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: The objective of this study was to explore the functional anatomy of the globus pallidus internus ( GPi ) by studying the effects of unilateral pallidotomy on parkinsonian ' off ' signs and levodopa - induced dyskinesias ( LID ) .
### Output Text:
The objective of this study was to explore the functional anatomy of the globus pallidus internus ( GPi ) by studying the effects of unilateral pallidotomy on parkinsonian ' off ' signs and <span class="chemical">levodopa</span> - induced dyskinesias ( LID ) . 