How does your brain process signals?

With advancements in EEG, PET, and fMRI researchers have now begun to gain a better understanding of which parts of the brain are responsible for processing stimuli from different sensory modalities.

"THERE ARE MULTISENSORY REGIONS IN THE BRAIN THAT ACT AS CONVERGENCE ZONES WHERE INPUTS FROM DIFFERENT SENSORY MODALITIES COMBINE, INTERACT, AND INFLUENCE EACH OTHER."

Rather than each structure having a single function, it is now understood that there are multisensory regions in the brain that act as convergence zones where inputs from different sensory modalities combine, interact, and influence each other1,2. 

This means that sensory signals from one source can influence how a signal from a different source is interpreted. For example, it has been proven that solutions that smell of strawberry are perceived to smell stronger when they are coloured red3, demonstrating how a visual cue can cause changes to your perceived sense of smell. This is called the cross-modal sensory effect.

 

Cross Modal Sensory Compensation Effect

 

How does this affect your cravings?

Amongst the five senses, the olfactory and gustatory systems are especially interconnected4,5,6. Together with the trigeminal system, they make up the chemosensory system, which is responsible for flavour perception7.

When you receive an olfactory stimulus, signals are sent from the olfactory bulb to the piriform cortex and then the orbitofrontal cortex. It has recently been discovered that the orbitofrontal cortex is one of the multisensory regions, which is predominantly responsible for taste and olfactory signals7,8,9.

One of the jobs of the orbitofrontal cortex is to determine the reward value10. Once a stimulus with a high reward value is identified, the brain’s reward circuitry, primarily composed of the dopamine system, is activated11,12. 

The key here is that whilst the orbitofrontal cortex discriminates between stimuli on the basis of valence as well as the intensity of the reward value13,14, it does not discriminate among the sensory modalities in which the stimuli are encoded15.

"THIS IS WHAT IS REFERRED TO AS THE CROSS-MODAL SENSORY COMPENSATION EFFECT; WHERE STIMULI FROM ONE SENSE CAN SATISFY THE DESIRE RELATED TO ANOTHER SENSE."

This is what is referred to as the cross-modal sensory compensation effect; where stimuli from one sense can satisfy the desire related to another sense6. Sensory-specific satiety does not require food to enter the gastrointestinal system and does not depend on the ingestion of calories17.

The powerful effect of the olfactory sense is often overlooked, however, it is particularly suited to achieving satiety as the body can adjust its responses rapidly to differing aromas18.

  1. Driver, Jon, Noesselt, Toemme (2008), “Multisensory Interplay Reveals Crossmodal Influences on “Sensory-Specific” Brain Regions, Neural Responses, and Judgments,” Neuron, 57 (1), 11–23.
  2. Van Atteveldt, Nienke, Murray, Micah M., Thut, Gregor, Schroeder, C.E. (2014), “Multisensory Integration: Flexible Use of General Operations,” Neuron, 81 (6), 1240–53.
  3. Zellner, Debra A., and Mary A. Kautz (1990), “Color Affects Perceived Odor Intensity,” Journal of Experimental Psychology: Human Perception and Performance, 16 (2), 391–97.
  4. Driver, Jon, Noesselt, Toemme (2008), “Multisensory Interplay Reveals Crossmodal Influences on “Sensory-Specific” Brain Regions, Neural Responses, and Judgments,” Neuron, 57 (1), 11–23.
  5. Van Atteveldt, Nienke, Murray, Micah M., Thut, Gregor, Schroeder, C.E. (2014), “Multisensory Integration: Flexible Use of General Operations,” Neuron, 81 (6), 1240–53.
  6. Zellner, Debra A., and Mary A. Kautz (1990), “Color Affects Perceived Odor Intensity,” Journal of Experimental Psychology: Human Perception and Performance, 16 (2), 391–97.
  7. Lundstrom, Johan N., Boesvelt, Sanne, Albrecht, Jessica (2011), “Central Processing of the Chemical Senses: An Overview,” ACS Chemical Neuroscience, 2 (1), 5–16.
  8. De Araujo, Ivan E.T., Rolls, Edmund T., Kringelbach, Morten L., McGlone, Francis, Phillips, Nicola (2003), “Taste-Olfactory Convergence, and the Representation of the Pleasantness of Flavour, in the Human Brain,” European Journal of Neuroscience, 18 (7), 2059–68.
  9. Gagnon, Lea, Vestergaard, Martin, Madsen, Kristoffer, Karstensen, Helena G., Siebner, Hartwig, Tommerup, Niels. (2014), “Neural Correlates of Taste Perception in Congenital Olfactory Impairment,” Neuropsychologica, 62, 297–305.
  10. Rolls, Edmund T. (2008), “Functions of the Orbitofrontal and Pregenual Cingulate Cortex in Taste, Olfaction, Appetite, and Emotion,” Acta Physiologica Hungarica, 95 (2), 131–64.
  11. Camerer, Colin, Loewenstein, George, Prelec, Drazen (2005), “Neuroeconomics: How Neuroscience Can Inform Economics,” Journal of Economic Literature, 43 (March), 9–64.
  12. Wise, Roy A. (2002), “Brain Reward Circuitry: Insights from Unsensed Incentives,” Neuron, 36 (2), 229–40.
  13. Hollerman, Jeffrey R., Tremblay, Leon, Schultz, Wolfram (1998), “Influence of Reward Expectation on Behavior-Related Neuronal Activity in Primate Striatum,” Journal of Neurophysiology, 80 (2), 947–63.
  14. Tremblay, Leon, Schultz, Wolfram (1999), “Relative Reward Preference in Primate Orbitofrontal Cortex,” Nature, 398, 704–08.
  15. Schultz, Wolfram (2002), “Getting Formal with Dopamine and Reward.” Neuron, 36 (2), 241–63.
  16. Biswas, Dipayan, Labrecque, Lauren, Lehmann, Donald, Markos, Ereni (2014), “Making Choices While Smelling, Tasting, and Listening: The Role of Sensory (Dis)similarity When Sequentially Sampling Products,” Journal of Marketing, 78 (1), 112–26.
  17. Rolls, Edmund T., Rolls, J.H. (1997), “Olfactory Sensory-Specific Satiety in Humans,” Physiology & Behavior, 61 (3) 461–73.
  18. Boesveldt, Sanne, de Graaf, Kees (2017), “The Differential Role of Smell and Taste For Eating Behaviour,” Perception, 46 (3-4), 307-319.