Simultanagnosia is where a person can’t perceive more than one object at a time. They literally cannot see the wood for the trees. There are two main types that differ depending on the location of the brain injury which has caused the syndrome.
Damage to the dorsal stream can cause dorsal simultanagnosia, where the patient cannot see two or more objects at the same time.
Damage to the ventral stream can cause ventral simultanagnosia, where the patient can see multiple objects, but can only identify one at a time.
The following is from p61 of the 1970 book Brain Damage and the Mind (ISBN 0140801405) by Moyra Williams, who describes a gentleman with dorsal simultanagnosia:
A sixty-eight-year old patient studied by the author had difficulty finding his way around because “he couldn’t see properly”. It was found that if two objects (e.g. pencils) were held up in front of him at the same time, he could see only one of them, whether they were held side by side, one above the other, or one behind the other.
Further testing showed that single stimuli representing objects or faces could be could be identified correctly and even recognized when shown again, whether simple or complex… If the stimuli included more than one object, only one would be identified at one time, though the other would sometimes “come into focus” as the first one went out…
If long sentences were presented, only the rightmost word could be read… If a single word covered as large a visual area as a sentence which could not be read, the single word was read in its entirety… If the patient was shown a page of drawings, the contents of which overlapped (i.e. objects were drawn on top of one another), he tended to pick out one and deny that he could see any others.
Recent evidence has suggested that although the unseen objects may not be consciously available, carefully designed psychological tests can detect they have been registered at some unconscious level.
The book Visual Agnosia by Prof Martha Farah covers a number of curious object perception disorders that occur after brain injury, including simultanagnosia.
The book’s webpage has a table of contents and some sample chapters freely available online.
Link to webpage for Visual Agnosia.