Intrinsic spatial knowledge about terrestrial ecology favors the tall for judging distance

Abstract

Our sense of vision reliably directs and guides our everyday actions, such as reaching and walking. This ability is especially fascinating because the optical images of natural scenes that project into our eyes are insufficient to adequately form a perceptual space. It has been proposed that the brain makes up for this inadequacy by using its intrinsic spatial knowledge. However, it is unclear what constitutes intrinsic spatial knowledge and how it is acquired. We investigated this question and showed evidence of an ecological basis, which uses the statistical spatial relationship between the observer and the terrestrial environment, namely, the ground surface. We found that in dark and reduced-cue environments where intrinsic knowledge has a greater contribution, perceived target location is more accurate when referenced to the ground than to the ceiling. Furthermore, taller observers more accurately localized the target. Superior performance was also observed in the full-cue environment, even when we compensated for the observers' heights by having the taller observer sit on a chair and the shorter observers stand on a box. Although fascinating, this finding dovetails with the prediction of the ecological hypothesis for intrinsic spatial knowledge. It suggests that an individual's accumulated lifetime experiences of being tall and his or her constant interactions with ground-based objects not only determine intrinsic spatial knowledge but also endow him or her with an advantage in spatial ability in the intermediate distance range.

Keywords: Human behavior; adaptation; ecology; perception.

Fig. 1. Proposed ecological basis of the visual system’s intrinsic bias.

(A) A dimly lit target in the dark is perceived at the intersection between the projection line from the eye and the intrinsic bias (curve). (B) The ground surface representation is approximated as a slant (dashed curve) when the floor is weakly delineated by texture elements, due to the integration of the intrinsic bias and external depth (texture) information. The ground representation is less slanted than that of the intrinsic bias, leading to the target location being more accurately judged. (C) An observer encounters multiple objects at various locations along a projection line as he or she interacts with the environment. Over time, one location (represented by the white circle) emerges as having the peak probability of encountering the most objects. (D) We propose that the visual system adopts the peak probability locations from all viewing directions to define its intrinsic bias. This provides the visual system with the “best guess” of where objects are located (that is, the intrinsic bias) when external visual information is impoverished. (E) Our ecological existence causes the intrinsic bias to skew toward the ground surface. (F) Prediction 1: There is an asymmetry in the shapes of the intrinsic biases in the upper and lower visual fields (yellow curves). (G) Prediction 2: A taller observer perceives a target with the same angular declination as farther than does a shorter observer. Note that the two targets in the figure have the same angular declination.

Fig. 2. Results of experiment 1 in the dark environment.

(A) The plus symbols depict the physical target locations. The triangles and circles plot the average judged locations of the taller (green) and shorter (blue) observers. The solid curves represent the intrinsic biases of the taller and shorter observers in the upper and lower visual fields. The dashed curves depict the predicted locations of the intrinsic biases of the taller and shorter groups in the upper visual field, had they been symmetrical to those in the lower visual field. (B) Plotting the average judged eye-to-target distance as a function of the physical angular declination/elevation reveals that judged distance was longer for the taller observers. (C) Plotting the average judged angular declination/elevation as a function of the physical angular declination/elevation reveals that judged direction was accurate.

Fig. 3. Results of experiment 2 in the reduced-cue environment.

(A) Side view of the dimly lit texture elements that delineated the floor and ceiling surfaces. (B) Top view of the same texture display with the test target locations added (green plus symbols). (C) The plus symbols depict the physical target locations. The triangles plot the average judged locations of the taller (green) and shorter (blue) observers. The solid curves represent the intrinsic biases of the taller and shorter observers in the upper and lower visual fields. The dashed curves depict the predicted locations of the intrinsic biases of the taller and shorter groups in the upper visual field, had they been symmetrical to those in the lower visual field. (D) Plotting the average judged eye-to-target distance as a function of the physical angular declination/elevation reveals that judged distance was longer for the taller observers. (E) Plotting the average judged angular declination/elevation as a function of the physical angular declination/elevation reveals that judged direction was accurate.

Fig. 4. Results of experiment 3 in the full-cue environment.

(A to C) Procedures for testing an observer on the Gilinsky successive equal-appearing intervals task while (A) standing on the ground, (B) standing on a box to raise the eye height by 30 cm, and (C) sitting on a chair to lower the eye height by 30 cm. (D) Plotting the average inferred distance as a function of the physical distance reveals that inferred distance was more accurate (closer to the equidistant dashed line) for the taller observers, even when they sat on a chair.