Visual input often arrives in a noisy and discontinuous stream, owing to head and eye movements, occlusion, lighting changes, and many other factors. Yet the physical world is generally stable; objects and physical characteristics rarely change spontaneously. How then does the human visual system capitalize on continuity in the physical environment over time? We believe that there is a 'continuity field' in which similar objects seen within a 15-second time frame are visually merged together to smooth out what would otherwise be a jittery perception of object features. This means that the visual system may be sacrificing accuracy for the sake of stable perception using a phenomenon known as serial dependence, or sequential dependence.
From Fischer and Whitney (Nature Neuroscience, 2014). Using an orientation judgment task, we were able to find that visual perception in humans in serially, or sequentially, dependent, meaning that the visual system uses prior and present input to inform perception at the present moment. Subjects viewed a series of randomly oriented gratings, as shown above, and reported the perceived orientation of each grating using an adjustment response. We found that perceived orientation was strongly and systematically attracted toward orientations seen over the last several seconds. Moreover, this effect was modulated by attention and was spatially tuned, occurring more strongly for successive stimuli that appeared nearby in space. This perceptual serial dependence is the basis for a previously unknown spatially and temporally tuned operator - the continuity field - that could facilitate perceptual continuity of orientation information over time. Although this bias manifests as a misperception, or perceptual error, it is adaptively imitating the stability of objects in the world to create visual continuity.
From Liberman, Fischer & Whitney (Current Biology, 2014). Not only do we see perceptual serial dependence in orientation judgments, but we have also seen that the perception of face identity is systematically biased towards identities seen up to several seconds prior. In a similar experiment to the one described above, we presented a random series of faces drawn from an identity morph continuum and measured the perceived identity of each face. A random target face was presented and each subject was asked to adjust a random face on the continuum to match the target face. The sequence of the experiment is pictured here in a short video clip.
In order to determine whether serial dependence operates on the level of identity, and not just low-level features, we also manipulated the viewpoint of the sequentially presented faces. We found that identity perception is, in fact, serially dependent across different face viewpoints. These results are consistent with a continuity field and show that the perception of faces, and not just features, is serially dependent. Therefore, the continuity field is object-selective, surviving changes in viewpoint, and reflects a mechanism that produces serially dependent perception of objects for the purpose of visual stability.