The aircraft was in clear air, descending through FL165, at an indicated airspeed of 270 kts, with the centre autopilot engaged. As the speedbrake handle was moved from the closed position the aircraft rolled rapidly from a wings level attitude to some 20 degrees right bank. The first officer disconnected the autopilot and recovered to wings level. The autopilot was subsequently re-engaged and operated normally. An Engine Indication and Crew Alerting System (EICAS) message (Spoilers), and associated spoiler indicator light, remained illuminated for the remainder of the flight. One flight attendent was slightly injured during the incident. Flight data recorder information revealed no unusual flight control inputs associated with the event. Ground inspection revealed that a fault event had been recorded by the No.2 Spoiler Control Module (SCM), but that fault could not be duplicated during ground testing. The operator carried out extensive troubleshooting of the complete system in conjunction with the manufacturer, without detecting any faults. The SCM was replaced as a precaution and the operator plans to install upgraded modules as they become available.