In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Thus , phasic signaling seems to be the critical mechanism by which dopamine enhances associative learning in healthy subjects and stroke patients .
### Output Text:
Thus , phasic signaling seems to be the critical mechanism by which <span class="chemical">dopamine</span> enhances associative learning in healthy subjects and stroke patients . 