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: Hypersensitivity to various auditory , tactile , and visual stimulation was present and shifts in the brainstem ambient power spectral frequency occurred in response to tactile stimulation .
### Output Text:
Hypersensitivity to various auditory , tactile , and visual stimulation was present and shifts in the brainstem ambient power spectral frequency occurred in response to tactile stimulation . 