Adult Brain Can Change, Study Confirms
It is well established that a child’s brain has a remarkable capacity for change, but controversy continues about the extent to which such plasticity exists in the adult human primary sensory cortex.
Drawings of visual objects by a stroke victim known as BL. When a stimulus appears just below BL’s blind area, the shape elongates upwards and into the blind area. He perceived circles as cigar shaped, squares as rectangles, and triangles as pencil shaped. (Credit: Graphic courtesy / Daniel Dilks, McGovern Institute for Brain Research at MIT, formerly of Johns Hopkins University)
Now, another fMRI study from MIT and Johns Hopkins University has used converging evidence from brain imaging and behavioral studies that seem to indicate that the adult visual cortex does indeed reorganize–and that the change affects visual perception.
The authors believe that as scientists find ways to use this adaptive ability, the work could have relevance to topics ranging from learning to designing interventions for improving recovery following stroke, brain injury, or visual disorders.
Animal studies conducted two decades ago and using single cell recording of neurons found that the adult animal brain can change, but shed little information about the adult human brain. In 2005, a functional magnetic resonance imaging (fMRI) study led by Professor Nancy Kanwisher at the McGovern Institute for Brain Research at MIT found evidence of plasticity in the visual cortex of adults with macular degeneration, an eye disease that deprives regions of the cortex of visual information.
But another fMRI study of macular degeneration found no such evidence, and an animal study using both single cell recordings and fMRI also questioned the 20-year-old animal work.
Lead author Daniel Dilks, a postdoctoral associate in Kanwisher’s lab who conducted the current work while a graduate student at Johns Hopkins in senior author Michael McCloskey’s lab, jumped into the fray when he found BL, a stroke patient.
BL’s stroke damaged the optic radiation fibers, which transmit information from the eye to the primary visual cortex, but the cortex itself remained intact. The damage eliminated input from the upper left visual field to the corresponding region of the primary visual cortex, thereby depriving a region of cortex and creating a blind area in the upper left visual field.
The researchers wanted to find out what happened to that deprived piece of cortex. “We discovered that it took on new functional properties, and BL sees differently as a consequence of that cortical reorganization,” explains Dilks.
BL had reported that things “looked distorted” in the lower left visual field (below his blind area). The researchers hypothesized that the distortions resulted from cortical reorganization in the deprived cortex. To isolate that distortion, they had BL fixate on a center dot while objects, such as squares, appeared in various parts of the visual field. As expected, BL saw nothing when a square appeared in his blind area.
But when the square appeared just below the blind area, he perceived the square as a rectangle extending upwards into the blind area. Likewise, he saw triangles as “pencil-like”, and circles as “cigar-like”.
Subsequent fMRI studies confirmed that the visually deprived cortex (representing the upper left visual field) was responding to information coming from the lower left visual field. The deprived cortex assumed new properties, a hallmark of plasticity, and that explained the visual distortions.
J Neurosci. 2007 Sep 5;27(36):9585-94.
Human adult cortical reorganization and consequent visual distortion.
Dilks DD, Serences JT, Rosenau BJ, Yantis S, McCloskey M.
Department of Cognitive Science, Johns Hopkins University, Baltimore, Maryland 21218, USA. firstname.lastname@example.org
Neural and behavioral evidence for cortical reorganization in the adult somatosensory system after loss of sensory input (e.g., amputation) has been well documented. In contrast, evidence for reorganization in the adult visual system is far less clear: neural evidence is the subject of controversy, behavioral evidence is sparse, and studies combining neural and behavioral evidence have not previously been reported. Here, we report converging behavioral and neuroimaging evidence from a stroke patient (B.L.) in support of cortical reorganization in the adult human visual system. B.L.’s stroke spared the primary visual cortex (V1), but destroyed fibers that normally provide input to V1 from the upper left visual field (LVF). As a consequence, B.L. is blind in the upper LVF, and exhibits distorted perception in the lower LVF: stimuli appear vertically elongated, toward and into the blind upper LVF. For example, a square presented in the lower LVF is perceived as a rectangle extending upward. We hypothesized that the perceptual distortion was a consequence of cortical reorganization in V1. Extensive behavioral testing supported our hypothesis, and functional magnetic resonance imaging (fMRI) confirmed V1 reorganization. Together, the behavioral and fMRI data show that loss of input to V1 after a stroke leads to cortical reorganization in the adult human visual system, and provide the first evidence that reorganization of the adult visual system affects visual perception. These findings contribute to our understanding of the human adult brain’s capacity to change and has implications for topics ranging from learning to recovery from brain damage.