Large manual pointing errors, but accurate verbal reports, for indications of target azimuth

Many tasks have been used to probe human directional knowledge, but relatively little is known about the comparative merits of different means of indicating target azimuth. Few studies have compared action-based versus non-action-based judgments for targets encircling the observer. This comparison promises to illuminate not only the perception of azimuths in the front and rear hemispaces, but also the frames of reference underlying various azimuth judgments, and ultimately their neural underpinnings. We compared a response in which participants aimed a pointer at a nearby target, with verbal azimuth estimates. Target locations were distributed between 20 degrees and 340 degrees. Non-visual pointing responses exhibited large constant errors (up to -32 degrees) that tended to increase with target eccentricity. Pointing with eyes open also showed large errors (up to -21 degrees). In striking contrast, verbal reports were highly accurate, with constant errors rarely exceeding +/-5 degrees. Under our testing conditions, these results are not likely to stem from differences in perception-based versus action-based responses, but instead reflect the frames of reference underlying the pointing and verbal responses. When participants used the pointer to match the egocentric target azimuth rather than the exocentric target azimuth relative to the pointer, errors were reduced.     

Exploring the process of progressive disorientation

While an increasing number of behavioral studies examining spatial cognition use experimental paradigms involving disorientation, the process by which one becomes disoriented is not well explored. The current study examined this process using a paradigm in which participants were blindfolded and underwent a succession of 70° or 200° passive, whole body rotations around a fixed vertical axis. After each rotation, participants used a pointer to indicate either their heading at the start of the most recent turn or their heading at the start of the current series of turns. Analyses showed that in both cases, mean pointing errors increased gradually over successive turns. In addition to the gradual loss of orientation indicated by this increase, analysis of the pointing errors also showed evidence of occasional, abrupt loss orientation. Results indicate multiple routes from an oriented to a disoriented state, and shed light on the process of becoming disoriented.     

Dynamic category structure in spatial memory

This study examines bias (constant error) in spatial memory in an effort to determine whether this bias is defined by a dynamic egocentric reference frame that moves with the observer or by an environmentally fixed reference frame. Participants learned the locations of six target objects around them in a room, were blindfolded, and then rotated themselves to face particular response headings. From each response heading, participants used a pointer to indicate the remembered azimuthal locations of the objects. Analyses of the angular pointing errors showed a previously observed pattern of bias. More importantly, it appeared that this pattern of bias was defined relative to and moved with the observer—that is, was egocentric and dynamic. These results were interpreted in the framework of a modified category adjustment model as suggesting the existence of dynamic categorical (nonmetric) spatial codes.     

The role of top-down knowledge about environmental context in egocentric distance judgments

Judgments of egocentric distances in well-lit natural environments can differ substantially in indoor versus outdoor contexts. Visual cues (e.g., linear perspective, texture gradients) no doubt play a strong role in context-dependent judgments when cues are abundant. Here we investigated a possible top-down influence on distance judgments that might play a unique role under conditions of perceptual uncertainty: assumptions or knowledge that one is indoors or outdoors. We presented targets in a large outdoor field and in an indoor classroom. To control visual distance and depth cues between the environments, we restricted the field of view by using a 14-deg aperture. Evidence of context effects depended on the response mode: Blindfolded-walking responses were systematically shorter indoors than outdoors, whereas verbal and size gesture judgments showed no context effects. These results suggest that top-down knowledge about the environmental context does not strongly influence visually perceived egocentric distance. However, this knowledge can operate as an output-level bias, such that blindfolded-walking responses are shorter when observers’ top-down knowledge indicates that they are indoors and when the size of the room is uncertain.

     

Two-category place representations persist over body rotations

We explored a system that constructs environment-centered frames of reference and coordinates memory for the azimuth of an object in an enclosed space. For one group, we provided two environmental cues (doors): one in the front, and one in the rear. For a second group, we provided two object cues: a front and a rear cue. For a third group, we provided no external cues; we assumed that for this group, their reference frames would be determined by the orthogonal geometry of the floor-and-wall junction that divides a space in half or into multiple territories along the horizontal continuum. Using Huttenlocher, Hedges, and Duncan’s (Psychological Review 98: 352-376, 1991) category-adjustment model (cue-based fuzzy boundary version) to fit the data, we observed different reference frames than have been seen in prior studies involving two-dimensional domains. The geometry of the environment affected all three conditions and biased the remembered object locations within a two-category (left vs. right) environmental frame. The influence of the environmental geometry remained observable even after the participants’ heading within the environment changed due to a body rotation, attenuating the effect of the front but not of the rear cue. The door and object cues both appeared to define boundaries of spatial categories when they were used for reorientation. This supports the idea that both types of cues can assist in environment-centered memory formation.     

Distraction shrinks space

Research investigating how people remember the distance of paths they walk has shown two apparently conflicting effects of experience during encoding on subsequent distance judgments. By the feature accumulation effect, discrete path features such as turns, houses, or other landmarks cause an increase in remembered distance. By the distractor effect, performance of a concurrent task during path encoding causes a decrease in remembered distance. In this study, we ask the following: What are the conditions that determine whether the feature accumulation or the distractor effect dominates distortions of space? In two experiments, blindfolded participants were guided along two legs of a right triangle while reciting nonsense syllables. On some trials, one of the two legs contained features: horizontally mounted car antennas (gates) that bent out of the way as participants walked past. At the end of the second leg, participants either indicated the remembered path leg lengths using their hands in a ratio estimation task or attempted to walk, unguided, straight back to the beginning. In addition to response mode, visual access to the paths and time between encoding and response were manipulated to determine whether these factors would affect feature accumulation or distractor effects. Path legs with added features were remembered as shorter than those without, but this result was significant only in the haptic response mode data. This finding suggests that when people form spatial memory representations with the intention of navigating in room-scale spaces, interfering with information accumulation substantially distorts spatial memory.     

Dissociation between location and shape in visual space

There are often large perceptual distortions of shapes lying on the ground plane, even in well-lit environments. These distortions occur under conditions for which the perception of location i saccurate. Four hypotheses are considered for reconciling these seemingly paradoxical results, after which 2 experiments are reported that lend further support to 1 of them--that perception of shapeand perception of location are sometimes dissociable. The 2 experiments show that whereas perception of location does not depend on whether viewing is monocular or binocular (when other distance cues are abundant), perception of shape becomes more veridical when viewing is binocular. This means that perception of shape is not fully constrained by the perceived locations of the vertices that define the shape.     

Active control of locomotion facilitates nonvisual navigation

In some navigation tasks, participants are more accurate if they view the environment beforehand. To characterize the benefits associated with visual previews, 32 blindfolded participants were guided along simple paths and asked to walk unassisted to a specified destination (e.g., the origin). Paths were completed without vision, with or without a visual preview of the environment. Previews did not necessarily improve nonvisual navigation. When previewed landmarks stood near the origin or at off-path locations, they provided little benefit; by contrast, when they specified intermediate destinations (thereby increasing the degree of active control), performance was greatly enhanced. The results suggest that the benefit of a visual preview stems from the information it supplies for actively controlled locomotion. Accuracy in reaching the final destination, however, is strongly contingent upon the destination's location during the preview.     

Is the anisotropy of perceived 3-D shape invariant across scale?

A number of studies have resulted in the finding of a 3-D perceptual anisotropy, whereby spatial intervals oriented in depth are perceived to be smaller than physically equal intervals in the frontoparallel plane. In this experiment, we examined whether this anisotropy is scale invariant. The stimuli were L shapes created by two rods placed flat on a level grassy field, with one rod defining a frontoparallel interval, and the other, a depth interval. Observers monocularly and binocularly viewed L shapes at two scales such that they were projectively equivalent under monocular viewing. Observers judged the aspect ratio (depth/width) of each shape. Judged aspect ratio indicated a perceptual anisotropy that was invariant with scale for monocular viewing, but not for binocular viewing. When perspective is kept constant, monocular viewing results in perceptual anisotropy that is invariant across these two scales and presumably across still larger scales. This scale invariance indicates that the perception of shape under these conditions is determined independently of the perception of size.     

Visually perceived location is an invariant in the control of action

We provide experimental evidence that perceived location is an invariant in the control of action, by showing that different actions are directed toward a single visually specified location in space (corresponding to the putative perceived location) and that this single location, although specified by a fixed physical target, varies with the availability of information about the distance of that target. Observers in two conditions varying in the availability of egocentric distance cues viewed targets at 1.5, 3.1, or 6.0 m and then attempted to walk to the target with eyes closed using one of three paths; the path was not specified until after vision was occluded. The observers stopped at about the same location regardless of the path taken, providing evidence that action was being controlled by some invariant, ostensibly visually perceived location. That it was indeed perceived location was indicated by the manipulation of information about target distance—the trajectories in the full-cues condition converged near the physical target locations, whereas those in the reduced-cues condition converged at locations consistent with the usual perceptual errors found when distance cues are impoverished.