(CNN) Whatever was behind the "sonic attacks" experienced by US government personnel in Havana, Cuba, starting in late 2016 remains a mystery -- but a new study published Tuesday looks inside the workers' brains for clues.

MRI brain scans from 40 patients -- 23 men and 17 women -- showed variations in brain structure and functional connectivity, which measures relationships among different brain regions, when compared with 48 other adults. The scans were taken between August 2017 to June 2018.

"There were group differences all over the brain," said study author Ragini Verma, professor of radiology and neurosurgery at University of Pennsylvania's Perelman School of Medicine. "Especially in an area called the cerebellum, which is also implicated in the kind of clinical symptoms that most of these patients were demonstrating, which is balance, eye movement, dizziness, etcetera."

Differences in connectivity were also observed in the brain's auditory and visuospatial areas, according to the study. However, the authors note that the clinical importance of these findings is uncertain, and they didn't have earlier MRIs of the patients to compare what their brains looked like before the incidents.

Moreover, these patterns don't fit a clear picture of a specific disorder, the authors say.

"It certainly does not resemble the imaging presentation of traumatic brain injury or concussion, although they present with clinical symptoms which are concussion-like," Verma said.

"It says something happened, and we need to look further, and that's about it."

Dr. Jamshid Ghajar, director of the Stanford Concussion and Brain Performance Center, said it was "remarkable" that researchers found differences between the brains of healthy controls and those involved in the Cuba incident, especially given "the differences within the population itself in terms of their symptoms and what kind of complaints they had."

"I think the jury's out on what caused it, but certainly these patients are complaining of symptoms and they've had measured impairments," said Ghajar, who was not involved in the new paper. "So something's going on, and I think it needs to be further investigated."

What did they hear?

Many of those stationed in Havana reported hearing "intensely loud" sounds coming from a specific direction, which they described as "buzzing," "grinding metal," "piercing squeals" and "humming," according to another study published by many of the same authors last year.

"The sounds were often associated with pressurelike or vibratory sensory stimuli," according to the study. "The sensory stimuli were likened to air 'baffling' inside a moving car with the windows partially rolled down."

One patient reported hearing two 10-second pulses, while others said they could hear the sound for more than 30 minutes, the report said.

recording obtained by the Associated Press and released in October became the first publicly reported audio sample said to be related to the attacks. (A pair of scientists noted the sound's similarities to the echoing call of a Caribbean cricket .)

Can sound really cause brain injury?

The noise itself is unlikely to have caused the symptoms directly, according to the earlier study, which noted that audible sound "is not known to cause persistent injury to the central nervous system."

"We actually don't think it was the audible sound that was the problem," Dr. Douglas Smith, an author on both studies and the director of the University of Pennsylvania's Center for Brain Injury and Repair, previously told CNN. "We think the audible sound was a consequence of the exposure."

"I know of no acoustic effect that would produce concussion-like symptoms; according to my research, strong effects on humans require loudness levels that would be perceived as very loud noise while exposed," Jürgen Altmann, a physics professor at Technischen Universität Dortmund in Berlin, previously told CNN.

Similarly, State Department and federal investigators have testified that they were unable to determine the source or cause of the ailments in Havana, stating only that they "were most likely related to trauma from a non-natural source."

What were the symptoms?

"If you took any one of these patients and put them into a brain injury clinic and you didn't know their background, you would think that they had a traumatic brain injury from being in a car accident or a blast in the military," Dr. Randel Swanson, another author on both studies and a specialist in brain injury rehabilitation at the University of Pennsylvania, previously wrote in the medical journal JAMA.

Swanson and his colleagues examined the patients and found a variety of symptoms including sharp ear pain, headaches, ringing in one ear, vertigo, disorientation, attention issues and signs consistent with mild traumatic brain injury or concussion.

In addition, a majority of the patients reported problems with memory, concentration, balance, eyesight, hearing, sleeping or headaches that lasted more than three months.

"It's like a concussion without a concussion," Swanson wrote.

Many reported feeling "mentally foggy" or "slowed" for months, the authors said. Some reported irritability and nervousness, with two meeting criteria for post-traumatic stress disorder. Poorer job performance was also observed.

Three people eventually needed hearing aids for moderate to severe hearing loss, and others had ringing or pressure in their ears. More than half needed to be prescribed medication in order to sleep or to deal with headaches. Many were, at least for a period of time, unable to return to work.

Doctors have noted that some of the patients' symptoms are not typically seen in a concussion, such as pain and ringing in only one ear. Also, while concussion patients often make a quick and full recovery, these patients experienced symptoms for months.

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Doctors remain baffled, while Cuban officials have vigorously denied that there were any targeted attacks on diplomats in Havana and said their symptoms could have been caused by other factors.

Meanwhile, more than a dozen Canadian diplomats who experienced similar symptoms while posted in Cuba are suing their government for millions

Officials have looked into similar cases in China. The US State Department expanded a health alert there after a series of supposed acoustic incidents left diplomatic personnel suffering injuries similar to those in Cuba.