sensation

• Sensitivity


  The more intense a stimulus, the more strongly it affects the relevant sense organs
• Absolute thresholds


  The minimum magnitude of a stimulus that can be reliably discriminated from no stimulus at all
• Minimum stimulus for different senses

  • Vision: A candle flame seen at 30 miles on a dark, clear night
  • Hearing: The tick of a clock at 20 feet under quiet conditions
  • Taste: One teaspoon of sugar in 2 gallons of water
  • Smell: One drop of perfume diffused into the entire volume of six rooms
  • Touch: The wing of a fly falling on your cheek from a distance of 1 centimeter
• Our sensory modalities are extremely sensitive to the presence of, or any change in, an object or event
• Psychophysical procedures


  Experimental techniques for measuring the relation between the physical magnitude of some stimulus and the resulting physiological response
• Photon


  Smallest unit of light energy
• Human vision is as sensitive as is physically possible
• A person can detect a flash of light that contains only 100 photons, only 7 of these 100 photons actually contact the critical molecules in the eye that are responsible for translating light into the nerve impulses that corresponds to vision
• Each of these 7 photons affects a different molecule. The critical receptive unit of the eye (a molecule), therefore, is sensitive to a single photon
• JND (Just Noticeable Difference)


  The minimum difference in stimulus magnitude necessary to tell two stimuli apart
• Measurement of change detection

  Determining how much stimulus intensity to be raised from some standard, defined to some arbitrary level, to distinguish the new, higher level from the base level
• Weber-Fechner Law


  The larger the value of standard stimulus, the less sensitive the sensory system is to changes in intensity. The intensity by which the standard must be increased to be noticed is proportional to the intensity of the standard
• Suprathreshold conditions


  Conditions in which stimulus intensity is above threshold
• Steven's Law


  Perceived psychological magnitude (ψ) is a power function of physical magnitude (Φ)
• Assumptions of Steven's Law

  • A jnd above some standard stimulus is some fixed percentage of the standard
  • Psychological intensity is approximately measured in units of jnd
• A power function with a less-than-1 exponent, such as corresponding to loudness, is concave down, and increasing level of physical intensity leads to progressively smaller increases in sensation
• A power function with a greater-than-1 exponent, such as corresponding to electric shock, is concave up, and increasing levels of physical intensity lead to progressively greater increases in sensation
• Signal detection theory


  Standard way of understanding how errors are made in many diverse situations
• Sensation


  Determined by the perceptual strength of the stimulus
• Bias


  Criterion set by the observer, for making a particular response
• Signal


  What the observer is trying to detect embedded in noise
• Noise


  Anything in the environment irrelevant to what the observer is trying to detect
• Expectations


  In a signal-detection situation, one of the factors that influence bias is expectation. The greater the observer's expectations that a signal will be present, the greater the observer's bias to respond yes
• Hit


  A response of correctly responding yes when a signal is present
• False alarm


  Response is incorrectly responding yes when only noise is present
• Hit rate


  Proportion of hit rate
• False-alarm rate


  Proportion of false-alarm-rate
• Inferences from hit rate and false-alarm rate

  • The observer does detect a signal only when the hit rate exceeds the false-alarm rate
  • If the hit-rate exceeds the false-alarm rate by a lot, we infer that sensitivity is high
  • If the hit rate exceeds the false-alarm rate only a little, we infer that the sensitivity is low
  • If the hit rate equals the false-alarm rate, we infer that sensitivity is zero
• Sensory coding


  How stimuli are transmitted from the sensory receptors to the brain
• Transduction


  Translating physical energy into electrical signals that can make their way to the brain
• Receptors


  Specialized nerve cells or neurons that pass electrical signals to connecting neurons until they reach the receiving area in the cortex
• Cortex


  Includes the primary motor area, the primary somatosensory area, the primary visual area, the primary auditory area, and the association areas
• Vision


  The physical stimulus is light, a form of electromagnetic energy
• Image forming system for vision

  • Cornea: transparent front surface of the eye that bends light inward
  • Lens: Focuses light on the retina, changes shape to focus on near or far objects
  • Pupil: Circular opening that varies in size to control amount of light entering the eye
  • Retina: Thin layer of tissue at the back of the eyeball
  • Fovea: Small region at the center of the retina where details are seen
• Rods

  Receptor cells best for seeing at night, operate at low intensities and lead to low resolution, colorless sensations
• Cones

  Receptor cells designed for seeing during the day, respond to high intensities and result in high-resolution sensations that include color
• Dark adaptation


  Changes in ability to see in the dark, like entering a dark theater from a bright street
• Light adaptation


  When the retina becomes light adapted and then placed in darkness, it becomes increasingly sensitive to light and the absolute threshold decreases
• Visual acuity


  The eyes' ability to resolve details
• Snellen acuity

  Acuity measured relative to a viewer who does not need to wear glasses. An acuity of 20/20 indicates that a normal person can read at a distance of 20 feet