Abstract
The ability to derive the facing direction of a spatially scrambled point-light walker relies on the motions of the feet and is impaired if they are inverted. We exploited this local inversion effect in three experiments that employed novel stimuli derived from only fragments of full foot trajectories. In Experiment 1, observers were presented with stimuli derived from a single fragment or a pair of counterphase fragments of the foot trajectory of a human walker in a direction discrimination task. We show that direction can be retrieved for displays as short as 100 ms and is retrieved in an orientation-dependent manner only for stimuli derived from the paired fragments. In Experiment 2, we investigated direction retrieval from stimuli derived from paired fragments of other foot motions. We show that the inversion effect is correlated with the difference in vertical acceleration between the constituent fragments of each stimulus. In Experiment 3, we compared direction retrieval from the veridical human walker stimuli with stimuli that were identical but had accelerations removed. We show that the inversion effect disappears for the stimuli containing no accelerations. The results suggest that the local inversion effect is carried by accelerations contained in the foot motions.
The stimuli were viewed binocularly at a distance of 80 cm as maintained by a chin-rest. A two-alternative, forced-choice direction discrimination paradigm was used where the observers' task was to decide the direction (left or right) in which the perceived entity (whatever it may be) seemed to face. Feedback was not given for observer responses.
For both groups, participants were first instructed on the task both verbally and by printed instructions on the computer screen. A practice block was then presented during which participants familiarized themselves with the task. After the practice block, participants completed the experiment proper.
For Group 1, the practice block comprised of 40 trials that consisted of left and right, and upright and inverted versions of the 10 gait cycle fragments of 200 ms length. The experiment proper consisted of 960 trials in total that were completed across two identical experimental blocks of 480 trials each. Each experimental block consisted of all possible combinations of the two directions, two orientations, 10 fragments, and three window lengths, repeated four times. Within each repetition, the 120 possible stimuli were presented in random order.
For Group 2, the practice block comprised of 20 trials that consisted of left and right, and upright and inverted versions of the five fragment pairs of 200 ms length. A total of 960 trials were completed for the experiment proper across two identical experimental blocks of 480 trials each. A single experimental block consisted of all possible combinations of the two directions, two orientations, five fragment pairs, and three window lengths, repeated eight times. Within each repetition, the 60 possible stimuli were presented in random order.
The resulting design of this experiment for both groups was a within-subject test of factors orientation (i.e., upright or inverted), fragment (Group 1) or fragment pair (Group 2), and fragment length.
Results obtained from Group 1 are illustrated in
Figure 2A. Overall performance was at chance level (0.45 proportion of responses consistent with the facing direction of the walker). It is evident that observers simply responded to the horizontal translatory displacement of the individual dots. For fragments 1–4 where dots moved in the direction in which the walker was originally facing, rates of responses that were consistent with the facing direction of the walker were high. For fragments 6–9 where dots moved in the opposite direction, observers consistently reported the opposing direction. For fragments 5 and 10 where the horizontal direction of the dot switches, responses depended on display length in a manner consistent with the assumption that observers based their responses on mere horizontal translation. Additionally, the data from this group show no inversion effect.
Statistics support the above observations. A 2 (test block) × 2 (orientation) × 10 (fragment) × 3 (length) repeated-measures analysis of variance (ANOVA) showed a significant main effect of fragment, F(9, 45) = 879.73, p < 0.001, a significant fragment × length interaction, F(18, 90) = 19.26, p = 0.007, but no effect of orientation, F(1, 5) = 0.33, p = 0.589, or length, F(2, 10) = 1.95, p = 0.222. All other interactions were not significant and performance between the two test blocks did not differ, F(1, 5) = 0.64, p = 0.460. Thus, data from the two blocks were pooled for all further analyses.
Tukey's post-hoc comparisons of the response rates of the 10 fragments, collapsed across orientation and length, indicated that the rates for fragments 2–4 did not differ ( p > 0.900 for all comparisons), but were higher than rates for all other fragments ( p < 0.020 for all). The rates of fragments 6–9 did not differ ( p > 0.900 for all), but were substantially lower than the rates of fragments 1, 5, and 10 ( p < 0.001 for all), which differed from each other ( p < 0.001 for all).
The fragment × length interaction was investigated with individual one-way ANOVAs that compared the three lengths for each fragment. The analyses yielded a significant main effect of length for fragments 5, F(2, 10) = 38.25, p < 0.001, and 10, F(2, 10) = 25.68, p < 0.001, only. Post-hoc Tukey comparisons of the three lengths for fragment 5 showed that the rate at 100 ms was significantly higher than rates at 150 ms and 200 ms ( p < 0.001 for both), and the rate at 150 ms was significantly higher than the rate at 200 ms ( p < 0.001). Comparisons for fragment 10 showed that while rates at 100 ms and 150 ms did not differ ( p = 0.574), they were significantly lower than the rate at 200 ms ( p < 0.001 for both).
The results for Group 2 are displayed in
Figure 2B. Overall performances were above chance level (0.59 proportion of responses consistent with facing-direction of the walker) with rates generally higher for the upright (0.64 proportion of responses) than for the inverted displays (0.53 proportion of responses). Closer inspection reveals that at both orientations, observers perceived the direction consistent with the facing direction of the walker for pairs 2/7, 3/8, and 4/9 (0.57–0.90 proportion of responses) with rates increasing systematically with increasing window length, but perceived the opposite direction for pair 5/10 (0.15–0.36 proportion of responses) with the rate of responses in the opposite direction increasing systematically with increasing window length. For pair 1/6, the perceived direction changed with changes in window length (0.25–0.70 proportion of responses).
Response rates for Group 2 were analyzed with a 2 (test block) × 2 (orientation) × 5 (fragment pair) × 3 (length) repeated-measures ANOVA. The analysis showed significant main effects of orientation, F(1, 11) = 361.98, p < 0.001, fragment pair, F(4, 44) = 158.00, p < 0.001, and length, F(2, 22) = 62.89, p < 0.001, a significant orientation × length interaction, F(2, 22) = 16.10, p < 0.001, and a significant fragment pair × length interaction, F(8, 88) = 11.98, p < 0.001. Performances between the two test blocks did not differ, F(1, 11) = 0.26, p = 0.621 and were thus pooled. A t-test using the pooled response rates confirmed that overall rates of responses consistent with the facing direction of the walker were higher than chance-level, t(11) = 7.712, p < 0.001.
Tukey's post-hoc comparisons of the five fragment pairs, collapsed across orientation and length, indicated that the rates for pairs 2/7 and 3/8 did not differ ( p = 0.820), and were both higher than the rate for pair 4/9 ( p < 0.005 for both). The rates of pairs 1/6 and 5/10 differed from those of all other pairs ( p < 0.001 for all comparisons). Finally, Tukey's comparisons of the three lengths revealed that rates at all three lengths differed ( p < 0.001 for all). Specifically, rates increased with increasing length.
The orientation × length interaction was analyzed with a one-way ANOVA that evaluated difference scores, obtained for each participant by subtracting rates of the inverted condition from those of the upright condition while collapsing for orientation and fragment pair, across the three lengths. The analysis showed a significant main effect of length, F(2, 22) = 16.16, p < 0.001. Tukey's post-hoc comparisons revealed that the inversion effect, measured in terms of the difference in rates of responses consistent with walker direction between upright and inverted orientations, was smaller for length 100 ms than for 150 ms and 200 ms ( p < 0.005 for both), and the inversion effects for 150 ms and 200 ms did not differ ( p = 0.194).
The fragment pair × length interaction was examined with individual one-way ANOVAs that compared the three lengths for each pair. The analyses showed a significant main effect of length for pair 1/6, F(2, 22) = 31.16, p < 0.001, pair 2/7, F(2, 22) = 10.21, p < 0.001, pair 3/8, F(2, 22) = 6.32, p = 0.007, pair 4/9, F(2, 22) = 4.57, p = 0.022, and pair 5/10, F(2, 22) = 13.31, p < 0.001. Tukey's comparisons for pair 1/6 indicated that rates at all three lengths differed ( p < 0.005 for all) with rates increasing with increasing window length. Comparisons for pair 2/7 indicated that the rate for 100 ms was significantly lower than the rates for 150 ms and 200 ms ( p < 0.003 for both), and the rates for 150 ms and 200 ms did not differ ( p = 0.967). Similarly for pair 3/8, the rate for 100 ms was significantly lower than the rates for 150 ms and 200 ms ( p < 0.017 for both), which did not differ ( p = 0.987). Comparisons for pair 4/9 showed that the rate at 100 ms was statistically lower than the rate at 200 ms only ( p = 0.020), although the rates for 150 ms and 200 ms did not differ ( p = 0.687). Finally, pair 5/10 showed a reversed trend from that of pairs 2/7 and 3/8. Specifically, the rate of responses consistent with the direction of the walker for 100 ms was significantly higher than the rates for 150 ms and 200 ms ( p < 0.001 for both), which did not differ ( p > 0.900).
The results for Group 1 are qualitatively different from those of Group 2. As expected, observers in Group 1 who were presented with displays derived from a single fragment merely based direction judgments upon the horizontal displacement of the individual dot elements. Across a full gait cycle, the displacement averages out and there is no net displacement. Accordingly, observers' overall response rates were at chance level. Interpreting the displays solely based upon dot translatory movement also explains the lack of an inversion effect in the results of Group 1.
In contrast to Group 1, the results for observers in Group 2 who were presented with displays derived from paired fragments show above chance-level response rates and a strong inversion effect. These results are consistent with the orientation-dependency for discriminating walking direction from scrambled displays derived from full walkers and corroborate the finding of the critical role of the foot motion (Troje & Westhoff,
2006). The response behavior of observers in Group 2 is qualitatively different from that of observers in Group 1 and cannot be explained by a simple linear integration of the responses given to the individual fragments.
Figure 2C re-plots the results of Group 1 collapsing the data over fragment pairs. Average response rates for these pairs are close to chance level and there is no inversion effect when the fragments are shown in isolation (Group 1). These data are very different from the data obtained when the fragment pairs are presented simultaneously (Group 2).
Critically, these results suggest that retrieving direction of motion from scrambled biological motion displays involves more than processing the local spatiotemporal cues inherent to the isolated motion of the foot. Rather, we must conclude that even within the context of a full scrambled walker display that cannot be resolved into a percept of coherent form, the foot's elemental cues are evaluated with respect to the motions of other elements, or at least to other parts of the same element's trajectory, as we have shown here. But, why are additional reference dots important?
Critical to the discussion of this question is the distinction between the direction of explicit translatory motion of an object (or a walker, in our case), and the implicit cues contained in the deformation of the body. We call the former extrinsic motion and the latter intrinsic motion (see also, Kersten,
1998, who used these terms in a slightly different, yet analogous context). In his early work, Johansson (
1973,
1974) demonstrated how the human visual system decomposes the kinematics of the display of a rolling wheel into common translatory (i.e., extrinsic) motion and deviant circular (i.e., intrinsic) motion in order to understand the mechanics of the scene. Presented alone, a point representing the rim of a rolling wheel is interpreted as moving along a cycloidal path (i.e., a path resembling periodic arches). Only when a second point is added (e.g., on the axle of the wheel) can observers perceive the rolling wheel. In the context of our stimuli, subtraction of the common translatory (extrinsic) component of the motion results in the absolute cyclic motion of the foot and retains the walker's intrinsic motion, but only if the full gait cycle can be considered. The qualitative difference in response behavior between the two groups in the present experiment is most likely due to a switch from the perception of solely extrinsic motion with the single fragments in Group 1 to the use of intrinsic motion cues with the paired fragments in Group 2. Only the latter matches the reality from which the stimuli were derived: the stationary walker that lacks any extrinsic motion but contains intrinsic motion cues revealing its facing direction.
It should be noted at this point that participants in this experiment were not informed as to the nature of the stimuli (i.e., that they were derived from a human walker). It is clear then, that the cues that are relevant for retrieving direction are independent of higher-level knowledge of the nature of the stimuli and particularly, of whether they derive from animate or inanimate objects. The exact nature of these cues however, and their contributions to the observed effects remain unclear.
Most of the fragment pairs employed in this experiment consist of a fragment that represents a fraction of the foot trajectory's stance phase and a fragment that represents a fraction of its swing phase. During the stance phase, the foot is planted on the ground and is more or less motionless. The motion of the dots that correspond to the stance phase is rather due to the motion of a panning camera that keeps the walker in the center of the display, or alternatively, to the motion of the belt of an invisible treadmill. The velocity of the dot is approximately constant and identical to the translatory (extrinsic) motion of the walker, but points in the opposite direction. The other dot, which represents a fragment from the swing phase exhibits a velocity profile that is much more variable. During the swing phase, the foot accelerates along both the horizontal and vertical dimensions due to muscle activity and gravitational acceleration. If the visual system could reliably identify stance-phase fragments and discriminate them from other types of fragments, it could safely assume that the walker is facing in the direction opposite to the direction in which the stance phase fragments move.
What, then, accounts for the inversion effect? Inversion, that is, the mirror flipping of stimulus about the horizontal axis, affects only vertical components of the trajectory. The linear, horizontal motion of the stance phase fragments is largely unaffected by inversion. We must conclude then that the observed inversion effect is due to vertical asymmetries in the swing phase segments. In principle, such asymmetries can be due to the polarity of vertical velocity, vertical acceleration, or even higher order derivatives. It is unlikely that the visual system is merely differentially sensitive to downward and upward vertical motion. In fact, this possibility is precluded by our results. For example, a superior sensitivity for downward vertical motion would predict higher rates of responses consistent with the facing direction of the walker for the paired displays containing an inverted fragment 1 than for displays containing its upright version. The results reflected the opposite pattern. Instead, we speculate that the visual system is sensitive to the vertical acceleration exhibited by the swing phase fragments. As noted earlier, such variations in velocity along the motion path, or trajectory forms, have been proposed to facilitate the perception of a variety of events (e.g., Bingham et al.,
1995).
In order to investigate the possible relationship between acceleration and the local inversion effect, we extended the use of our novel foot displays to a larger number of stimulus samples (foot motions of other animals) in
Experiment 2. In this next experiment, we created paired fragment stimuli (as those presented to Group 2 in
Experiment 1) from the foot motions of a human walker, human runner, cat, and pigeon and presented them at both upright and inverted orientations to observers in a direction discrimination task.
If acceleration carries the local motion-based inversion effect in biological motion perception, the size of the effect should vary with different animal foot motions which undoubtedly carry very different motion profiles. In both Chang and Troje (
2008) and Troje and Westhoff (
2006), there were no significant differences among the inversion effects for the different animals tested. Note however that in those experiments, the foot dots were presented within the context of a full walker. The paired fragment stimuli used in
Experiment 1 display solely foot-specific motion information and may be more sensitive to any differences in orientation effects among different stimulus types.
As in
Experiment 1, the data were analyzed both in terms of the proportions of responses consistent with the facing direction of the walker from which the stimuli were derived and in terms of
d′ sensitivity measures derived from these rates. Both analyses yielded identical results and the results are presented here in terms of the proportions of responses consistent with walking direction.
A comparison of the mean proportion of responses consistent with the facing direction of the walker for each stimulus type at both orientations, collapsed across fragment pairs is shown in
Figure 4. An inspection of the response rates indicated that the rates were higher for the cat (upright = 0.64 proportion of responses; inverted = 0.58 proportion of responses) than for the human walker (upright = 0.61 proportion of responses; inverted = 0.47 proportion of responses), human runner (upright = 0.66 proportion of responses; inverted = 0.42 proportion of responses), and pigeon (upright = 0.55 proportion of responses; inverted = 0.46 proportion of responses) stimuli. Overall, rates for the upright stimuli (0.62 proportion of responses) were higher than rates for the inverted stimuli (0.48 proportion of responses). As can be seen in
Figure 4 however, the difference in rates between upright and inverted orientations varied with stimulus type: the difference was largest for the runner stimuli and smallest for the cat stimuli.
Figure 5 shows the mean proportion of responses consistent with walking direction of five fragment pairs at the two orientations separately for each stimulus type. From this figure, it is evident that for certain stimulus types, the difference in rates between upright and inverted orientations also depended on the particular fragment pair.
The above observations are supported by statistical analyses. The data were first entered in a 2 (block) × 4 (type) × 2 (orientation) ANOVA. The analysis showed significant main effects of stimulus type, F(3, 57) = 12.60, p < 0.001, orientation, F(1, 19) = 143.95, p < 0.001, and a significant type × orientation interaction, F(3, 57) = 21.00, p < 0.001. As the performance between the two test blocks did not differ, F(1, 19) = 0.04, p = 0.851, data from the two blocks were pooled for further analyses.
Tukey's post-hoc comparisons of the different stimulus types showed that overall, the proportions of responses consistent with walking direction were higher for the cat stimuli than for all other stimulus types ( p < 0.001 for all). Average rates for the walker, runner, and pigeon did not differ ( p > 0.200 for all).
Individual comparisons of the two orientations for each stimulus type indicated that the rates for upright stimuli were higher than for inverted stimuli for all stimulus types (Bonferroni-corrected t-tests, p < 0.002 for all). The type × orientation interaction was subsequently analyzed with a one-way ANOVA on difference scores, obtained for each participant by subtracting rates for the inverted stimuli from those of the upright stimuli. The analysis showed a significant main effect of stimulus type, F(3, 57) = 21.00, p < 0.001. Tukey's post-hoc comparisons indicated that the inversion effect, measured in terms of the difference between rates for the upright and inverted stimuli, was larger for the runner stimuli than all other types ( p < 0.001 for all). The inversion effects for the walker and pigeon stimuli did not differ ( p = 0.188) but the inversion effect for the walker was significantly larger than the one for the cat ( p = 0.006). The comparison between the inversion effects for the pigeon and cat was not significant ( p = 0.507).
Individual two-way ANOVAs were used to analyze the different fragment pairs between the two orientations for each of the four stimulus types. The analysis for the human walker revealed significant main effects of orientation, F(1, 19) = 73.00, p < 0.001, and pair, F(4, 76) = 113.07, p < 0.001, and a significant orientation × pair interaction, F(4, 76), p = 0.029. Tukey's comparisons of the different pairs indicated that while response rates for pairs 1/6 and 5/10 did not differ ( p = 0.798), they were significantly lower than the rates for all other pairs ( p < 0.001 for all comparisons). The response rate for pair 2/7 was not different from the rate for pair 3/8 ( p = 0.277), but was significantly higher than the rate for pair 4/9 ( p < 0.001). The comparison between pairs 3/8 and 4/9 was not significant ( p = 0.089). The orientation × pair interaction was analyzed with Tukey's comparisons of upright and inverted orientations for each pair. The analyses revealed that the rates for upright stimuli were higher than those for inverted stimuli for pairs 2/7, 3/8, 4/9, and 5/10 ( p < 0.001 for all), but not pair 1/6 for which the two orientations did not differ ( p = 0.113).
A comparable ANOVA for the runner showed significant main effects of orientation, F(1,19) = 75.37, p < 0.001, and pair, F(4,76) = 35.51, p < 0.001, but no interaction. Tukey's comparisons of the different fragment pairs showed that while the rates for pairs 2/7, 3/8, and 4/9 did not differ ( p > 0.800 for all), they were significantly higher than the rates for pairs 1/6 and 5/10 ( p < 0.001 for all), which did not differ ( p = 0.146).
Similarly, the ANOVA for the cat revealed significant main effects of orientation, F(1,19) = 13.81, p = 0.002, and fragment pair, F(4,76) = 120.79, p < 0.001, and no interaction. Here, Tukey's comparisons of the different fragment pairs revealed that the rates for pairs 2/7, 3/8, and 4/9 did not differ ( p > 0.600 for all comparisons), but were significantly higher than the rates of pairs 1/6 and 5/10 ( p < 0.001 for all). The rate for pair 1/5 was also significantly higher than the rate for pair 5/10 ( p < 0.001).
The ANOVA for the pigeon showed significant main effects of orientation, F(1,19) = 48.89, p < 0.001, and fragment pair, F(4,76) = 61.25, p < 0.001, and a significant orientation × pair interaction, F(4,76) = 19.68, p < 0.001. Tukey's comparisons of the different fragment pairs indicated that while the rates for pairs 1/6, 2/7, and 3/8 did not differ ( p > 0.200 for all), they were significantly higher than the rates of pairs 4/9 and 5/10 ( p < 0.001 for all). In addition, the rate for pair 4/9 was significantly higher than the rate for pair 5/10 ( p < 0.001). The interaction was analyzed with Tukey's tests of upright versus inverted orientations per each fragment pair. The analyses revealed that the rates were higher for upright versions than for inverted versions of pairs 3/8, 4/9, and 5/10 ( p < 0.030 for all). The two orientations did not differ for pairs 1/6 ( p = 0.092) and 2/7 ( p = 0.274).
Finally, in
Table 1, we calculated the average vertical accelerations both in real world coordinates and screen coordinates, and a measure of the inversion effect for each of the fragment pairs that comprised the stimuli in this experiment. Here, the inversion effect is quantified by a difference score, computed by subtracting the proportion of responses consistent with walking direction for the inverted stimulus from the proportion of responses consistent with walking direction for the upright stimulus. We subsequently compared the inversion score for each stimulus to the absolute difference in vertical acceleration between its two constituent fragments by means of a linear regression analysis.
Figure 6 shows the regression analyses for the full stimulus set and for the stimulus set with two outliers (open circles) removed. Although no relationship is evident when the full set is considered (
r 2 = .01;
p = 0.659), a significant positive correlation between the inversion score and absolute difference in vertical acceleration becomes evident once the outliers are removed (
r 2 = 0.35;
p = 0.009).
Table 1 Average vertical accelerations and inversion score for each fragment pair. Sign of acceleration value indicates direction of acceleration. Positive and negative acceleration values indicate downward and upward accelerations, respectively. Arrows indicate vertical direction of the motion. Inversion score was computed by subtracting the proportion of responses consistent with the facing direction of the walker from which the stimulus was derived for the inverted orientation from that for the upright orientation.
Table 1 Average vertical accelerations and inversion score for each fragment pair. Sign of acceleration value indicates direction of acceleration. Positive and negative acceleration values indicate downward and upward accelerations, respectively. Arrows indicate vertical direction of the motion. Inversion score was computed by subtracting the proportion of responses consistent with the facing direction of the walker from which the stimulus was derived for the inverted orientation from that for the upright orientation.
Type | Fragment
pair | Real world vertical
acceleration (m/s 2) | Screen vertical
acceleration (deg/s 2) | Inversion
score |
Human walker | 1 / 6 | 0.03 ↑ / −1.35 ↓ | 0.10 ↑ / −4.01 ↓ | 0.05 |
2 / 7 | 9.45 ↓ / 0.21 ↑ | 27.95 ↓ / 0.63 ↑ | 0.20 |
3 / 8 | 0.06 ↓ / −0.47 ↑ | 0.19 ↓ / −1.39 ↑ | 0.17 |
4 / 9 | −4.16 ↓ / −1.06 ↑ | −12.30 ↓ / −3.13 ↑ | 0.15 |
5 / 10 | 0.24 ↓ / −3.45 ↑ | 0.71 ↓ / −10.20 ↑ | 0.14 |
Human runner | 1 / 6 | 4.66 ↑ / −1.67 ↓ | 13.79 ↑ / −4.95 ↓ | 0.18 |
2 / 7 | 8.56 ↓ / −3.79 ↑ | 25.32 ↓ / −11.21 ↑ | 0.25 |
3 / 8 | 2.05 ↓ / −4.17 ↑ | 6.06 ↓ / −12.33 ↑ | 0.33 |
4 / 9 | −4.54 ↓ / −1.32 ↑ | −13.43 ↓ / −3.92 ↑ | 0.25 |
5 / 10 | −1.59 ↓ / 1.82 ↑ | −4.72 ↓ / 5.39 ↑ | 0.22 |
Cat | 1 / 6 | 0.77 ↑ / −2.65 ↓ | 2.28 ↑ / −7.85 ↓ | 0.05 |
2 / 7 | 2.34 ↑ / −0.16 ↑ | 6.94 ↑ / −0.48 ↑ | 0.03 |
3 / 8 | 1.09 ↓ / 0.53 ↑ | 3.23 ↓ / 1.58 ↑ | 0.02 |
4 / 9 | 2.26 ↓ / 0.02 ↓ | 6.68 ↓ / 0.05 ↓ | 0.06 |
5 / 10 | −1.24 ↓ / −2.66 ↑ | −3.68 ↓ / −7.88 ↑ | 0.11 |
Pigeon | 1 / 6 | −4.13 ↓ / 6.33 ↑ | −12.22 ↓ / 18.73 ↑ | −0.05 |
2 / 7 | 0.65 ↑ / 20.41 ↑ | 1.92 ↑ / 60.41 ↑ | −0.04 |
3 / 8 | −0.95 ↑ / 5.87 ↓ | −2.81 ↑ / 17.36 ↓ | 0.25 |
4 / 9 | −4.54 ↑ / −6.99 ↓ | −13.43 ↑ / −20.69 ↓ | 0.23 |
5 / 10 | −8.74 ↑ / −6.98 ↓ | −25.86 ↑ / −20.65 ↓ | 0.07 |
As in
Experiments 1 and
2, the data were analyzed both in terms of the proportions of responses consistent with the facing direction of the walker from which the stimuli were derived and in terms of
d′ sensitivity measures derived from these rates. Again, both analyses yielded identical results. As in
Experiments 1 and
2, the results are presented in terms of proportions of responses consistent with walking direction.
The mean proportion of responses consistent with walking direction of the two stimulus types for the five fragment pairs at upright and inverted orientations are presented in
Figure 7. An examination of the mean response rates for upright versus inverted orientations collapsed across stimulus type and fragment pair revealed that in general, the rates for upright stimuli (0.56 proportion of responses) were higher than the rates for inverted stimuli (0.52 proportion of responses). However, a closer inspection of the means separately for the two stimulus types reveals that the rates for upright displays were higher than the rates for the inverted displays for the natural stimuli only.
Response rates were analyzed with a 2 (type) × 2 (orientation) × 5 (fragment pair) repeated-measures ANOVA that showed a significant main effect of orientation, F(1, 19) = 7.15, p = 0.015, a significant main effect of fragment pair, F(4, 76) = 28.33, p < 0.001, and a significant type × orientation interaction, F(1, 19) = 13.11, p = 0.002. The main effect of type and other interactions were not significant.
Tukey's post-hoc comparisons of the different fragment pairs, collapsed over the two stimulus types and two orientations showed that while the rates for pairs 2/7, 3/8, and 4/9 did not differ ( p > 0.500 for all comparisons), they were significantly higher than the rates for pairs 1/6 and 5/10 ( p < 0.001 for all), which did not differ ( p = 0.219).
The type × orientation interaction was analyzed with Bonferroni-corrected, two-tailed t-tests comparing upright and inverted orientations for each stimulus type. The analyses revealed that the rates were higher for upright displays than for inverted displays for the natural stimuli only ( p < 0.001; upright = 0.58 proportion of responses; inverted = 0.49 proportion of responses). Upright and inverted versions of the constant speed stimuli did not differ ( p = 0.335; upright = 0.53 proportion of responses; inverted = 0.55 proportion of responses).
This research was supported by the Canada Foundation for Innovation (CFI), the Ontario Innovation Trust (OIT), a NSERC Discovery grant, the NCAP program of the Canadian Institute for Advanced Research, and the Canada Research Chair program.
Commercial relationships: none.
Corresponding author: Nikolaus Troje.
Address: Department of Psychology, Queen's University, Kingston, Ontario K7L 3N6, Canada.