Julien , the roots of emotions are visceral activity along with limbic and cortico-thalamic  feedback . Each self generated emotion such as worry is accompanied by cardiorespiratory pattern changes associated with subconscious  autonomic nervous system changes . These changes promote the ongoing worry . Comparing that to a transient response from looking at an angry face. Cortico-thalamic feedback from vision is more superficial and does not involve a visceral subconscious  activation. Consequently the anatomic changes and breathing pattern stays the same. 

You ask a very interesting question about the external validity of laboratory studies on emotion.    I say emotion is as emotion does:   If you are interested in sadness, then there should be some way of validating that you have indeed assessed sadness, e.g. by overt behaviors, subtle facial muscle patterning, cardiovascular responses, etc.    The response to external stimuli  also depends on the person responding.  For example, an angry face might make a person laugh, depending on the face.  (She's so cute when she's angry etc etc...).  So if you want to study anger in the laboratory, make sure you are really pissing your participants off before you go to publish!  

Thank you both. So how pertinent is my inquire into individual differences with regards to emotional stimulation (i.e. if a certain picture or stimuli is truly invoking the emotional state i desire) if how John P. Kline said: "she's so cute when she's angry" may interfere with my hypothesis testing abilities in the context of emotional induction in the experimental setting and also if i want to avoid like Ravinder Jerath described: a " superficial" representation of emotional influences at a neuronal level? 

Also important to remember levels of analysis, and not fall victim to "greedy reductionism," or the tendency to assume that all complex behavior can be explained by patterned neural activity, which always begs the question:  What's doing the patterning?  Homunculi, infinite regress, etc etc.... Such fun!  :)  

I agree to some extent that "greedy reductionism" (love the term) should be avoided. But shouldn't some level of communication be applied between each level. And also, is it not so that taking only behavioral accounts of behavioral phenomena, may give rise to a circular argument (i.e. behavior do what behavior does)?  I fear these questions will lead us astray from the validity of emotional induction in experimental settings, but it is fun.

The following recent publications would further clarify how our brain processes anxiety 

In the first article we show through color illustrations how breathing patterns change the mind body response and how we can use the voluntary changes in breathing elicit cardiorespiratory synchronization to effectively treat anxiety .

https://www.researchgate.net/publication/274965480_Self-Regulation_of_Breathing_as_a_Primary_Treatment_for_Anxiety

The second article clarifies the layers of brain and body oscillations based on evolution and neurophysiology responsible for the emotional response. In endogenous response this system kicks in . By just looking at angry face the breathing pattern does not change . 

 http://journal.frontiersin.org/article/10.3389/fnhum.2015.00248/full

Article Self-Regulation of Breathing as a Primary Treatment for Anxiety

You express doubts about our ability to induce emotions experimentally, but this is something that has often been done in experiments of affect, usually by films constructed specifically to evoke particular moods or emotional responses. Here is one of such papers studying experimentally elicited moods, but a search should yield many more.

Stanley, D. J., & Meyer, J. P. (2009). Two-dimensional affective space: a new approach to orienting the axes. Emotion, 9(2), 214-237.