Andrew Newberg, an M.D. and radiologist at the University of Pennsylvania, published in 2006 his study: The measurement of regional cerebral blood flow during glossolalia: A preliminary SPECT study. Newberg and colleagues used imaging technology to look at the brains of Pentecostal Christians speaking in tongues. They then looked at their brains when they were singing gospel music. They found that those practicing glossolalia (tongues-speaking) showed decreased activity in the brain’s language center, compared with the singing group. The imaging results are suggestive of people’s description that they do not have control of their own speech when speaking in tongues. Newberg said scientists believe that speech is taken over by another part of the brain during glossolalia, but did not find it during the study."
This jives with the definitions of tongues-speaking given by Gordon Fee and N.T. Wright.
Fee writes: “Contrary to the opinion of many, spiritual edification can take place in ways other than through the cortext of the brain. Paul believed in an immediate communing with God by means of the S/spirit that sometimes bypassed the mind; and in [1 Corinthians 14] vv. 14-15 he argues that for his own edification he will have both." (Gordon Fee, The First Epistle to the Corinthians, 657)
I love N.T. Wright's description of the gift-manifestation of "tongues": “’Tongues’ refers to the gift of speech which, though making sounds, and using apparent or even actual languages, somehow bypasses the speaker’s conscious mind. Such speech is experienced as a stream of praise in which, though the speaker may not be able to articulate what precisely is being said, a sense of love for God, of adoration and gratitude, wells up and overflows. It is like a private language of love.” (N.T. Wright, Paul for Everyone: 1 Corinthians, 182)
Newberg's study is explained and reviewed by Frederick L. Ware in "Can Religious Experience Be Reduced to Brain Activity? The Place and Significance of Pentecostal Narrative." (In Science and the Spirit: A Pentecostal Engagement with the Sciences, Kindle location 1668)