Research Spotlight: Visual Recognition with Only a Single Hemisphere?

Marlene Behrmann, PhD

Humans are capable of dramatic feats of visual perception, recognizing a single object, face, word or scene in under 250 milliseconds with little effort and extraordinary accuracy. Dr Marlene Behrmann, who joined the Department of Ophthalmology at the University of Pittsburgh in July 2022, is determined to shed some light on how visual perception is accomplished by the human brain and how this ability evolves over the lifespan. One scientific approach she adopts is to study the recognition abilities of individuals whose visual processing is compromised as a result of, for example, cortical stroke, traumatic brain injury or tumor. This is akin to a reverse engineering approach: study a system when parts are dysfunctional and make inferences about the normal operation of the device.

Recently, Behrmann has received a RO1 grant from the National Eye Institute entitled “Reorganization of visual function in patients with posterior cortical research: Selectivity and plasticity.” In the context of this funding, Behrmann is currently conducting investigations with children aged 5 to 21 who have undergone surgical resection, for example, lobectomy (resection of a portion of a cortex) or hemispherectomy (removal of a hemisphere) for the treatment of pharmaco-resistant epilepsy. The primary question is whether the preserved cortical tissue is capable of ‘taking over’ the functions which would have been subserved by the resected tissue and Behrmann’s focus is on the cortical visual system and the extent to which it is plastic or malleable. Behrmann and her collaborators (PhD and MD PhD students) along with Dr Christina Patterson, epileptologist at Children’s Hospital, adopt both detailed visual behavioral testing (visual psychophysics) and neuroimaging (structural and functional MRI) to address this issue.

These studies explore the nature and extent of impairments in visual behavior in the paediatric patients and compares their findings against those of typically developing age-matched controls and matched patients who have also had surgical resections of nonvisual cortical areas. Despite the removal of a significant amount of visual cortex, the patients, who have large unilateral resections including visual cortex, typically exhibit a remarkable degree of recovery of visuo-cognitive function, particularly if the resection is performed in early childhood. This restitution of function is all the more surprising given the distinct patterns of lateralization of the two cerebral hemispheres in the normal brain. For example, in one recent paper with children with hemispherectomy, Behrmann et al. showed, unsurprisingly, that the patients have persistent hemianopia. Intriguingly, though, their visual performance was better than one might have predicted given that they have only a single hemisphere. In this study, participants sat in front of a computer screen and a pair of words or a pair of faces was presented. Participants simply had to judge whether the two instances were the same or different. Typically, faces are processed better by the right hemisphere and words by the left hemisphere. Surprisingly, with a single hemisphere, these patients performed at about 80% accuracy and this was true for both stimulus types (words and faces) and irrespective of whether the right or left hemisphere had been resected. Although the patients’ performance was statistically inferior to that of the control group, that they were able to perform relatively well reflects the plasticity of the hemispheres and the flexibility in recruitment of the preserved hemisphere for both kinds of visual tasks.

Many other issues are being addressed in this research, including the changes in the neural basis of cortical visual function from pre- to post-surgery, and longitudinally thereafter, the specific mechanisms that give rise to recovery, the potential modulatory effects of brain regions outside the visual system, and possible biographical factors predicting recovery (e.g., age, side and size of resection). Shedding light on the neural and computational mechanisms of visual cortex may usefully inform and constrain novel approaches for intervention and rehabilitation of individuals with visuoperceptual disorders.