As Bryan Krause indicates in the comments, the statement that 'One third of the cortical surface is dedicated to vision processing' is disputable. Especially given that fact that there's evidence that even the primary visual cortex is involved in multimodal processing (MacPherson, 2018). Nonetheless, it is a valuable statement as it makes clear that a lot of our cortex can process visual stimuli and that it is an important sense for us.
If you need a reference for the one third statement, you can use Kupers & Ptito (2014) or (MacPherson, 2018). Note that these references are secondary literature, i.e., they are not research papers where they actually calculated the cortical surface involved in visual processing.
Fig. 1. Functional areas of the cortex. Source: Boundless Physiology Textbook
- MacPherson, Sensory Substitution and Augmentation, Proceedings of the British Academy 2018
- Kupers & Ptito, Neurosci Biobehav Rev (2014); 41: 36–52