It's important to keep in mind that diminished occurrence of discursive thinking does not necessarily correlate with an inactive brain. Rather, neural resources are diverted away from verbal processes and toward attentional processes, which require a fair amount of activity to maintain.
Only one empirical study to date has been conducted with regard to jhana/dhyana and the brain. It utilizes one subject (Leigh Brasington, known for his extensive meditation experience and writings on the jhanas) and was published by Hagerty et al. (2013). A preliminary paper was submitted in 2008 and is easily accessible online. I can summarize the recently published results.
For the purposes of this summary, data are reported in terms of brain activity as measured through standard functional magnetic resonance imaging (fMRI) of blood oxygen level dependent (BOLD) signals. Both anatomical names and the associated Brodmann's Areas (BA) are summarized below. Results conform to subjective report by the subject.
Progression through the four rupa jhanas/dhyanas and the four arupa jhanas/dhyanas (eight in total) in the subject reliably resulted in:
- Dimming of external awareness as measured through decreased activity in visual (BA 17, 19) and auditory (BA 41, 42) processing areas.
- Fading of internal verbalization as measured through decreased activity in Broca's Area (BA 44, 45) and Wernicke's Area (BA 39, 40).
- Alteration in one's sense of personal boundaries as measured through decreased activity in orientation areas (BA 5, 7).
- Highly focused attention as measured through increased activity in the Anterior Cingulate Cortex (ACC) (BA 32, 33).
- Experience of ecstatic joy as measured through increased activity in the Nucleus Accumbens (NAc) and Medial Occipital Frontal Cortex (Med OFC).
Note: Interestingly, the NAc is a major part of the dopamine reward system, which is implicated in addiction. Thus, it is possible that the pleasure induced by jhana/dhyana through increased activity of the NAc may become addictive for the practitioner.
- Less rhythmic movement as measured through decreased activity in the Somatosensory Cortex (BA 1, 2, 3), Primary Motor Cortex (BA 4), and Cerebellum.
Of course, this is merely a summary, and it is encouraged (especially for those familiar with neuroimaging studies and the emerging field of Contemplative Science) to read the full peer-reviewed article:
Hagerty, M. R., Isaacs, J., Brasington, L., Shupe, L., Fetz, E. E., & Cramer, S. C. (2013). Case study of ecstatic meditation: fMRI and EEG evidence of self-stimulating a reward system. Neural Plasticity, 2013