Carl D. Hopkins
Lecture 20. Principles of Sensory Coding
All of an animal’s knowledge of its physical world is obtained through the action of its
sense organs which convert four principle attributes of a stimulus into a neural code.
These attributes are
modality, location, magnitude, and timing.
The neural code of
modality is determined by the particular type of sense organ which is activated as a
labeled line. Location
is encoded as the position of the activated receptor over the body
surface which is represented as a topographic map of the sensory surface. Stimulus
magnitude is encoded according by the timing or the frequency of nerve impulses or
physiological rules that apply to all sensory modalities. The timing
of the stimulus is
encoded by receptors which differ in adaptation rates. Combined, the sense organs use
neural codes to inform the brain about the
Purves et al., (2008) Neuroscience, 4th edition, Chapter 9. (Somatic Senses: Touch and
Proprioception), pp. 207-229
Be able to:
Explain the meaning of each of these terms: Transduction, Encoding,
Adaptation, Labeled Line, Integrate and Fire Model, Rate Coding, Lateral
Inhibition, Receptive Field, Sensory Map.
Explain how the nervous system is able to keep track of all these different
parameters defining an external stimulus: what, where, when, and how much?
The focus today is on the somatosensory system and our sense of touch. In this lecture,
we will consider the following key concepts, more or less in sequence:
encoding; adaptation; labeled lines; receptive fields; and
convergence, map formation, and lateral inhibition.
First, let us review the three natural ways that a neuron becomes activated: (1) by
synaptic inputs, (2) by endogenous firing (pacemaker activity), or (3) by activating
sensory receptors. In this lecture we will see how the sense organs tell us
happened in the environment (the identification of the event),
of a stimulus
there was (determining the magnitude of the stimulus),
it was in space, and
it occurred. All sensory events are encoded in the universal language of the nervous
system: all or none action potentials. In this lecture we review how these signals are
encoded and how the brain interprets these sensory codes.
We begin our discussion with the somatosensory system, and the mechanoreceptors
that give us a sense of touch. Mechanoreceptors were some of the first sensory
receptors to be explored physiologically (Adrian and Umrath, 1925).
Fig. 1 shows a diagram of a Paccinian corpuscle illustrating the physiological processes
of transduction, encoding, and adaptation. The Paccinian corpuscle consists of a