7.2: General Principles of Sensory Systems

GENERAL PRINCIPLES OF SENSORY SYSTEMS

Although each of our senses are quite different in the type of stimulation that they respond to, there are many principles that can be generalized across most, if not all sensory systems.

The table below displays the main aspects of the sensation and perception process that takes place across all sensory systems in some way. Some of the steps of the process are more involved for certain systems compared to others, but they all include the four main steps of stimulus input, sensation, perception, and action. There are a number of specific terms for structures and processes included in this table and the terminology list following it. Much more detail on all of these terms and processes will be presented in the sections for each individual sensory system that follows.

Common Aspects of all Sensory Systems

	Stimulus Input→	Sensation →	Perception →	Action
General process	Sensory systems attend to & translate specific stimuli	Stimuli are converted (transduced) into neural messages	Sensations are integrated to produce perceptions	Perceptions lead to changes in behavior
Specific mechanisms of action	Accessory structures and selective attention are involved with initial stimulus processing	Specialized Sensory Receptor Neurons are utilized for sensory transduction	Initial Perception of stimulus properties occurs in "primary" sensory cortical areas; Holistic Perception occurs in higher cortical areas	Behavioral changes include physical behaviors, "mental" or psychological changes, and physiological changes
Sensory Systems Vision	Stimulus: Light waves - Photons Accessory Structure: Structures of the Eyeball	Sensory Receptor Neurons: Rods Cones	Initial Perception: Lines of contrast Holistic Perception: Combination of visual stimulus characteristics (depth, color, movement, etc.)
Audition	Stimulus: Sound Waves – Air vibrations Accessory Structure: Structures of the outer and middle ear	Sensory Receptor Neurons: Auditory Hair Cells	Initial Perception: Distinct tones (i.e. pitches) Holistic Perception: Combination of auditory stimulus characteristics (loudness, pitch, timbre)
Somatosensory	Stimulus: Varied – Touch, Pressure, Temperature, Pain, etc. Accessory Structure: Varied – Pacinian Corpuscles, Merkel Discs, Free Nerve Endings, etc.	Sensory Receptor Neurons: Somatosensory Neurons	Initial Perception: Varied – Touch, Pressure, Temperature, Pain, etc. Holistic Perception: Combination of somatosensory stimulus characteristics (touch, pressure, pain, etc.)

Sensation and Perception Terminology

The following terms are used in most sensory systems and describe key neural elements of the sensation and perception of stimuli.

Sensory transduction

Our sensory systems work by converting different types of stimuli in the environment (i.e. visible light, sound waves, chemical molecules) into action potentials in the nervous system. This conversion is called sensory transduction and occurs in all sensory systems.

Sensory receptors

Sensory transduction begins at the sensory receptors. Each sensory system has specialized cells that are able to detect the environmental stimuli. Photoreceptors detect light, chemical receptors in the tongue and nose detect odors and taste, mechanoreceptors detect touch, and hair cells detect sound.

Receptor Potentials

We have learned about postsynaptic potentials in neurons, receptor potentials are similar membrane potential changes that happen in sensory receptors in response to a stimulus.

Receptive fields

Receptive fields are easiest to understand in the visual and somatosensory systems. The receptive field for a neuron is the region of the retina or skin where a stimulus (light or touch) will evoke a response in the neuron. Receptive fields in the auditory system can consist of a certain frequency of sound and/or the location of sound in space.

Receptive fields can vary in size and shape depending on the characteristics of neuron (i.e. type, location in body, location in pathway). Receptive fields become more complex as information travels to the brain.

Lateral Inhibition

Lateral inhibition is a process used by sensory systems to enhance the perception of signals, particularly at edges, points, or other changes in the stimulus. It occurs because overlapping receptive fields can inhibit each other. This inhibition enhances the perceived differences between the stimulus and the area not stimulated.

Neural Coding

There are a number of different ways in which the nervous system encodes complex information. Two that are common within the sensory systems are line coding and population coding.

Labeled Line Coding

In the labeled line coding of information, one cell encodes for one type of sensory quality. Pain is a good example of this. If a pain receptor is activated, the resulting sensation will be pain, regardless of the manner in which the receptor is stimulated. In other words, the sensory neurons are specifically tuned to one sensory stimulus. If that receptor-cell type was dysfunctional, the sensation will not be perceived. For example, there is a mutation that prevents sodium channels in pain receptors (but not other cell types) from working. When this mutation occurs, the subject cannot feel pain.

Population Coding

In populating coding, one cell can encode more than one sensory modality, and it is the combination of many cells that make up the perception. An example of this is color vision. Each color photoreceptor is most sensitive to a specific color (blue, green, or red), but a range of wavelengths can elicit changes in firing rates in the neuron. Therefore, the responses from a population of color photoreceptors must be combined to perceive the full spectrum of color.

Higher level processing of taste and olfaction also uses population coding – sometimes the sense of smell is needed in addition to the sense of taste to fully perceive a flavor. Have you ever been congested from a cold and food just doesn’t taste the same? That’s due to this combining of the senses for a full perception.

Pathways

In general, the route sensory information takes from the periphery to the central nervous system is similar among most of the systems. Environmental stimuli become encoded by a specialized receptor in the periphery. Information then enters the central nervous system via the spinal cord or brainstem and relays through the thalamus, a structure that sits deep in the forebrain. The only sensory system that does not relay through the thalamus is the olfactory system. The thalamus then sends projections out to the primary cortical regions for each sensory system.

Thalamic "relay" ++

It’s common to hear that sensory information “relays” through the thalamus on the way to the cortex (for example, in the paragraph above). This language can give the impression that the thalamus is only responsible for making sure the sensory signal gets from periphery to the cortex. This greatly underestimates the thalamic role. The thalamus is known to contribute to the processing and modification of the sensory signal.

Attributions:

1. General Principles of Sensory Systems section was created by Alan Keys, Ph.D., Sacramento City College, Sacramento, CA.
2. Sensation and Perception Terminology section was adapted by Alan Keys, Ph.D. from Foundations of Neuroscience by Casey Henley and is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.