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: AIM : Experimental and clinical studies have revealed that hippocampal DBS can control epileptic activity , but the mechanism of action is obscure and optimal stimulation parameters are not clearly defined .
### Output Text:
AIM : Experimental and clinical studies have revealed that hippocampal DBS can control epileptic activity , but the mechanism of action is obscure and optimal stimulation parameters are not clearly defined . 