Dear Doctor: My boyfriend laughs when I turn down the car stereo when I’m trying to find a new address, but I’ve seen his dad do the same thing when he’s changing lanes on the freeway. Why does the quiet make you feel like you can see better?

Dear Reader: Your boyfriend may have fun teasing you, but we’d be surprised if there haven’t been similar instances in his own life.

The scenarios you describe, which require you to evaluate incoming data and make split-second decisions, suck up a lot of mental energy. Researchers refer to this as “cognitive load.” Eliminating a distraction such as sound can help with concentration by reducing the cognitive load. That’s because, much like a computer, our brain’s capacity to engage in multiple tasks is finite.

To understand why, it helps to appreciate the complexities of the human brain. It’s made up of three major parts -- the cerebrum, cerebellum and brainstem. The largest and topmost part is the cerebrum, which is divided into left and right hemispheres. It deals with learning, emotions, reasoning and fine motor skills, and it interprets the flood of data from the five senses. Tucked beneath is the much smaller cerebellum, which oversees muscle movement, balance and posture. Acting as a relay center between the two and the spinal cord is the brainstem, which also oversees the staying-alive stuff we don’t think about but can’t live without. This includes breath, heartbeat, swallowing, sleep cycles and temperature control.

The three areas of the brain, plus the spinal cord, make up the central nervous system. It regulates movement, thought and emotion. It’s why you’re able to find that new address. But while it seems as though we can concentrate on multiple things at once, the brain handles tasks sequentially. It’s just making the switch so swiftly, in mere nanoseconds, that it feels like we’re multi-tasking. Once you’re hunting for a street sign, though, or gauging car lengths in fast-moving traffic, all of the collecting and interpreting of data by the central nervous system reaches critical mass. That cognitive load we talked about earlier gets to be too great. Turning down the radio removes enough incoming data to make it easier to concentrate.

A small study in Sweden looked into this phenomenon several years ago. Researchers had each of 32 participants perform an easy visual task and a challenging visual task. They were also instructed to ignore the audio -- a random sequence of sounds -- that was playing. The subsequent MRI tests revealed that when participants concentrated on a visual task, there was a decrease in the responsiveness of the auditory nerves. Basically, the brain itself was turning down the volume.

The takeaway here is that when our inner mute button can’t adequately remove the auditory distraction, our brain kicks in and we decide to turn down the car stereo. We’re adjusting our environment so the most important task -- finding the address or safely merging onto the freeway -- has a better chance of getting accomplished.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Hello dear readers, and welcome to the first letters column of autumn. Are we alone in wondering how summer flew by so quickly? Here at Ask the Doctors headquarters, you’ve kept our mailboxes full, so we’re diving in.

-- We got a lot of mail after a column about hemorrhoids, with writers ranging in age from a new mother in her 20s to older adults. The common theme in their advice to fellow hemorrhoid sufferers? Moisture.

A woman in her 70s, writing from Oklahoma, finds relief in the tub. “One way I've found to keep hemorrhoids from being so painful is to take warm baths,” she writes. “For me, this works much better than any medicine I have found.”

A man in South Carolina, who has struggled for decades with the maddening itch, says his doctor suggested a bidet. “I ordered one that attaches to the toilet for about $40,” he writes. “I have not had any itching since, so it’s worth a try!”

-- A reader with dentures wonders about the potential health effects of the adhesive that keeps the devices in place. “I sometimes have to use the adhesive twice a day in order to eat meals, or else I wind up biting the insides of my bottom lip,” she writes. “Is daily ingestion of denture adhesive harmful?”

Some denture adhesives contain zinc, a mineral that our bodies need for good health. However, an excess of zinc over time can cause health problems, including nerve damage, often in the hands and feet. There have been some reports of zinc toxicity among denture wearers, but in those cases the person used two or more tubes of a zinc-based adhesive per week. According to product instructions, a single tube is meant to last seven to eight weeks.

In this particular case, it’s possible that the dentures no longer fit properly, which can happen due to bone shrinkage. We recommend seeing your dentist. Your existing dentures may need to be relined, or you may need to be fitted for new ones.

-- In response to a column about Lyme disease, a reader asks whether tests have improved since 1996, the year she had a run-in with a deer tick. The answer is yes, today’s tests are more accurate. However, it takes several weeks for antibodies to Lyme disease bacteria to develop, so it’s possible to get a false negative if one is tested too soon. It takes four to six weeks after infection for the Lyme test to show an accurate positive result.

-- We’ll close with a thought from a reader in Pennsylvania who thought something was missing in a recent column about mindfulness and meditation:

“As a former geriatric nurse, I have often seen patients calmed by prayer, a Bible reading and a Scripture quote as they face a serious operation or death,” she wrote. “I think this deserves to be mentioned in reference to the calming and beneficial effect it promotes.”

As always, thank you for your thoughts, kind words and ideas. We look forward to hearing from you, and will see you for another letters column next month.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: I just read about a study where people who were depressed actually had a different gut microbiome than people who weren’t depressed. Is that true? Does your gut determine how you feel?

Dear Reader: The recognition of the link between emotions and the gut is so ancient, it’s been hardwired into our language. There’s the gut feeling, going with your gut, butterflies in your stomach, a sinking feeling, bad news that’s a punch to the gut, having a gut response, or finding something to be nauseating, sickening or gut-wrenching.

Now, new research suggests that not only does the brain affect the gut, but that the connection actually goes both ways. Specifically, that the composition of the gut microbiome may play a role in how someone feels. This is a new and sometimes controversial field of research, and it is the subject of (occasionally fractious) debate.

The study you refer to may be one published in February in the scientific journal Nature Microbiology, which focused on 1,054 people from Belgium. Of those, 173 were either diagnosed with depression or did poorly on a questionnaire that asks participants to rate their quality of life.

When the composition of participants’ microbiomes were analyzed and compared, researchers found an interesting difference. Specifically, people with depression lacked two types of microbes, known as Coprococcus and Dialister, which were present in the guts of those who were free from depression.

Data in the Nature study were compared with a study of 1,064 Dutch people in which researchers found that the same two microbes were missing from the guts of those who either reported or were diagnosed with depression. In addition, the two different groups with depression had greater numbers of a certain microbe believed to be involved in Crohn’s disease.

This adds weight to the theory that inflammation has a role in mental health. The specific reason that these two missing microbes affect depression is unclear. One promising line of thought, according to the researchers, is that that Coprococcus has been linked to dopamine, a brain chemical that influences feelings of pleasure and satisfaction. The microbe also makes a substance that acts as an anti-inflammatory.

Such studies can suggest cause-and-effect relationships, but they shouldn’t be considered “proof” of a connection. They don’t prove them.

It’s fascinating that the depressed people in the studies lacked certain microbes in their guts, but more research is needed to show direct causality. The good news is that each new study adds to our understanding and sets the stage for new research to come. The hope is that, over time, this line of inquiry will lead to new therapies such as microbiome profiling or precision bacterial supplements, which will offer new avenues of relief to people living with depression.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)