach the cortex.

  • Layers of the LGN:

    • Magnocellular Cells: Motion and flicker.

    • Parvocellular Cells: Colour and detail.

    • Koniocellular Cells: Blue-yellow colour contrast.

  • Experiment Examples: Researchers use vertical/horizontal gratings to stimulate specific pathways in visual experiments.

 

 

4.    Primary Visual Cortex (V1):

  • Over 50% of the cortex is dedicated to vision.

  • Retinotopy: Each part of the visual field is mapped onto a specific location in V1.

  • Hubel & Wiesel Experiments: Studied orientation selectivity in the visual cortex, using visual stimuli to identify neuron responses.

  • Plasticity in Visual Development: Early experiences shape visual cortex development, as shown in the Blakemore & Cooper (1970) experiment with kittens.

5.    Beyond V1:

  • Two Visual Processing Streams:

    • Dorsal Stream (Where): Processes spatial awareness and movement.

    • Ventral Stream (What): Processes object identification and recognition.

  • Example of Motion-Colour Interaction: The Suchow & Alvarez (2011) illusion shows how motion can reduce the salience of colour changes.

6.    Neural Plasticity & Critical Periods:

  • Neural pathways are shaped by early experiences, and there are critical periods for development.

  • Example: Kittens raised in environments with only vertical or horizontal lines showed no response to orientations do not present during development, demonstrating "use it or lose it" principle.

7.    Hierarchy of Processing:

  • Processing becomes increasingly complex along the visual pathway, from simple feature detection in the retina to more abstract processing in higher brain areas like V1 and beyond.