Formed visual hallucinations are seen in lesions of:
Frontal lobe
Temporal lobe
Occipital lobe
Parietal lobe
The lesions of temporal lobe can cause all types of hallucinations (auditory, visual, olfactory etc). Formed visual hallucinations are elaborate visual hallucinations (e.g a patient may repo seeing a tree with green leaves which has a parrot sitting and a man standing near it) . The presence of elaborate visual hallucinations should raise a strong doubt of an organic cause (i.e. a pathology of brain parenchyma), specifically a temporal lobe pathology.
Basics of Psychiatry