People who are unable to visualize the faces of loved ones could have aphantasia, which is a newly defined condition to describe people who are born without a 'mind's eye'.
Cognitive neurologist professor Adam Zeman from the University of Exeter and colleagues describe the experience of such people.
One of the subjects, Tom Ebeyer, 25, from Ontario, Canada, keenly felt a sense of loss when he realized at the age of 21 that his girlfriend could visually "see" things in her mind's eye in a way that he could not.
"I began to feel isolated, being unable to do something so central to the average human experience. The ability to recall memories and experiences, the smell of flowers or the sound of a loved one's voice," said Tom.
He said the condition had severely affected his relationships as he is unable to recall shared experiences.
"After the passing of my mother, I was extremely distressed in that I could not reminisce on the memories we had together. I can remember factually the things we did together, but never an image," he added.
Niel Kenmuir, 39, from Lancaster in Britain, first realized he could not visualize images at primary school.
"I can remember not understanding what 'counting sheep' entailed when I couldn't sleep. I assumed they meant it in a figurative sense," Kenmuir said.
Niel is an avid reader but avoids books with vivid landscape descriptions as they bring nothing to mind for him.
"This intriguing variation in human experience has received little attention. Our study revealed an interesting dissociation between voluntary imagery, which is absent or much reduced in these individuals, and involuntary imagery, for example in dreams, which is usually preserved," Zeman said.
The findings were published in the journal Cortex.