The Left Side of Your Face Looks Distorted!
The rare syndrome of unilateral left prosopometamorphopsia -- in which the left half of well-known and unfamiliar faces looks distorted -- was reported by Trojano et al. (2009) in a neuropsychological case study:
D.G., a 24-year-old right-handed housewife with 8 years of formal education, suddenly developed severe migraine, confusional state, and blurred vision, mainly in her right visual hemifield, after child delivery. Consciousness returned normal and migraine subsided in a few hours, but the visual disturbances persisted longer.Repeated EEG recordings showed theta-wave slowing over posterior regions of the left hemisphere. ... Repeated MR did not disclose pathological areas. Brain SPECT (performed 30 min after the injection of 740 MBq of Tc-99m HMPAO) showed reduced blood flow in the inferior and lateral cortex of the left occipital lobe.The patient's MRI scan appeared normal but a SPECT (single photon emission computed tomography) scan revealed diminished cerebral blow flow in the left lateral/inferior occipital lobe, as shown below.
Fig. 1 (adapted from Trojano et al., 2009). Axial brain SPECT scans showing reduced blood flow in the inferior and lateral cortex of the left occipital lobe (regional blood flow reduced by 25.8% with respect to contralateral cortex).
D.G. was seen in the clinic 6 months after her stroke, and her right visual field was intact (meaning she did not have damage to the contralateral, or left, primary visual cortex). However,
...she complained that the left half of people's faces (the part on her right side) appeared “out of shape”. D.G. claimed that “the left eye looks elongated towards left ear, the nose appears to be bended towards left cheek and the mouth towards the chin” irrespectively of whether she looked at familiar or unknown people, or even at herself in a mirror. Nonetheless, she reported to be able to recognize relatives and famous people by face, and also to visualise familiar faces in her mind without distortions.
Fig. 2 (Trojano et al., 2009). Patient's drawing of a face to depict her own subjective complaint.
She did not appear to have impairments in other visuospatial perceptual skills, and she could recognize famous faces as well as the control participants did, ruling out prosopagnosia (a face recognition deficit). Interestingly D.G. did not report distortions in the appearance of non-face objects. In a series of experiments, she was impaired in matching pairs of chimeric faces differing in their left halves, but not in a similar condition with chimeric cars. To explain this unusual deficit with a left hemisphere lesion (the right hemisphere is thought to be better at face recognition), the authors speculated that
early stages of face perception proceed in parallel in the two hemispheres and only at late stages the right hemisphere integrates information gathered from both parts of the stimulus to build up an unitary face representation. Replication of such findings and neurofunctional investigations on patients with PMO are warranted to directly verify neural dynamics related to unilateral defects in face processing.
Trojano, L., Conson, M., Salzano, S., Manzo, V., & Grossi, D. (2009). Unilateral left prosopometamorphopsia: A neuropsychological case study. Neuropsychologia, 47 (3), 942-948 DOI: 10.1016/j.neuropsychologia.2008.12.015.
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