Goals

Thermal sensations (warmth and cold) are directly related to thermoregulatory responses and consequently to metabolic processes in the body and mechanisms of energy balance. Thermal stimuli can be beneficial when the levels of stimulation are equal to the body’s metabolic needs. Extremes of thermal stimulation, however, can disturb body temperature regulation. Furthermore, extreme thermal stimulation commonly produces pain, and the noxious sensations of pain and irritation often signal current or imminent damage to body tissues.

The chemical senses – taste and smell – play a critical role in guiding the intake of food and thus in nutrition and energy balance. Foods and beverages are beneficial (indeed, crucial for survival) as long as nutrition and caloric intake are consistent with the body’s physiological needs and metabolic expenditures. Inadequate intake of food threatens health when caloric intake falls short of energy expenditure. But excessive intake of food can also become harmful, when caloric intake exceeds energy expenditure.

To understand sensory processing of thermal, noxious, and chemosensory stimuli, research relies on psychophysical analysis of sensory and higher-level (cognitive) processes in perception. Analysis of people’s ability to detect, distinguish, and identify stimuli makes it possible to draw inferences about underlying neural mechanisms, inferences that then can be tested through direct physiological investigations. These psychophysical and physiological studies both emphasize mechanisms of central neural integration and interaction. Elucidating processes of neural integration and interaction is central to understanding: (a) the perceptions of temperature, irritation, and pain, because these perceptions commonly result from interactions among effects of mechanical, thermal, and noxious stimulation; and (b) the perception of flavor, because flavor results from the central combination of gustatory, olfactory, and somatosensory information when food is taken into the mouth. Central neural processes underlie selective attention, which determines how efficiently and effectively different sensory inputs are processed. Finally, central processes underlie learning, which determines how people come to identify and respond appropriately to potentially harmful or beneficial stimuli in the environment.

Current Research

People’s sensory systems are constantly barraged at any moment by stimuli activating several of the senses, sometimes all of them. Contrary to the long-standing view that each sensory system largely operates independently, integration and interaction of multisensory signals is actually the rule rather than the exception. The perception of warmth, cool, heat, and pain, for example, often result from complex interactions among the outputs of receptors that respond to thermal, mechanical, and noxious stimuli. And the perception of flavor involves the integration of gustatory, somatosensory, and olfactory signals. Gustatory qualities, such as sweet, sour, salty, and bitter, arise from stimulation of taste receptors on the tongue. Somatosensory qualities arise from thermal, mechanical, or nociceptive stimulation of receptors on the tongue. And, of special importance, olfactory qualities in flavors arise from molecules that reach the olfactory epithelium through the back of the mouth and that help us distinguish, for example, chocolate from honey. Pierce scientists are actively studying the integration and interaction of multisensory signals in skin sensations and in taste and flavor perception. Psychophysical research is investigating excitatory and inhibitory interactions among mechanical, thermal, and noxious stimuli, interactions between temperature and taste, and interactions among the gustatory and olfactory components of flavors.

In the face of the barrage of sensory information, attention serves as a primary mechanism by which the central nervous system selects a subset of stimuli to process most effectively and efficiently. Consequently, Pierce scientists are currently using psychophysical methods to understand how people can attend selectively to somatosensory stimuli presented to different regions of the body and to the perceptually different components of chemosensory stimuli when the stimuli are tasted (put in the mouth) and when they are smelled (sniffed).

Perception and sensory-guided behavior ultimately reflect the activity in very large numbers of neurons, of neuron ensembles, in the central nervous system. An important component of the program involves the use and ongoing development of new and improved methods to monitor, in real time, the activity of very large numbers of neurons. Current research uses three approaches to do this: One uses functional magnetic resonance imaging (fMRI) to identify regions of the brain activated by environmental stimuli; methods of neuroimaging are being used in particular to investigate the brain mechanisms of flavor perception. A second approach uses electrodes inserted in large numbers of neurons to record simultaneously the nerve potentials. And the third monitors changes in voltage that are associated with neural firing; by inserting into neurons proteins that fluoresce in response to the changes in voltage, the neural responses can be measured indirectly, by optical methods. This multifaceted approach enhances our ability to identify the neural structures and patterns of neural responses that underlie both the perception of environmental stimuli and the behavioral responses to them.