Sensation and Perception: An Introduction

• Sensation and perception are the processes through which stimuli in the environment are experienced
• Sensation involves the sense organs
• Perception involves the interpretation and organization of sensations

Learning Objectives

• Differentiate between sensation and perception
• Explain the nature of perception and its scope
• Explain the process of perception
• Identify factors affecting perception
• Describe the laws of perceptual organization
• Summarize the most common types of perceptual constancies
• Explain the basis of perceptual illusion

Introduction

• Psychology is the scientific study of behavior and mental processes.
• Sensation and perception are very important mental processes
• Psychologists apply sensation and perception principles to diverse areas like defense, robotics, health, and sports.

Sensation

• Philosopher Thomas Reid (1710-1796) distinguished between sensation and perception
• Sensations: activities of sense organs experienced in consciousness
• Perception: depends on sensation, the perceiver is aware of objects or events in their environment
• Sensation: awareness due to stimulation of a sense organ
• Perception: the organization and interpretation of sensations
• There are six senses with corresponding organs:
  • Seeing (eyes)
  • Hearing (ears)
  • Smelling (nose)
  • Touching (skin)
  • Taste (tongue)
  • Sensing body position (proprioception and kinesthesia)
• Sensory receptors give visual, tactile, auditory, and olfactory information
• Senses accomplish transduction
  • Stimuli are detected by receptor cells
  • Converts to electrical impulses
  • Carried to the brain
• Human senses translate physical energy into electrical signals via specialized receptor cells
• The signals are transmitted to the brain via specialized sensory nerves
• Sensation relates to initial contact between organism and environment, focusing on sensory stimulation and input registration
• Perception is the process of interpreting and organizing received information for conscious experience of objects and relationships

Human Senses and Physical Energy

• Our senses (eyes, ears, skin, nose, tongue) provide sensations of vision, hearing, skin senses, smell, and taste.
• Physical energy which emanates from objects (light, sound waves, heat, touch) are the stimuli that create different sensations
• No physical energy = no stimuli = no sensation.

Process of Sensation

• Physical energy becomes stimuli that is received by sense organs via specialized receptor cells
• The energy converts to electrical impulses - this is called transduction
• Specialized receptor cells translate physical energy into electrical impulses (transduction)
• Electrical impulses travel from sense organs along nerve fibers to the central nervous system
• Ends in the appropriate area of the cerebral cortex
• Sensation is the reception of stimuli
• Sensation is the transmission of messages to the cerebral cortex

Our Senses: Vision

• The eyes are like a color television or camera
• Light enters through a small hole and passes through a lens that focuses on a photosensitive surface
• Vision is managed through the cornea, pupil, iris, and retina
• Receptor cells transmit information to the brain via the optic nerve
• Cones sense color
• Rods sense black and white
• Rods and cones are distributed on the retina at 100+ million and 6 million respectively
• Rods and cones aid in light or dark adaptation
• Adaptation from bright to dim light is managed by rods and cones
  • Chemicals in rods and cones build up in dim light with greater concentration than when stimulated by bright light
  • Cones adapt faster than rods in the dark
  • Rods are more sensitive to light compared to cones when fully adapted
• Cones are in the center, rods are on the edge of the retina
• Rods become active when looking away from a dim light in darkness

Visual Acuity

• Shape of a person's eyeball affects discrimination of details in the field of vision
• If an eyeball is too big, the lens focuses the image in front of the retina, near objects are clear, and far objects are blurred (nearsightedness)
• If an eyeball is too short, the lens focuses the image behind the retina, far objects are in sharp focus, and close objects are indistinct (farsightedness)
• Visual acuity is the ability to discriminate details in the field of vision properly
• Visual acuity decreases with age.

Blind Spot

• The spot where the nerves of the eye converge to form the optic nerve is called the blind spot
• The blind spot has no visual acuity
• Optic nerves connect the eyes to the brain from the back wall of the eyeball
• Effects of blind spots are compensated by head movement or use of the other eye.

Hearing

• Ears facilitate the sensation of hearing
• Ears detect sounds from the external world
• Sound changes air pressure by vibration
• Changes are registered through the ears
• Three main characteristics of sound: pitch, loudness, and timbre Pitch
• Highness or lowness determined by the frequency of vibration of waves
• Louder sounds are created by increased vibration Loudness
• Amplitude of sound waves
• Volume increase on television increases amplitude of vibrations and increases loudness Timbre
• Quality of sound from a particular sound source
• Different instruments will sound different playing the same note due to timbre
• Outer, middle, and inner ear help in auditory functioning

Smell

• Vision, hearing, smell, taste, and skin senses tell us about objects and events close to our body
• Vision, hearing, and smell are receptive systems that enlarge our world by responding to a stimuli at a distance
• Smell is in many ways most primitive
• Receptor cells present in the nose are stimulated to deliver the sense of smell
• Provides info about chemicals suspended in the air that excite receptors at the top of the nasal cavity
• Humans have ~50 million olfactory receptors Dogs
• Dogs possess ~200 million receptors
• More sensitive smell
• Used in dogsquad to detect crime and criminals.
• Carlson (1998): human olfactory receptors can detect substances with molecular weights between 15 and 300
• Sense of smell in humans is most primitive Olfaction
• More effective in other species
• Certain animals secrete chemical substances called pheromones to trigger reactions in other members of their kind
• Olfaction also serves as primitive communication

Taste

• Taste is related to smell
• Tastes primarily depend on taste buds scattered across the upper surface and side of the tongue
• Taste bud structure: each contains several receptor cells Humans: ~10,000 taste buds
• Chickens have 24
• Catfish have ~175,000, distributed all over body
• Only four basic tastes: sweet, salty, sour, and bitter
• While eating, you are not aware of only the taste but of smell, texture, temperature, and pressure
• Sensitivity to salt is highest on the tip and sides of the tongue
• Sour is detected on the sides of the tongue
• Bitter is detected on the back of the tongue
• Each basic taste quality is associated with taste receptors.
• Sweet is produced by various sugars and saccharin
• Number of taste buds on decreases with age resulting in older people being less sensitive to taste

Skin Senses

• Sensory receptors provide various sensations: pressure, warmth, cold and pain
• Sensitivity acute - hands, fingers, lips, tongue
• The skin at different spots are not uniformly sensitive to stimuli that produce different sensations.
• Touched with a pointed object = pressure or touch
• Lips, fingers, hands, and tip of tongue are most sensitive
• Arms, legs, and body trunk less sensitive
• Receptors: heat, cold, temperature, and pain
• Temperature receptors concentrated on the trunk
• Cold receptors 6x more concentrated than heat receptors
• Pain: produced as a result of tissue injury
• Extreme stimulation of any sense organ may cause pain (bright light, loud noise, high/low temperature)

Kinesthetic Sense

• Provides information about positions and movements of muscles and joints
• Kinesthesia: sense that gives us information about the location of our body parts with respect to one another and allows us to perform movement
• Kinesthetic receptors reside in muscles and joints
• Enable maintenance of balance
• Allows distinction between objects of different weights by lifting
• Track body movement and body position in relation to gravity

Perception: Nature and Scope

• Perception is the interpretation of our experience
• Selectively attend to stimuli
• Attention filters unnecessary sensory information
• Attention depends on internal and external factors
  • Internal: interest, motivation, needs, preparatory factors
  • External: intensity, size, repetition, contrast, novelty, movement

Stages of Perception

Three steps:

• Selection - brain has limited capacity, selects stimuli
• Organization - mentally arrange in a meaningful pattern
• Interpretation- meaning is assigned to the organized stimuli

Theoretical Approaches to Perception

• Two approaches explain perception in pattern recognition: Top-Down Processing
• Experience driven process
• Stimuli do not have sufficient information to be interpreted meaningfully
• Relies on experiences to interpret stimuli Bottom-Up Processing
• Data driven process
• Stimuli carry sufficient information to be interpreted
• Does not rely on experiences
• Top-down = existing knowledge organizes stimuli features
• Bottom-up = analyses smaller features and constructs the complete picture

Role of Attention in Perception

Selective attention

• Focus on stimulus of interest
• Filters irrelevant stimuli Sustained attention
• Attending to a stimulus for a longer period of time
• Prevents distraction

Laws of Perceptual Organization: Gestalt Principles

• Gestalt = form
• Perception involves tendency to seek form or pattern
• Basic premise: ‘whole is different from the sum of its part'
• Organization (or structured whole/Gestalten) gives different meaning to perception Various Laws: Law of Figure-ground Relationship
• Tendency to segregate our world in the form of figure and ground
• A figure (image) is always seen against the background.
• Figure image has clear definite shape and is remembered easily
• Ground image is shapeless Law of Proximity
• Grouping close objects together
• Stimuli closer together perceived as belonging to one group Law of Good Figure/Pragnanz/Symmetry
• Gestalt principle
• Tendency to organize stimuli to make the figure balanced or symmetrical
• Group in the simplest and stable shape Law of Continuation
• Tendency to perceive figures in continuation rather than in parts
• Perception of lines Law of Common Fate
• Things are organized according to movement together in a group
• Stimuli moving in similar directions perceived as belonging to the same group. Law of Closure
• Completes our perception
• Fills gaps in stimuli Law of Similarity
• Grouped together according to similarity
• Group circles based on colors.

Factors Affecting Perception

• Central determinants found to affect process of perception: expectation, emotions, stimulus characteristics, past experiences, and cultural background.

Effect of Motivation or Need as Perceptual Determinants

• A person who is hungry is likely to notice food odors more often as opposed to a person who is not hungry

Effect of Expectation or Perceptual Expectancy

• The ambiguous stimulus is perceived as a number or letter based on context

Effect of Emotions

• Emotions can affect perception
• If perceptual task is irrelevant to emotions, it hinders your performance
• If perceptual task is relevant to emotions it facilitates performance

Effect of Stimulus Characteristic

• Stimuli different in sound, taste, look, or feel grab our attention more
• According to evolutionary psychology, this property has a survival purpose to help humans in identify danger

Effect of Experience

• Prior experience shapes perception

Effect of Culture

• Interpretation of a stimulus changes with the change in the culture

Perception of Depth

• Depth perception: visual ability to perceive the world in three dimensions and judge the distance of an object
• Not present at birth but develops early in age + learned through experience
• Distance perception: how we determine the distance of an object.
• Monocular cues (one eye)
• Binocular cues (two eyes)

Monocular Cues

• Information that the brain receives from one eye
• Weaker than binocular cues
• Used by painters to give 3D perspective Relative Size
• Distance of an object perceived based on relative size
• Premise: if two objects approximately similar size, the object perceived as larger is closer Texture Gradient
• Perception of the change in the gradient or degree of texture
• Objects nearer have rough/distinct texture
• Objects farther have less distinct/smooth texture Arial Perspective
• Objects perceived at a distance with the presence of haze
• Haze: atmospheric dust particles, fog or water vapor Linear Perspective
• A point on the horizon based on the convergence of straight lines Interposition/Occlusion
• overlapped or obscured = father away

Accommodation

• Accommodation: size of lenses adjusts based on the distance
• Lenses become thicker when object closer
• Lenses become thinner when object at distance

Motion Parallax

• Focuses on the critical point that as one moves about in the environment
• Objects at a different distance move at different speeds

Binocular Cues

• Received from both eyes
• More powerful than monocular cues
• Stereopsis: the process of gaining binocular cues to assess depth Retinal Disparity (binocular parallax)
• Humans have two eyes, separated by the distance of average 6.3 cm
• Close objects result in greater difference of the retinal image Convergence
• Eyes make an angle while focusing (convergence angle)
• Distance/nearby objects have different convergence angles

Movement Perception

• Ability to judge the direction and speed of a moving object
• Retinal motion- image on the retina moves when object moves Motion after-effect (MAE)
• Fixate gaze on a moving object
• Stationary object seen as moving in the opposite direction Induced movement
• Smaller stationary body surrounded by larger moving body
• Smaller body perceived to move in the direction opposite to the larger body Apparent movement:
• Also called as phenomenal motion,
• When stationary stimuli are presented in succession, it is perceived in a motion.
• Perceived motion occurs without any energy movement across the receptor surface. Phi-Perception-
• Succession of decorative lights turned in sequence Stroboscopic Effect
• Movie projector places successive pictures of moving scene on a screen Autokinetic Effect
• Staring/fixating at a stationery spot of light in a completely dark room results in the spot appearing to move

Size Perception

• Involves judging the size of the stimuli
• Judged correctly even with the change in distance
• Size-distance invariance hypothesis
• Familiar size hypothesis
• Direct perception hypothesis

Size-distance invariance hypothesis (SDIH)

• The perceived size of a stimulus is proportional to perceived distance
• If distance information available interpreted based on retinal image
• If distance information not available: size judged based on visual angle

Familiar Size Theory/Cue:

• This cue is used to judge not only size but also the distance and depth of the stimuli.
• We know the visual angle for a stimulus decreases with the decrease in the distance.
• Our brain uses this information (visual angle) along with our previous information of the size of the targeted stimulus and determines its actual size, distance and depth.

Theory of Direct Perception:

• i) there is no perceptual representation of size correlated with the retinal size of the object,
• ii) perceived size and perceived distance are independent functions of information in stimulation, and
• iii) perceived size and perceived distance are not causally linked, nor is the perception of size mediated by operations combining information about retinal size and perceived distance.

Errors in Perception: Illusion

• Misinterpreting stimuli
• Discrepancy between awareness and a stimulus Muller-Lyer Illusion:
• Two straight lines of same length appear different
• arrow-head illusion, Ponzo Illusion:
• Two converging straight lines distort size of two identical lines drawn across
• Railway illusion Ebbinghaus Illusion:
• Optical illusion of relative size perception
• Titchener circles The Ames Room Illusion:
• Optical illusion, leads to distortion of relative size Moon Illusion:
• Perception of the moon having different sizes at horizon and zenith
• The type of illusion is that of a shape or area. Poggendorff Illusion:
• When on ablique line is intercepted by a blank area defined by two vertical parallel lines, the two resulting segments of the oblique line do not appear to be in a straight line.