👁️Perception Unit 1 – Sensory systems and processing

Key Concepts and Terminology

• Sensation involves detecting physical energy from the environment and encoding it as neural signals
• Perception interprets, organizes, and consciously experiences sensory information
• Transduction converts physical energy into electrical signals in the nervous system
• Sensory receptors are specialized cells or neurons that respond to specific types of stimuli
• Sensory thresholds determine the minimum intensity of a stimulus required for detection
  • Absolute threshold is the smallest detectable level of a stimulus
  • Difference threshold (just noticeable difference) is the smallest detectable change in a stimulus
• Sensory adaptation occurs when responsiveness to a constant stimulus decreases over time
• Signal detection theory explains how the presence of a stimulus is determined amid background noise

Sensory Receptors and Transduction

• Sensory receptors are specialized to detect specific types of stimuli (light, sound, pressure, chemicals)
• Receptors transduce physical energy into electrical signals through changes in membrane potential
• Receptor potential is a graded potential that varies with stimulus intensity
• Generator potential is a graded potential that triggers action potentials in sensory neurons
• Sensory neurons transmit action potentials to the central nervous system for processing
• Sensory receptors are classified based on their location and the type of stimulus they detect
  • Exteroceptors detect external stimuli (eyes, ears, skin)
  • Interoceptors detect internal stimuli (blood pressure, gut distension)
  • Proprioceptors detect body position and movement (muscle spindles, joint receptors)
• Sensory receptors adapt to prolonged stimulation, reducing their response over time

Neural Pathways and Signal Processing

• Sensory pathways consist of a series of neurons that relay information from receptors to the brain
• Ascending pathways carry sensory signals from the periphery to higher brain regions
• Descending pathways modulate sensory processing and control motor responses
• Thalamus is a key relay station for most sensory pathways, directing signals to specific cortical areas
• Primary sensory cortices (visual, auditory, somatosensory) receive and process modality-specific information
• Association cortices integrate information from multiple sensory modalities and higher cognitive processes
• Sensory information undergoes serial and parallel processing in the brain
  • Serial processing occurs in a step-by-step manner along a single pathway
  • Parallel processing involves simultaneous processing in multiple pathways or brain regions
• Lateral inhibition enhances contrast and sharpens sensory representations (Mach bands)

Sensory Modalities Overview

• Vision detects light and processes color, form, depth, and motion
  • Photoreceptors (rods and cones) in the retina transduce light into neural signals
  • Visual pathways include the lateral geniculate nucleus and primary visual cortex
• Audition detects sound waves and processes pitch, loudness, and localization
  • Hair cells in the cochlea transduce sound vibrations into neural signals
  • Auditory pathways include the cochlear nucleus, inferior colliculus, and primary auditory cortex
• Somatosensation includes touch, temperature, pain, and proprioception
  • Mechanoreceptors, thermoreceptors, and nociceptors in the skin detect various stimuli
  • Somatosensory pathways include the dorsal column-medial lemniscus system and spinothalamic tract
• Gustation (taste) detects chemicals dissolved in saliva and is categorized into five basic tastes (sweet, salty, sour, bitter, umami)
• Olfaction (smell) detects airborne chemicals and is capable of discriminating numerous odors
  • Olfactory receptors in the nasal cavity bind to odorant molecules
  • Olfactory bulb and piriform cortex process olfactory information

Perceptual Organization and Integration

• Gestalt principles describe how the brain organizes sensory elements into meaningful patterns
  • Proximity, similarity, and continuity group elements that are close, similar, or aligned
  • Closure and common fate complete incomplete figures and group moving elements
• Figure-ground segregation distinguishes objects (figures) from their backgrounds
• Perceptual constancy maintains stable perception despite changes in sensory input
  • Size constancy, shape constancy, and color constancy
• Depth perception uses monocular (pictorial) and binocular (stereopsis) cues to estimate distance
• Multisensory integration combines information from different sensory modalities to enhance perception
  • McGurk effect demonstrates the influence of visual cues on speech perception
• Top-down processing uses prior knowledge and expectations to influence perception
• Bottom-up processing relies on sensory input to build perceptual representations

Attention and Selective Processing

• Attention selectively focuses on specific aspects of sensory input while ignoring others
• Selective attention filters relevant information and suppresses irrelevant information
  • Cocktail party effect demonstrates the ability to focus on one conversation in a noisy environment
• Divided attention involves processing multiple stimuli or tasks simultaneously
• Attentional blink reflects a temporary impairment in detecting a second target after identifying a first target
• Feature integration theory proposes that attention binds separate features into unified objects
• Priming occurs when exposure to one stimulus influences the response to a subsequent stimulus
• Attentional capture occurs when salient or unexpected stimuli automatically draw attention
• Inattentional blindness is the failure to notice an unexpected stimulus when attention is focused elsewhere

Individual Differences and Disorders

• Sensory thresholds and perceptual abilities vary among individuals
• Synesthesia is a condition in which stimulation of one sensory modality evokes experiences in another modality
• Agnosia is an inability to recognize or identify objects despite intact sensory processing
  • Visual agnosia, auditory agnosia, and tactile agnosia
• Phantom limb syndrome involves the perception of sensations in a limb that has been amputated
• Neglect syndrome is a failure to attend to or respond to stimuli on the contralesional side of space
• Hallucinations are perceptual experiences in the absence of external stimuli
• Illusions are misperceptions or distortions of sensory input
  • Müller-Lyer illusion demonstrates the influence of context on perceived line length
• Perceptual learning improves sensory discrimination through training and experience

Real-World Applications and Case Studies

• Sensory substitution devices translate information from one modality to another (visual-to-tactile, auditory-to-visual)
• Cochlear implants restore hearing by directly stimulating the auditory nerve in individuals with severe hearing loss
• Prosthetic limbs provide sensory feedback to improve motor control and embodiment
• Virtual reality and augmented reality rely on manipulating sensory input to create immersive experiences
• Sensory marketing uses visual, auditory, and olfactory cues to influence consumer behavior
• Perceptual expertise develops through extensive training in specific domains (radiologists, wine tasters, bird watchers)
• Case studies of individuals with unique perceptual experiences or deficits provide insights into sensory processing
  • The man who mistook his wife for a hat (visual agnosia)
  • The colorblind painter (achromatopsia)
  • The human echolocator (using sound to navigate)