Timing, or temporal coding, refers to spike codes in which the specific timing of spikes rather than the overall firing rate encodes information. One often-studied version of temporal coding is the synchronization of spikes across neurons. Synchronization of neuronal spiking has been shown to occur in a number of brain regions and has been related to function in a number of instances. An advantage of temporal coding is that it can convey information more quickly than can rate coding, inasmuch as it does not require averaging, which takes time. Moreover, rate coding and temporal coding are not mutually exclusive, inasmuch as overall firing rates can be varied while synchronous events are superimposed. Such multiplexing of codes may increase the information transmission capacity of neuronal pathways.
Sensory neurons encode information about stimuli. In the process of sensory transduction, one or more aspects of the stimulus must be encoded in a way that can be interpreted by the CNS. The encoded information is an abstraction based on (1) which sensory receptors are activated, (2) the responses of sensory receptors to the stimulus, and (3) information processing in the sensory pathway. Some stimulus parameters that can be encoded include sensory modality, location, intensity, frequency, and duration. Other aspects of stimuli that are encoded are described in relation to particular sensory systems in later chapters.
A sensory modality is a class of sensation. For example, sustained mechanical stimuli applied to the skin result in sensations of touch or pressure, and transient mechanical stimuli may evoke sensations of flutter or vibration. Other cutaneous modalities include cold, warmth, and pain. Vision, audition, taste, and smell are examples of noncutaneous sensory modalities. The specific sensory receptors define the normal energy associated with the modality of a sensory pathway. For example, the visual pathway includes photoreceptors, neurons in the retina, the lateral geniculate nucleus of the thalamus, and the visual areas of the cerebral cortex (see ). The normal means of activating the visual pathway is light striking the retina. However, mechanical stimulation (e.g., pressure on the eyeball) or electrical stimulation of neurons in the visual pathway also produce a visual sensation. Thus neurons of the visual system can be regarded as a labeled line, which, when activated by whatever means, results in a visual sensation.
The location of a stimulus is signaled by activation of the particular population of sensory neurons whose receptive fields are affected by the stimulus. The information may be encoded in the CNS as a neural map. For example, a somatotopic map is formed by arrays of neurons in the somatosensory cortex that receive information from corresponding locations on the body surface (see ). In the visual system, points on the retina are represented by neuronal arrays that form retinotopic maps (see
Intensity may be encoded in a number of ways. Because action potentials have a uniform magnitude, some sensory neurons encode intensity by their frequency of discharge (rate coding). The relationship between stimulus intensity and response can be plotted as a stimulus-response function. For many sensory neurons, the stimulus-response function approximates an exponential curve with an exponent that can be less than, equal to, or greater than 1. Stimulus-response functions with fractional exponents characterize many mechanoreceptors. Thermoreceptors, which detect changes in temperature, have linear stimulus-response curves (exponent of 1). Nociceptors, which detect painful stimuli, may have linear or positively accelerating stimulus-response functions (i.e., the exponent for these curves is 1 or greater). The positively accelerating stimulus-response functions of nociceptors help explain the urgency that is experienced as the pain sensation increases.
Another way in which stimulus intensity is encoded is according to the number of sensory receptors that are activated. A stimulus at the threshold for perception may activate only one or only a few primary afferent neurons of an appropriate class, whereas a strong stimulus of the same type may recruit many similar receptors. Central sensory neurons that receive input from sensory receptors of this particular class would be more powerfully affected as more primary afferent neurons discharge. Greater activity in central sensory neurons may be perceived as a stronger stimulus.