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: We have studied the regional cerebral blood flow ( rCBF ) changes induced by the execution of a finger - to - thumb opposition motor task in the supplementary and primary motor cortex of two groups of parkinsonian patients on L - dopa medication , the first one without L - dopa induced dyskinesia ( n = 23 ) and the other with moderate peak - dose dyskinesia ( n = 15 ) , and of a group of 14 normal subjects .
### Output Text:
We have studied the regional cerebral blood flow ( rCBF ) changes induced by the execution of a finger - to - thumb opposition motor task in the supplementary and primary motor cortex of two groups of parkinsonian patients on <span class="chemical">L - dopa</span> medication , the first one without <span class="chemical">L - dopa</span> induced dyskinesia ( n = 23 ) and the other with moderate peak - dose dyskinesia ( n = 15 ) , and of a group of 14 normal subjects . 