Mind, Brain

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Mind, Brain Behavior

• Monday
• February 10, 2003

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Sensory Systems

• Sensory modalities
• Vision, hearing, touch, taste, smell
• Submodalities building blocks that constitute
  elementary senses
• Form more complex sensations.
• Example taste includes sweet, sour, salty,
  bitter

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Intensity of Sensation

• Depends on the strength of the stimulus.
• Sensory threshold -- lowest detectable intensity.
• Change with experience, fatigue, context
• Example threshold for pain is raised during
  sports.
• Just noticeable difference (JND) minimal
  perceptible difference in strength between two
  stimuli.

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Webers Law

• The JND will be different for different sensory
  modalities.
• Webers Law expresses the sensitivity of a
  sensory system to differences between two
  stimuli
• JND K x S
• where K is a constant and S is a reference
  stimulus.

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Duration of Sensation

• Duration of a sensation depends on both the
  duration of the stimulus and its intensity.
• Adaptation -- intensity of sensation diminishes
  if a stimulus persists for a long time.

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Source of a Stimulus

• Sensations can be localized, within the body or
  in space (in the case of sound or vision).
• Two-point threshold the minimum detectable
  distance between two stimuli.
• 1 mm on the fingertips

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Transduction

• Two stages
• Transduction the stimulus affects the receptor
  of a neuron and causes local depolarization or
  hyperpolarization.
• Neural encoding -- the change in the neuron
  results in an action potential.
• Each receptor is tuned to a narrow range of
  stimuli.
• Tuning curve the sensitivity of the receptor.

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Receptor Fields

• Primary sensory neurons have receptor fields.
• Receptor neurons provide input to second-order
  neurons.
• Relay nuclei transmit sensory information along
  sensory pathways in the brain.
• Thalamus is the relay for virtually all sensory
  pathways to the cortex.
• Projection neurons send their axons to the next
  relay nucleus in the pathway.

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Parallel Pathways

• Different submodalities send information along
  separate and parallel pathways at the same time.
• Submodalities of somatic sensation
• Touch, limb position, pain, temperature
• Carried by two parallel pathways
• Parallel pathways converge in the cortex, in
  higher-order sensory cortical regions.

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How is Information Encoded?

• Patterns of discharge of action potentials
• Frequency code -- strength is conveyed by
  frequency (faster rates of firing)
• Population code strength is also conveyed by
  how many neurons are responding within a group
• Slow vs fast adapting receptors differentiate
  duration of a stimulus.
• Labeled line code -- modality is encoded by the
  particular receptor type.

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Sharpening of Stimuli

• Lateral inhibition a mechanism that combines
  excitatory and inhibitory components to enhance a
  signal.
• The difference between stimulation of one part of
  a receptive field and another is strengthened.
• Reduces the likelihood that a stimulus at the
  edges of a field will activate a relay neuron.
• Enhances the acuity of the sensory system.

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Modality-Specific Mechanisms

• In addition to these shared properties, different
  sensory modalities perform different functions
  and have mechanisms for doing so.
• Specific receptors in somatic sensory system are
  specialized for different kinds of input.
• Population codes are most important to taste and
  smell.
• Timing and intensity of signals is important to
  localizing sounds in space.