Manual matching of perceived surface orientation is affected by arm posture: evidence of calibration between proprioception and visual experience in near space

Pedal and haptic estimates of slant suggest a common underlying representation

It is well known that people verbally exaggerate the slant of visually perceived geographical, virtual, and man-made hills. More recently it has been shown that haptic and verbal estimates of slant result in similar exaggerations, supporting the proprioception calibration hypothesis-that similar biases exist in both verbal estimates of visually perceived slant and proprioceptively perceived hand orientation. This seems to point to a common underlying representation of slant. However, it is unclear if and how manual proprioceptive estimates might be relevant for perception of ground surface slant or how this might translate to pedal perception of surface orientation. In the current work we tested whether pedal perception is systematically connected to a representational system shared by haptic and visual perception. We did this by having people orient their foot to four different orientations of a ramp (Experiment 1) or to a staircase (Experiment 2) and compared these to estimates made using a free hand measure as well as to verbal estimates. Our results show that verbal, haptic, and pedal measures of visually perceived surface orientation all result in similar estimates of slant and do so across different slanted surfaces. This suggests that verbal and haptic proprioceptive estimates tap into a representational system of visually perceived surface orientation that is relevant for walking up various surface orientations.

Response Modality vs. Target Modality: Sensory Transformations and Comparisons in Cross-modal Slant Matching Tasks

Humans constantly combine multi-sensory spatial information to successfully interact with objects in peripersonal space. Previous studies suggest that sensory inputs of different modalities are encoded in different reference frames. In cross-modal tasks where the target and response modalities are different, it is unclear which reference frame these multiple sensory signals are transformed to for comparison. The current study used a slant perception and parallelity paradigm to explore this issue. Participants perceived (either visually or haptically) the slant of a reference board and were asked to either adjust an invisible test board by hand manipulation or to adjust a visible test board through verbal instructions to be physically parallel to the reference board. We examined the patterns of constant error and variability of unimodal and cross-modal tasks with various reference slant angles at different reference/test locations. The results revealed that rather than a mixture of the patterns of unimodal conditions, the pattern in cross-modal conditions depended almost entirely on the response modality and was not substantially affected by the target modality. Deviations in haptic response conditions could be predicted by the locations of the reference and test board, whereas the reference slant angle was an important predictor in visual response conditions.

Does perceived angular declination contribute to perceived optical slant on level ground?

When one looks at a spot on level ground, the local optical slant (i.e., surface orientation relative to the line of sight) is geometrically equivalent to the angular declination (i.e., sagittal visual direction relative to horizontal). In theory, angular declination provides an unbiased proximal source of information for estimating optical slant on level ground. Two experiments were conducted to investigate whether human visual systems take advantage of this information. An aspect ratio task was used as an implicit measure for assessing perceived optical slant. Participants gave verbal estimates of the perceived aspect ratio of an L-shaped arrangement, formed by three balls on level ground or on slanted surfaces (hills). Gaze direction was held horizontal when viewing the stimuli on hills. Experiment 1 examined two optical slants (22° to 35°) at relatively short viewing distances (3.1 to 11.5 m), while Experiment 2 tested a shallow optical slant (6°) at relatively long viewing distances (5.7 to 17.2 m). No significant difference in perceived aspect ratio was found between the level-ground and the hill conditions in either experiment. These findings suggest that angular declination does not contribute to perceived optical slant on level ground. It seems that the perception of optical slant and of gaze declination are independent, and this may be because the two variables are normally used jointly to determine a higher order perceptual variable-geographical slant.

Distances on hills look farther than distances on flat ground: Evidence from converging measures

Distances on hills are judged as farther than when the same distance is presented on the flat ground. The hypothesized reason for this difference is because perception is influenced by the increased effort required to walk up a hill than to walk the same distance on flat ground. Alternatively, distances presented up a hill might be judged as farther for other, nonperceptual reasons such as bias from demand characteristics. To test whether distances on hills are perceived as farther or are merely judged as farther, we used a variety of measures, including visual matching and blindwalking tasks, and found similar effects across all measures. This convergence is consistent with a perceptual explanation. Second, we mined our data with the goal of making recommendations for future research on this paradigm. Although all of the perceptual measures used showed similar effects, visual matching was the only measure that had good intrasubject reliability. We recommend that future research on this action-specific effect could use any measure unless the research is geared towards individual differences, in which case, only the visual matching measure of perceived distance should be used.

Haptic adaptation to slant: No transfer between exploration modes

Human touch is an inherently active sense: to estimate an object's shape humans often move their hand across its surface. This way the object is sampled both in a serial (sampling different parts of the object across time) and parallel fashion (sampling using different parts of the hand simultaneously). Both the serial (moving a single finger) and parallel (static contact with the entire hand) exploration modes provide reliable and similar global shape information, suggesting the possibility that this information is shared early in the sensory cortex. In contrast, we here show the opposite. Using an adaptation-and-transfer paradigm, a change in haptic perception was induced by slant-adaptation using either the serial or parallel exploration mode. A unified shape-based coding would predict that this would equally affect perception using other exploration modes. However, we found that adaptation-induced perceptual changes did not transfer between exploration modes. Instead, serial and parallel exploration components adapted simultaneously, but to different kinaesthetic aspects of exploration behaviour rather than object-shape per se. These results indicate that a potential combination of information from different exploration modes can only occur at down-stream cortical processing stages, at which adaptation is no longer effective.

Palm board and verbal estimates of slant reflect the same perceptual representation

People verbally overestimate the orientation of slanted surfaces, but accurately estimate or underestimate slanted surfaces using a palm board. We demonstrate a fundamental issue that explains why the two different values typically given for palm board and verbal/visual matching estimates express similar perceptual representations of slanted surfaces. The fundamental problem in studies measuring palm board and verbal estimates is that the "measure"-either (1) reproducing a verbally given angle or the orientation of a slanted surface with an unseen hand or (2) verbally or visually estimating a visually perceived surface-has always been confounded with the "surface"-either using (1) a palm board or (2) a hill or ramp. Although reproduction has exclusively been used with palm boards in these studies, at the same time verbal estimation or visual matching has exclusively been used with hills/ramps. In three experiments, we showed that verbally estimating palm board orientations produces overestimates by a factor of 1.5, whereas reproducing the orientation of the surface of a ramp to verbally given angles produces gains of ~0.6. These values are similar to those seen for verbal overestimates of slanted surfaces, and to palm board gains for near surfaces and the relative palm-board-to-verbal gains for outdoor hills, respectively. Eliminating this confound eliminated the difference previously seen across surfaces. We discuss how and why different measures should produce different results if we overestimate slant in general and perceptually represent slant in the same way, both haptically and visually.

Do Individual Differences and Aging Effects in the Estimation of Geographical Slant Reflect Cognitive or Perceptual Effects?

Several individual differences including age have been suggested to affect the perception of slant. A cross-sectional study of outdoor hill estimation (N = 106) was analyzed using individual difference measures of age, experiential knowledge, fitness, personality traits, and sex. Of particular note, it was found that for participants who reported any experiential knowledge about slant, estimates decreased (i.e., became more accurate) as conscientiousness increased, suggesting that more conscientious individuals were more deliberate about taking their experiential knowledge (rather than perception) into account. Effects of fitness were limited to those without experiential knowledge, suggesting that they, too, may be cognitive rather than perceptual. The observed effects of age, which tended to produce lower, more accurate estimates of hill slant, provide more evidence that older adults do not see hills as steeper. The main effect of age was to lower slant estimates; such effects may be due to implicit experiential knowledge acquired over a lifetime. The results indicate the impact of cognitive, rather than perceptual factors on individual differences in slant estimation.

Manual anchoring biases in slant estimation affect matches even for near surfaces

People verbally overestimate hill slant by ~15°-25°, whereas manual estimates (e.g., palm board measures) are thought to be more accurate. The relative accuracy of palm boards has contributed to the widely cited theoretical claim that they tap into an accurate, but unconscious, motor representation of locomotor space. Recently, it was shown that a bias that stems from anchoring the hand at horizontal prior to the estimate can quantitatively account for the difference between manual and verbal estimates of hill slant. The present work extends this observation to manual estimates of near-surface slant, to test whether the bias derives from manual or visual uncertainty. As with far surfaces, strong manual anchoring effects were obtained for a large range of near-surface slants, including 45°. Moreover, correlations between participants' manual and verbal estimates further support the conclusion that both measures are based on the same visual representation.

Perceived azimuth direction is exaggerated: Converging evidence from explicit and implicit measures

Recent observations suggest that perceived visual direction in the sagittal plane (angular direction in elevation, both upward and downward from eye level) is exaggerated. Foley, Ribeiro-Filho, and Da Silva's (2004) study of perceived size of exocentric ground extent implies that perceived angular direction in azimuth may also be exaggerated. In the present study, we directly examined whether perceived azimuth direction is overestimated. In Experiment 1, numeric estimates of azimuth direction (-48° to 48° relative to straight ahead) were obtained. The results showed a linear exaggeration in perceived azimuth direction with a gain of about 1.26. In Experiment 2, a perceptual extent-matching task served as an implicit measure of perceived azimuth direction. Participants matched an egocentric distance in one direction to a frontal extent in nearly the opposite direction. The angular biases implied by the matching data well replicated Foley et al.'s finding and were also fairly consistent with the azimuth bias function found in Experiment 1, although a slight overall shift was observed between the results of the two experiments. Experiment 3, in which half the observers were tilted sideways while making frontal/depth extent comparisons, suggested that the discrepancy between the results of Experiment 1 and 2 can partially be explained by a retinal horizontal vertical illusion affecting distance estimation tasks. Overall the present study provides converging evidence to suggest that the perception of azimuth direction is overestimated.

What do hands know about hills? Interpreting Taylor-Covill and Eves (2013) in context

Hills appear much steeper than they are. Although near surface slant is also exaggerated, near surfaces appear much shallower than equivalently slanted hills. Taylor-Covill and Eves (2013) propose a new type of palm orientation measuring device that provides outputs that accurately reflect the physical slants of stairs and hills from 19 to 30° and also seems to accurately reflect the slants of near surfaces (25-30°). They question the validity of the observations of Durgin, Hajnal, Li, Tonge & Stigliani (2010), who observed that palm boards grossly underestimated near surfaces. Here I review our recent work on the visual and haptic perception of near surface orientation in order to place Taylor-Covill and Eves' arguments in context. I note in particular that free hand measures of real surfaces in near space show excellent calibration, but free hand measures show gross exaggeration for hills. This leads to the question of the grounds for preferring a mechanical device to a freely wielded hand. In addition I report an investigative replication of the crucial observations that led to our concerns about the value of palm boards as measures of perception and note the specific methodological details that we have accounted for in our procedures. Finally, I propose some testable hypotheses regarding how better-than-expected haptic matches to hills may arise.

Humans have precise knowledge of familiar geographical slants

Whereas maps primarily represent the 2-dimensional layout of the environment, people are also aware of the 3-dimensional layout of their environment. An experiment conducted on a small college campus tested whether the remembered slants of familiar paths were precisely represented. Three measures of slant (verbal, manual, and pictorial) were collected in 2 different between-subject conditions (perception and memory) for 5 familiar paths on the campus of Swarthmore College, ranging in slant from 0.5° to 8.6°. Estimates from memory and from perception did not differ for any of the measures. Moreover, estimates from all measures, though different in mean value, were correlated within participant, suggesting a common underlying representation was consulted in all cases.

On the anisotropy of perceived ground extents and the interpretation of walked distance as a measure of perception

Two experiments are reported concerning the perception of ground extent to discover whether prior reports of anisotropy between frontal extents and extents in depth were consistent across different measures (visual matching and pantomime walking) and test environments (outdoor environments and virtual environments). In Experiment 1 it was found that depth extents of up to 7 m are indeed perceptually compressed relative to frontal extents in an outdoor environment, and that perceptual matching provided more precise estimates than did pantomime walking. In Experiment 2, similar anisotropies were found using similar tasks in a similar (but virtual) environment. In both experiments pantomime walking measures seemed to additionally compress the range of responses. Experiment 3 supported the hypothesis that range compression in walking measures of perceived distance might be due to proactive interference (memory contamination). It is concluded that walking measures are calibrated for perceived egocentric distance, but that pantomime walking measures may suffer range compression. Depth extents along the ground are perceptually compressed relative to frontal ground extents in a manner consistent with the angular scale expansion hypothesis.

A comparison of two theories of perceived distance on the ground plane: The angular expansion hypothesis and the intrinsic bias hypothesis

Two theories of distance perception-ie, the angular expansion hypothesis (Durgin and Li, 2011 Attention, Perception, & Psychophysics 73 1856-1870) and the intrinsic bias hypothesis (Ooi et al, 2006 Perception 35 605-624)-are compared. Both theories attribute exocentric distance foreshortening to an exaggeration in perceived slant, but their fundamental geometrical assumptions are very different. The intrinsic bias hypothesis assumes a constant bias in perceived geographical slant of the ground plane and predicts both perceived egocentric and exocentric distances are increasingly compressed. In contrast, the angular expansion hypothesis assumes exaggerations in perceived gaze angle and perceived optical slant. Because the bias functions of the two angular variables are different, it allows the angular expansion hypothesis to distinguish two types of distance foreshortening-the linear compression in perceived egocentric distance and the nonlinear compression in perceived exocentric distance. While the intrinsic bias is proposed only for explaining distance biases, the angular expansion hypothesis provides accounts for a broader range of spatial biases.

The social psychology of perception experiments: hills, backpacks, glucose, and the problem of generalizability

Experiments take place in a physical environment but also a social environment. Generalizability from experimental manipulations to more typical contexts may be limited by violations of ecological validity with respect to either the physical or the social environment. A replication and extension of a recent study (a blood glucose manipulation) was conducted to investigate the effects of experimental demand (a social artifact) on participant behaviors judging the geographical slant of a large-scale outdoor hill. Three different assessments of experimental demand indicate that even when the physical environment is naturalistic, and the goal of the main experimental manipulation was primarily concealed, artificial aspects of the social environment (such as an explicit requirement to wear a heavy backpack while estimating the slant of a hill) may still be primarily responsible for altered judgments of hill orientation.