What does "they tend within and between classes" mean in 'The Irreducible Mind'? I am a philosophy student but I'm also interested in philosophy of mind, a field that overlaps issues in psychology and neuroscience these days. This passage is taken from a similar multidisciplinary book but this particular passage is mentioning various neuroimaging techniques. I have difficulty understanding what is meant by the embolden phrase. I am not sure whether &quot;within and between&quot; has a technical or simply a semantic/grammatical significance. In either case I remain confused. And hence I was not sure which site if at all is appropriate for asking clarification.
Within page 17 of the book Irreducible Mind: Toward a Psychology for the 21st Century by Kelly, et al. (2007), it says...

The third development was the advent and maturation of the functional
neuroimaging technologies mentioned in the Introduction, which enable
us to observe directly, without opening up the skull , the activity of
the working human brain. Two principal classes of such methods are
currently available, although others are under development.12 The
first provides measures of the electric and magnetic fields directly
generated by neuroelectric activity in populations of suitably located
and oriented cortical neurons (electroencephalography [EEG]; magnetoencephalography [MEG]).
The second provides measures of hemodynamic or metabolic consequences
of neural activity such as local changes of blood flow, glucose
consumption, or blood oxygenation (especially positron emission
tomography [PET]; and functional magnetic resonance imaging [fMRI]).
All these technologies are complex and expensive, and they tend within
and between classes to have complementary strengths and weaknesses
(see, e.g., Nunez &amp; Silberstein, 2000; Toga &amp; Mazziotta, 1996; Wikswo,
Gevins, &amp; Williamson, 1993). Together, they undoubtedly constitute a
major methodological advance for cognitive neuroscience. Indeed,
scarcely an issue now goes by of any cognitive neuroscience journal
that does not contain one or more papers featuring images outfitted
with colored spots identifying regions of &quot;significant&quot; brain
&quot;activation&quot; produced by some stimulus or task.13

I first posted the passage to esl and english.se but the answers I got were not definitive.
An answerer on esl.se told me it may have to do with within-subject and between-subject experiment methods or convey a similar meaning.
On english.se I was told it could simply be the common phrasal verb &quot;tend to&quot; with &quot;within and between&quot; breaking it up as a parenthetical clause with commas missed. That is it can be rewritten as &quot;they tend, within and between classes, to have...&quot;
In either case I don't know what &quot;classes&quot; the author is talking about. Are these classes of the imaging devices he enumerates? This latter question could be quite technical. Still excuse me if this was not the right question for this site.
References
Kelly, E. F., Kelly, E. W., Crabtree, A., Gauld, A., &amp; Grosso, M. (2007). Irreducible mind: Toward a psychology for the 21st century. Plymouth, UK: Rowman &amp; Littlefield.
This is kinda unclear so I'm not sure my reading is the correct one. But for me the two classes are the electric/magnetic class of imaging techniques and the hemodynamic/metabolic class of imaging techniques, and each class has multiple methods within it. It's a world that looks like this: {[EEG,MEG],[PET,fMRI]}. Within a single class, the multiple methods have complementary strengths and weaknesses, at the same time, the two classes have complementary strengths and weaknesses relative to each other. Within a class, you can be looking at the same type of information but have different trade-offs in terms of temporal and spatial resolution, or image quality and expense, depending on the method. Between classes, you're looking at fundamentally different types of information, so you get a different (complementary) picture of the brain's activity.
Squashing both of those contrasts down into one sentence, you have "they tend within and between classes to have complementary strengths and weaknesses"

Probably.