When science journalist Carl Zimmer wrote a 2010 article in Discover magazine about English neurologist Adam Zeman's case study of a man who couldn't visualize people or things, the professor was approached by 21 people who saw themselves in the article and wanted to learn more.

Now Zeman and colleagues at the University of Exeter Medical School are reporting in the journal Cortex that the condition could affect as many as 2.5 percent of the population.

They're calling it "aphantasia," though Zeman insists in an interview with the BBC that it is not a disorder but rather a "variability of human experience" where most of us "spend our lives with imagery hovering somewhere in the mind's eye, which we inspect from time to time." Says one man of his childhood insomnia: "I couldn't see any sheep jumping over fences, there was nothing to count." Others with the condition can't visualize the faces of their friends and family.

The newly described condition is something that affects some people from birth, which is different from the experience first described by Zeman in Discover about a patient who lost the ability to visualize after a heart procedure.

In patients with congenital aphantasia, Zeman says that a network of regions distributed throughout the brain but implicated in visualization may function differently, not from a traumatic event but from birth.

Similarly, people can be born with hyperaphantasia, or the ability to visualize in acute detail. To better understand the neurological underpinnings of this range of visual experiences, Zeman is working on a year-long project—The Eye’s Mind—that involves artist Susan Aldworth, art historian John Onians, and philosopher Fiona Macpherson.

(Speaking of visualization, see why Roseanne Barr is going blind.)

This article originally appeared on Newser: Some People Are Born Without a 'Mind's Eye'

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