Sensation and Perception

Sensation vs. Perception

Sensation

• The process of using our sensory systems (hearing, sight, taste, smell) to detect stimuli in our environment

Perception

• How we consciously recognize, organize, and identify sensory stimulus.Involves bottom-up and top-down processing

Sensory Receptor Cells

Sensory receptor cells 

• Specialized cells that convert a specific form of environmental stimuli into neural impulses

Sensory transduction

• The process of converting a specific form of environmental stimuli into a neural impulse that our brain can read

• Steps:

1. Stimulus detection

2. Receptor activation

3. Generation of electrical signals

4. Transmission to nervous system

5. Processing in the CNS

6. Perception and response

Absolute Threshold

• The smallest amount of stimulation necessary for a stimulus to be detected 50% of the time

• Changes with age (as we get older)

Difference Threshold (just noticeable difference) - the minimal difference needed to notice a difference between two stimuli

Signal detection theory - the response to a signal in every situation depends on an individual's ability to differentiate between the signal and noise, and their response criteria

Inattentional blindness - occurs when an individual fails to perceive an unexpected stimulus in plain sight, as a result of a lack of attention rather than visual defects 

The Chemical Senses: Smell (Olfactory)

• Odurants - airborne chemicals that are detected as odours

• Olfactory receptor neurons - sensory receptor cells that convert chemical signals from odorants into neural impulses that travel to the brain

Smell and Taste Disorders

• Ageusia - inability to taste

• Anosmia - inability to detect odours

• Hyposmia - reduced ability to smell

• Reflex Epilepsy - a seizure occurs only after exposure to a specific odour

• Migraine Headaches - specific odours can trigger migraines

Types of Tasters

• Non-tasters - people who are unable to taste the chemical propylthiouracil (PROP), a bitter compound

• Medium tasters - people with an average number of taste buds; can taste the bitter PROP as a median level

• Supertasters - people who are extremely sensitive to some tastes, have a high number of taste buds, and are highly sensitive to PROP; women more likely than men to be supertasters

Tactile or Cutaneous Senses

• The tactile or somatosensory system is a combination of skin senses, including;

  • Pressure

  • Touch

  • Temperature

  • Vibration

  • Pain

Somatosensory Receptors in the Skin

• Free nerve endings

  • Located near surface of the skin

  • Function: detect touch, pressure, pain, temperature

• Meissner’s corpuscles

  • Located in fingertips, lips, and palm

  • Function: transduce info about sensitive touch

• Merkel’s discs

  • Located near the surface of the skin

  • Function: transduce info about light to moderate pressure against the skin

• Ruffini’s end-organs

  • Located deep in the skin

  • Function: registers heavy pressure and movement of the joints

• Pacinian corpuscles

  • Located deep in the skin

  • Function: respond to vibrations and heavy pressure

Steps to Perceiving Touch

• Fast pathway (myelinated pathway)

  • Sharp, localized pain is felt quicker because it travels along myelinated neurons to the brain

• Slow pathway (unmyelinated pathway)

  • These inputs communicate with brain regions involved in processing emotions, paine perceived is often more burning and sharp pain

Disorders of the Tactile Senses

• Chronic pain

  • Lasts longer than 12 weeks

• No pain

  • Inability to detect pain or temperature or produce tears

• Phantom pain

  • Hallucinations of touch, pressure,pain in body part that is gone

Gate Control Theory of Pain

• The gate: the spinal cord, has a mechanism that either lets the signals of pain pass or blocks them to the brain 

The Auditory Sense: Hearing

• Sound waves - vibrations of the air in frequency

• Frequency - number of cycles per second in a wave

  • We hear best in the 2000-5000 hz range 

• Amplitude - magnitude (height of a wave)

  • Determines loudness (measured in decibels) 

How the ear hears

• Sound Waves enter the outer ear

• Waves hit the tympanic membrane (eardrum)

• Waves pass in to the middle ear, contains three bones

  • Ossicles: maleus (hammer), incus (anvil), stapes (stirrup)

• Stapes hit the oval window causing a wave to form in the fluid in the cochlea

• The fluid detects the basilar membrane, bending the hair cells that transduce the fluid sound wave into electrical activity

• Hair cells move and neural impulses are created and sent to the brain

Disorders of the Auditory Senses

• Deafness - loss of hearing

  • Genetic or caused by infection

• Conductive deafness - sound waves are unable to be transferred to inner ear

• Sensorineural deafness - damage to the inner ear or auditory nerve

• Tinnitus - ringing in the ear

  • 1 in 200 people

Sense of Sight:

• 70 percent of all sensory receptors in our body are in the eyes

Vision and the Electromagnetic Spectrum

How the eye works

• Light enters the eye

• Muscles in the iris adjust pupil size to control the amount of light allowed in

• Muscles also change the shape of the lens to bring the object into focus

• The lens focuses the light on the retina, multi-layered sheet of nerve cells

• Photoreceptors in the retina transduce the light waves into neural impulse

Rods and Cones

• Rods

  • Detect light

  • Used for periphery and night vision 

  • Not as acute as cones

  • Many more rods than cones

• Cones

  • Used for central and colour vision 

  • Very acute 

  • The fovea (centre of retina) contains all cones

  • Not as many cones

  • Concentrated in the centre

Visual Pathway

• Optic nerve contains the axon of 1 million ganglion cells that exit the eye via blind spot and project to the thalamus

The “What” Pathway

• Helps us determine the identity of an object

• Visual agnosia - damage to the “what” pathway (can’t visually recognize objects)

• Prosopagnosia - a form of visual agnosia in which people cannot recognize faces

The “Where” Pathway

• Locating Objects in space

• Hemi-neglect - damage to the “where” pathway; people ignore one side of their visual field

  • Damage to right side of “where” pathway experience negative effects on left side of visual field and vice versa

Gestalt Theory of Visual Processing

• How we perceive and interpret complex images and scenes

• The brain organizes visual information into meaningful wholes, or “gestalts,” rather than just perceiving a collection of individual elements

• 5 Laws:

  • Proximity

  • Similarity

  • Continuity

  • Closure

  • Figure-Ground

Visual Impairments and Loss

• Cataracts - clouding of the lens of the eye; affects acuity and color vision

• Retinopathy - damage to the small blood vessels; leaks and causes blurred vision, spotting, or floaters

• Glaucoma - fluid pressure builds up inside the eye, damaging the optic nerve; blurred vision and the loss of peripheral vision

• Macular degeneration - inability to see objects clearly; distorted vision and dark spots in centre of vision

• Hyperopia - focusing the image behind the retina; difficulty in seeing objects close up

• Myopia - focusing the image in front of the retina; difficulty in seeing object

• Synesthesia: a neurological condition that causes people to experience one sense through another

  • Two nerve activating simultaneously

  • Seeing letters or associating sounds with colours and letters/numbers