6 - Illusions, Hallucinations, Disorders of Higher Cortical Function

Question 1

Illusions and Hallucinations

Answer 1

1. Illusion = false perception of visual stimuli
2. Hallucination = subjective perception of an object or event when no sensory stimulus is present
   
   1. ​Formed = real objects
   2. Unformed = light, spots, geometric patterns, dots
3. Patients with dementia or altered sensorium (delirium, hypnosis) prone to hallucinations
4. Reduced visual acuity or visual field often leads to positive visual phenomenon

Question 2

Optical Causes of Illusions

Answer 2

1. Tear film disturbances = ghosting of images
2. Irregular K surface
3. PI = dysphotopsia and shadowing
4. Oil-droplet cataract, PSC
5. s/p PCIOL = temporal visual field disturbances such as arc of light or a dark area termed positive or negative dysphotopsia

Question 3

Retinal Causes of Illusions

Answer 3

1. ERM, ME, MH, Subretinal disease
2. Cause changes in position of RPE
3. Metamorphopsia = perceptual distortion of linear objects appearing curved or discontinuous
4. Micropsia — retinal elements pushed together causing perceived image shrinkage
5. Macropsia — retinal elements stretched causing perceived image enlargement
6. Dyschromatopsia — Viagra, digoxin, Vit A def, choroidal ischemia, retinal ischemia

Question 4

Optic Nerve Cause of Illusion

Answer 4

1. Pulfrich phenomena — after episode of demyelinating ON relative conduction delay. Objects moving perpendicular to line of sight actually moving toward or away from patient.

Question 5

Cortical Cause of Illusion

Answer 5

1. V1 — primary visual cortex or
2. V2-V5 associative areas
3. Teleopsia (objects appear too distant)
4. Peleopsia (objects appear to close)

Question 6

Ocular Causes of Hallucination

Answer 6

1. PVD = photopsia which may be apparent in dark light with saccades
2. Outer retinal disease photopsias — more constant. CAR, MEWDS, AZOOR, Birdshot

Question 7

Optic Nerve Causes of Hallucination

Answer 7

1. Phosphenes – ON produces phosphenes by eye movement. Sound-induced photisms — sensations of color or light induced by a loud noise 2/2 discharge from LGN which is responsive to sound as well

Question 8

Cortical Causes of Hallucination

Answer 8

1.