Chapter 4 - Sensation and Perception
Illusion, Sensation, and Perception
Illusion: A perception in which the way a stimulus is perceived does not match its physical reality.
Sensation: The detection of physical energy by sense organs, which then sends information to the brain.
Perception: The brain's interpretation of raw sensory inputs.
Transduction: The process of converting external energy or substances into electrical activity within neurons.
Sense Receptors: Specialized cells responsible for converting external stimuli into neural activity specific to a sensory system.
Sensory Adaptation: A process through which activation is greatest when a stimulus is first detected, allowing for diminished response over time.
Key Concepts:
Perception ≠ reality.
Psychophysics: The study of how sensory stimuli are perceived based on their physical characteristics.
Absolute Threshold: The lowest intensity of a stimulus that can be detected 50% of the time.
Just Noticeable Difference (JND): The smallest difference in intensity of a stimulus that can be detected.
Weber’s Law: Principle stating that there is a constant proportional relationship between the JND and the original stimulus intensity.
Signal Detection Theory: A theory about how stimuli are detected under various conditions, applied in areas such as lie detection and security screening.
Sensation and Perception in Depth
Naive Realism: The belief that the world is exactly as it appears is incorrect; our interpretation of stimuli is shaped by various factors.
Following concepts illustrate the relationship between sensation and perception:
Retina converts light waves into electrical impulses.
Sensory receptors—rods, cones, taste buds, hair cells—play crucial roles in sensation.
Example of sensory adaptation: Not noticing a fragrance after being exposed to it for a few minutes.
The brain actively constructs perception rather than merely recording sensory input; shaped by past experiences, expectations, and context.
Gustav Fechner (1860): Pioneer in the study of psychophysics.
Example: Detecting a candle flame from 50 km away on a clear night.
Example: Noticing a softest increase in sound volume on headphones.
Example: Detecting a 2g difference in weight depends on the original weight.
Four Possible Outcomes in Signal Detection:
Hit: Correctly detecting a stimulus.
Miss: Failing to detect a present stimulus.
False Alarm: Detecting a stimulus that is not present.
Correct Rejection: Correctly identifying the absence of a stimulus.
Attention and Perception
Synesthesia: A condition where people experience cross-modal sensations; e.g., seeing colors when hearing sounds.
Selective Attention: The process of focusing on specific sensory input while ignoring others.
Inattentional Blindness: Failing to notice stimuli right in front of us when attention is focused elsewhere (e.g., the Invisible Gorilla experiment).
Change Blindness: Noticing major changes in visuals during disruptions, like during a video cut.
Divided Attention: Splitting attention between tasks reduces accuracy (e.g., texting while driving).
Light is electromagnetic radiation, with properties including:
Wavelength: Determines hue (color).
Amplitude: Determines brightness.
Purity: Affects saturation (vividness).
Parts of the Eye:
Pupil: Circular hole for light entry.
Cornea: Transparent structure that focuses light.
Lens: Changes shape to focus images (accommodation).
Sir Francis Galton (1880): Coined the term synesthesia.
Mechanisms of Vision
Visual System:
Short Wavelength: Violet/blue; Long Wavelength: Red.
Color perception is psychological; we see light waves based on receptor stimulation.
Eye Functions:
Various parts control light entry and focus, adapting to different lighting conditions.
Retina: Converts light into neural activity.
Fovea: Center of the retina for sharp vision.
Acuity: Sharpness of vision depended on rods and cones in the retina.
Rods: Allow for low light vision.
Cones: Enable color vision.
Blind Spot: Area without rod or cone receptors.
Optic Nerve: Transmits information from retina to brain.
Sclera: Protective outer layer of the eye.
Aqueous and Vitreous Humours: Maintain shape of eyeball and nourish retina.
Nearsightedness (Myopia): Inability to see far; Farsightedness (Hyperopia): Inability to see close.
Visual Processing and Color Theories
Transduction Process for Light:
Light → Rods/Cones → Bipolar Cells → Ganglion Cells → Optic Nerve → Brain.
Feature Detector Cells: Detect lines and edges, foundational for visual perception.
Theories of Color Vision:
Trichromatic Theory: Color perception based on three primary colors sensitivity (red, blue, green).
Opponent-Process Theory: Colors perceived in opposing pairs (red-green; blue-yellow).
Modern View: Both theories apply at different processing levels: Trichromatic at receptor level; Opponent-Process at neural pathway level.
Visual Pathways and Disorders
Primary Visual Cortex (V1): Located in the occipital lobe.
Visual Association Areas (V2-V5): Specific areas for color and motion processing.
Ventral Stream: Responsible for object recognition (what).
Dorsal Stream: Handles spatial location and action guidelines (where/how).
Disorders Related to Vision
Visual Agnosia: Inability to recognize objects despite being able to see.
Optic Ataxia: Difficulty accurately reaching for seen objects.
Blindsight: Cortical blindness where individuals respond to unseen stimuli.
Color Blindness: Commonly red-green deficiency, especially in males.
Auditory System: Mechanics of Hearing
Frequency (Hz): Number of sound wave cycles per second; correlates to pitch (high vs. low).
Amplitude (dB): Measure of sound wave height/intensity, related to loudness.
Timbre: Quality of sound that makes sources (e.g., instruments) sound unique.
Outer Ear:
Pinna: Amplifies and funnels sound to the tympanic membrane.
Middle Ear Components:
Ossicles: Malleus (hammer), Incus (anvil), and Stapes (stirrup) transmit vibrations.
Process of Hearing
Cochlea: Spiral-shaped organ involved in hearing containing hair cells.
Transduction in Hearing:
Sound → Mechanical vibration → Fluid wave in cochlea.
Movement bends hair cells.
Ion channels open, causing electrical potential and neurotransmitter release.
Signals travel through the auditory nerve to the brain.
Place Theory: Specific locations on the basilar membrane match tones with certain pitches.
Frequency Theory: Rate of neuron firing represents pitch.
Volley Theory: Neurons alternate firing to represent higher frequencies.
Localization of Sound: Achieved via interaural time, intensity, and phase difference.
Disorders Related to Hearing
Conductive Hearing Loss: Issues in outer/middle ear; impedes sound transmission.
Sensorineural Hearing Loss: Damage to hair cells or the auditory nerve;
Tinnitus: Ringing or buzzing sound without an external source.
Presbycusis: Age-related loss of high-frequency hearing.
Cochlear Implants: Device to stimulate the auditory nerve for sensorineural loss.
Chemical Senses: Smell and Taste
Both systems use Chemoreceptors: Neurons responsive to specific chemicals.
Smell Detection Steps:
Odor molecules enter the nasal cavity dissolving in mucus.
Bind to olfactory receptor neurons in the olfactory epithelium.
Signals transmitted to the olfactory bulb and primary olfactory cortex.
Pheromones: Chemical signals affecting behavior; presence of vomeronasal organ (VNO).
Taste Detection Steps:
Food molecules dissolve in saliva at taste pores on the tongue.
Bind to taste receptor cells in taste buds.
Signals travel to thalamus and gustatory cortex.
Primary Taste Qualities
Traditionally five basic tastes:
Sweet: Sugars; energy source.
Sour: Acids; spoilage detection.
Salty: Sodium ions; electrolyte balance.
Bitter: Alkaloids; poison avoidance.
Umami: Glutamate; protein detection.
Flavour: Combination of taste, smell, texture, temperature, and pain (e.g., spiciness).
Supertasters: Higher sensitivity due to more taste buds; Non-tasters: Lower sensitivity.
Touch, Temperature, and Pain
The Somatosensory System sends signals about touch, temperature, and pain through afferent neurons to the brain.
Pain Fibers:
A-delta Fibers: Fast, sharp pain.
C Fibers: Slow, dull pain.
Temperature Detection: Through free nerve endings responsive to heat and cold.
Pain Pathway: Signals from pain receptors travel through the spinal cord to various brain regions.
Emotional and cognitive factors play crucial roles in pain perception and tolerance.
Integration of Touch, Proprioception, and Balance
Proprioceptors: Feedback on limb position.
Vestibular System: Monitors head position and movement.
Gate Control Theory of Pain: Non-painful input can potentially close the pain “gate” in the spinal cord, reducing perception of pain.
Perceptual Set and Context Effects
Top-Down Processing: Brain interprets sensory information based on stored knowledge and expectations.
Perceptual Constancies: Maintaining stable perception of objects despite changes in conditions (size, shape, color).
Gestalt Principles: Our brain organizes stimuli into coherent wholes based on principles like proximity, similarity, continuity, closure, and symmetry.
Depth Perception: Judging distance using binocular and monocular cues.
Illusions show how perception is influenced by context and brain interpretation.
Conclusion
Subliminal Perception: Exists but does not significantly influence behavior in a meaningful way.
Cross-Modal Perception: Integration of different sensory inputs leads to a coherent perceptual experience, demonstrating the brain's complex processing capabilities.