General Definition of Sensory Coding
Sensory coding is a type of information processing that occurs in nervous systems and can be thought of as four separate yet related phenomena:
- Reception, whereby specialized sensory receptors absorb physical energy from sensory stimuli.
- Transduction, which involves the conversion of this physical energy into electrical energy in the form of neuronal firing.
- Coding, which is the correspondence between specific parameters of the stimulus and specific parameters of the neuronal firing that represents it.
- Awareness, the possible conscious perception of encoded sensory stimuli.
Philosophical and Experimental Origins
The British empiricists John Locke, George Berkeley and David Hume proposed that all knowledge is based on sensory experience: vision, hearing, taste, smell and touch. This led French philosopher Auguste Comte to argue that the study of behavior should be a subdiscipline within biology, and that the rules of operation of the mind should be derived from objective observation.
Ernst Weber, Gustav Fechner, Hermann Helmholtz, and Wilhelm Wundt were the first experimental psychologists. They showed that the senses differ in their modes of reception, but involve the same three stages of processing: a physical stimulus, transformation of the stimulus into nerve impulses, and a response to this signal after perception or conscious experience. Early sensory psychophysics studies by Weber and Fechner showed that sensory systems always transmit four basic types of information: modality, location, intensity and timing. Along with Helmholz and von Frey, these psychophysicists used experimental data about the sensitivity of sensory systems to formulate mathematical laws that predicted the relationship between stimulus magnitude and sensory discrimination.
Fundamental Attributes of Sensory Experience
Sensory experience has four fundamental attributes that are encoded by specialized subcategories of neurons within the nervous system.
- A modality is a general class of stimulus defined by the type of energy that the stimulus transmits and by the specialized receptors that sense this energy.
- The location of a stimulus is given by the set of sensory receptors within a sensory system that are activated by that stimulus. Sense organs often have a topographic distribution of receptors so as to provide information about, say, the size and position in space of the stimulus.
- The amplitude of responses of each receptor gives the intensity of a stimulus, which is a function of the total amount of energy conferred by the stimulus to the receptor.
- The timing of stimulation is given by the onset and end of the receptor response, i.e. the speed of energy gain and loss in the receptor. Thus the firing pattern of active sensory neurons encodes both the intensity and time course of stimulus presentation.
Encoding Stimulus Features
Encoding Stimulus Location
Information regarding the site of stimulation on the body or the location of a stimulus in space, the size and shape of objects, and the fine detail of a stimulus or of the environment are crucially represented by the spatial arrangement of stimulated receptors in a sense organ. The spatial area within which stimulation excites a sensory neuron is referred to as its receptive field.
In vision and somatic sensation, this receptive field confers a specific topographic location to the sensory output of the corresponding sensory neuron. A receptor will only respond to stimulation within its receptive field, and a stimulus larger than a single receptive field will activate neighboring receptors. Thus stimulus size affects the number of receptors that are stimulated. The resolution of a sensory system can be a function of its receptor density. Finer resolution of spatial detail is possible with denser receptor populations since each receptor will have a more restricted receptive field. Receptor density, however, is not uniform throughout a sensory sheet. The resulting differences in the density of afferent inputs to the central nervous system leads to discrepancies in the topographic representation of various body parts in central nervous system maps: densely innervated body parts occupy the largest areas, while sparsely innervated parts occupy the smallest areas.
The receptors for hearing, taste and smell are spatially distributed according to the sensitivity, or energy spectrum, of these modalities. Auditory receptors are organized according to the sound frequency to which they respond, while chemoreceptors on receptive surfaces in the nose and on the tongue are distributed based on their particular chemical sensitivity.
Encoding Stimulus Intensity
The intensity or amount of sensation experienced is a function of stimulus strength. The sensory threshold is the lowest stimulus strength a subject can detect. These thresholds are usually determined statistically through exposure of a subject to a series of stimuli of random amplitude. The percentage of times that a subject reports detecting the stimulus is plotted against stimulus amplitude, yielding a psychometric function. The sensory threshold is conventionally the stimulus amplitude detected in half the trials. The sensitivity of receptors for a modality limits the sensory threshold; threshold energy is tied to the minimum stimulus amplitude that generates action potentials in a sensory nerve.
Afferent fibers increase their discharge frequency as stimulus intensity increases. This is because stimulus amplitude influences the activity of sensory receptors. Sensory stimuli alter the membrane potential in a pattern that generates a digital pulse code whereby action potential frequency encodes the amplitude of receptor potentials. Thus higher frequency action potentials result from larger receptor potentials which have been elicited by higher-amplitude stimuli.
After an action potential has fired, a neuron undergoes an absolute refractory period during which action potentials cannot be generated due to inactivation of sodium channels. The next spike fired by the receptor's axon will occur late in the refractory period or at its end. A receptor potential of greater amplitude will cause the neuron to reach its firing threshold earlier, thereby reducing the interspike interval. Moreover, stronger stimuli can also activate a greater number of receptors, i.e. the responsive receptor population size encodes stimulus intensity. This phenomenon is referred to as a population code and generally hinges on the existence of variable sensory thresholds among the individual receptors of a given sensory system.
Encoding Stimulus Duration
Changes in the frequency of action potentials encode the temporal properties of a stimulus. Receptors often indicate the rate of increase or decrease in stimulus intensity via rapid alterations of their firing rate. Cessation of firing can indicate the end of stimulus presentation. Adaptation is the process whereby sensory receptors decrease their responses and sensation is reduced or disappears as a result of prolonged stimulus presentation without change. Such receptor adaptation is a likely neural mechanism whereby subjects gradually lose awareness of constant stimuli.
There exist both slowly and rapidly adapting receptors. Thus sensory systems can detect contrasts in stimuli, that is to say changes in the temporal and spatial patterns of stimuli. Velocity and acceleration, the time derivatives of stimuli that indicate motion, are detected by rapidly adapting receptors that fire with rates proportional to the speed of motion and stop when stimuli come to rest. Similarly, certain visual and somatosensory neurons detect spatial contrasts via sensitivity to edges: they fire faster when abrupt changes occur in their receptive fields, as opposed to when spatial properties remain constant.
There are five principle sensory modalities that were first identified in ancient times: hearing, vision, touch, taste and olfaction. In modern times, the somatic senses of pain, temperature, itch and proprioception, as well as the vestibular sense of balance, are also included.
Sensory receptors, their central pathways, and corresponding target areas in the brain constitute a sensory system. The sensory receptor converts stimulus energy into electrical energy, a processing mechanism found in all sensory systems. This receptor generated electrical signal is called the receptor potential, and the transformation of stimulus energy into electrical energy is called stimulus transduction. The morphology of a sensory receptor is usually specific and selective for transduction of a particular form of energy, a phenomenon referred to as receptor specificity. This property of receptor specificity is the basis of the labeled line code, the key coding mechanism for the modality of the stimulus. Receptor selectivity for a particular type of stimulating energy indicates that the corresponding axon is a line of communication specific for a given modality and carries information only about that type of stimulus. Sensory receptors make connections with and activate particular structures within the central nervous system. Thus a given modality is represented by a set of neurons that are connected to a specific class of receptors and constitute a sensory system.
Humans have four classes of receptors, each of which responds selectively to one form of energy: mechanical, chemical, thermal or electromagnetic. In the somatosensory system, mechanoreceptors permit touch, proprioception and joint position, while hearing and balance occur through mechanoreceptors in the inner ear. Mechanoreceptors sense physical deformation of and pressure on tissue, which is transduced into electrical energy by the physical impact of the stimulus on cytoskeleton-linked cation channels in the membrane. The resulting deformation of stretch-sensitive channels increases ion conductances that depolarize the receptor. Pain, itch, taste and olfaction occur through chemoreceptors, which generally respond to the appropriate ligand with a depolarization. Dermal thermoreceptors sense temperature in the body and surrounding environment. Retinal photoreceptors respond with hyperpolarization to electromagnetic energy.
The major modalities each have several submodalities, since each of the four receptor classes described above is not homogeneous, but rather consists of various different receptor subtypes that are specialized to a limited stimulus energy range. Thus receptors effectively act as filters for a narrow bandwidth of energy, and exhibit tuning to a unique stimulus that activates the receptor at low energy, also known as the receptor's adequate stimulus. Data from physiological experiments can be used to generate a tuning curve that displays the receptor's sensitivity range and preferred stimulus energy band, at which it is activated by the lowest possible stimulus amplitude.
It should be noted that while sensory neurons are generally sensitive to only one type of stimulus, the specificity is not absolute: for example, retinal photoreceptors may be activated by very strong mechanical stimulation.
Signals from individual sensory neurons are integrated and converge as they are processed at higher levels in the central nervous system. Receptors project to first-order neurons in the central nervous system, which subsequently project to second-order and higher order-neurons. Relay nuclei process sensory information and determine if it is sent to cortex. Noise and sporadic neural activity are filtered out to generate a stronger signal. Within relay nuclei, the convergence of connections allows processing of sensory signals in conjunction with activity from adjacent input channels. The receptive fields of neurons in relay nuclei are defined by the population of neurons that synapse onto them and are therefore larger and more complex than in receptors. Inhibitory interneurons within relay nuclei function to increase contrast between stimuli, thereby enhancing spatial resolution. Sensory information is further processed in the thalamus and cerebral cortex, with signal enhancement occurring throughout sensory processing pathways. A given sensory stimulus may or may not ultimately be consciously perceived by the subject.
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