A right-frontal region critical for identifying 3-D objects

We present an intriguing case of a left-handed 20-years-old patient, HS, with a right frontal tumor. The patient underwent awake surgery. During it parts of his brain regions around the tumor were electrically stimulated using an electrical probe. Such electrical stimulation disrupts the functioning of brain regions around the spot where the probe is applied. When such stimulation was applied to the posterior parts of the patient’s right frontal operculum, the patient could not identify line-drawings of a pyramid, and misidentified it as “two-triangles” or as “parallelogram”. The patient could identify it correctly when the stimulation was switched-off. The patient's surgery was terminated. He was then treated with radiotherapy. The patient is currently in remission.
We reasoned that this deficit could have been due to the patient being unable to see or infer third-dimension from two-dimensional drawings of such objects. To test this, we conducted an fMRI experiment on the patient two-years after the surgery. The patient was shown a series of pictures. Some of these were pictures of two-dimensional shapes like squares, others were easily discernible three-dimensional objects e.g. cylinders. Crucially, we used a set of pictures that were of three-dimensional objects but their being three-dimensional was difficult to discern e.g. picture of cylinder taken from such an angle that the shape appears like a rectangle and requires effort to discern as a cylinder. Areas involved in a cognitive process activate more when the task is difficult and more of that process is needed. We therefore predicted that regions involved in 3-D vision will be more activated when discerning 3D hard-to-discern objects compared to when discerning 3D easy-to-discern objects. We found a significant cluster of activation at the site where electrical stimulation during operation had impaired his capacity to identify a pyramid. We also localized HS’ language network. We found that his language regions were in the left-hemisphere despite his being left-handed. This also showed that his inability to identify the pyramid is unlikely to be due to disruption of some language region.
Brain regions critical for 3-D vision are unclear. Our finding suggests that a region within the right frontal operculum may be causally necessary for such vision, thus may shed light on the neural basis of perception of 3-D objects.