The 'thing' in question looked like half a slave collar, but instead of going around the neck, it seemed to be fused along the nape.

A haunted look appeared in Chopper's eyes. "They called it a puppet-controller implant. Th-they put on me... t-to test... it-it makes me do whatever they tell me to." The traumatized reindeer started shaking as the memories of his time as a test subject resurfaced.

The action seemed to snap Chopper back into his 'Doctor' mode. At least, he stopped falling apart and bundled his patient back into bed instead. "What are you doing getting out of bed? Don't push yourself! Whatever you did to your own heart earlier saved your life, but there was already irreversible damage!"