(CN) – First U.S. Embassy personnel in Cuba heard a noise, then they became dizzy and had trouble concentrating. The State Department described the event as a sonic attack, but conflicting evidence surrounding Havana Syndrome has led others to question this conclusion – and the results of brain imaging released Tuesday only deepen the mystery.

Research published Tuesday by the American Medical Association retroactively combs through brain scans obtained during treatment in an attempt to describe the neurological impact – if anything happened at all.

When the Associated Press sent reporters to record the sound, all they found were crickets.

Previous research published by the American Medical Association in 2018 described symptoms as “similar to those found in persistent concussion syndrome, yet there was no evidence of blunt head trauma.”

Without attempting to conclude one way or another what gave U.S. government workers headaches – if anything at all – researchers sought to document how patient brains differed from the brains of other men and women of similar age and education who did not work in Cuba during that time period.

Researchers compared MRI scans conducted at University of Pennsylvania’s Center for Brain Injury from 40 government workers who were in Cuba during that period, with 48 scans from “demographically similar healthy controls” looking for potential markers from “exposure to uncharacterized directional phenomena of unknown etiology, manifesting as pressure, vibration or sound.”

Twenty-three men and 17 women were treated at the University of Pennsylvania where MRIs took place, between four and 403 days after they were potentially exposed to the force. The median patient age was 40 years old. Four patients could not be located to give consent to be included in the study.

In addition to overall white and gray matter totals, researchers compared microstructures and functional connectivity affecting vision, hearing, and executive control.

Compared to healthy controls, researchers observed “significant differences in whole brain white matter volume, regional gray and white matter volumes, cerebellar tissue micro structural integrity, and functional connectivity in the auditory and visuo spatial subnetworks but not in the executive control subnetwork,” according to the study.

While patients who experienced Havana Syndrome exhibited a decrease in white brain matter volume, there seemed to be no difference in gray matter volume, which is where synapses occur.

Researchers described gray matter microstructures and functional connectivity in patients as “nonnormal,” but found little difference between overall network or functional connectivity between patients and the control group.

The brain scans did not look like the conventional images of traumatic brain injury or stroke, nor did they “form an anatomical basis for a specific behavioral dysfunction observed in other brain disorders,” the researchers said.

“The areas implicated in the patients’ brains, namely the cerebellum as well as the visuospatial and auditory networks, align with the neurological symptoms that were observed in the patients,” said lead author Dr. Ragini Verma, a professor of radiology and head of the Diffusion and Connectomics in Precision Healthcare Research imaging lab at Penn, in a statement.

But because researchers don’t have data on what workers were exposed to or for how long, it’s hard to draw conclusions. There is no way to distinguish if the changes in brain anatomy are due to unknown phenomena or the healing process following such an event.

In their conclusion, researchers neither confirm nor deny directed energy weapons had been used except to the say that the phenomena is worth further study.

“These findings may represent something not seen before,” said co-author Dr. Douglas H. Smith, a professor of teaching and research in neurosurgery, in a statement.