Start with these:
Eagleman DM, Kagan AD, Nelson SN, Sagaram D, Sarma AK (2007). A standardized test battery for the study of Synesthesia. Journal of Neuroscience Methods. 159: 139-145
Ward, J., Simner, J., Simpson, I., Rae, C., Del Rio, M., Eccles, J. A., & Racey, C. (2024). Synesthesia is linked to large and extensive differences in brain structure and function as determined by whole-brain biomarkers derived from the HCP (Human Connectome Project) cortical parcellation approach. Cerebral Cortex, 34(11)
Tomson SN, Avidan N, Lee K, Sarma AK, Tushe R, Milewicz DM, Bray M, Leal SM, Eagleman DM (2011). The genetics of colored sequence synesthesia: Suggestive evidence of linkage to 16q and genetic heterogeneity for the condition. Behavioural Brain Research. 223(2011):48-52
Tomson SN, Narayan M, Allen GI, Eagleman DM (2013). Neural networks of colored sequence synesthesia. Journal of Neuroscience. 33(35):14098-106.
Eagleman DM, Kagan AD, Nelson SN, Sagaram D, Sarma AK (2007). A standardized test battery for the study of Synesthesia. Journal of Neuroscience Methods. 159: 139-145
Ward, J., Simner, J., Simpson, I., Rae, C., Del Rio, M., Eccles, J. A., & Racey, C. (2024). Synesthesia is linked to large and extensive differences in brain structure and function as determined by whole-brain biomarkers derived from the HCP (Human Connectome Project) cortical parcellation approach. Cerebral Cortex, 34(11)
Tomson SN, Avidan N, Lee K, Sarma AK, Tushe R, Milewicz DM, Bray M, Leal SM, Eagleman DM (2011). The genetics of colored sequence synesthesia: Suggestive evidence of linkage to 16q and genetic heterogeneity for the condition. Behavioural Brain Research. 223(2011):48-52
Tomson SN, Narayan M, Allen GI, Eagleman DM (2013). Neural networks of colored sequence synesthesia. Journal of Neuroscience. 33(35):14098-106.