Physiology of Behavior: Chapter 6 - Vision

Chapter 6: Vision

Principles of Sensation and Perception

  • Sensation: The act of detecting an external stimulus.

  • Perception: The organization and interpretation of sensations.

  • Transduction: The process of changing an external stimulus (energy) into neural signals. In vision, this is performed by photoreceptors (specialized neurons that convert light energy into neural signals).

Topics Covered

  • Light

  • The Eye

  • Visual Pathways

  • Perceiving Color

  • Perceiving Form

  • Perceiving Distance

  • Perceiving Orientation and Movement

Light

  • Visible light is a narrow band of the electromagnetic spectrum.

  • Three Dimensions of Light Important for Vision:

    • Hue: The dominant wavelength of light.

    • Brightness: The intensity of light.

    • Saturation: The relative purity of light.

The Eye

Anatomy

  • Components:

    • Cornea

    • Iris

    • Pupil: Opening in the iris allowing light into the eye.

    • Lens: Focuses light onto the retina (lens accommodation).

    • Vitreous Humor: Large fluid-filled area behind the lens.

    • Retina: Rear of the eye containing three cell layers (including photoreceptors).

    • Fovea: Center of the retina, rich in cones, providing the best visual acuity.

    • Optic Disk: Blind spot.

  • Six Extraocular Muscles: Control eye movements.

Retinal Layer Anatomy

  • Layers of Retina:

    • Photoreceptor Layer: Contains rods and cones.

    • Bipolar Cell Layer: Contains bipolar and other support cells; integrates signals from photoreceptors to ganglion cells.

    • Ganglion Cell Layer: Contains ganglion cells that send signals to the brain.

Photoreceptors: Cones and Rods

  • Cones:

    • Most prevalent in the fovea (6 million).

    • Provide information about hue (color vision), excellent acuity, and require moderate to high light levels.

  • Rods:

    • Most prevalent in peripheral retina (120 million).

    • Sensitive to low levels of light (night vision) and provide monochromatic information with poor acuity.

Phototransduction Mechanism

  • Photoreceptors contain photopigments that react with light:

    • Depolarization: Occurs when Na+ channels are kept open by cGMP.

    • In light: cGMP is broken down, causing hyperpolarization, leading to less neurotransmitter release (glutamate).

ON and OFF Bipolar Cells

  • ON Bipolar Cells:

    • Have metabotropic glutamate receptors.

    • Become depolarized with less glutamate in light and release glutamate to ganglion cells.

  • OFF Bipolar Cells:

    • Have ionotropic receptors, remain activated in the dark, and respond less in light.

Visual Pathways Overview

  • Optic Nerves: Comprised of axons from ganglion cells.

  • Optic Chiasm: Where optic nerves converge; information split by visual fields—left visual field to the right brain and right visual field to the left brain.

  • Signals pass from optic chiasm to the lateral geniculate nucleus (LGN) of the thalamus and onto the primary visual cortex (V1).

Lateral Geniculate Nucleus (LGN)

  • Consists of six layers of neurons:

    • Layers 1, 4, and 6 receive input from the contralateral eye.

    • Layers 2, 3, and 5 receive input from the ipsilateral eye.

  • Functional systems:

    • Magnocellular Layers: Analyze motion.

    • Parvocellular Layers: Analyze form and color.

    • Koniocellular Sublayers: Process color.

Striate Cortex

  • Primary Visual Cortex (V1):

    • Highly organized with 6 layers.

    • Receives input from the LGN, combining visual stimuli from various sources beyond single ganglion cell inputs, responding to features larger than the field of a single ganglion cell.

Extrastriate Cortex

  • Important for further processing visual information:

    • Dorsal Stream: Processes object location and movement.

    • Ventral Stream: Processes object identity and color.

Perceiving Color

Role of Cones

  • Three types of cones correspond to:

    • Blue

    • Green

    • Red

  • Color blindness results from deficiencies in the cones' photopigments.

Role of Ganglion Cells

  • Retinal ganglion cells utilize an opponent-color system with pairs of opposing colors:

    • Red-green cells,

    • Yellow-blue cells,

    • Other cells detect black & white.

Perceiving Form

Ventral Stream Function

  • Neurons in the inferior temporal cortex recognize specific shapes and patterns.

  • Visual Agnosia: Damage may impair abilities to recognize items despite intact visual acuity.

Perceiving Distance

Depth Perception

  • Crucial for accurately locating objects:

    • Monocular vision: Based on pictorial cues.

    • Binocular vision: Involves stereopsis and retinal disparity, assisting in precise depth perception.

Effects of Damage

  • Damage to the posterior parietal lobe can lead to loss of depth perception and monocular vision.

Perceiving Orientation and Movement

Striate Cortex

  • Most neurons respond selectively to line orientation.

Extrastriate Cortex Movement Processing

  • Area V5 responds to motion, and damage can result in akinetopsia (loss of motion perception).

These notes encapsulate the extensive study material on vision physiology, covering foundational concepts to anatomical details and neurological pathways necessary for understanding human vision.