Response Time

The time elapsed between the presentation of a stimulus and the user's physical response, typically measured in milliseconds. In scanning-based AAC, response time (often called Tact) is the interval between when an item is highlighted and when the user activates the switch. Response time comprises perceptual processing (recognizing the target), cognitive processing (deciding to act), and motor execution (pressing the switch). Understanding response time distributions helps clinicians set appropriate scanning delays and enables adaptive systems to detect fatigue, anticipation errors, or mismatched scanning speeds.