Mind, Brain & Behavior Monday February 10, 2003

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

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.

Weber’s Law  The JND will be different for different sensory modalities.  Weber’s 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.

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.

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

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.

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.

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.

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.

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.

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.