Sensory Processes
Josée L. Jarry, Ph.D., C.Psych.
Introduction to Psychology
Department of Psychology
University of Toronto
May 28, 2003
Sensation & Perception
• Sensation– Experience associated with stimuli – The initial steps by which the sense organs and
neural pathways take in stimulus information.
• Perception– Subsequent organizing of information – Meaningful interpretation of information
The Process of Sensation• Physical stimulus
– Matter or energy impinging on the sense organs.
• Physiological response– Pattern of electrical activity that occurs in the sense
organs, the nerves, and the brain as a result of the stimulus.
• Sensory experience– The subjective, psychological sensation - sound,
taste, sight - that is experienced by the individual whose sense organs have been stimulated.
Basic Anatomy of Human Senses• Receptors
– Specialized structures that respond to physical stimulus by producing electrical changes that can initiate neural impulses
• Sensory neurons– Carry neural impulses from the receptors to the
central nervous system
• Sensory areas– Specific areas of the cerebral cortex devoted to
specific senses.
Transduction Common to All Senses
• Transduction– The process by which a receptor cell produces an
electrical change in response to a physical stimulus
• Receptor potential– In response to a stimulus, the membrane of the
receptor cell depolarizes which leads to action potentials in the axons of sensory neurons.
• Quantitative & Qualitative dimension
Stimulus Quantity and Quality
• Senses respond not only to a particular class of stimulus energy
• The also respond to variations in that energy
• All forms of energy vary along at least two dimensions
• Quantitative dimension– Concerns the amount or intensity of energy present
• Qualitative dimension– Concerns the precise kind of energy present
Coding of Stimulus Quantity & Quality
• Coding– Pattern of action potential sent to the brain that
preserves the quantity and quality of a stimulus.
• Coding of stimulus quantity– Stronger stimuli produce larger receptor potentials– Produce faster rates of action potentials in sensory
neurons.
• Coding of stimulus quality– Different receptors within any given sensory tissue
are tuned to different forms of energy.
Sensory Adaptation
• Change in sensitivity that occurs when a sensory system is either stimulated or not stimulated for a length of time.
• Absence of stimulation– Sensory system becomes temporarily more sensitive– Responds to weaker stimuli
• Presence of stimulation– Sensory system becomes temporarily less sensitive– Requires stronger stimuli to produce a response.
Pain• Other senses communicate information
about the external world
• The experience of pain comes from one’s own body
• Pain is not only a sense but it is also a drive
• People are motivate to avoid and/or reduce pain
• Pain has survival value
Neural Pathways of Pain
• Anatomically related to the cutaneous senses
• Free nerve endings– The sensitive terminals of pain neurons are not
surrounded by special capsules or end organs as are the endings of touch and temperature receptors
– Free nerve endings can be found in all body tissues from which pain is sensed, from the skin to the pulp of the teeth.
Two Types of Peripheral Pain Neurons
• A-delta fibers– Thick, myelinated, fast conducting neurons– Mediate the feeling of initial fast, sharp, highly
localized pain.
• C fibers– Very thin, unmyelinated, slow-conducting– Mediate slow, dull, more diffuse, often burning
pain.
Central Pain Pathways: Fast Pain
• Fast pain and A-delta fibres– A-delta fibres synapse on cells in the spinal
cord that lead to an area of the thalamus called the ventrobasal complex
– ventrobasal complex also receives neurons that mediate touch
– sends its output to the somatosensory cortex– allows us to localize where pain originates
Central Pain Pathways: Slow Pain
• Slow pain and C fibres– C fibres synapse on cells in the spinal cord – Relays to a midline nucleus in the thalamus and– to the limbic system– responsible for motivational and emotional
aspects of pain– Those connections are important for the
interpretation of pain.
Neural and Chemical Inhibition of Pain
• Gate control theory
• Ronald Melzack and Patrick Wall (1965, 1982)– For pain to be experienced, input from
peripheral pain neurons must pass through a gate located at the point where they enters the spinal cord and lower brain stem.
Pain-inhibiting System
• Periaqueductal gray (PAG)– PAG neurons have excitatory connections with
inhibitory interneurons in the spinal cord– These inhibitory interneurons prevent ascending
neurons to relay pain messages to the brain
• Endorphins– The spinal cord inhibitory interneurons releases
endorphins– Endorphins are inhibitory neurotransmiters