If counting sheep is an abstract concept, or you are unable to visualise the faces of loved ones, you could have aphantasia - a newly defined condition to describe people who are born without a "mind's eye".
Some people report a significant impact on their lives from being unable to visualise memories of their partners, or departed relatives.
Others say that descriptive writing is meaningless to them, and careers such as architecture or design are closed to them, as they would not be able to visualise an end product.
Cognitive neurologist Professor Adam Zeman, at the University of Exeter Medical School, has revisited the concept of people who cannot visualise, which was first identified by Sir Francis Galton in 1880.
A 20th century survey suggested that this may be true of 2.5% of the population - yet until now, this phenomenon has remained largely unexplored.
Visualisation is the result of activity in a network of regions widely distributed across the brain, working together to enable us to generate images on the basis of our memory of how things look.
These regions include areas in the frontal and parietal lobes, which "organise" the process of visualisation, together with areas in the temporal and occipital lobes, which represent the items we wish to call to the mind's eye, and give visualisation its "visual" feel.
An inability to visualise could result from an alteration of function at several points in this network. This problem has been described previously following major brain damage and in the context of mood disorder.
The recent research came about by chance when 21 people contacted Prof Zeman after reading an article on his previous research and realising they had never been able to imagine.
Now, Prof Zeman and his team are conducting further studies with those affected to find out more about why some people are born with poor or diminished visual imagery ability.
"This intriguing variation in human experience has received little attention," he said.
"Our participants mostly have some first-hand knowledge of imagery through their dreams.
"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."
Prof Zeman and colleagues describe these patients' experience in a paper just published in the journal Cortex.