You use paired commas to set off a parenthetical.

> Findings show that there is increased blood flow to the brain.

If you want to insert "during the initial stages of love" as a parenthetical, you set it off with commas, hence:

> Findings show that<b>,</b> during the initial stages of love<b>,</b> there is increased blood flow to the brain.

This has nothing to do with the fact that the parenthetical happens to be preceded by the word "that". For example:

> Findings show, during the initial stages of love, there is increased blood flow to the brain.
>
> Findings show, during the initial stages of love, that there is increased blood flow to the brain.

Notice the commas are still there, still setting off the parenthetical. These are a bit awkward, but still perfectly legal grammatically.