The inferior parietal lobule, including the partially overlapping temporoparietal junction ("IPL/TPJ"), is activated during complex functions, such as theory-of-mind and episodic memory retrieval. It also seems to be important in lower-order functions such as the automatic shifting of attention to unexpected but potentially important stimuli or the feeling of residing in one's own body. Hundreds of functional MRI studies, using diverse conditions, have reported regions in the IPL/TPJ "light up" during a range of behaviors. It seems like information from almost the entire brain converges on the IPL/TPJ. There has been much discussion on the topic of what the IPL/TPJ does.
We have done a series of studies in which we addressed one topical question: can we identify subdivisions that are involved in different behaviors and functions? Please see the Publications & CV page for references to publications related to the IPL/TPJ research line.
Resolving subdivisions within the TPJ/IPL
In our first study, published in the Journal of Neuroscience (2015), we used independent component analysis (ICA), a technique that can be applied on fMRI data to identify functional subdivisions, based on brain activity recorded during a scan. In contrast to most other studies, we applied ICA on a limited region instead of the whole brain - the IPL/TPJ ("local ICA"). This approach increased our ability to detect small subdivisions. We found five subdivisions in the right hemisphere and four in the left. In another study, published in Cerebral Cortex (2017), we found an additional posterior subdivision. The different subdivisions had unique functional connectivity patterns, indicating that they may be associated with different functions and behaviors.
Pairing subdivisions with behavioral functions
To verify that the subdivisions discovered in the studies described above were relevant to brain function, we scanned subjects while they performed five different tasks probing social, memory and attentional functions previously associated with the IPL/TPJ. After collecting the fMRI data, we used the local ICA method to isolate the subdivisions and measured which of them were associated with different behaviors. This study, published in eNeuro (2016), showed that theory-of-mind activity (false belief task), target detection (oddball attention task), and attentional reorienting (Posner task) activated different subdivisions. We also found one subdivision that was universally activated in all five tasks, indicating that there is a region in the IPL/TPJ that may have a less specific, overarching function. Subsequently, in a study published in PNAS (2016), we reported that this region was associated with subjective awareness, independent of attention.
Sex differences in the IPL/TPJ
As part of an autism study funded by the Simons Foundation (see also Autism page), we are currently testing for sex differences in the organization of the IPL/TPJ, using local ICA and connectivity analyses. We are using high-resolution fMRI data acquired at a very high field strength (7T), which gives us a greater chance to discover subtle differences in IPL/TPJ topography. We are also studying sex differences in children with ADHD.