\subtitle doesn't work in article document class

If you used scrartcl instead of article, the command \subtitle would immediately work as expected. scrartcl belongs to the KOMA-Script classes which are extended replacements of the LaTeX standard classes. I strongly recommend KOMA-Script. For reasons have a look at:

• Most useful additions in KOMA-Script

Your document is considerably large. An article-like class may be ok (scrartcl), but if you need a chapter-like sectioning you could consider to use report (or scrreprt, which is a KOMA class).

You might be interested in the answers to this question:

• What are your favorite document classes and what do you use them for?
• What are the available “documentclass” types and their uses?

To supplement Stefan's answer, the core LaTeX article class does not have a \subtitle macro. You can therefore either do something like

\title{Title \\ Subtitle}

or consider the titlepage environment. Other document classes, notably KOMA-Script, do have a separate \subtitle macro available.

A fully adapted possibility for the subtitle generation in the document class article includes reducing the fontsize in the subtitle, as @Seamus mentioned above, but relatively (!), and increasing the distance between title and subtitle. Therefore my suggestion is following:

\documentclass{article}
\usepackage{relsize}

\title{Experimental Test\\[0.2em]\smaller{}Stimuli generation}

\begin{document}
    \maketitle
    My document
\end{document}

If you want a "fully" adaptable title: relative size and line separation can be adapted

\documentclass{article}
\usepackage{relsize}


\newcommand{\subtitlerelsize}{1} %relative size: integer value
\newcommand{\subtitlelinesep}{0.2em} %line separation: a LaTeX length

\title{Experimental Test\\[\subtitlelinesep]%
    \smaller[\subtitlerelsize]{}Stimuli generation}

\begin{document}
    \maketitle
    My document
\end{document}