The sensory consequences of intentional actions (action effects) are often judged to be less intense compared to identical but externally generated stimuli. This phenomenon is normally explained in terms of predictive forward models within the sensorimotor system which partially inhibit predictable sensory feedback. An unsettled question is whether merely observing another agent performing a predictable action may also trigger a forward model with attendant sensory attenuation, or alternatively, if a self-generated motor signal is necessary. I conducted three experiments to investigate this question using a visual speed discrimination task. Participants judged which of two moving stimuli was faster. The first stimulus was initiated by the participant's own key press (Self), another person's key press (Other), or the computer program (Computer), and had a fixed speed. The second stimulus was always initiated by the computer and had a variable speed. The point of subjective equality (PSE) was compared for each condition. In Experiment 1 participants performed the task at their own pace. The Self condition was judged to be slower than the Other or Computer conditions, while the latter two did not differ. To control for the possibility that self-initiated movements were more temporally predictable and/or less attended than movements by other agents, in Experiment 2 the pace was controlled by go signals, and a green light followed every human or computer action to indicate that a movement was about to begin. Compared to Experiment 1, the PSE increased in all conditions, but the Self condition was still judged to be slowest and the Computer condition the fastest, while the Other condition was in between. In Experiment 3 the predictability of the action effects was manipulated independently from the agent who produced them, in order to investigate whether expectation similarly attenuates the intensity of Self and Computer-initiated action effects. Participants used two keys to initiate moves in two directions (left or right). In the Predictable group, the direction of the move matched the direction of the key press 80% of the time. In the Unpredictable group, the directions only matched 50% of the time. Self moves were only attenuated in the Predictable group. I conclude that sensory attenuation is influenced by a combination of private and shared or publicly available information, and that the influence of public information may be particularly tuned to biological agents. Furthermore action effects must be predictable to become attenuated.
Read
