Temporal strategies for neural coding
of sensory information
Peter Cariani Eaton Peabody Laboratory of Auditory Physiology
Massachusetts Eye and Ear Infirmary
243 Charles St, Boston, MA 02114 USA
CC-AI, 1995, Vol. 12, No. 1-2, Special Issue on Self Reference in Biological and Cognitive Systems, L. Rocha Editor
Potential strategies for temporal neural processing in the brain and their implications for the design of artificial neural networks are considered. Current connectionist thinking holds that neurons send signals to each other by changes in their average rate of discharge. This implies that there is one output signal per neuron at any given time (scalar coding), and that all neuronal specificity is achieved solely by patterns of synaptic connections. However, information can be carried by temporal codes, in temporal patterns of neural discharges and by relative times of arrival of individual spikes. Temporal coding permits multiplexing of information in the time domain, which potentially increases the flexibility of neural networks. A broadcast model of information transmission is contrasted with the current notion of highly specific connectivity. Evidence for temporal coding in somatoception, audition, electroception, gustation, olfaction and vision is reviewed, and possible neural architectures for temporal information processing are discussed.
The human brain is by far the most capable, the most versatile, and the most complex information- processing system known to science. For those concerned with problems of artificial intelligence there has long been the dream that once its functional principles are well understood, the design and construction of adaptive devices more powerful than any yet seen could follow in a straightforward manner. Despite great advances, the neurosciences are still far from understanding the nature of the "neural code" underlying the detailed workings of the brain. i.e. exactly which information-processing operations are involved.
If we choose to view the brain in informational terms, as an adaptive signalling system embedded within an external environment, then the issue of which aspects of neural activity constitute the "signals" in the system is absolutely critical to understanding its functioning. It is a question which must be answered before all others, because all functional assumptions, interpretations, and models depend upon the appropriate choice of what processes neurons use to convey information. The role of the time patterns of neural discharges in the transmission and processing of information in the nervous system has been debated since the pulsatile nature of nervous transmission was recognized less than a century ago. Because external stimuli can be physically well-characterized and controlled, the encoding of sensory information has always played a pivotal role in more general conceptions of neural coding.