A missed opportunity to study odor-evoked emotional memories

The researchers of Can a study exclude the limbic system and adequately find how we process value? published another study. In this 2015 human study, subjects were monitored with fMRI scans while making choices on the identity and pleasantness of rewarding food odors.

I feel that the researchers missed quite a few good opportunities to advance science. Instead of making peripheral assessments of limbic system areas and citing numerous other studies, they could have included emotional content in their study and drawn their own conclusions.

Consider these opportunities:

  • Wouldn’t the odors used in the study such as chocolate cake and pizza and strawberry and potato chips – and other “comfort” foods – potentially be associated with emotional responses?
  • Don’t most humans have memories that include pleasant food odors?
  • Wouldn’t it have been informative to ask the subjects during fMRI scans to identify what emotions were evoked by the pleasant food odors?
  • Wouldn’t these resultant fMRI scans be expected to potentially show more strongly activated limbic system areas, given the hippocampus’ position as the seat of emotional memories?
  • Wouldn’t the additional emotional responses and memories and subsequent limbic system area activations potentially influence the subjects’ value judgments?

Instead, the researchers peripherally included limbic system areas in the study. The supplementary material included passages such as:

“Identity-specific value signals were found in not only the OFC, [orbitofrontal cortex] but also the ACC [anterior cingulate cortex] and hippocampus.”

Like the previous study, the current study’s focus was to provide evidence that areas of the cerebrum were in control when people made value judgments. The term “value” in the current study meant:

“the pleasantness of the odor.”

Like the previous study, areas of the limbic system weren’t addressed until the tail end of the supplementary material. The researchers cited other studies in an attempt to dismiss the role of the ACC in making value judgments, then said:

“Although we are unable to distinguish between these alternative explanations, our findings suggest that value-related signals in ACC—whether signed or unsigned—are specific to the identity of the expected outcome.”

Since the current study found that “identity” was encoded by cerebral areas, the above sentence was written to nudge the reader into inferring that the cerebrum dominated value judgments of “the pleasantness of the odor.”

The researchers similarly cited other studies in the last paragraph instead of specifically discussing how they studied the participation of the hippocampus part of the limbic system. They then speculated that the hippocampus’ contributions to value judgments in the current study were explained by the referenced studies:

“We speculate that the hippocampus is involved in retaining sensory-based information about specific rewards, which may be linked to value-based representations in OFC for later consolidation.”

Like the previous study, the researchers were begrudgingly diverted away from their focus on cerebral areas when they were forced to acknowledge the limbic system’s contributions to value judgments of “the pleasantness of the odor.”

http://www.pnas.org/content/112/16/5195.full “Identity-specific coding of future rewards in the human orbitofrontal cortex”

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