Gibson (1954; 1979) suggested that psychophysical performance in a laboratory setting should be more representative of perception in the real world when observers are tested with more realistic stimuli. The downward bias in acceleration detection suggests that the ecological relevance of direction influences sensitivity. In a similar vein, outside the laboratory we generally see complex optic flow more often than pure translation (although we do still see lateral motion in the natural environment). Radial optic flow tends to occur whenever we move through the environment while looking straight ahead, although it is less commonly experienced than combinations of radial, translational, and rotational optic flow, which arise from changes in self-motion and object trajectory.
Nevertheless, for the purpose of Experiment 5, we used the simplest form of optic flow that human observers can experience outside the laboratory so that we could control the dot parameters and make direct comparisons of psychophysical performance between motion pattern types. Experiment 5 revealed that sensitivity to the presence of
acceleration is higher for radial motion than for horizontal motion, and there is no
difference between acceleration and deceleration detection overall. This suggests that the type of motion pattern viewed affects how well we can detect the presence of
acceleration. Given that the rate of radial optic flow can help to inform the observer about his or her rate of movement through the environment (Prokop, Schubert, & Berger, 1997), having a higher sensitivity to radial acceleration may have important implications for navigating and interacting with objects.
The functional hierarchy of how motion is processed in the visual system may also help to explain the radial bias in acceleration perception. As discussed in Chapter 1, simpler aspects of motion such as local components (e.g., temporal and spatial frequencies) tend to be processed relatively earlier in the visual pathway, for example in areas as early as V1 (Hubel & Wiesel, 1968; Singh et al., 2000), than more complex features, such as form, depth, and heading (Andersen, 1997; Andersen et al., 1990; Maunsell & Van Essen, 1983; Van Essen & Gallant, 1994). In contrast, the coding of complex motion patterns occurs later on, in areas such as MST, by neurons that have larger receptive fields tuned to specific patterns, such as radial, translational, and rotational motion (Duffy & Wurtz, 1991a; Duffy & Wurtz, 1991b; Tanaka, Fukada, & Saito, 1989; Tanaka & Saito, 1989). Moreover, higher order areas, such as MST and VIP, are involved in processing heading information in optic flow (Bremmer, Duhamel, Ben Hamed, & Graf, 2002; Britten & van Wezel, 2002; Duffy & Wurtz, 1995; Zhang & Britten, 2011), and MST has been reported to show greater activation in response to radial optic flow than to translation (Smith et al., 2006). Therefore, it is possible that the self-motion cues in radial motion may induce the recruitment of those higher-order areas and, as a result, this greater processing power may be responsible for the difference in acceleration detection between horizontal and radial motion.
An interesting result arose when we compared the data from Experiments 5 and 6. In Experiment 5 participants were free to move their eyes around the visual field once the random dot stimuli were presented. In comparison, in Experiment 6 they were required to fixate the centre of the screen at all times. Despite differences in task instructions, dot parameters, and aperture size (the aperture size in Experiment 5 was more than twice as large as the two used in Experiment 6), acceleration detection thresholds are very similar between Experiments 5 and 6 (on average, 29 % and 28 %, respectively). It is unclear why performance is so consistent between these different experimental conditions. One might wonder whether observers attended more to the centre of the visual field than the periphery in Experiment 5, which would have resulted in limited tracking. Moreover, one might argue that it is more difficult to track radial motion than horizontal motion because dots move in all cardinal and oblique directions, which may also encourage observers to attend to certain areas, such as the centre, instead of the whole field. However,
Experiment 6 revealed that there is no difference in acceleration sensitivity between the central and peripheral areas of optic flow. In addition, observers tend to make vergence eye movements (Busettini, Masson, & Miles, 1997), as well as optokinetic nystagmus and smooth pursuit (Niemann, Lappe, Büscher, & Hoffmann, 1999) in response to radial motion. Moreover, Niemann et al. found that observers can track individual dots in radial optic flow well when instructed to (OKN tends to be elicited when observers are
instructed to attend to the whole field). Furthermore, a lack of tracking does not seem consistent with our findings that smooth pursuit improves motion sensitivity
(Experiments 1 and 3), given that we demonstrated acceleration sensitivity is much higher for radial motion than horizontal motion in Experiment 5.
Another possibility is that participants may have not followed the instructions to maintain fixation in the centre of the display in Experiment 6, especially because we did not use a fixation cross during the random dot stimuli presentations in order to avoid relative motion cues. Consequently, as we did not record eye movements to ensure that participants fixated the centre of the screen, one might argue that the similarity in performance could actually be the result of tracking. However, this is unlikely because participants are generally able to maintain fixation when instructed to do so, especially if they are experienced observers (Braun et al., 2008), which most of the participants in
Experiment 6 were. Moreover, several participants reported that the task was difficult to do because of the instruction to maintain fixation, which indicates that they were
adhering to the task instructions in Experiment 6. Therefore, instead, it is possible that radial acceleration perception is relatively insensitive to eye movements (and possibly aperture size) in general.