Tasting and smelling as active, exploratory sensory processes.

Abstract

Tasting and smelling in terrestrial vertebrates usually involve intermittent, active sampling of the chemosensory environment. This active sampling is produced by ingestive and/or respiratory behavior initiated by the central nervous system. Vascular and secretory factors modulate access to chemosensitive regions. For olfaction, per se, respiratory rhythms provide the basic pattern of intermittent odorant input. In humans and many other species, sniffing alters the respiratory pattern and moves relatively brief, higher-flow-rate samples toward the olfactory epithelium. Sniffing may be triggered by nonchemosensory events or by odorants obtained during normal respiration and can be stereotyped with regard to rate and duration of the sniffs. Within the central nervous system, processing of olfactory input can differ as a function of its temporal relationship to respiratory and sniffing rhythms. For gustation, ingestion of food and liquids provides the major source of stimulation. Since these ingestive acts are intermittent, although often repetitive and stereotyped, natural taste stimulation also is necessarily both patterned and distributed in time. Swallowing and the interingestion interval that follows tend to return the taste receptors toward a salivary baseline. During the drinking of liquid by humans, retention of a bolus in the mouth is much briefer than intersip intervals. Simulated drinking with alternating flows of artificial saliva and a sufficiently different stimulus yields constant taste intensity. Conversely, unchanging gustatory stimulation produces reductions in intensity.