November 8, 1999
Volume 52, No. 11
Researchers find brain link between sight and touch
For the first time, researchers have verified that the part of the brain involved in processing the sense of sight is also necessary for the sense of touch. Results of an Emory study confirming the role of visual cortex in tactile perception were reported in a recent issue of Nature.
"The findings are relevant to understanding not only how the brain normally processes sensory information but also how such processing is altered in conditions such as blindness, deafness or numbness, and ultimately to improving methods of communication for individuals afflicted with these disorders," said lead author Krishnankutty (Krish) Sathian, joint associate professor of neurology and rehabilitation medicine and a faculty member in Emory's neuroscience program.
Until recently, scientists believed separate brain regions processed information gathered by the various senses. This view is now being challenged.
"The kind of interaction among the senses identified in our work may be more common than generally appreciated," Sathian said. "Recent findings that the visual cortex is active during Braille reading in the blind are perhaps less surprising if viewed in this context."
In one of Sathian's laboratory tests, a grooved object is impressed onto the fingertips of human volunteers. With their eyes closed or blindfolded, the subjects attempt to distinguish via touch the orientation of the grooves, i.e., the direction in which they run.
"People who performed this task told us they were visualizing 'with the mind's eye' the orientation of the grating on the fingertip, suggesting to us that visual imagery facilitates this tactile task," Sathian said. "I had thought for some time that visual imagery might be involved in tactile perception. We decided to see if we could obtain more direct evidence for this."
Sathian's group then used positron emission tomography to show that a region of the cerebral cortex associated with sight is engaged when humans attempt to distinguish orientation via delicate touch, as in the fingertip grating task (NeuroReport, 1997). But whether this region was truly necessary for tactile performance was unclear.
The current study aimed to answer this question. Researchers asked normally sighted volunteers to perform a series of tactile discrimination tasks designed to quantify tactile abilities, including the fingertip grating task.
Fourteen subjects performed the tasks while researchers used harmless transcranial magnetic stimulation to transiently block the function of various parts of the brain region associated with the sense of sight (the occipital cortex) and touch (the somatosensory cortex).
When a key region of visual cortex (the parieto-occipital cortex) was blocked, subjects were significantly less successful in discriminating the orientation of the grating via touch, according to first author of the paper Andro Zangaladze, now a resident in nurology at Thomas Jefferson University Hospital in Philadelphia, who conducted the research while a fellow in Sathian's lab. However, performance on the spacing task was unaffected, implying that the effect was selective for orientation. In contrast, blocking somatosensory cortex interfered nonselectively with performance of both tasks.
"Together with the subjective reports of visual imagery in this task and the associated parieto-occipital cortical activation noted previously, our findings support the proposal that visual processing facilitates normal tactile discrimination of orientation," the authors wrote. "Perhaps this is related to the fact that we generally rely on the visual system for orientation discrimination. Thus, involvement of the visual cortex may be beneficial when macrogeometric features such as orientation are to be discriminated, but not for microgeometric features such as texture."
Co-authors of the study include Charles Epstein and Scott Grafton, both associate professors of neurology.
"This study exemplifies the kind of collaboration that is so characteristic of Emory," Sathian said. "Apart from my own background in sensory physiology and psychophysics, the work drew on the expertise of Dr. Zangaladze in human electrophysiology, of Dr. Epstein in transcranial magnetic stimulation and of Dr. Grafton in functional brain imaging."