Chapter 4

Synesthesia

• Synesthesia is a condition in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway.

Sensation vs. Perception

• Sensation: The process by which our sensory receptors and nervous system receive and represent stimulus energies from our environment.

• Perception: The process of organizing and interpreting sensory information, enabling us to recognize meaningful objects and events.

• Difference: Sensation is about detecting stimuli, while perception is about understanding and making sense of those stimuli.

Transduction

• Transduction: The conversion of one form of energy into another. For each sense:

  • Vision: Light waves converted to neural impulses via photoreceptors in the retina.

  • Hearing: Sound waves converted to neural impulses by hair cells in the cochlea.

  • Touch: Pressure, temperature, and pain detected by skin receptors converted to neural impulses.

  • Taste: Chemical stimuli from food interact with taste buds, converted to neural impulses.

  • Smell: Olfactory neurons detect airborne molecules, transducing them into neural impulses.

Psychophysics

• Psychophysics: The study of the relationship between physical stimuli and the sensations and perceptions they produce.

• Absolute Threshold: The minimum intensity of a stimulus that can be detected 50% of the time.

• Just Noticeable Difference (JND): The smallest difference in stimulus intensity that a specific sense can detect.

• Weber’s Law: The principle stating that the JND is a constant proportion of the original stimulus intensity.

Signal Detection Theory & Sensory Adaptation

• Signal Detection Theory: A theory predicting how and when we detect the presence of a faint stimulus amid background noise.

• Sensory Adaptation: The diminished sensitivity to a stimulus as a consequence of constant exposure to that stimulus.

Vision

• We perceive light waves, which have three properties: amplitude (brightness), wavelength (color), and purity (saturation).

• Parts of the Eye:

  • Cornea: The eye's outer layer that helps focus light.

  • Pupil: The opening that lets light enter.

  • Iris: The colored part of the eye that controls the pupil size.

  • Lens: Focuses light onto the retina.

  • Retina: Contains photoreceptors (rods and cones) that transduce light into neural impulses.

• Accommodation: The process by which the eye adjusts its lens to focus on near or distant objects.

• Rods vs. Cones:

  • Rods: More sensitive to light, function well in low light, and do not detect color.

  • Cones: Function in bright light, responsible for color vision, concentrated in the fovea, which provides sharp vision.

• Blind Spot: The area of the retina lacking photoreceptors where the optic nerve exits the eye.

• Color Perception: Three types of cones respond to different wavelengths of light (red, green, blue). We perceive more than three colors through combination and interpretation of these signals.

• Color Afterimage: A visual sensation that remains after the stimulus is removed, due to the photoreceptors adapting to overstimulation.

• Feature Detectors: Neurons that respond selectively to specific features of a stimulus (e.g., edges, angles).

• Perceptual Constancy: The ability to perceive objects as stable despite changes in sensory input.

• Gestalt Principles: We perceive things as organized wholes, following rules like simplicity, closure, continuity, similarity, proximity, and common fate. Figure-Ground refers to the organization of the visual field into objects that stand out from their surroundings.

• Monocular Depth Cues: Cues available to each eye alone (e.g., relative size, linear perspective, texture gradient, interposition, relative height).

• Binocular Disparity: The difference in images between the two eyes, which helps with depth perception.

• Change Blindness: The phenomenon of not noticing changes in a scene.

• Inattentional Blindness: Failure to notice a fully visible but unexpected object because attention was engaged on another task.

Hearing

• We perceive sound waves, characterized by frequency (pitch), amplitude (loudness), and complexity (timbre).

• Parts of the Ear:

  • Pinna: Outer ear that captures sound.

  • Auditory Canal: Transmits sound to the eardrum.

  • Eardrum: Vibrates in response to sound.

  • Ossicles: Tiny bones that amplify sound.

  • Cochlea: Spiral-shaped organ where sound waves are transduced into neural impulses.

  • Hair Cells: Sensory receptors in the cochlea that convert mechanical energy from sound into neural signals.

  • Basilar Membrane: Structure in the cochlea that contains hair cells.

• Transduction in Hearing: Sound waves vibrate the eardrum, which moves the ossicles, sending fluid waves through the cochlea where hair cells transduce sound waves into neural impulses.

• Pitch Perception:

  • Place Code: Explains high-frequency sounds based on the location on the cochlea where hair cells are activated.

  • Temporal Code: Relates to low-frequency sounds by the timing of neural impulses. Temporal code works well for low frequencies; place code is more effective for high frequencies.

• Locating Sounds: Determining the location of a sound involves comparing the time and intensity of sounds reaching both ears.

Other Senses

• Haptic Perception: The ability to perceive and identify objects through touch, involving receptors in the skin that respond to pressure, temperature, and pain.

• Pain Receptors: Two types—fast (sharp, immediate pain) and slow (dull, aching pain).

• Gate Control Theory: Suggests that pain perception is controlled by a “gate” mechanism in the spinal cord that can either block or allow pain signals to reach the brain, influenced by psychological factors.

• Vestibular System: The system in the inner ear that helps maintain balance and spatial orientation; linked to motion sickness due to conflicts between visual and vestibular information.

• Smell: Detected through airborne chemicals via olfactory neurons, allowing us to identify about 1000 different odors despite having only around 350 olfactory receptor neurons (ORNs) through a combinatorial coding.

• Taste: Taste buds on the tongue detect five tastes: sweet, salty, sour, bitter, and umami, each with specialized receptors.