2) Perception II

Cognitive Psychology 1 - Perception II: Brightness and Colour

• The stimulus for vision is visible light, a band of energy within the electromagnetic spectrum; its wavelength (difference in peaks of the electromagnetic waves) and intensity/luminance (amount of photons).

• Light can be described as consisting of small packets of energy called photons.

• Light intensity/luminance is associated with brightness perception. Different wavelengths of light are associated with different colour perceptions.

• Brightness and colour are perceptual properties, not physical ones.

• Light interacts with the environment through absorption, reflection, and transmission.

• Different materials reflect and absorb different wavelengths of light, affecting our perceptions of brightness and colour.

• The human eye has a single chambered structure with a convex cornea and lens projecting an image onto the retina.

• This structure enables directional sensitivity—the ability to detect the spatial structure of light rather than just its total amount.

• Photoreceptors, including rods and cones, transduce light into electrical signals.

Two Types of Photoreceptors: Rods and Cones

• Rods are located primarily in the peripheral retina and are sensitive to low light levels. They can detect single photons.

• Cones are concentrated in the centre of the retina (fovea) and require higher light levels (daylight) and respond to light.

• Cones have 3 different photopigments, each sensitive to different wavelengths of light (short, medium, and long).

The Visual Pathways

• Visual information is transmitted from the retina to the brain via the optic nerve.

• The pathway includes the optic chiasm, lateral geniculate nucleus (LGN), and primary visual cortex (V1).

Brightness Perception

• Light intensity is linked to perceived brightness (higher intensity regions usually appear brighter).

• Brightness perception is influenced by both bottom-up and top-down processes.

• Bottom-up factors include: light intensity and processes like light/dark adaptation and lateral inhibition.

• Top-down factors include the brain's use of knowledge about how light interacts with objects, including identifying shadows.

Brightness Perception - Light/Dark Adaptation

• The retina's sensitivity is constantly adjusted to compensate for changes in mean luminance.

• Sensitivity is reduced with high light intensity and increased in low light.

• Light/dark adaptation refers to this process. It dictates how contrast (the ratio of object's luminance to the background/mean luminance) affects brightness perception, playing a crucial role in brightness constancy.

Brightness Perception - Negative Afterimages

• Light/dark adaptation produces illusions.

• For example, staring at a red object might create a green afterimage.

Brightness Perception - Lateral Inhibition

• Lateral inhibition is an early information processing form in the retina.

• Retinal ganglion cells receive excitatory and inhibitory input from neighboring photoreceptors (thus arranged in a centre-surround) influencing perception of edges and borders of objects.

• The effect of lateral inhibition can be seen in the Hermann grid illusion.

Brightness Perception - Top-Down Influences

• Visual systems use knowledge about how light interacts with three-dimensional objects to perceive brightness (e.g., shadows affecting brightness constancy).

• This process can lead to perceived errors in 2D images portraying 3D scenes, such as the checker-shadow illusion.

Colour Perception

• Colour is a perceptual property associated with the wavelength of light.

• Normal human colour perception relies on the relative output of three types of cones (trichromacy).

• Colour perception is influenced by bottom-up processing (e.g., photopigments in cones) and top-down processing (e.g., knowledge of the environment).

Colour Perception - Trichromacy

• Cone photoreceptors contain one of three different photopigments, each with different wavelength sensitivities: S-cones (blue), M-cones (green), and L-cones (red).

• The relative outputs of these three cone types allow for unambiguous signalling of wavelength.

Colour Perception - Variations from Trichromacy

• Monochromacy: individuals with either 0 or 1 functioning cone type, resulting in complete colour blindness.

• Dichromacy: individuals with only 2 functioning cone types (protanopia, deuteranopia, tritanopia).

• Anomalous trichromacy: a more common form of colour perception deficiency, characterized by a defect in one of the cone types. Individuals with such differences are sometimes diagnosed using Ishihara colour tests.

Colour Perception - Opponency

• Retinal ganglion cells receive excitatory and inhibitory input from different cone types, creating distinct red/green and blue/yellow pathways.

Colour Perception - Negative Afterimages

• Colour opponency is demonstrated through negative afterimages (e.g., staring at red creates a green afterimage).

The "Lilac Chaser" Revisited

• This illusion is a demonstration of colour perception, likely due to retinal processing and adaptation and lack of constancy.

Colour Perception - Top-Down Influences

• Visual systems account for the intensity and composition of light hitting different surfaces, affecting colour perception. This can create illusions where the same wavelength of light is perceived differently due to how light interacts with and reflects on objects.

#TheDress Revisited

• This phenomenon highlights individual differences in colour perception and variations in the brain's interpretation of the interplay of light reflected off objects and the surrounding light.

Summary of the Learning Objectives

• Understand the stimulus for vision and basic structure of the human visual system

  • Visible light is a band of energy
  • Wavelength and intensity are components
  • Eye structure (cornea, lens, retina, rods, cones)

• Describe the factors that shape the perception of brightness

  • Bottom-up: Light intensity, adaptation, lateral inhibition
  • Top-down: Knowledge of environment (shadows)

• Understand the principles of normal and abnormal colour perception

  • Trichromacy (3 cones)
  • Variations (monochromacy, dichromacy, anomalous trichromacy) and tests (Ishihara)

Explore the fascinating aspects of visual perception in this quiz. Test your knowledge on the nature of light, its interaction with the environment, and how the human visual system processes brightness and color. Dive deep into concepts like color constancy and the roles of different photoreceptors.