Sarah Goodwin

Kathi Ovnic
Holly Korschun
October 4, 1999

EMBARGOED FOR RELEASE until 2 p.m., E.D.T., Wed., Oct. 6, 1999


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 University study confirming the role of visual cortex in tactile (touch) perception are reported in this week's issue of the journal 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," says lead author Krishnankutty (Krish) Sathian, M.D., Ph.D., associate professor of neurology and joint associate professor of rehabilitation medicine at the Emory University School of Medicine and a faculty member in the interdisciplinary Neuroscience Program at Emory University. 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," Dr. Sathian says. "Recent findings that visual cortex is active during Braille reading in the blind are perhaps less surprising if viewed in this context."

In one of the tasks used in Dr. Sathian's laboratory, a grooved object is impressed onto the fingertip (grating) 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 the grooves run (along vs. across the finger).

"People who performed this task told us they were visualizing 'with themind's eye' the orientation of the grating on the fingertip, suggesting to us that visual imagery facilitates this tactile task," Dr. Sathian says. "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."

The Emory group then used positron emission tomography (PET) 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. The researchers asked normally sighted volunteers to perform a series of tactile discrimination tasks designed to quantify tactile abilities, including the fingertip grating task. Subjects were asked to tell either the orientation (direction) of the grating (along or across the finger) or whether the grooves were wide or narrow (spacing task). The 14 subjects performed the tasks while researchers used harmless transcranial magnetic stimulation to transiently block the function of, first, various parts of the brain region associated with the sense of sight (the occipital cortex), and then, the brain region mediating the sense of 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, M.D., Ph.D., now a resident in neurology at Thomas Jefferson University Hospital in Philadelphia, who conducted the research while a fellow in Dr. Sathian's laboratory at Emory. 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 on 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 write. "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 M. Epstein, M.D., and Scott T. Grafton, M.D., both associate professors in Emory's neurology department.

"This study exemplifies the kind of collaboration that is so characteristic of Emory," Dr. Sathian says. "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."

The work was supported by research grants to Dr. Sathian from the NationalInstitute of Neurological Disorders and Stroke and the National Eye Institute of the National Institutes of Health.