Lecture 2: Color Vision and Perception

Trichromatic Theory: Originated first, proposes three types of cones sensitive to different wavelengths:
- Short-wavelength cones (S-cones) are most responsive to blue light.
- Medium-wavelength cones (M-cones) are most responsive to green light.
- Long-wavelength cones (L-cones) are most responsive to red light.
Opponent Process Theory: Suggests that some colors are perceived in opposing pairs:
- Pairs include red vs. green, blue vs. yellow, and black vs. white.
- This theory explains phenomena such as afterimages, where staring at a color leads to seeing its opponent after looking away.
Evidence for these theories is gained through psychophysical experiments contrasting color perception from different stimuli.

Trichromatic theory substantiated via physiological data from cone cells in the retina, where cones are selectively responsive to specific wavelengths.
The opponent process theory is confirmed by physiological evidence from ganglion cells, which exhibit activity that reflects the opponent color relationships; for instance, certain ganglion cells may increase firing rates in response to red light while decreasing for green light.

Experiment: Using colored lenses (red, green) over the eye can show chromatic adaptation.
- After prolonged exposure, removing the lenses leads to altered color perception, revealing adaptation effects.
Mechanism: Prolonged exposure to one color, like red, temporarily reduces the sensitivity of corresponding neurons; thus, once vision returns to normal, colors may appear shifted in the opposite direction (e.g., everything may seem greenish).

Observed in experiments where colors appear to alter based on surrounding colors, demonstrating a visual phenomenon.
- Highlighted by examples where identical colors appear different depending on context.
- Example: A gray square surrounded by a vibrant multi-colored background may appear tinted (e.g., slightly blue or yellow) due to the influence of surrounding hues.

Discovered by Celeste McCullough in the 1960s, this effect represents long-lasting aftereffects showing how visual adaptation to color can persist.
Adaptation based on horizontal and vertical orientation leads to sustained shifts in color perception when visual focus changes or the head is turned.

Staring at a colored stimulus may produce afterimages, shedding light on the opponent processes.
Color constancy explains how objects retain the same perceived color, regardless of changes in lighting or context due to adjustments made by the visual system; for example, a white shirt appears white both in sunlight and under artificial light.

Cancellation Experiments: Measure how much of one color is needed to cancel the perception of another, revealing the strength of color mechanisms.
- Such experiments show that various combinations yield curves indicating the effectiveness of cancellation, aiding in understanding color processing.

Ganglion Cells: Serve as the primary neurons for processing color by forming excitatory and inhibitory connections depending upon the wavelength sensitivity of cones.
- The concept of ‘neural tinker toys’ illustrates how combinations of excitation and inhibition combine to give rise to the rich tapestry of color perception.

Common types include:
- Protanopia: Absence of red cones, affecting red-green discrimination.
- Deuteranopia: Absence of green cones, leading to similar disorientation in color perception.
- Tritanopia: Absence of blue cones, less common and often leading to difficulties distinguishing blue from yellow.
Color deficiencies have implications for daily life, aesthetics, and certain occupations (e.g., professions in arts and sciences), with more prevalence in males due to genetic inheritance patterns.

Subjective Color: Refers to colors experienced through stimulation, like spinning disks, perceived by the brain without corresponding direct wavelength changes.
- This defines instances where the brain fills in perceived colors based on context and stimuli, even when those colors aren't physically present.

The complex interplay of physiological mechanisms, adaptation processes, and perceptual organization play crucial roles in shaping our color experience.
Ongoing research is essential to delve deeper into understanding the intricasies of phenomena like color constancy and subjective experiences in everyday life.

Recommended reading on sensory perception and the concept of ‘Umwelt’ is essential for insight into the limitations of human perception when juxtaposed with experiences of other species.