+0
It seems that "it" here refers to "it is hard for patients with this (depression) symptom to imagine the optimistic results of an action." But it is not the equivalent of "it."

Context:

Clinicians refer to this slowing down of motivation in depressed patients as "psychomotor retardation." According to Deisseroth, who is also a practicing psychiatrist, patients may experience this symptom mentally, finding it hard to envision the positive results of an action, or, he said, they may feel physically heavy, like their limbs just do not want to move.
"This is one of the most debilitating aspects of depression, and motivation to take action is something that we can model in animals. That's the exciting opportunity for us as researchers," said Deisseroth, who also holds the D.H. Chen Professorship.
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Hi,

. . . patients may experience this symptom mentally, finding it hard to envision the positive results of an action . . .

It refers to to envision the positive results of an action.

The sentence could be rewritten as
. . . patients may experience this symptom mentally, finding to envision the positive results of an action hard to envision . . .

Here's a simpler example.
It is easy to make a mistake.
To make a mistake is easy.

For more information, read here about cleft sentences.

http://www.sil.org/linguistics/GlossaryOfLinguisticTerms/WhatIsACleftSentence.htm
http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv149.shtml

Clive
Comments  
It has no antecedent; it is merely a dummy.
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 Clive's reply was promoted to an answer.
They’re just placeholders, Clive. They don’t refer to anything, as anaphoric pronouns do.

?To make a mistake is easy to make a mistake. Emotion: tongue tied
Hi,

. . . patients may experience this symptom mentally, finding it hard to envision the positive results of an action . . .

It refers to to envision the positive results of an action.

The sentence could be rewritten as
. . . patients may experience this symptom mentally, finding to envision the positive results of an action hard . . .. . .

Here's a simpler example.
It is easy to make a mistake.
To make a mistake is easy.

For more information, read here about cleft sentences.

http://www.sil.org/linguistics/GlossaryOfLinguisticTerms/WhatIsACleftSentence.htm
http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv149.shtml

Clive

PS I reworded the red part to eliminate the repetition of 'to envision'. Sorry. careless mistake!
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