Study Notes on Sensation and Perception

Key Learning Goals

• Explain how stimulus intensity is related to absolute thresholds and JNDs.

• Summarise signal-detection theory.

• List the three properties of light and the aspects of visual perception they influence.

• Describe the role of the lens, pupil, and retina in the functioning of the eye.

• Trace the routing of signals from the eye to the brain, and explain the brain's role in visual information processing.

• Distinguish between two types of color mixing, compare main theories of color vision, and explain individual variations in the perception of color.

4.1 The Visual System: Essentials of Sight

• Humans are predominantly visual animals, often equating sight with trustworthiness (e.g., "seeing is believing").

• Visual perception is fundamentally complex, reflecting a distinction between what is sensed (light) and what is perceived (meaning attached to visual stimuli).

  • Example: Seeing a hug between a person and a sibling, recognizing the person, but interpreting the emotional weight of that action can vary based on context.

4.1.1 The Stimulus: Light

• Light: A form of electromagnetic radiation that travels as waves at the speed of light.

• Physical properties of light:

  • Amplitude: Height of light waves; affects brightness perception.

  • Wavelength: Distance between wave peaks; affects color perception.

  • Purity: Refers to mix variations of light; affects color saturation (richness).

  • Saturation: Relative amount of whiteness in a color; as whiteness decreases, saturation increases.

• Humans perceive only a small portion of the electromagnetic spectrum, called the visible spectrum (wavelengths identified from approximately 400 to 700 nm).

• Comparison with other species:

  • Insects can see ultraviolet light; fish and reptiles can detect infrared wavelengths.

4.1.2 Stimulus Intensity and Thresholds

• Threshold: The point at which a stimulus triggers a response.

  • Absolute threshold: The minimum stimulus intensity needed for detection (e.g., sound being heard when the radio volume is increased).

  • Philosophical discussions arise regarding the absolute nature of these thresholds, suggesting gradual changes can lower thresholds.

• Just Noticeable Difference (JND):

  • The smallest difference in stimulus intensity that a person can detect, varies subjectively among individuals (e.g., taste test with sugar).

• Signal-Detection Theory:

  • A framework that explains how stimuli are detected based on both the psychological state of the individual and the stimulus intensity itself.

  • This theory acknowledges that factors like personal experience, motivation, and biases impact perception and detection of stimuli.

4.1.3 The Eye: A Living Optical Instrument

• Structure of the eye includes:

  • Cornea: The front, transparent layer that helps focus light.

  • Lens: Adjusts focusing through the process called accommodation, where shape changes to focus on near or distant objects.

  • Nearsightedness (myopia): Difficulty seeing distant objects because light focuses in front of the retina.

  • Farsightedness (hyperopia): Difficulty seeing close objects because light focuses behind the retina.

  • Iris: The colored part of the eye that controls the size of the pupil, regulating light entry.

  • Pupil: Opening that adjusts based on light intensity and focus; can constrict or dilate accordingly.

  • Retina: Light-sensitive layer where images are formed and processed before being sent to the brain.

  • Contains rods (for low-light vision) and cones (for color vision).

4.1.4 The Retina: Signals to the Brain

• Neural Processing in the Retina:

  • The retina processes visual information via a network of specialized cells and compiles it into neural signals that travel via the optic nerve to the brain.

  • Blind spot: Area at the optic disk where the optic nerve exits the eye, lacking sensory receptors (normally compensated for by the other eye).

  • Visual Receptors:

  • Rods: Enable vision in low light, concentrated on the outer areas, involved in peripheral vision.

  • Cones: Responsible for color vision and sharp detail, concentrated in the fovea.

4.1.5 Visual Information Processing

• Visual information is transmitted through the optic nerves to the optic chiasm, where half of each optic nerve crosses to the opposite side of the brain, ensuring signals from both eyes are merged in the respective hemispheres.

• Pathways:

  • Primary pathway relays signals to the thalamus before reaching the primary visual cortex in the occipital lobe.

  • Secondary pathways involve the superior colliculus and deal with integrating multisensory information and spatial orientation.

• Feature Detectors: Neurons in the visual cortex respond specifically to certain visual features like edges and lines, forming the basis for recognizing complex stimuli.

• Two processing streams:

  • Ventral stream (the "what" pathway) processes object identity (form and color).

  • Dorsal stream (the "where" pathway) processes spatial location and movement.

4.1.6 Viewing the World in Colour

• Color perception is primarily influenced by wavelength; humans can perceive millions of colors largely through primary combinations of red, green, and blue.

• Color Mixing:

  • Subtractive Mixing: Removal of wavelengths (common with pigments).

  • Additive Mixing: Combination of wavelengths (common with light sources).

• Theories of Color Vision:

  • Trichromatic Theory: Proposed that three types of cones with varying sensitivity to different wavelengths enable color perception.

  • Opponent Process Theory: Suggests color perception is controlled by opposing pairs (red-green, blue-yellow).

• Psychological effects include color blindness, which often relates to specific deficiencies in these color receptor systems.

Conclusion

These notes summarize key aspects of visual sensation and perception, detailing the complex mechanisms underlying how humans interpret visual stimuli and the physiological processes involved. Understanding both the physical properties of stimuli and individual differences in perception remains crucial in psychology.