Vision_Part 2

Chapter Overview: Visual Processing and Color Vision

1. Primary Visual Cortex (V1)

  • Location: Occipital cortex

  • Function: Processing visual information from retinal ganglion cells

  • Action potentials recorded using electrodes

2. Receptive Fields in Primary Visual Cortex

  • Cells in the primary visual cortex exhibit "on" and "off" receptive fields, which are similar in function and organization to those found in ganglion cells. These receptive fields are important for the processing of visual information. In detail:

  • On Cells: Respond positively to light stimulation in their receptive field center, increasing their firing rate when light is present.

  • Off Cells: Respond negatively to light stimulation in their receptive field center, decreasing their firing rate when light is present.

Simple Cells in the primary visual cortex are a specialized type of neuron that particularly contributes to the perception of visual stimuli through their unique characteristics:

  • Function: They are primarily responsible for orientation detection, meaning they can detect and respond to the angle at which a visual stimulus appears in their receptive field.

  • Excitation Mechanism: Simple cells exhibit excitation when bars of light or edges oriented in specific angles enter their receptive field. The cell's response can vary significantly depending on the precise orientation of the bar, allowing for complex visual processing.

  • Inhibition Mechanism: These cells are also subject to inhibition when light falls on the surrounding areas of their receptive fields, which further refines their response to edges and contours in visual stimuli. This characteristic enables the visual cortex to enhance contrast and discern object boundaries effectively.

  • Spatial Frequency Sensitivity: Simple cells can be sensitive to different spatial frequencies, meaning they can detect fine details as well as broader, more diffuse patterns, essential for recognizing shapes and textures in the visual environment.

  • Simple Cells:

    • Function: Orientation detection

    • Excitation: Bars of light at specific orientations

3. Response to Movement

  • Simple visual cortex cells respond to the direction of moving bars, with movement to the left or right leading to a decrease or inhibition of their response.

4. Ventral and Dorsal Streams

4.1 Ventral Stream (What Pathway)

  • Location: Temporal lobe

  • Neuron Response: Excited by combinations of visual features

  • Function: Object Recognition (size, texture, color)

    • Important for recognizing complex visual patterns (e.g., faces)

4.2 Dorsal Stream (Where/How Pathway)

  • Location: Parietal lobe

  • Neuron Response: Processes visual information for action

  • Function:

    • Locating objects and determining how to use them

    • Object recognition via touch and action

    • Involvement of mirror neurons in grasping intentions and goals

5. Effects of Damage in Visual Streams

5.1 Ventral Stream Damage

  • Symptoms:

    • Inability to recognize objects by sight

    • Can replicate complex visuals but struggles with identifying them

    • Can utilize structural features for movement guidance

5.2 Dorsal Stream Damage

  • Symptoms:

    • Recognizing objects but difficulty representing locations

    • Difficulty converting vision into action

    • Impairments in navigating and visually tracking moving objects

6. Color Vision

6.1 Basics of Color Perception


Color Vision - Cones and Trichromatic Theory

Three Types of Cones

  • S-cones (short wavelength cones): Sensitive to short wavelengths, primarily around 420 nm, responding best to blues and violets.

  • M-cones (medium wavelength cones): Sensitive to medium wavelengths, primarily around 530 nm, responding best to greens.

  • L-cones (long wavelength cones): Sensitive to long wavelengths, primarily around 560 nm, responding best to reds.

Each cone type has photopigments that are sensitive to specific ranges of light wavelengths due to the unique composition of opsins within the photoreceptor cells.

Trichromatic Theory

  • According to the Trichromatic Theory, color perception arises from the relative activation of these three types of cones.

  • The human eye can perceive a broad spectrum of colors, thanks to the combination of responses from the three cone types covering wavelengths from approximately 350 nm (ultraviolet) to 700 nm (infrared).

  • The brain interprets colors based on the ratio of activity across these cones. For example, a burst of activity in L-cones combined with moderate M-cone activity might be interpreted as yellow.

  • This theory explains color discrimination and how individuals perceive color. However, it does not fully account for color vision phenomena like afterimages and color constancy, which are addressed by alternative theories such as the Opponent Process Theory.

  • Trichromatic Theory:

    • Each cone type responds to different wavelengths (350 nm to 700 nm)

  • Ratio of cone activation determines perceived color

6.2 Color Blindness

  • Types: Lack of red or green photopigments

  • Red-Green color blindness: More common in males due to X-linked genes

7. Afterimages and Opponent Process Theory

7.1 Afterimages

  • Concept: Rebound effect and fatigue effect explain afterimages

  • Physiological basis linked to ganglion neuron response rates

7.2 Opponent Process Theory

  • Color coding cells respond differently to opponent colors

    • Four basic colors: related to the firing rates of ganglion cells

    • Translates trichromatic responses into an opponent color system