One can also define an intersubject functional connectivity (ISFC), which measures the response of functional connectivity shared across subjects. The ISFC is estimated by calculating the Pearson correlation coefficient between a region's whole-brain functional connectivity in a single subject and the average whole-brain functional connectivity of that region across all remaining subjects.
Similarly, constraints on the functional connectivity between two brain regions for a given subject can be inferred from ISFC.
Generally, intersubject analyses quantify the extent to which a given functional measurement (brain activity or connectivity) in one subject statistically differs from the expected distribution of the measurement in other subjects. To infer generalized constraints on the activity of a single brain region, we compute the ISC – a measure of reliability of the stimulus-driven response across subjects without any a priori knowledge of the temporal composition of that response \cite{hasson2004intersubject}.
Techniques to assess intersubject correspondence in brain activity and connectivity offer a powerful means to discriminate between brain regions that adhere to generalized constraints of functional architecture and brain regions that break such constraints in order to contribute to subject-specific behaviors.
Together, measures of ISC and ISFC can be used to tease apart and interpret relationships between activity and connectivity that accompany the evolution of cognitive processes over time, such as those supporting human learning. We use the ISC and ISFC to assess the degree to which each region's activity and connectivity are constrained across subjects during the course of learning, and we operationalize our study by testing 3 specific hypotheses. First, we hypothesized that subject-general constraints on activity and connectivity would be predominantly located in 3 general areas: (i) motor cortex, consistent with the shared demands of finger movements necessary to press buttons on the response box \cite{bassett2011dynamic,mattar2017network}, (ii) visual cortex, consistent with the shared demands of cognitive processing necessary to parse the visual stimuli of the novel objects, and (iii) other areas previously associated with the learning of value, including lateral occipital cortex \cite{persichetti2015value}. Second, we hypothesized that the coherence of stimulus-induced activity would place constraints on brain activity and connectivity during early learning, but that subject-specific activity and connectivity patterns would dominate later learning when greater neural real estate was available for contemporaneous, non-task-related processing as well as processing reflecting subject-specific learning strategies. Third and finally, we hypothesized that the extent to which a subject's brain activity or connectivity obeys subject-general constraints would be related to learning performance. Efforts to address these hypotheses could serve to more fundamentally elucidate dynamic constraints on task-dependent activity and functional connectivity during learning.