Physiological and psychophysical evidence for temporal coding of sensory qualities in different modalities is considered. A space of pulse codes is outlined that includes 1) channel-codes (across-neural activation patterns), 2) temporal pattern codes (spike patterns), and 3) spike latency codes (relative spike timings). Temporal codes are codes in which spike timings (rather than spike counts) are critical to informational function. Stimulus-dependent temporal patterning of neural responses can arise extrinsically or intrinsically: through stimulus-driven temporal correlations (phase-locking), response latencies, or characteristic timecourses of activation. Phase-locking is abundant in audition, mechanoception, electroception, proprioception, and vision. In phase-locked systems, temporal differences between sensory surfaces can subserve representations of location, motion, and spatial form that can be analyzed via temporal cross-correlation operations. To phase-locking limits, patterns of all-order interspike intervals that are produced reflect stimulus autocorrelation functions that can subserve representations of form. Stimulus-dependent intrinsic temporal response structure is found in all sensory systems. Characteristic temporal patterns that may encode stimulus qualities can be found in the chemical senses, the cutaneous senses, and some aspects of vision. In some modalities (audition, gustation, color vision, mechanoception, nocioception), particular temporal patterns of electrical stimulation elicit specific sensory qualities.
Keywords
GENERAL CLASSES OF NEURAL PULSE CODESThe neural coding problem in perception involves the identification of the neural correlates of sensory distinctions [1][2][3][4][5][6][7][8]. Sensory information can be encoded in patterns of neurons that respond (channel codes) or in temporal relations between spikes (temporal codes). Temporal codes can be further subdivided into time-of-arrival codes that rely on relative spike timings across neurons and temporal pattern codes that rely on internal patterns of spikes that are produced (Table 1). Here we review psychophysical and neurophysiological evidence that bears on temporal codes in perception.Temporal codes utilize stimulus-dependent time structure in neural responses. This time structure can be produced either through phase-locking or intrinsic response characteristics. The simplest time-of-arrival code compares spike arrival times between two neurons to convey the time difference between them, irrespective of the temporal structure internal to each spike train. In contrast, temporal pattern codes utilize this internal time structure to convey information. Temporal pattern codes utilize interspike intervals [1,8], interval sequences [9], and timecourses of discharge [10,11] to convey information. Both time-of-arrival and temporal pattern codes permit multiplexing of different kinds of information [10], though time-division [12], frequency-division [13,14] and codedivision [9] schemes. Some evidence for temporal codin...