Prothalamic, derived from the term "thalamic," means relating to, located in or resembling the thalamus, a portion of the brain. As this term is quite specific, antonyms can be a bit tricky to come by. However, some possible antonyms might include words like "extrathalamic", meaning outside or beyond the thalamus, "cortical," relating to or located in the cerebral cortex, which is the outermost layer of the brain, or "peripheral," referring to parts of the nervous system that are located outside the brain and spinal cord. Other synonyms for "prothalamic" might include "diencephalic," "intrathalamic," or "thalamic-related.