MCAT Behavioral Science - Chapter 2: Sensation and Perception

Objectives

1. Sensation vs. Perception
   • Sensory Receptors
   • Thresholds
   • Signal Detection Theory
   • Adaptations
2. Vision
   • Structure and function of the eye
   • Visual pathways and processing
3. Hearing
   • Structure and function of the ear
   • Auditory pathways
   • Hair cells
4. Other Senses
   • Smell, Taste, Somatosensation, Kinesthetic sense
5. Object Recognition
   • Gestalt principles

Sensation vs. Perception

• Sensation: Conversion of physical, electromagnetic, auditory, etc., information into electrical signals in the nervous system. Performed by receptors in PNS and sent to CNS.
• Perception: Processing of sensory information to make sense of its significance. Combines external sensory experience and internal brain activities.

Sensory Receptors

• Neurons respond to stimuli, triggering electrical signals
• Encode multiple aspects of stimuli (e.g., photoreceptors for light)
• Types include:
  • Photoreceptors: Respond to electromagnetic waves
  • Hair cells: Movement of fluid in the inner ear
  • Nociceptors: Painful or noxious stimuli
  • Thermoreceptors: Changes in temperature
  • Osmoreceptors: Osmolarity of the blood
  • Olfactory receptors: Volatile compounds
  • Taste receptors: Dissolved compounds
• Pathways: Sensory ganglia -> central nervous system -> projection areas in the brain

Thresholds

• Absolute Threshold: Minimum stimulus energy to activate a sensory system
• Threshold of Conscious Perception: Minimum stimulus energy needed for conscious perception
• Difference Threshold: Minimum difference in stimulus magnitude to perceive a difference
• Weber's Law: Constant ratio between the change in stimulus magnitude and its original magnitude

Signal Detection Theory

• Perception affected by non-sensory factors like experience, memory, motives, expectations
• Response Bias: Tendency to respond to a stimulus in a certain way due to non-sensory factors
• Trials: Catch trials (signal present) and noise trials (signal absent)
• Outcomes: Hits, misses, false alarms, correct negatives

Adaptations

• Change in sensitivity over time, physiological (sensory) or perceptual (psychological) adjustments

Vision

Structure and Function of the Eye

• Cornea: Gathers and focuses incoming light
• Anterior and Posterior Chambers: Anterior (in front of iris), Posterior (between iris and lens)
• Iris: Colored part, controls pupil size (dilator and constrictor pupillae muscles)
• Lens: Refracts incoming light
• Retina: Contains photoreceptors (rods and cones)
  • Cones: Color vision, fine details
  • Rods: Light/dark detection, night vision
• Bipolar and Ganglion Cells: Transmit information to the optic nerve
• Amacrine and Horizontal Cells: Important for edge detection

Visual Pathways and Processing

• Optic Chiasm: Nasal fibers cross, temporal fibers do not
• Optic Tracts: Visual field information organized post-chiasm
• Pathway: Optic Chiasm -> lateral geniculate nucleus (LGN) of thalamus -> visual cortex (occipital lobe)
• Parallel Processing: Simultaneous analysis of color, shape, motion
  • Feature Detection: Specialized cells for color (cones), shape (parvocellular cells), motion (magnocellular cells)

Hearing

Structure and Function of the Ear

• Outer Ear: Pinna (auricle), external auditory canal, tympanic membrane (eardrum)
• Middle Ear: Ossicles (malleus, incus, stapes), Eustachian tube (equalizes pressure)
• Inner Ear: Bony labyrinth (cochlea, vestibule, semicircular canals), membranous labyrinth (filled with endolymph)
  • Cochlea: Hearing apparatus (Organ of Corti)
  • Vestibule: Linear acceleration, orientation (utricle, saccule)
  • Semicircular Canals: Rotational acceleration

Auditory Pathways

• Begins in cochlea -> Vestibulocochlear nerve -> brainstem -> medial geniculate nucleus (MGN) of thalamus -> auditory cortex (temporal lobe)
• Includes superior olive (localizes sound) and inferior colliculus (startle reflex)

Other Senses

Smell

• Olfactory Chemoreceptors: Located in olfactory epithelium
• Pathway: Olfactory nerves -> olfactory bulb -> olfactory tract -> higher brain regions, limbic system

Taste

• Basic Tastes: Umami, sour, sweet, bitter, salty
• Taste Buds: Papillae, sensitive to dissolved compounds
• Pathway: Taste buds -> brainstem -> thalamus -> higher brain regions

Somatosensation

• Four modules: Pressure, vibration, pain, temperature
• Receptors:
  • Pacinian corpuscles: Deep pressure, vibration
  • Meissner corpuscles: Light touch
  • Merkel cells: Deep pressure, texture
  • Ruffini endings: Stretch
  • Free nerve endings: Pain, temperature
• Key Concepts:
  • Two-point threshold: Minimum distance for two distinct stimuli
  • Physiological zero: Normal skin temperature (86-97°F)
  • Gate theory of pain: Spinal cord can preferentially forward signals to reduce pain perception

Kinesthetic Sense (Proprioception)

• Ability to tell the position of body parts in space, critical for coordination, balance, and mobility

Object Recognition

Bottom-Up vs. Top-Down Processing

• Bottom-Up Processing: Data-driven, combines individual sensory stimuli into a cohesive image
• Top-Down Processing: Conceptually driven, uses memories and expectations to recognize the whole object
• Perceptual Organization: Uses both processing types to create a complete picture

Gestalt Principles

• Law of Proximity: Elements close together are perceived as a unit
• Law of Similarity: Similar objects are grouped together
• Law of Good Continuation: Elements following the same pathway are grouped together
• Subjective Contours: Perceiving shapes not present in stimulus
• Law of Closure: Perceived as complete figures even if incomplete
• Law of Prägnanz: Perceptual organization is as regular, simple, and symmetric as possible

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Note: Refer to specific figures or diagrams related to eye and ear anatomy for detailed visualization.