We never forget our loved ones once they have passed away.
But for as many as six in ten of us, the memories become something much more tangible.
According to a study, most people who have lost a partner will see, hear or sense them in some way.
Researchers said the level of these 'hallucinatory experiences' - for example seeing a loved one in their old chair or hearing them call their name - was 'strikingly high'.
They said the phenomenon is much more common than we might think because many bereaved people are reluctant to report their experiences for fear of being looked upon as mentally unwell.
The team at University of Milan said: 'Post-bereavement hallucinatory experiences (PBHEs) are abnormal sensory experiences that are frequently reported by bereaved individuals without a history of mental disorder.
'Overall, evidence suggests a strikingly high prevalence of PBHEs - ranging from 30 per cent to 60 per cent - among widowed subjects, giving consistence and legitimacy to these phenomena.'
The researchers, whose study was published in the Journal of Affective Disorders, came to their conclusions after compiling the results of all previous peer-reviewed, English language research that has been carried out on PBHEs.
Jacqueline Hayes, an academic at the University of Roehampton, has also studied PHBEs for many years, but prefers to call them experiences of continued presence (ECPs) because of the negative connotations the term 'hallucination' can have. She has carried out extensive interviews with people of all ages and backgrounds across the UK, who have lost spouses, parents, children, siblings and friends.
She said: 'People report visions, voices, tactile sensations, smells, and something that we call a sense of presence that is not necessarily related to any of the five senses.
'They happen involuntarily, and, for example, not while someone is deliberately "remembering".
'They are always significant to the bereaved and continue some aspect of the relationship with a loved one; sometimes they also magnify it.
'For example, someone who experienced a problematic relationship with her mother while she was alive now experiences hostility through hearing her mother's voice.
'I found that these experiences could at times be healing and transformative, for example hearing your loved one apologise to you for something that happened - and at other times foreground the loss and grief in a painful way.
'People's selves are not separate from others, particularly not significant others. It is therefore quite natural that these close relationships continue after death, and that interactions may occur as before.
'It would in fact be quite strange if such interactions, that we come to expect as part of our everyday lives, suddenly stop.'
Some researchers have theorised that the experiences are similar to flashbacks experienced by sufferers of post-traumatic stress disorder, but Dr Hayes said they are more complicated than that.
'[ECPs] can occur for many years afterwards, and even when the bereaved are no longer experiencing trauma and they are usually not in the form of flashbacks but can be quite new experiences,' she said.
'Whether they are helpful or unhelpful depends on the nature of the relationship with the deceased.'
Many who have had positive encounters with their deceased loved ones say they have been soothed to sleep or been given the encouragement to achieve a difficult task.
Some say they have even been helped to complete a mundane chore, such as a man whose grandmother - who had been dead for four years - told him to fix the kitchen waste disposal system for his grandfather who was finding the task very stressful.
Dr Hayes added: 'The form [ECPs] take also fits the relationship with the deceased. It's like they walk on to the stage, on cue, and play the part the bereaved would expect them to.'
- Daily Mail