Here's a graph of the fMRI time series from one region of interest in one subject.
I won't go into the mathematical details unless you want me to, but in general, we can use these data to quantify how much a region's activity changes between periods of leftward attention and periods of rightward attention.
What we end up with is a number that's negative if the region is more active during left attention,
and positive if the region is more active during right attention.

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`Physiological Studies of Attention in Autism: Implications for Autistic Cognition and Behaviour', Matthew Belmonte, 26 January 2002