While a single statistical map is already informative, often it is necessary to combine the information of two (or several) different analyses together (e.g. by applying a mask or restricting the statistic by the minimum value across two or more maps) to come to more reliable evidence for an effect.
This wiki page is meant as a quick guideline for how to use NeuroElf features when different maps are to be combined.
To follow the steps presented on this page, you should
Also, you should know about what kind of map combination and/or computation you would like to perform so that you can actually make use of NeuroElf's features.
Given that NeuroElf offers both scripting and GUI-based, both of these options will be described below.
For now, three major types of map combinations are discussed on this page:
A conjunction (as described above) is defined by setting the combined value to the larger of the two available significance estimates (alpha, p), which corresponds to the smaller of the two statistical values, given that the two particles follow the same distribution using the same degrees of freedom parameter(s).
This functionality is implemented in the conjval and conjvalp functions, and can be used either in a scripted fashion (manually or by calling the VMP::ComputeFormula method) or via the GUI formula button. A possible script excerpt would look like this:
% we assume that a VMP with 2 maps is loaded and that % - the two maps use the same type of statistic with % - the same D.F. parameter (no need to convert to p-values) % first we create a third map within the VMP container vmp.Map(3) = vmp.Map(1); % then we can rename this map vmp.Map(3).Name = [vmp.Map(1).Name ' (conj) ' vmp.Map(2).Name]; % and then we apply the conjunction formula vmp.Map(3).VMPData = conjval(vmp.Map(1).VMPData, vmp.Map(2).VMPData); % in case the VMP is also loaded (or to be loaded) in the GUI vmp.Browse; neuroelf_gui('setcstatmap', 3);
Alternatively the ComputeFormula method of the VMP object can be used instead:
% using the same assumptions as above vmp.ComputeFormula('conjval(#1, #2)');
This call will automatically add a map at the end and use the first map in the formula as a template. And this behavior is also mimicked in the Version of the GUI. Upon clicking the
compute-formula (slanted f) button, a dialog appears allowing to enter: