Define stimuli as a dictionary
Example:
self.create_fixation(color='white')
line1 = visual.Line(self.win, name='line1')
line2 = visual.Line(self.win, fillColor='DarkRed')
self.s = {
'fix': self.fixation,
'stim1': [visual.ImageStim(self.win, name='stim1')],
'stim2': GroupStim(stimuli=[line1, line2], name='lines')
}