"Background activity" is usually a "frozen phrase." There are dozens of Internet references to this term in connection with EEG (see a few below). The Spanish is a poor translation from English, because "arreactivo" should agree with "actividad." (This is only slightly different from Henry's suggestion, but it's the way it is really said.)

... to regional primary (eg, contusions) or secondary (eg, ischemia) lesions; (c)
analysis of background activity and its reactivity to stimuli, relevant for ...

www.springerlink.com/index/ATG741BCBNT58R6C.pdf
There were no epileptiform discharges. There was no response of the
background activity to noxious stimuli or photic stimulation. ...
www.neurologyindia.com/article.asp?issn=0028-3886;
year=2003;volume=51;issue=2;spage=266;epage=268;aulast

... narrow, slightly irregular, and nonreactive pupils ... Painful stimuli produced purposeful,
defensive reactions ... showed a delta background activity with superimposed ...
www.neurology.org/cgi/content/full/56/8/1080