Sensation and Perception

Sensation versus Perception

• Sensation: beginning stages

• Perception: latter, interpretation stages

• Prior knowledge, memory, current emotional state all influence how sensations are translated into our knowledge of reality

Stages of Conversion in Sensation

1. Accessory structures modify physical stimulus

   • Lens of the eye

   • Outer ear modifies sound

2. Transduction: physical energy is picked up by receptors and converted into neural energy

   • Cells in the retina respond to light energy

   • Receptors respond best to changes in energy levels, otherwise adaptation occurs

3. Sensory nerves send transduced neural energy to the brain

   • Thalamus (relay station)

   • Different parts of the cortex (e.g., visual or auditory cortex)

4. Sensation is produced once message reaches the brain

Measuring the Senses and Thresholds - Psychometrics

• Noise: random excitation or inhibition of neurons that either increases or decreases the sensed intensity of a physical stimulus

• Implications of noise:

  • Repeated presentations of the same physical level of intensity do not always produce the same internal sensation.

  • Doubling a physical signal does not always produce a doubling of sensation.

  • Absolute threshold: lowest level of intensity at which a person detects a stimulus 50% of the time

Absolute Threshold

• No Noise

  • Physical intensity: 1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0

• With Noise

  • Inhibitory Noise

  • Excitatory Noise

  • Physical intensity: 1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0

Weber’s Law

• Difference threshold: smallest amount of change in a stimulus before a change is detected

• Weber Fraction: difference threshold increases in proportion to the standard

• Weber's law: change in intensity of stimulus/intensity of standard = C OR ∆I/I = C

Hearing

• Sound waves

  • Frequency: gives the pitch (Hertz)

  • Amplitude: gives the volume (decibels)

  • Complexity (timbre): "nature" of the sound (e.g., instruments in music)

The Eye

• Photoreceptors in the retina

• Photopigment: substance contained in photoreceptors that breaks down in response to light

• Two types of photoreceptors:

  • Rods: vision in dim light, no color information, contain rhodopsin

  • Cones: vision in bright light, color information, contain iodopsin (three different types)

Colour Vision: The Trichromatic (Young/Helmholtz) Theory

• Colours of the spectrum: unique wavelengths of light or combinations of unique wavelengths

• Primary colours:

  • Pigments: red, blue, and yellow (subtractive mixture)

  • Light: red, blue, and green (additive mixture)

• Cones sensitive to:

  • Short wavelengths: blue

  • Medium wavelengths: green

  • Long wavelengths: red

• Combinations of iodopsin breakdown produce sensation of other colors on the spectrum

Support for Trichromatic Theory

• Dichromatic color-blindness: some people have only two types of iodopsin

• Monochromatic color-blindness: only have one type of iodopsin

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• Problems for Trichromatic Theory

  • Dichromats can see yellow

  • Yellow is the sensation when both red and green iodopsin is bleached

  • How can both be bleached if dichromats don't have either green or red iodopsin?

  • Other theories (e.g., opponent process theory) have been proposed to work in conjunction with trichromatic theory

  • Visual receptor types are organized in opponent pairs: blue/yellow, red/green, black/white

  • Can cause color afterimages

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• Touch: the Tactile Sense

  • Three different receptors involved with touch

    • Temperature

      • It's relative

      • At 32 degrees Celsius (physiological zero), neither warmth nor cold is felt

      • Above that temperature, we feel warmth; below that temperature, we feel cold

      • However, if skin temperature is raised or lowered, what is sensed as hot or cold changes

    • Pressure

      • It's relative

      • Changes in pressure are felt

      • Large adaptation

      • Don't feel our wristwatch pressing on our skin except for only a short while

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• From Photoreceptor to Sensation ("seeing")

  • Information from rods and cones is conveyed to ganglion cells (via bipolar cells) which send info up the optic nerve to the brain

  • Ganglion cells are particularly important for feature analysis

  • The place where optic nerve leaves the eye is known as the optic disk (blind spot)

  • All information from the eye meets at the optic chiasm which re-routes it to the thalamus, and then to the visual cortex (in the occipital lobe)

  • Information from left visual field goes to the right cerebral hemisphere; information from the right visual field to the left cerebral hemisphere

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• Pain

  • Pain pathways with neurotransmitters: Substance P produces pain; Endorphins decrease it

  • Stimulating parts of the brain with electrodes can release endorphins to reduce pain

  • Endorphins are released in anticipation of pain as well (classical conditioning)

  • Acupuncture may work by stimulating endorphins

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• Smell - the Olfactory Sense

  • Pheromones (e.g., androstenone) are chemicals secreted in the body which produce a physiological response

  • Dogs, monkeys, rodents all respond to pheromones

  • Do humans?

  • Women are asked to dance more often if ovulating

  • Some evidence that women on the same dormitory floor "sync up" menstrual cycles (menstrual synchrony)

  • Mechanism may be pheromones

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• Olfaction: High Adaptation

  • Over time, the ability to detect odor drops to about 30%

  • 8% of people lose the sense of smell: lose interest in sex (tightly linked)

  • Olfaction is tightly linked to taste (inability to smell during a bad cold reduces taste sensation)

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• Taste

  • Main sensors are taste buds: bitter, salty, sour, and sweet (possibly umami - the taste of glutamate)

  • Taste sensation may operate similarly to color sensation (Trichromatic Theory)

  • Different ratios of output from a few different types of receptor cells can produce a multitude of sensations

  • Color: 3 different types of receptors

  • Taste: 4 different types of receptors (possibly a fifth)

  • Other factors such as olfaction, mood, and texture add to the flavor of food

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• Tastes on The Tongue

  • Sweet

  • Sour

  • Salty

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• Papillae on the Surface of the Tongue: A Taste Bud

  • Taste buds are found on the surface of the tongue

  • A blowup of a vertical cross-section near the surface of the tongue shows that the taste buds are located in taste trenches between the tiny bumps called papillae

  • Taste cells

  • Taste bud