"The 10 percent of the brain myth asserts that humans generally use only 10 percent (or some other small percentage) of their brains... By extrapolation, it is suggested that a person may harness this unused potential and increase intelligence" (Wikipedia).
I've heard this myth said by Rabbanim in shmuzzen to teach us that one should aim to use 100% of their brain to learn Torah. To be Milamed zechut, although 100% is definitely not a possibility, we do have to breathe and do other brain functions.
"What is true is that the brain has an extraordinary capacity for change known as plasticity... the vast majority of us certainly have untapped potential" (Centre for Educational Neuroscience, see more on Wikipedia).
One thing I do not think has been studied is the possibility to not use the brain for basic human needs like pain management and use it for other things instead.
הָהוּא מִינָא דְּחַזְיֵיהּ לְרָבָא דְּקָא מְעַיֵּין בִּשְׁמַעְתָּא, וְיָתְבָה אֶצְבְּעָתָא דִידֵיהּ תּוּתֵי כַּרְעָא וְקָא מָיֵיץ בְּהוּ, וְקָא מַבְּעָן אֶצְבְּעָתֵיהּ דְּמָא. אֲמַר לֵיהּ: עַמָּא פְּזִיזָא, דְּקַדְּמִיתוּ פּוּמַּיְיכוּ לְאוּדְנַיְיכוּ, אַכַּתִּי בְּפַחְזוּתַיְיכוּ קָיְימִיתוּ. בְּרֵישָׁא אִיבַּעְיָא לְכוּ לְמִשְׁמַע, אִי מָצִיתוּ — קַבְּלִיתוּ, וְאִי לָא — לָא קַבְּלִיתוּ. אֲמַר לֵיהּ: אֲנַן
The Gemara relates that a heretic saw that Rava was immersed in studying halakha, and his fingers were beneath his leg and he was squeezing them, and his fingers were spurting blood. Rava did not notice that he was bleeding because he was engrossed in study. The heretic said to Rava: You impulsive nation, who accorded precedence to your mouths over your ears. You still bear your impulsiveness, as you act without thinking. You should listen first. Then, if you are capable of fulfilling the commands, accept them. And if not, do not accept them. He said to him: About us,
דְּסָגֵינַן בִּשְׁלִימוּתָא כְּתִיב בַּן: ״תֻּמַּת יְשָׁרִים תַּנְחֵם״, הָנָךְ אִינָשֵׁי דְּסָגַן בַּעֲלִילוּתָא כְּתִיב בְּהוּ: ״וְסֶלֶף בּוֹגְדִים יְשָׁדֵּם״.
who proceed wholeheartedly and with integrity, it is written: “The integrity of the upright will guide them” (Proverbs 11:3), whereas about those people who walk in deceit, it is written at the end of the same verse: “And the perverseness of the faithless will destroy them.”
We see from this Gemara that one can be so engrossed in Torah, that regular brain functions like feeling pain don't work. I have also heard a couple of stories from recent times following this theme. When I googled it to write this, the first one I found was this
About seven years before his passing, Harav Ovadia was complaining of terrible stomach pains, and his son Rav David accompanied him to the hospital. The doctors ordered a battery of tests, and they uncovered an issue that required immediate laparoscopic surgery. But at his advanced age, and considering his weak overall medical condition, the doctors felt that it was dangerous to place him under anesthesia. “It will him take several weeks to recover from the anesthesia, ” they explained. “That’s when he’ll come out of it. ” They told Rav David that they would do the half hour surgery without anesthesia, but they warned him that the procedure would be extremely painful. ”We’re going to summon several staff members just to hold him down, ” they said, “and you must remain outside because you won’t be able to handle the sight of him suffering so much. If you want to remain right outside the operating theater, you may, but be prepared for his shrieks of pain. ” Rav David asked the doctors to describe to the Rav what the procedure would be like, so that he wouldn’t be shocked when the pain began. “B’seder, ” Harav Ovadia sighed upon hearing the plan. “If this is a pain that I have no choice but to suffer, it should be kaparat avonot (an atonement for my sins). But please bring me my Gemara and I’ll try to distract myself from the pain as much as possible. ” “I brought him his Gemara – he was learning Bava Batra at the time — and I left the room, ” relates Rav David. “A large group of doctors and other hospital staff entered the operating room, but I did not hear a single sound emanating from within. Finally, after about half an hour, the doctors came out and said, ‘We were shocked —‘ he didn’t utter a peep.’ ”I went inside and asked, Abba, how are you feeling?’ He looked up at me and said, slightly irritated, Ask the doctors how long they’re going to make me wait. Let them start already!’ ”He was so utterly engrossed in his learning that he had not felt them operating.” (Achim)
It may even be possible that one can use 100% of their brain for Torah, at that point their physical body would completely stop functioning and they would become like an angel. The Navi states that Eliyahu Hanavi went up to heaven in a fiery chariot. He is considered still alive.
Although we are not reaching the level of Eliyahu, Rava or Hacham Ovadia. May we all use more of our brains for Torah.