MRI study indicates the adult brain reorganizes after stroke
Gwynne D. Koch
The results of a four-year study of a stroke patient have provided further evidence of the plasticity of the adult human brain and may aid in the development of therapies for rehabilitation from brain injury, stroke and macular degeneration.
Daniel D. Dilks of the McGovern Institute for Brain Research at MIT in Cambridge, Mass., and his colleagues at Johns Hopkins University in Baltimore and at the University of California, Irvine, conducted behavioral and neuroimaging studies of a 51-year-old man six months after he suffered a stroke. Structural MRI showed that the stroke damaged the nerve fibers that normally provide input from the upper left visual field to a region of the primary visual cortex, effectively creating a blind area, but caused no injury to the cortex itself.
The patient was subjected to a series of visual and tactile shape-perception tasks, including judging the height and width of geometric shapes, to confirm preliminary testing results that indicated distorted perception of shapes presented in the lower left visual field. As shown in Figure 1, the subject reported the shapes as vertically elongated upward into the blind upper left visual field. Results from these tasks revealed that the patient’s perceptual distortion was selective to vision and affected only the vertical dimension of objects.
Figure 1. A patient with localized neural damage caused by stroke described geometric shapes presented in his lower left visual field as vertically elongated, suggesting that the adult human brain can reorganize and change an area’s functioning after injury. As shown, a circle appeared as a cigar shape extending upward, a square looked like a tall rectangle, and a triangle with the point facing down was perceived as a pencil standing upright. Images reprinted with permission of the Journal of Neuroscience.
Besides the behavioral experiments, neuroimaging was conducted to map his brain activity while he passively viewed a random sequence of white wedges flickering at a rate of 8 Hz, presented for 5 s on a black background in one of 12 locations corresponding to the positions of numbers on a clock. Eye-tracking performed during the scanning ruled out the possibility that large, systematic eye movements influenced the MRI results.
Images were acquired with a Philips Intera 3T scanner using a magnetization-prepared rapid acquisition gradient echo T1-weighted sequence. Whole-brain echoplanar functional images were acquired in 35 transverse slices with a head coil from MRI Devices Corp. of Waukesha, Wis. Software from Brain Innovation BV of Maastricht, the Netherlands, was used for functional MRI analyses.
The patient responded the same as the four control subjects to stimuli presented in the upper and lower right (positions 1R and 6R, respectively) and very lower left (position 6L) visual fields. However, as shown in Figure 2, wedges in position 5L activated the region of the visual cortex typically affected by images presented in the upper left. Very little response to stimuli presented in the blind area was observed. After emerging from the scanner, the subject provided a verbal description of what he saw that was consistent with the functional MRI data.
Figure 2. To map patterns of neural activation, a wedge stimulus was randomly displayed in 12 locations during functional MRI scanning (a). Stimuli presented in the right visual field activate the left hemisphere, while stimuli in the left visual field activate the right. The inflated cortical surface of the test subject’s right (affected) hemisphere (b) shows a different pattern of activation than that of the four control participants (c), unlike the left (unaffected) hemisphere, which shows similar patterns. The upper and lower boundaries of the primary visual cortex are demarcated by solid black lines.
Because specific regions of the brain respond to objects in particular areas of a person’s visual field, the scientists were able to determine that the cortex representing the upper left section had changed. That region no longer responded to stimuli in the upper left visual field, but instead responded to items in the lower left. The researchers concluded that neurons in the visually deprived region were activated by images that normally affected an adjacent region, demonstrating brain plasticity, or capacity for reorganization.
They are exploring whether changes in the adult cortex can occur sooner than six months and are testing patients who have visually deprived cortexes resulting from macular degeneration to corroborate their findings. They also are investigating the underlying neural mechanisms for cortical reorganization.
Journal of Neuroscience, Sept. 5, 2007, pp. 9585-9594.
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