When Nina Strohminger was a teenager, her grandmother had dementia. “Before she got sick, she was not a very nice person,” Strohminger said. “One of the first things that went when this disease was taking hold is she became really, really nice. I just remember her stopping me one day and saying ‘Nina, your skin is so beautiful,’ and I was like ‘what is happening?’”

We spoke on the phone, both of us traveling—I paced outside of a Starbucks in Brooklyn while she packed her things to move from Durham, NC, where we both lived at the time, to New Haven, CT, where she’d be starting as a postdoctoral researcher at Yale University. (Full disclosure: Strohminger is a friend and collaborator of mine.)

“She didn’t seem like the same person,” Strohminger went on. “She just seemed completely different.” This question, what makes someone seem like who they are?, is a fundamental one that philosophers and psychologists have always struggled to answer. Some thinkers have suggested that it’s our bodies that define us, while others argue that it’s our memories. In a forthcoming paper with University of Arizona philosopher Shaun Nichols, Strohminger advances a novel solution: What makes us who we are is our moral character—whether we’re honest, what sorts of things we value, and how well we treat our grandchildren.

Strohminger’s research depends in part on an extremely rare type of brain cell, the spindle neuron. Outside of the sharpest mammals, like dolphins, elephants, and great apes, no other animal shares them with us. Typical neurons form dense canopies with their neighbors, connecting with one another across several clustered branches; spindle neurons, however, are skinny and long, arranged in parallel, with only one branch for receiving and transmitting information at either end. These neurological rarities appear in regions located near the front of the brain and around our temples—regions associated with self-control, emotion, and social behavior.

Functional imaging of the brain regions they populate suggests that spindle neurons are integral to our emotional and social lives; they help make us human. Frontotemporal Dementia (FTD), a neurodegenerative disease second only to Alzheimer’s in prevalence, destroys more than 70 percent of them.