Madeleines are made from eggs, butter, sugar, flour, vanilla and lemon. Here’s a recipe.

 

 

Marcel Proust has stuck them into our minds as the prototypical memory stimulator. Here’s how it works, as explained by Gordon M. Shepherd in Chapter 20 of his Neurogastronomy: How the brain creates flavor and why it matters, Columbia University Press 2012. I quote:

Activating Proust’s Brain

We start with the knowledge that the taste of a madeleine must be mostly due to its smell. The stimulus for Proust’s taste experience was therefore the odors emanating from the mixture of pastry crumbs soaked in tilleul, the aromatic lime-scented infusion made from linden blossoms. What, then, might these smells have been?

It is sometimes argued that Proust dithered over exactly what kind of biscuit it was that stimulated his reverie, but it really does not matter. A traditionally made madeleine, in addition to possessing odor molecules that arise from the butter and eggs, would include several types of “aroma essences.” Flavors of foods are enhanced by heating and dissolving in water, which increase the vapor pressure so that volatile molecules are released into the air or within the mouth. Thus, as children learn, a humble pastry gives off its aromas with greater effect when its crumbs are dissolved in hot liquid. The aromas in a madeleine would include vanilla and several types of related odor molecules in the lemon, such as citral and limonene, which belong to the terpenes, a family of essential oils secreted by plants. As described in chapter 4, they are highly volatile, consisting of 5 carbon atom units linked together in various shapes and with various reactive functional groups such as esters, alcohols, and acids. The other source of olfactory stimulation in Proust’s concoction was the tilleul, which contributed its own scent.

How do these molecules give rise to a smell perception? As we have seen, receptor molecules in the fine hairlike cilia lying in the mucus are stimulated by the smell molecules as they are inhaled into the nose (the orthonasal route), and also by the smell molecules released from within the mouth that rise into the nasal cavity from the back of the mouth (the retronasal route). It is by this latter route, after the narrator has taken the brew into his mouth, that the smell molecules are released and carried by the warm and humid air of his nasopharynx to his olfactory sensory cells.

The smell molecules, absorbed into the mucus, act on receptor molecules in the cilia membranes. These in turn initiate the cascade of microkicks from one signaling molecule to the next to change a membrane protein formed around a tiny channel that lets electric charges flow through it. This alters the electrical potential across the cell membrane, leading to the discharge of impulses in the cell that is conveyed through its long fiber (axon) to the first relay station in the brain.

The narrator’s mouthful of crumb-laden tea thus activates a range of receptors tuned to the different volatile components, leading to impulse discharges that carry the information to the brain. But in addition to activating impulses, the signaling cascade in the receptor cells also contains a number of pathways for controlling the sensitivity of the sensory response. Repeated stimulation brings about desensitization of the secondary messenger pathway. In Swann’s Way, Proust appears to be describing precisely this effect: “I drink a second mouthful, in which I find nothing more than in the first, then a third, which gives me rather less than the second. It is time to stop; the potion is losing its magic. It is plain that the truth I am seeking lies not in the cup but in myself. The drink has called it into being, but does not know it, and can only repeat indefinitely, with a progressive diminution of strength, the same message which I cannot interpret, though I hope at least to be able to call it forth again . . . I put down the cup and examine my mind.”

Desensitization of a secondary response is well known in the experimental literature and is often referred to as sensory adaptation. It is a very general phenomenon that occurs any time a given nerve cell or neural pathway is stimulated repeatedly. As discussed in chapter 8, the logic of this is that the nervous system is not constructed to register every sensory stimulus imposing on it, but only those that signal a sudden change from a former state. It is these that carry the most critical information, until a different stimulus occurs.

It seems clear that desensitization of the initial sensory mechanism takes place during Proust’s initial repeated attempts to conjure up the “truth.” However, a close reading of the text indicates that several other neural processes are likely also occurring. A second process is adaptation in the neural pathways that process the odor information to give rise to the odor perception; these would occur in the pathways of the brain flavor system. Adaptation may also occur in the pathways that link an odor perception per se to the systems underlying odor memory. Finally, there are systems related to the narrator’s vision of the “truth” — the systems involved in the storage of the visual memories and their retrieval. The fading of the “truth” may thus be due to multiple mechanisms of adaptation, in addition to the desensitization of the receptors. But to assess this further, we need to ask what is the nature of the odor perception elicited by the potion.

Impulses in the fibers from within the sensory neurons give rise to spatial patterns of activity within the first brain related station, the olfactory bulb. These patterns are the smell images of the information carried in the smell molecules (chapters 4-10), which are projected to the olfactory cortex where they form a content addressable memory of the smell object (chapter 11), and are sent from there to the orbitofrontal cortex (chapter 12) to be combined with other sensory and motor systems to form the perception of smell and flavor (chapters 13-18). 

It is this flavor image that was recognized by Proust’s brain, at first only indistinctly, as being part of a more complex memory that initially seemed beyond recall. The flavor image of the tea-soaked madeleine is thus metonymic for the complex multisensory image of the town of Combray.

Smell, Emotion, and Memory Recall

The direct access of the smell pathway to these forebrain mechanisms is essential for understanding the nature of Proust’s olfactory-evoked experience. We have indicated the cortical mechanisms involved in Proust’s cognitive, perceptual response (chapter 18). This direct olfactory connection to the forebrain provides insight into the heightened degree of the emotional state evoked by the odor stimuli, the strength of the voluntary search for the missing “truth,” and the overwhelming quality of the “involuntary” memory finally brought forth.

The emotions evoked by the madeleine are central to the whole theoretical edifice of the madeleine episode . . . Brain research can best provide insight into the question “Whence did it come?” We have explained how, from the olfactory cortex, the pathway for perception is directed towards the prefrontal neocortex. But the olfactory cortex also gives rise to multiple pathways that connect directly to the so-called limbic regions of the brain that are involved in the mediation of both memories and emotions.

The key structures include the hippocampus, a central organizing node for single-event “episodic” memories, and the amygdala, which, in parallel with the orbitofrontal cortex, is involved in stimulus reinforcement association learning. 

So now you know.

 

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