oh, foreigners can 'get' them alright...but i think they will actively use them slightly differently...BECAUSE different sounds have different connotations in different cultures, and while it is perfectly possible to understand how others use them...when you expres yourself, that is such a mixed bag of just...stuff...lots of emotions and connotations and things you're not really aware of, but it's always there, resonating along the edges of the the 'official'/'visible' meaning...

my advice would be to exclude non-native speakers, it is bound to distort the result. and even if it doesn't...which would have to be proven in another study...you leave yourself wide open to critique if you do not take the native/non-native speaker thing into account...someone is bound to attack you for it. just spare yourself the hassle...