Simultanagnosia is impaired perception of multi-element or multipart visual displays, such that pictures are described in a piecemeal manner. Recognition of single objects is preserved; this is likened to having a fragment or island of clear vision which may shift from region to region.
Two types of simultanagnosia are described:
An attentional limitation preventing more than one object being seen at a time; although superficially similar to apperceptive visual agnosia, with which it has sometimes been classified, patients with dorsal simultanagnosia can recognize objects quickly and accurately, but unattended objects are not seen. There may be inability to localize stimuli even when they are seen, manifest as visual disorientation. Reading is severely impaired. Patients may grope, as though blind. Dorsal simultanagnosia is associated with bilateral posterior parietooccipital lesions, and is one feature of Balint’s syndrome.
A limitation in the number of objects which can be recognized in unit time, i.e., there is no primary recognition problem in that individual shapes can be recognized. Ventral simultanagnosia is most evident during reading which is severely impaired and empirically this may be the same impairment as seen in pure alexia; otherwise deficits may not be evident, unlike dorsal simultanagnosia. Ventral simultanagnosia may be a form of associative visual agnosia. It is associated with left inferior temporo-occipital cortical lesions.
Coslett HB, Saffran E. Simultanagnosia: to see but not two see. Brain
1991; 114: 1523-1545
Farah MJ. Visual agnosia: disorders of object recognition and what theytell us about normal vision. Cambridge: MIT Press, 1995