As the probe makes contact with the surface of the brain, the patient frowns and says, “A little sound in my ear.” The surgeon asks which side, and the patient states, “Left.” The surgeon questions what the noise sounded like, and the patient responds, “Like hitting a chair or some other object.” The surgeon moves the probe posteriorly along the sylvian fissure, and the patient speaks up once more: “My left ear—sounded like wind going through the trees.”—The Cerebral Cortex of Man, by Wilder Penfield and Theodore Rasmussen (New York: The MacMillan Company; 1950)
This interaction occurred in the operating room of Wilder Penfield, MD, and is one example of the auditory sensations he elicited from hundreds of patients during neurosurgery. For this research, Dr. Penfield is world renowned, having contributed significantly to our understanding of the central auditory nervous system. Due to the importance of his work to audition, we are highlighting some of his studies in this article.
Wilder Graves Penfield was born in Spokane, WA, in 1891 and died in 1976. He attended Princeton University, where his goal was to obtain a Rhodes Scholarship. He achieved this prestigious scholarship through his tenacity in the study of biology and athleticism on the football field. With the scholarship, Dr. Penfield pursued his medical degree at the University of Oxford and Johns Hopkins University.
During his studies at Oxford, he became fascinated by neurology, thanks to his greatest influence, Sir Charles Sherrington, who convinced Dr. Penfield that the nervous system was the “undiscovered country” (Leitch A: A Princeton Companion; 1978). Penfield was so swept up by this field that he returned to Oxford for graduate work in neurophysiology after completing his medical studies and his neurosurgery training under Harvey Cushing, MD.
In this way he became a “neurologist-in-action,” (Leitch A: A Princeton Companion; 1978), conducting neurosurgery upon patients as well as engaging in neurological research. After years of work, Dr. Penfield incorporated these two domains of neurology under one roof, forming the Montreal Neurological Institute. He conducted much of his research while director of the institute, which has continued the tradition of conducting cutting-edge central auditory research.
MAPPING THE BRAIN
The experiments of Dr. Penfield began with his exploration of epilepsy. He is credited with perfecting the surgical treatment of the disorder (Leitch A: A Princeton Companion; 1978).
During the surgery, Dr. Penfield would use an electrical current to induce an epileptic episode in a patient, which became a valuable method of investigating cortical function. A seizure in different areas of the brain will elicit different sensations and movements depending on the functional structures stimulated. (Penfield W: The Excitable Cortex in Conscious Man. Liverpool: Liverpool University Press; 1958.)
Patients were given local anesthesia and kept conscious for the operation. Dr. Penfield would use a probe with an electrical current of one to three volts to stimulate certain areas of the cortex. After each stimulation, the patient would tell him any sensation or movement they felt throughout the body. He would then make a note on a small square of paper, placing it on the surface of the cortex at the point that induced that sensation.
In this manner, he would map the brain to discover the precise origin of the patient's epilepsy and could then remove that part without damaging other healthy structures. This technique became known as the Montreal Procedure and is still used today (Speyrer, JA: A Review of an Article by Wilder Penfield, MD).
Over decades of performing these procedures, Dr. Penfield was able to sketch out what is now known as the sensorimotor homunculus, a visual depiction of which areas of the sensory and motor strips represent different areas of the body. Through this approach, he was also able to obtain valuable information regarding stimulation of the auditory cortex and the temporal lobe.
LATERALIZATION OF FUNCTION
Given Dr. Penfield's influence on auditory science, we will now detail some of his experiments that were critically important to the advancement of the field.
Electrical stimulations were conducted throughout the brain, including the temporal lobe and auditory cortex, to interesting effect. During these stimulations, some patients reported hearing buzzing, ringing, chirping, and knocking. (Roeser RJ, Valente M, Hosford-Dunn H: Audiology: Diagnosis. New York: Thieme Medical Publishers Inc.; 2000.) Others claimed that the surgeon's voice changed in pitch and volume during stimulation.
In a later study with neurosurgeon Phanor Perot, Dr. Penfield noted that when the left auditory cortex of certain patients was stimulated, patients reported hearing people speaking or shouting, while when the right auditory cortex was stimulated, patients reported hearing music or singing (Roeser RJ, Valente M, Hosford-Dunn H: Audiology: Diagnosis). While this observation, only occurred in five percent of patients, it has proved influential to the medical community's understanding of the lateralization of brain function.
Dr. Penfield stimulated the temporal lobe along the length of the superior temporal gyrus, receiving reports of auditory sensation from patients. Other patients said they felt like they could not hear or as though something were placed over their ears during stimulation; however, they were still able to hear and understand words that were spoken to them (Penfield W: The Cerebral Cortex of Man). These responses are quite informative for the understanding of cortical function.
In these experiments, it was reported that patients “heard” sounds as through the ear contralateral to the site of stimulation (Penfield W: The Cerebral Cortex of Man). This observation lends credence to what we now know about these neural sensory pathways crossing between hemispheres before reaching the cortex.
This invasive technique for determining brain function was a precursor to the fMRI; it was a groundbreaking approach that directly explored how the patient feels or responds when a certain part of the brain is activated. The procedure also may be the earliest form of intraoperative monitoring. While we now have physiologic measures monitored during surgery, Dr. Penfield was able to accomplish a similar result simply by asking patients what they felt.
Over the course of his long career, Wilder Penfield made significant contributions to the knowledge of temporal lobe function and auditory anatomy. His work aided in localizing the auditory areas of the cortex, though these areas are not yet completely delineated. In spite of Dr. Penfield being a neurosurgeon, his work had significant implications for the field of audiology.
For more information on Wilder Penfield, MD, and his research on auditory cortex stimulation, consider reading:
* Penfield W, Phanor Perot. The brain's record of auditory and visual experience: a final summary and discussion. Brain 1963;86:595-696.
* Penfield Wilder, Roberts Lamar. Speech and Brain Mechanisms. Princeton: Princeton University Press; 1959.
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