observers should perceive the speed elicited by such stimuli as being approximately the same regardless of the projected distance (Figure 9). The reason for the different outcome generated by a Bayesian framework compared to the predictions made by empirical ranking is based in how each approach conceptualizes the goal of vision [35]. A Bayesian framework assumes that a motion percept is determined by the most likely 3-D speed that generated the stimulus, the implied goal being to link percepts with the specific physical characteristics of sources in the world underlying a stimulus sequence. As indicated in Figure 1, however, the inverse optics problem precludes direct access to the properties of the physical world, making the expressed goal of a Bayesian framework impossible to achieve as formulated. In contrast, the method of empirical ranking predicts motion percepts based on the full range of past experience rather than a particular state of the world, and these conform closely to the observed psychophys- ical functions. Further implications arising from an empirical strategy of motion perception If the visual system has indeed evolved to link projected images with objects according to accumulated behavioral feedback, then any attempt to understand vision in terms of the properties of images alone should fail. As illustrated in Figure 1, the inverse problem implies that to be successful any account of visual perception must be based on the empirical relationships between images and their possible sources. The evidence here suggests that this information accumulates in visual system circuitry over Perceiving Motion Empirically evolutionary and individual time, giving rise to perceptions that represent the movements of objects in terms of their biological utility rather than the speeds and trajectories the objects actually have. Considered in this way, the discrepancies between the measured properties of a retinal stimulus sequence and perceived motion are due to the visual processing strategy that evolved to contend with the inverse problem. Some recent neurobiological evidence consistent with this interpretation of vision comes from optical imaging of striate and extrastriate visual cortex. Thus, it has been shown that stimuli moving in different directions and at different speeds can elicit the same pattern of neuronal activity [34]. At the same time, there continues to be much debate over the hierarchical concept of visual cortical organization generally and the proper interpretation of striate and extrastriate processing in particular [36–38], These and other observations [39] are all consistent with an empirical strategy of sensory processing. Supporting Information Supporting Information S1 Found at: doi:10.1371/journal.pone.0006771.s001 (0.03 DOC) MB Figure S1 Figure S1. Comparison of the functions predicted by empirical ranking with the results of psychophysical testing for reference stimuli of 4u and 6u. The presentation is similar to the illustration of the 2u results in Figure 6. (A) The cumulative distribution data from Figure 5 re-plotted to indicate the predicted motion percepts for a 4u reference stimulus as a function of image Figure 9. Bayesian predictions of the psychophysical results elicited by image sequences traversing different projected distances at the same speed. To predict the relevant percepts, the mean of each posterior in Figure 8 was calculated (indicated by the corresponding colored points). In contrast to the observed psychophysical results, a Bayesian model predicts little or no change of perceived motion in response to a given image speed (2.6u/s, 3.9u/s, 6.5u/s, 7.8u/s, or 10.4u/s) traversing different projected distances (1u–7u) on the image plane (cf. Figure 7B; see also Figure 6C). doi:10.1371/journal.pone.0006771.g009 PLoS ONE | www.plosone.org 10 August 2009 | Volume 4 | Issue 8 | e6771