Chapter 4: Sensory, Attentional and Perceptual Processes (Notes)

Nature and Varieties of Stimulus

• The external environment contains a wide variety of stimuli. Some stimuli are visible, some are audible, some are smells, tastes, or tactile sensations.
• Human beings have a set of sense organs (sense modalities) to deal with these stimuli. Five external sense organs: eyes, ears, nose, tongue, and skin. Two deep senses: kinesthetic (body position and movement) and vestibular (balance and orientation).
• Seven sense organs register ten different varieties of stimuli (e.g., brightness, color, loudness, temperature, texture).
• The initial experience of a stimulus is called sensation: the detection and encoding of physical stimuli by a sensory organ. Sensation refers to immediate basic experiences like “hard”, “warm”, “loud”, and “blue”.
• Each sense organ is highly specialized for a particular kind of information; hence, each is a sense modality.
• Some receptors are visible (e.g., eyes, ears) while others lie inside the body and require tools to observe.
• The process of sensation, attention, and perception are interrelated and form the basis of cognition: knowledge of the world arises when stimuli are registered, attended to, and interpreted.
• The chapter introduces receptors, attention, and perception, including how we may be deceived by certain stimuli (figures and pictures).

Sense Modalities and Sensation

• Sensation is the initial registration of stimuli by sense organs and encoding of that information as neural signals.
• The human body has ten basic modalities registered by seven sense organs (eyes, ears, nose, tongue, skin, kinesthetic, vestibular).
• Receptors are specialized for different stimulus forms; they collect information from both external and internal environments.
• Absolute and Difference Thresholds are key concepts in understanding how stimuli are registered.

Functional Limitations of Sense Organs

• Sense organs function within a limited range of stimulation; they require stimuli to be of optimal intensity to be noticed.
• Absolute Threshold (AL): the minimum stimulus intensity required for detection 50% of the time. It varies across individuals and situations.
• Example: in a sugar-water experiment, the AL is the minimum number of sugar granules required to perceive sweetness on 50% of occasions.
• Difference Threshold (DL): the minimum difference between two stimuli that can be detected 50% of the time.
• Example: the minimum change in sweetness (∆I) necessary for the water to be perceived as sweeter than the previous sample, on 50% of trials.
• Important relation: to study sensation, both AL and DL for different modalities (visual, auditory, etc.) must be considered, as sensation depends on stimulus characteristics and neural pathways.
• If there is damage to receptors, neural pathways, or brain areas, sensation can be partially or completely lost.

Attentional Processes

• Attention is the process by which certain stimuli are selected from a crowded field of sensory input.
• Attention includes alertness (readiness to respond), concentration (focus on a specific object or event), and search (actively looking for a target among distractors).
• Attention involves effort allocation and has a focus (focal point) and a fringe (periphery of awareness).
• Everyday life shows that attention can be allocated to multiple stimuli, especially with highly practiced tasks (automatic processing).
• Automatic processing characteristics: (i) occurs without intention, (ii) unconscious processing, (iii) little or no conscious thought.
• The process is not simply a single mechanism; there are different forms and theories of attention.

Box 4.1: Divided Attention

• Divided attention occurs when a person attends to more than one task at a time; highly practiced activities can be performed with less attention.
• Examples include driving while talking, listening to music, wearing sunglasses, etc., where some tasks become automatic.
• Automatic processing enables multitasking but is not perfect and depends on practice and task similarity.

Types and Theories of Attention

• Selective Attention: choosing a limited number of stimuli from a larger set to process further.
  • Our perceptual system has limited capacity; not all stimuli can be processed.
  • Factors influencing selective attention include characteristics of the stimuli (external) and factors within the person (internal).
• External factors (stimulus-driven):
  • Size, intensity, motion, novelty, complexity – larger, brighter, moving, novel, and moderately complex stimuli attract attention.
  • Rhythmic auditory stimuli attract more attention than verbal narratives; sudden, intense stimuli capture attention.
• Internal factors (person-driven):
  • Motivational factors (biological/social needs) affect attention; e.g., hunger makes faint smells noticeable, exam situations focus attention on instructions.
  • Cognitive factors (interest, attitude, preparatory set) influence which stimuli are attended to; preparatory set primes a readiness to respond to certain stimuli.
• Theories of Selective Attention
  • Filter theory (Broadbent, 1956): a bottleneck allows through only one stimulus to higher processing, screening out others at the filtering stage.
  • Filter-attenuation theory (Triesman, 1962): unattended stimuli are not completely blocked; their strength is attenuated, allowing some weak stimuli (e.g., one’s own name) to pass through.
  • Multimode theory (Johnston & Heinz, 1978): attention is a flexible system that selects stimuli at three stages: (1) sensory representations, (2) semantic representations, (3) combined representations; early selection (stage 1) requires less effort than late selection (stage 3).
• Sustained Attention (Vigilance)
  • The ability to maintain attention on a task or object over long periods.
  • Examples: air traffic controllers, radar operators; signals may appear unpredictably and missed detections can be dangerous.
• Factors Influencing Sustained Attention
  • Sensory modality: auditory signals often yield better sustained attention than visual signals.
  • Clarity of stimuli; sustained attention improves with clearer stimuli and less ambiguity.
  • Temporal factors: regular intervals aid attention; irregular intervals reduce it.
  • Spatial factors: cues appearing in fixed locations are easier to attend to than those appearing randomly.
• Box 4.2: Span of Attention (Miller’s Magic Number)
  • Span of attention is the amount of information a person can grasp from a complex array at one moment.
  • Typically around seven items, plus or minus two: 5–9 items.
  • This limit helps explain everyday phenomena (e.g., the number plate formats designed for legibility).
• Box 4.3: ADHD (Attention Deficit Hyperactivity Disorder)
  • ADHD is characterized by impulsivity, excessive motor activity, and poor sustained attention; more common in boys.
  • Not solely biological; social-psychological factors (home environment, family pathology) contribute.
  • Treatments include:
  • Medications such as Ritalin (methylphenidate) to reduce hyperactivity and distractibility and to improve concentration; side effects can include slowed growth.
  • Behavioral management programs that use positive reinforcement and structured materials to maximize immediate feedback and success.
  • Cognitive-behavioral training: self-instruction strategies (stop, think, then do) to shift attention and promote reflective behavior; effects can be relatively stable over time.

Perceptual Processes

• Perception is the process of recognizing, interpreting, and giving meaning to information from sense organs; it involves learning, memory, motivation, emotions, and other cognitive processes.
• Perception is constructive: individuals interpret sensory input through their own motives, expectations, prior experiences, cultural knowledge, and attitudes.
• Processing Approaches in Perception
  • Bottom-up processing: recognition begins with sensory components and builds up to a whole; emphasis on stimulus features and mental construction.
  • Top-down processing: recognition begins with the whole (perceiver’s expectations) and identifies components; emphasis on the perceiver and prior knowledge.
  • Both processes interact; perception is a product of both stimulus properties and the perceiver’s cognitive framework.

The Perceiver

• Motivations and expectations influence perception: needs guide interpretation of ambiguous stimuli.
• Cultural background and experiences shape perceptual interpretation: different environments and learning histories alter perception.
• Examples:
  • Hunger influences perception of food in ambiguous images (hungry individuals more likely to perceive food objects).
  • People from pictureless or pictorially limited environments may struggle with pictorial depth cues; Eskimos may distinguish among nuanced snow types; hunters and gatherers may interpret depth differently than urban dwellers.
• Expectancies and perceptual sets: people tend to see what they expect, even if it does not match external reality.
• Cognitive styles influence perception: field-dependent vs. field-independent styles
  • Field-dependent individuals perceive the world holistically, whereas field-independent individuals perceive it analytically.
  • Visual tests (e.g., hidden triangle) differentiate individuals by speed to locate figures; quick finders = field-independent; slow finders = field-dependent.
• Figure-ground organization: our perception tends to separate figures from their backgrounds; this is the most primitive level of perceptual organization.

Principles of Perceptual Organisation (Gestalt)

• Gestalt view: we perceive visual fields as organized wholes (configurations) rather than as a mere collection of parts; the whole has properties not present in the parts.
• Pragnanz (Good Figure): the brain organizes stimuli into the simplest, most stable form.
• Figure-ground segregation: the primitive form of organization where some elements stand out as figures while others recede as background.
• The following laws describe how we organize visual fields into coherent wholes:
  • The Principle of Smallness: smaller areas tend to be seen as figures against a larger background.
  • The Principle of Symmetry: symmetrical areas are seen as figures against asymmetrical backgrounds.
  • The Principle of Surroundedness: areas surrounded by others are perceived as figures.
  • The Principle of Proximity: objects close to each other are grouped together.
  • The Principle of Similarity: similar items are perceived as belonging together.
  • The Principle of Continuity: elements that form a continuous pattern are perceived as a single group.
  • The Principle of Closure: we tend to fill gaps and perceive complete figures rather than incomplete parts.
• Figure-ground examples include Rubin’s vase and various line-dot patterns used to illustrate these principles.

Perception of Space, Depth, and Distance

• The visual field is three-dimensional in real life, even though retinal images are two-dimensional, due to depth perception.
• Depth perception relies on monocular cues (one eye) and binocular cues (two eyes).
• Monocular cues (also called pictorial cues) are useful when viewing with one eye and are often used by artists to convey depth in paintings. Important monocular cues include:
  • Interposition (Overlap) and Relative Size: nearer objects occlude farther objects; smaller retinal images imply greater distance.
  • Linear Perspective: parallel lines appear to converge with distance.
  • Texture Gradient: density of texture elements increases with distance.
  • Motion Parallax (kinematic cue): as we move, nearer objects move faster than distant ones.
  • Relative Height: farther objects tend to appear higher in the visual field.
  • Relative Clarity: distant objects appear less clear than near objects.
  • Aerial Perspective: the atmosphere causes distant objects to look hazier or bluer.
  • Light and Shade: highlights and shadows provide information about distance.
  • Accommodation: the lens changes shape to focus; the brain uses this information to gauge distance.
• Binocular cues (physiological cues) require both eyes and provide strong depth information:
  • Retinal Disparity (binocular disparity): the two retinas receive slightly different images due to eye separation; greater disparity indicates closer objects.
  • Convergence: the inward turning of the eyes when focusing on near objects; greater inward turning signals nearer objects.
• Other cues include:
  • The vanishing point at the horizon (perspective cues) where parallel lines appear to converge with distance.
  • Texture gradient, accommodation, motion parallax, and convergence all contribute to depth perception in real-world contexts.

Perceptual Constancies and Illusions

• Perceptual Constancies: stable perceptions of objects despite changes in sensory input.
  • Size Constancy: objects are perceived as having a constant size despite changes in retinal image size with distance.
  • Example: approaching a friend, retinal size changes, but perceived size remains roughly constant.
  • Formal idea: the perceived size remains stable as the distance grows; the retinotopic image changes but perception does not.
  • Brightness Constancy: perceived brightness remains the same despite changes in illumination.
  • Example: a white sheet looks white in sunlight and indoors; blacks also stay black under varying lighting.
  • Shape/Form Constancy: recognition of an object’s shape remains constant even when the retinal image is distorted by orientation.
  • Example: a plate looks circular though its retinal image may be elliptical from angle.
• Illusions
  • Illusions are misperceptions where sensory input is misinterpreted, often due to contextual cues.
  • Some perceptual illusions are universal (e.g., Müller-Lyer illusion, Vertical-Horizontal illusion), while others vary across individuals or cultures.
  • Phi-phenomenon: apparent motion produced by successive flashing lights; requires certain brightness, size, spatial gaps, and temporal contiguity to evoke motion.
  • Illusions demonstrate that perception is an active constructive process influenced by stimuli and prior experience.

Socio-Cultural Influences on Perception

• Perception is influenced by cultural and environmental contexts; perceptual organization can vary across cultures.
• Research on visual illusions across cultures shows differences:
  • Segall, Campbell, and Herskovits found African subjects more susceptible to vertical-horizontal illusion, while Western subjects showed greater susceptibility to Müller-Lyer illusion.
  • People living in dense forests (Africans) often perceive verticality (long trees) and overestimate vertical dimensions; Western environments with right-angle structures yield different depth cues.
• Hudson’s studies with Africans showed difficulties recognizing objects depicted in pictures and interpreting depth cues in pictorial representations among those with no prior exposure to pictures.
• Sinha and Mishra conducted studies across diverse cultural settings (foresters, villagers, urban workers) showing that interpretation of pictures and depth cues are strongly related to cultural experiences.
• Practical implication: perceptual skills and interpretation of visual information are learned through cultural exposure and context, not universal and fixed across all populations.

Expectations, Cognitive Styles, and Perceptual Sets

• Perceptual Sets: readiness to perceive in a particular way based on expectations; people tend to see what they expect to see.
• Cognitive Styles – Field-Dependent vs. Field-Independent
  • Field-Dependent: perceive the world holistically; rely on external reference frames.
  • Field-Independent: perceive the world analytically; decompose scenes into smaller parts.
• Tests and illustrations (e.g., hidden figures) differentiate individuals by susceptibility to global vs. local processing.

Perception in Everyday Life: Summary of Key Mechanisms

• Perception is a blend of bottom-up processing (stimulus-driven) and top-down processing (concept-driven).
• The perceiver’s goals, prior knowledge, and cultural background shape interpretation.
• Perceptual organization (Gestalt laws) describes why we perceive organized wholes rather than discrete elements.
• Depth and space perception depend on monocular and binocular cues and perceptual constancies that stabilize perception across changes in lighting, distance, and angle.
• Illusions reveal the constructive nature of perception and the role of context in interpretation.

Key Terms

• Absolute threshold, Binocular cues, Bottom-up processing, Depth perception, Difference threshold, Divided attention, Figure-ground segregation, Filter theory, Filter-attenuation theory, Gestalt, Monocular cues, Perceptual constancies, Phi-phenomenon, Selective attention, Sustained attention, Top-down processing, Visual illusions.

Concepts and Formulas

• Absolute Threshold (AL): the minimum stimulus intensity required to be detected 50% of the time.
• Difference Threshold (DL): the minimum change in a stimulus that can be detected 50% of the time.
• Span of Attention: typically 7 ± 2 items; Miller’s magic number.
• Monocular Cues (examples): Interposition, Relative Size, Linear Perspective, Texture Gradient, Motion Parallax, Accommodation, Relative Height, Light and Shade, Aerial Perspective.
• Binocular Cues (examples): Retinal Disparity, Convergence.
• Perceptual Constancies: Size, Brightness, Shape (Form) Constancy.
• Illusions: Universal vs. Personal; Phi-phenomenon.

Review Questions

• Explain the functional limitations of sense organs.
• Define attention. Explain its properties.
• State the determinants of selective attention. How does selective attention differ from sustained attention?
• What is the main proposition of Gestalt psychologists with respect to perception of the visual field?
• How does perception of space take place?
• What are the monocular cues of depth perception? Explain the role of binocular cues in the perception of depth.
• Why do illusions occur?
• How do socio-cultural factors influence our perceptions?

Review Activities and Projects

• Collect ten advertisements from magazines. Analyse the content and message, focusing on attentional and perceptual factors used to promote the product.
• Create a toy model for visually impaired and sighted children to compare tactile and visual perception of a horse/elephant; perform a cross-modal description activity.
• Use a second toy model (parrot) and have visually challenged children draw it from memory; compare with sighted children's drawings.

Project Ideas

• Illusions are examples of nonveridical perceptions; categorize them as universal vs. culture-specific and discuss implications.
• Discuss how socio-cultural factors influence perceptual familiarity with and salience of stimuli, and perceptual inference habits across cultures.

Summary (condensed)

• Our knowledge of the world depends on sensation, attention, and perception, which are interrelated.
• Sense organs have functional limits; AL and DL quantify detection and discrimination thresholds.
• Attention selects and processes only a subset of stimuli; selective and sustained attention are core modes, with division possible for practiced tasks.
• Perception constructs meaning from sensory input, guided by bottom-up and top-down processes, the perceiver’s motivations, cultural experiences, and expectations.
• Gestalt principles explain how we organize visual information into meaningful wholes and how figure-ground segregation occurs.
• Depth and space perception rely on monocular and binocular cues, with perceptual constancies stabilizing perception across changing conditions.
• Illusions reveal the constructive nature of perception and the influence of context.
• Socio-cultural factors shape perceptual experience and interpretation; perceptual skills are not universal.