The extrapolation of occluded motion: basic mechanism and application

Predicting the future states of moving objects that are hidden by an occluder for a brief period is of paramount importance to our ability to interact within a dynamic environment. This phenomenon is known as motion extrapolation (ME). Numerous gaps in the literature can be found disregarding the mechanisms involved in ME of which the current thesis attempts to address. Behavioural experiments usually utilize a prediction-of-motion paradigm, which requires participants to make a direct estimation of the time-to-contact (TTC). In this task, the initial trajectory of a target stimulus is presented, which then becomes occluded, observers are then asked to respond when they believe the target has reached a marked point behind that occluder without it ever actually reappearing (Tresilian, 1999; Rosenbaum, 1972). Alternatively, other experiments have adopted a timing discrimination task in which participants are required to indicate whether a moving target, following occlusion, reappears ‘early’ or ‘late’ (Makin, Poliakoff & El-Deredy, 2009; Makin, Poliakoff, Ackerley & El-Deredy, 2012). Experiments In the first part of this thesis, I investigated whether the visual memory system is active during the extrapolation of occluded motion and whether it reflects speed misperception due to the well-known illusion such as the apparent slower speed of low contrast object or large size object (Thompson 1982; Epstein 1978). Results revealed that with a TTC task observers estimate longer time to contact with low contrast and large stimuli compared to high contrast and small stimuli respectively. Note that the stimuli in both conditions are moving at equal speed. Therefore, the illusion of the apparent slower speed with low contrast and large stimuli remains in the visual memory system and influences motion extrapolation. Chapter III aims to investigate the interaction between real motion and motion extrapolation. Gilden and colleagues (1995) showed that motion adaptation affects TTC judgment showing that real motion detectors are somehow also involved during ME. A step further that I made was to investigate the effect of brief motion priming and adaptation, occurring at the earliest levels of the cortical visual streams, on time-to-contact (TTC) estimation of a target passing behind an occluder. By using different exposure times of directional motion presented in the occluder area prior to the target’s disappearance behind it, my aim was to modulate (prime or adapt) extrapolated motion of the invisible target, thus producing different TTC estimates. Results showed that longer (yet sub-second) exposures to motion in the same direction of the target produced late TTC estimates, whereas shorter exposures produced shorter TTC estimates, indicating that rapid forms of motion adaptation and motion priming affect extrapolated motion. My findings suggest that motion extrapolation might occur at the earliest levels of cortical processing of motion, where these rapid mechanisms of priming and adaptation take place. In Chapter IV of my thesis, I explore not only the visual factors of motion extrapolation, but also the timing mechanisms involved and their electrophysiological correlates. The first question is whether the temporal processing is required for accurate ME, and whether this is indexed by neural activity of the Contingent Negative Variation (CNV). A second question is, whether there is a specific electrophysiological correlates that highlight the shifting from real motion perception to motion extrapolation. In this electroencephalographic experiment, participants were adapted with a moving texture (Gilden et al., 1995). The adaptation with the moving texture could bias and modify temporal processing. Participants made a direct estimation of Time to Contact, which showed that classic adaptations were able to bias temporal judgments and modulate the amplitude of the CNV, suggesting a complex feedforward-feedback network between low- and high level cortical mechanisms. Finally, a negative defection (N190) was found, for the first time, as a neurophysiological correlate in the temporal-occipital electrodes in the right and left hemisphere for the rightwards and leftwards ME respectively, indicating the involvement of motion mechanisms of intermediate cortical level in ME. Chapter V aims to show at distinguishing between extrapolation, and interpolation of occluded motion. Extrapolation is the ability to extract the trajectory, speed and direction of a moving target that becomes hidden by an occluder, thanks to the information extracted from the visible trajectory. Interpolation is a similar phenomenon, i.e. from the visible trajectory one can extract speed and direction as in Extrapolation. The main difference is that for interpolate visible cue are needed along the invisible trajectory. If the occluder is invisible and the occluded trajectory is symmetrical respect to a visible cue, one can connect these cues (spatial points) in order to form a spatio-temporal map and infer where and when the target will reappear. This is not possible in absence of visible cues such as in extrapolation condition. In a new task, observers were required to press a button as fast as possible (reaction time) when they saw a moving target reappearing from an invisible occluder. Results showed that observers could even anticipate the reappearance of an object moving behind the occluder. However, only in some circumstances: i) when the occluder was not positioned over the blind spot but in retinal areas that project to the visual cortex; ii) with an entirely invisible occluder the visible motion before occlusion had to be presented and iii) visual-spatial cues had to signal the center of the invisible trajectory. When these conditions are given, observers can use the spatial information given by the point of disappearance, the visible cue that represented the center of the invisible trajectory, then infer the point of reappearance by symmetry. Therefore having a set of discrete spatial positions (and its cortical representation) in which the moving occluded target will be in a certain moment of time, it is convenient to interpolate this point in order to create a spatio-temporal map to infer where and when the object will be (saliency map). I consider this process of motion interpolation as an amodal filling-in process. The last part of my thesis involved a practical application of ME. Participants cannot interpolate when the moving target passes in a zone over retinal areas that do not project to the visual cortex (blind spot). In this case, observers perform a true reaction time and do not anticipate the response. Patients with Macular Degeneration cannot see with their fovea since it is damaged. Therefore, that part of the retina does not project to the visual cortex anymore. In a task in which they have to press a response button when a moving target disappear into or reappear from their scotoma, we predict that they cannot anticipate the response to the reappearance of the target. Five patients with macular degeneration were therefore instructed to press a button when they see a moving target disappear into and reappear from their scotoma. Patients repeated this task several times with different linear trajectories of the target. Connecting the point in space in which a patient presses the button, it was possible to draw the shape and the size of the scotoma with a software. The size of the scomota found with this experiment was compared with that measured with a Nidek MP-1. A linear correlation of R2 about of 0.8 was found between the Nidek MP-1 and scotoma measured connecting the point in which patients reported to see the target reappear from their scotoma. Therefore, this software which was written by me (considering its limits) may become a useful tool to obtain a reliable perimetry in a given situation in which an expensive machine such as the MP-1 is not available.