Sensation and Perception

Sensation and Perception

• Sensation involves stimulus detection
• Perception involves organizing and giving meaning to sensory input
• Sensory organs translate stimuli into nerve impulses, a process called transduction
• The brain subsequently interprets this information

Absolute Threshold

• The intensity at which a stimulus can be detected 50% of the time
• A lower absolute threshold indicates higher sensitivity

Signal Detection Theory

• Examines perception as a decision-making process, influenced by factors like fatigue, stimulus importance, and expectations
• Involves participants indicating whether they perceived a stimulus
• In signal detection theory there are four possible outcomes: hit, miss, false alarm, and correct rejection

Difference Threshold

• The smallest difference in stimulation that can be detected 50% of the time
• Also known as the just noticeable difference (JND)
• Weber’s Law states that the JND is proportional to the intensity of the original stimulus
• The JND is a proportion, not a constant amount
• For weight, the JND is about 1/50; so for 50 gm, a 1 gm difference is needed and for 500 grams, a 10 gram difference is needed

Sensory Adaptation

• Sensory adaptation involves diminishing sensitivity to an unchanging stimulus
• Sensory adaptation occurs across all sensory modalities
• This process frees senses to be more sensitive to changes in the environment, offering adaptive value

Vision

• Vision perceives a small part of the electromagnetic spectrum, from 700 nm (red) to 400 nm (violet)

Anatomy of the Visual System

• Cornea: A transparent protective structure
• Pupil: An adjustable opening that controls the amount of light entering the eye
• Lens: An elastic structure that focuses light by thickening for nearby objects and thinning for distant objects
• Retina: Contains photoreceptors that transduce light energy into electrical impulses

Myopia and Hyperopia

• Myopia (nearsightedness) occurs when the eyeball is longer than normal, causing the lens to focus light in front of the retina, making seeing far away objects difficult
• Hyperopia (farsightedness) occurs when the eyeball is too short, causing the lens to focus light behind the retina, making seeing close-up objects difficult

Photoreceptors in the Retina

• Cones: Responsible for color and detail and function best in high illumination, concentrated in the fovea
• Rods: Function best in low illumination and are 500 times more sensitive to light than cones, located mostly concentrated in the periphery of the retina except for in the fovea

Neural Pathways in Vision

• Rods and cones synapse with bipolar cells, which then synapse with ganglion cells
• Cones typically have a one-to-one connection, while many rods connect to a single bipolar cell
• Axons of ganglion cells form the optic nerve, creating a 'blind spot' where the optic nerve exits the eye

Visual Transduction

• Transduction involves converting stimulus characteristics into nerve impulses
• Visual transduction involves action of photopigments
• In visual transduction, absorption of light changes the rate of neurotransmitter release
• A greater rate of release equates to a stronger signal passed on

Brightness and Dark Adaptation

• Some animals have a 'dual vision system' that allows them to see in both daylight and at night
• Dark adaptation is progressive improvement in brightness sensitivity in low illumination

Cone and Rod Adaptation

• In low light conditions, vision relies on rods
• Rods and cones adapt differently in dark adaptation
• Cones adapt after 10 minutes, reaching maximum sensitivity
• Rods adapt after 30 minutes
• Rods can detect light intensities 1/10,000 as great as before dark adaptation

Rod Sensitivity to Red Light

• Rods are not sensitive to wavelengths of red light
• Rods remain dark adapted under red light, allowing for better transition from red light to dim or dark conditions

Trichromatic Theory of Colour Vision

• States there are three types of color receptors in the retina: blue, green, and red
• Visual system combines activity from these cells
• Colours perceived by additive mixture of impulses
• White colour is produced, if all are equally activated

Limitations of Trichromatic Theory

• Cannot explain why red-green colour-blind individuals cannot perceive yellow (red + green = yellow)
• Fails to explain 'afterimages'

Afterimages

• Afterimages are caused by staring at one colour leading to neural fatigue
• The effect of this fatigue is a 'rebound' of the receptors; the receptor fires with an opponent opposite reaction

Opponent-Process Theory of Colour vision

• Each opponent-process cone responds to two different wavelengths
• Opponent-process cone types: red or green / blue or yellow / black or white

Dual-Process Theory of Colour Vision

• Combines trichromatic and opponent process theories for colour vision
• States that 3 cone types sensitive to short (blue), medium (green), and long (red) wavelengths stimulate opponent-process reactions
• Opponent processes occur in ganglion cells, neurons in relay stations, & visual cortex

Colour Deficient Vision

• Trichromats have normal colour vision
• Dichromats are deficient in one colour system (red-green being the most common)
• Monochromats are sensitive to black-white only

Audition

• Stimulus for hearing is sound waves
• Sound waves are described through frequency and amplitude

Sound Waves

• Frequency: Number of cycles per second (Hertz) or pitch
• Amplitude: Amount of compression and expansion of molecules that is related to loudness.
• Decibels (dB) are a logarithmic scale that measures sounds
• Every increase of 10 = 10 fold increase in loudness.
• Restaurant sound levels measure 50dB, rock band sound levels measure 120dB and jet sound levels measure 140dB

Anatomy of Hearing

• Outer Ear: Contains the pinna for earrings & sunglasses
• Middle ear: amplifies sound
• Inner Ear: Where basilar membrane is located

How the Middle Ear Works

• Vibrations cause the malleus, incus, and stapes (ossicles) to vibrate
• Ossicles amplify sound more than 30X
• Oval window moves in and out

Inner Ear and Sound Receptors

• The inner ear contains the cochlea
• Basilar membrane of the cochlea moves as the oval window moves
• On top is the organ of Corti
• Hair cells act as sound receptors

Theories of Pitch and Loudness Coding: Frequency Theory

• Nerve Impulses 'match' frequency of wave (1000 Hz = 1000 impulses per second)
• Works only for frequencies under 4000 Hz

Theories of Pitch and Loudness Coding: Place Theory

• Specific frequencies peak at certain places on basilar membrane
• Specific areas of the cochlea are processed in specific areas of the cortex (Frequency Map)
• High frequencies peak near oval window
• Low frequencies peak farther down

Sound Localization

• Binaural hearing (using two ears) helps localize sound
• Timing of sounds: sounds arrive at closest ear first and uses differences in arrival time
• Intensity of sounds: sound arrives at closest ear will be more intense, and uses differences in intensities

Chemical Senses: Taste and Smell

• Chemical sense relies on chemical molecules, not a form of “energy” for stimulation

Taste

• Chemical receptors called taste buds, comprise of several receptor cells
• Around 9,000 taste buds are grouped in different regions
• Taste buds are located on edges & back of tongue
• Taste responds to 4 basic qualities: sweet, sour, salty, and bitter

Smell

• Receptors line upper nasal cavity; about 40 million receptors
• Receptor sites resemble neurotransmitter binding site; odour molecules ‘lock’ into certain sites
• Receptors send messages to olfactory area in brain
• Pheromones: chemical signals found in natural body scents

Sense of Touch

• The sense of touch comprises four tactile sensations: pressure, pain, warmth, cold
• Skin, the largest organ in the body, contains a variety of receptor structures
• Pain and temperature are detected by free nerve ending receptors beneath skin's surface
• Touch and pressure are detected through nerve fibers at base of hair follicles
• Touch sense sends messages to somatic-sensory cortex

Body Senses

• Kinesthesis provides feedback about muscles & joint positions
• Kinesthesis' Receptors = nerve endings in muscles, joints, tendons

Perception: Creation of Experience

• Bottom-up processing: Analysis of individual stimulus elements that is re-combined into a unified whole
• Top-down processing utilises existing knowledge, concepts, ideas, expectations

Role of Attention

• Involves 2 processes: focusing on certain stimuli and filtering out other information
• Demonstrated in shadowing experiments : Listen to 2 messages delivered through earphones (one through each earphone), focus on one and ignore other.
• You can recall information from attended message, can not recall information from non-attended message.

Inattentional Blindness

• Failure of unattended stimuli to register in consciousness is inattentional blindness
• Inattentional blindness can feel like 'looking' without 'seeing'

Environmental and Personal Factors in Attention

• Both play a role in attention
• Stimulus characteristics: intensity, novelty, movement, contrast, repetition

Gestalt Principles of Perceptual Organization

• Gestalt = pattern, shape, form
• Wholes we see are more than the sum of their parts
• Perception is governed by laws that determinedhow things are grouped together

Perceptual Organization

• Figure-Ground: a perceived visual field is divided into a figure (main focus) and ground (background)
• Figure: simple, distinct shape while foreground is foreground & background

Gestalt Laws of Organization

• Gestalt laws of organization are rules by which visual scenes are organized
• Similarity: Similar items belong together
• Proximity: Elements that are close together belong together
• Closure: Close open edges and percieve boundaries
• Continuity: Elements that are linked to form continuous lines

Perception and Hypothesis Testing

• The role of perceptual schema, mental representations, allows for classification of stimuli
• Perception is the best interpretation based on knowledge and experience

Perceptual Set

• Perceptual Set is a readiness to perceive stimuli in particular way
• It is based on expectations and prior information
• Psychological context is created to use in top-down processing

Perceptual Constancies

Types of perceptual constancies

• Size: Walk to your car- Image on your retina gets larger but you still percieve your car the same size
• Shape: Friend's face has different shape when you look at their profile but you still percieve the the same as when you look at their face
• Brightness: Color of your jacket may look different in fullsun light but you know the colour hasnt changed

Depth Perception

• Image on the retina is 2 dimensional
• Because we have 2 eyes, Image on the retina is 2 dimensional
• Monocular Cues means depth you can see perceived with one eye
• Binocular Cues means depth you can see perceived with two eyes

Monocular Depth Cues

• Texture: Objects farther away have finer texture (less detail)
• Clarity: Objects farther away are hazier
• Relative Size: More distant objects are smaller
• Motion Parallax: When wemove - nearby objects move fasterin opposite direction than more distant objects
• Light & Shadow
• Linear Perspective: Parallel lines converge in distance
• Interposition: Closer objects are in ‘front’ of others
• Height in horizontal plane: Further objects are higher in the horizontal plane

Binocular Depth Cues

• Based on binocular disparity: Two eyes receive different visual images
• Feature detectors analyze differences
• Convergence: Feedback from ocular muscles while focusing on something distant and then close

Perception of Movement

• Primary cue = image movement across retina
• It also utilizes movement against a background
• Apparent motion is the illusion of motion
• Stroboscopic motion (phi phenomenon) is is illusions such motion pictures

Perceptual Development

• Interplay of biology & experience is key
• Biology shows newborns do not crawl onto ‘deep end' of visual cliff
• Experience shows we adapt to 'upside down' environment

Critical periods

• Critical periods is time period during which certain experiences must occur in order for perception to develop normally
• If visual problems are not corrected early, there is a permanent problem