Perception and hallucinations

2. Causes of visuoperceptual difficulties

The ageing process can bring about many difficulties, particularly those relating to sight and visuoperception. These include:

  • seeing things less sharply (blurring)
  • needing more time to adapt to changes in light levels (eg when going from a dark room into sunlight)
  • the area in which objects are seen (the ‘visual field’) getting smaller, and loss of peripheral vision (being able to see things outside of the direct line of vision)
  • problems with depth perception (the ability to judge the distance of or to an object and see in three dimensions)
  • shadowing from small shapes floating in the visual field (known as ‘floaters’).

Eye conditions that can affect visuoperception include cataracts, glaucoma, macular degeneration and retinal complications from diabetes. These can all result in changes such as blurring, distortion, partial loss of visual field and, in some cases, blindness. They can also cause hallucinations known as Charles Bonnet syndrome.

Charles Bonnet syndrome occurs when people with deteriorated vision start to see things that aren’t there (visual hallucinations). There are two main types of hallucination: simple repeated patterns or complex images of people, landscapes or objects. These hallucinations are caused by the person’s deteriorating sight and not from any other condition.

For more information on hallucinations see ‘Hallucinations in people with dementia’.

A stroke can also cause someone to have problems with their vision. They may lose the ability to see things directly ahead (central vision loss) or lose peripheral vision (visual field loss). This can happen in both eyes at the same time. They may also have problems with eye movement and how the brain processes visual signals.

Certain medications can cause or contribute to problems with vision. They include drugs for cardiovascular problems, non-steroidal anti-inflammatory drugs, antibiotics, drugs for Parkinson’s disease, and eye medications.

Specific types of dementia can also damage the visual system and cause visuoperceptual difficulties. These include Alzheimer’s disease, Parkinson’s disease dementia, dementia with Lewy bodies and vascular dementia. Rarer forms of dementia, such as posterior cortical atrophy (PCA), can also cause visuoperceptual difficulties.