The differential effects of lateral midbrain lesions on the two phases of formalin pain

Lesions were placed in the lateral midbrain (LM) to determine the effects of disruption of paleo- and neospinothalamic systems, at the level of the midbrain, on the biphasic tonic pain response induced by formalin. During the first phase of formalin pain, subjects with LM lesions exhibited significantly less pain responding than did unoperated controls or subjects with lesions at another site, the habenula. During the second phase, there were no significant differences in pain responding among the three groups. The results suggest that the two phases are mediated by different neural substrates and specifically that Phase 2 appears to involve either neural substrates caudal to our LM lesions or sites rostral to the lesions but accessible via a more medial path.