What is the meaning of negative second for a Morlet wavelet? I kind of familiar with wavelet transform (I just used it in code but do not have a deep understanding about it). 
I've been reading an article and came across this sentence:


  where 4Fs is the length of the
  Morlet wavelet with -2 to 2 second support with sampling frequency Fs


and I can't figure out what the -2 second means. By the way the article link is: Deep Convolutional Neural Networks for Brain Computer Interface using Motor Imagery
Continuous wavelets with symmetric envelope are often described, by convention, on a symmetric time interval: $[-T,T]$. The Gaussian being of infinite support, this means it is truncated.

This is only a matter of convention, as the wavelet shape is translated and dilated over the whole time axis. One could thus specify as well the same wavelet on the interval $[0,2T]$.