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: Using functional magnetic resonance imaging ( fMRI ) in normal volunteers , we studied the gabapentin - induced modulation of brain activity in response to nociceptive mechanical stimulation of normal skin and capsaicin - induced secondary hyperalgesia .
### Output Text:
Using functional magnetic resonance imaging ( fMRI ) in normal volunteers , we studied the <span class="chemical">gabapentin</span> - induced modulation of brain activity in response to nociceptive mechanical stimulation of normal skin and <span class="chemical">capsaicin</span> - induced secondary hyperalgesia . 