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: Electro - oculography , electroretinography , visual evoked potentials , and multifocal electroretinography in patients with vigabatrin - attributed visual field constriction .
### Output Text:
Electro - oculography , electroretinography , visual evoked potentials , and multifocal electroretinography in patients with <span class="chemical">vigabatrin</span> - attributed visual field constriction . 