Brain imaging in a patient with hemimicropsia

Abstract

Hemimicropsia is an isolated misperception of the size of objects in one hemifield (objects appear smaller) which is, as a phenomenon of central origin, very infrequently reported in literature. We present a case of hemimicropsia as a selective deficit of size and distance perception in the left hemifield without hemianopsia caused by a cavernous angioma with hemorrhage in the right occipitotemporal area. The symptom occurred only intermittently and was considered the consequence of a local irritation by the hemorrhage. Imaging data including a volume-rendering MR data set of the patient's brain were transformed to the 3-D stereotactic grid system by Talairach and warped to a novel digital 3-D brain atlas. Imaging analysis included functional MRI (fMRI) to analyse the patient's visual cortex areas (mainly V5) in relation to the localization of the hemangioma to establish physiological landmarks with respect to visual stimulation. The lesion was localized in the peripheral visual association cortex, Brodmann area (BA) 19, adjacent to BA 37, both of which are part of the occipitotemporal visual pathway. Additional psychophysical measurements revealed an elevated threshold for perceiving coherent motion, which we relate to a partial loss of function in V5, a region adjacent to the cavernoma. In our study, we localized for the first time a cerebral lesion causing micropsia by digital mapping in Talairach space using a 3-D brain atlas and topologically related it to fMRI data for visual motion. The localization of the brain lesion affecting BA 19 and the occipitotemporal visual pathway is discussed with respect to experimental and case report findings about the neural basis of object size perception.

Ipsilateral visual illusion after unilateral posterior cerebral artery infarction: A report of two cases

“…The transience of ipsilateral visual illusions in both patients may have occurred because the core area for the size constancy in the left hemisphere was spared and only parts of the forceps major were involved. However, this supposition is not consistent with the results of previous case reports in which hemimicropsia or hemimacropsia was restricted to contralateral homonymous segments of the visual field, regardless of the lesion side [7][8][9]. These observations suggest that each hemisphere was in charge of size processing only for contralateral stimuli.…”

“…Deficits in size constancy in our cases were explained by central misinterpretation of the received afferent inputs because they were binocular with hemifield involvement. However, our findings were unique in that our patients showed deficits in the size constancy of the ipsilateral hemifield, while previous studies reported cases in which unilateral occipitotemporal lesions caused contralateral micropsia or macropsia [7][8][9]. It is possible that visual illusions in our patients are not truly "ipsilateral" to the lesion.…”

“…Their visual illusion was characterized by zooming in, macropsia or micropsia, and these symptoms appeared to be related to deficits in size constancy [7,8]. Size constancy is a phenomenon in which an object appears to retain its size despite substantial variations in the viewing distance.…”

A Blueprint for Target Motion: fMRI Reveals Perceived Sequential Complexity to Modulate Premotor Cortex

“…Note that intact size-constancy perception does not necessarily depend on intact motion perception, as evident from patient studies. Thus, an isolated misperception of the size of objects (in one hemifield), a socalled hemimicropsia, has been reported in a patient with a very focal lesion in x ϭ 37.5; y ϭ 68; z ϭ 1 (Kassubek et al, 1999). The authors suggest an area V4 affection, since this patient was not impaired in motion perception, which would have been the case if the lesion was in area MT.…”

Observing and engaging in purposeful actions with objects influences estimates of their size

“…Size estimates involving the grasping of the nut in the example above would be under dorsal control. The second system, the ventral stream, is involved intimately in object recognition and, in that role, appears to assess different aspects of the size of an object than does the dorsal stream (Kassubek et al, 1999). In the example above, the ventral stream would control functions related to the selection of appropriately sized materials.…”