Are food cravings the body’s way of telling us that we are lacking nutrients?

—J. SHELTON, OGDEN, UTAH

Peter Pressman of the Cedars-Sinai Medical Center in Beverly Hills, Calif., and Roger Clemens of the University of Southern California School of Pharmacy explain:

A hankering for particular foods is not linked to any obvious nutrient insufficiency. But other biological factors appear to be at work.

Researchers have employed functional magnetic resonance imaging (fMRI) to explore the neural basis of such appetites. The images suggest that when somebody is pining for a certain fare, brain components in the amygdala, anterior cingulate, orbital frontal cortex, insula, hippocampus, caudate and dorsolateral prefrontal cortex are activated. A network of neural regions may be involved with the emotion, memory and chemosensory stimuli of food yens.

Desire for chocolate offers an example. The treat’s constituents may influence satiation or alter our longing for it by affecting mood-influencing chemicals in the brain, such as phenylethylamine, tyramine, serotonin, tryptophan and magnesium.

Additional factors such as simple carbohydrate content may amplify a food’s appeal or even attenuate depression. More support for a nutrition-neurological connection comes from research that shows that administration of naloxone, an agent that blocks opiate receptors in the brain, appears to inhibit the consumption of sweet, high-fat foods—chocolate among them. Studies of cannabinoids, which commonly occur in marijuana, have shed more light on the neurochemistry of selective appetite. In addition, research has identified an entire spectrum of gut neuropeptides with elaborate central nervous system feedback and influence on satiety.

Some studies suggest that chocolate craving, especially among women, may partly result from a sense of deprivation or a reaction to stress, hormonal fluctuation and modulation of neuropeptide concentrations. Culture has an influence as well. Spanish women, for example, eat relatively large quantities of chocolate and exhibit limited craving for that sweet. In contrast, American women consume less yet present a strong “chocophilic” tendency.

What causes feedback in a guitar or microphone?

Robert L. Clark of the Pratt School of Engineering at Duke University offers this answer:

Several mechanisms can lead to the unpleasant shriek known as feedback. Let us deal first with the simple case of a microphone and an amplified speaker. Feedback occurs when a “loop” closes between an input (the microphone) and output (the amplified speaker). The sound radiated from the amplified speaker reaches the microphone and is subsequently amplified again and again, until it saturates and can no longer amplify the input. This excessive ratio of output to input, called gain, occurs at a particular frequency and arises from many factors. These can include the distance between the microphone and the speaker, the directional design of the microphone and speaker, the influence of reflective surfaces within the acoustic environment, and the presence of additional microphones and amplified speakers. To reduce gain, an equalizer can adjust the signal amplification.

When a microphone is used with an acoustic guitar, the amplified speaker closes the loop between the input and output when the radiated sound from the speaker reaches the guitar. In such cases, the guitar starts vibrating excessively at a particular frequency (typically between 100 and 200 hertz), or the room itself can begin to resonate, producing an audible tone. A similar mechanism occurs in electric guitars. Structural vibrations induced by acoustic feedback can magnify the signal generated by sensors embedded in the guitar to “pick up” its sound, leading to the instability of feedback.