Original ArticlesEvolved navigation theory and the environmental vertical illusion
Introduction
It is often assumed that perceptual mechanisms function identically across most environmental features, especially basic visual perception of line or surface orientation, length, and location. However, evolved navigation theory (ENT) is a research approach that predicts specializations in the perception or navigation of environmental features that are reliably associated with navigational costs over evolutionary time (Jackson, 2005, Jackson & Cormack, 2007a). ENT is primarily an application of signal detection theory to navigation costs over evolutionary time as a means to predict the type and magnitude of specific navigational adaptations. A more general approach of looking at how decision consequences pose selection across domains has been summarized by Haselton and Buss (2000) as error management theory.
ENT focuses on understanding the selective forces particular to navigation as a means to predict unknown locomotor, visual, and other navigational adaptations. For example, vertical and inclined surfaces present a navigation risk of falling that is unmatched in horizontal surfaces. Falls of a few meters produce serious injuries that would be exceedingly rare when navigating a few horizontal meters. Vertical navigation likely posed a strong selective factor in the evolution of terrestrial organisms, certainly those with arboreal ancestors, such as humans. Thus, an implication under ENT is that some perceptual or navigational mechanisms might be specialized to lessen the risks of falling.
One possible method of establishing navigational route preferences is by exaggerating distance perceived from costlier routes because organisms prefer nearer navigational goals (Somervill & Somervill, 1977). A process that exaggerated perceived vertical surface length could thereby decrease vertical navigation in the presence of less costly alternatives. This would result in vertical surfaces perceived as longer than equidistant horizontal surfaces, which would decrease vertical navigation frequency and its associated falling costs. This simple mechanism could dynamically weight navigational decisions in real time across all surfaces by exaggerating perceived length based upon initial surface length and orientation. Such a mechanism captures both navigation difficulty and a primary predictor of falling costs: distance from the ground. This would also flexibly weight navigational decisions by cost without outright prevention of vertical navigation, which is beneficial for organisms such as humans, who have derived important benefits from vertical navigation over evolutionary time.
A previous investigation of ENT-derived predictions demonstrated that participants unknowingly overestimated vertical surface length at a magnitude corresponding to the potential falling risk (Jackson & Cormack, 2007a). However, many surfaces cast images that are oriented vertically with respect to the observer's head or eyes (i.e., egocentrically vertical) — including surfaces with trivial falling risk. For example, looking down at the distance from one's feet to a distant point ahead on horizontal ground casts an egocentrically vertical image on the retinae, yet poses negligible falling risk. Although researchers commonly assume that the retinal image largely determines perceived distance, it obviously does not predict falling costs accurately enough to decrease them.
The feature that does predict falling cost is environmental, or exocentric, verticality: the extent to which a surface parallels the direction of primary gravitational force. In order for the vertical overestimation derived from ENT to result in appropriate falling cost avoidance, it should exaggerate exocentrically vertical surface length, with little regard for egocentric verticality.
We addressed this question in the current study by comparing real-world distance estimates across effectively equal egocentric images that nonetheless had different exocentric orientations corresponding to very different falling costs. We predicted from ENT that participants would overestimate distance only from exocentrically vertical surfaces because such surfaces posed distinct falling costs over evolutionary time.
We also varied stimulus length in the current study, which we predicted could affect the hypothesized overestimation in one of two ways. First, participants might overestimate by a constant percentage of the stimulus length (i.e., Weber's law) because such a simple algorithm might provide sufficient falling cost avoidance. Alternatively, participants might overestimate by an ever-greater magnitude as stimulus length increases because longer vertical surfaces at these distances pose both greater likelihood and overall cost of falling.
Section snippets
Methods
Thirty-eight introductory psychology participants reporting normal (20:20) or corrected-to-normal vision estimated distances in an outdoor testing site. Fig. 1 schematically illustrates participants' distance estimates.
Results
Participants overestimated exocentrically vertical distances only and did so to a large, increasing degree as stimulus length increased (see Fig. 2). Distances that appear vertical on the retinae were only overestimated if they were vertical in the environment, with longer distances overestimated by greater magnitudes. Participants underestimated exocentrically horizontal distances to a slight degree that paralleled accuracy.
All six estimates significantly differ from both accuracy [the least
Discussion
As predicted from ENT, participants overestimated exocentrically vertical distances and did not overestimate exocentrically horizontal distances — despite the fact that actual distance was equal across both orientations and that egocentric orientation and image size were highly similar. We title this the environmental vertical illusion.
We found the environmental vertical illusion at a very large magnitude for a previously unknown psychological process that likely occurs constantly throughout
Acknowledgments
We thank Mike Domjan for research materials; Jenée James Jackson, Tasha Beretvas, Clarke Burnham, Randy Diehl, and Joe Horn for suggestions; Zach Keeton and Germaine Ho for collecting data; and reviewers for comments. We also acknowledge direction on citations by editor Martie Haselton.
References (26)
- et al.
A scenario analysis of ladder fall accidents
Journal of Safety Research
(1991) - et al.
Follow-up investigations of slip, trip and fall accidents among postal delivery workers
Safety Science
(1999) Natural selection and the regulation of defenses: A signal detection analysis of the smoke detector principle
Evolution and Human Behavior
(2005)- et al.
- et al.
The vertical–horizontal illusion in a visually-rich environment
Perception and Psychophysics
(1967) - et al.
Judging distance across texture discontinuities
Perception
(2003) The senses considered as perceptual systems
(1966)The ecological approach to visual perception
(1979)- et al.
Error management theory: A new perspective on biases in cross-sex mind reading
Journal of Personality and Social Psychology
(2000) Horizontal and vertical distance perception: The discorded-orientation theory
Perception and Psychophysics
(1996)
Human spatial orientation
Falling towards a theory of the vertical–horizontal illusion
Evolved navigation theory and the descent illusion
Perception and Psychophysics
Cited by (34)
When humans can fly: Imprecise vertical encoding in human 3D spatial navigation
2022, Behavioural Brain ResearchCitation Excerpt :Such anisotropy in spatial memories is consistent with the pattern of the vertical-horizontal illusions (i.e., overestimation of the length of a vertical line compared with the same length of a horizontal line) in visual perception [25]. Such anisotropy in spatial memories could be due to humans’ larger horizontal visual field than their vertical visual field [23], higher risks of falling in the vertical dimension than horizontal dimension [16], or organizing memories of a multilevel building into regions according to horizontal levels so that the distance between two locations across regions (i.e., levels) appears to be longer than the same distance between two locations within a region (level) [4,13,24]. However, the above anisotropy in 3D spatial memories (i.e., human participants overestimated vertical distance but underestimated horizontal distance) only indicates differences in encoding vertical and horizontal information; it does not necessarily indicate a horizontal advantage (i.e., more precise coding horizontally) in 3D spatial memories.
The influence of locomotory style on three-dimensional spatial learning
2018, Animal BehaviourEvolved navigation theory and the plateau illusion
2013, CognitionCitation Excerpt :The away estimates were shorter than both the actual stimulus and observers’ perceptions of a surface without falling risks (i.e. ground). Underestimation in order to increase safe navigation extends directly from the ENT concept of overestimation in order to decrease risky navigation outlined in previous literature (Jackson, 2005, 2009, 2013; Jackson & Cormack, 2006, 2007, 2008, 2010; Jackson & Willey, 2011). Although the visual setting of Experiment 1 did not generate a clear underestimation, we observed the expected underestimation across all expected distances in Experiment 2, plausibly generated by the presence of clear navigation risks.
Evolved navigation theory and horizontal visual illusions
2011, CognitionCitation Excerpt :Current participants did not overestimate surfaces without falling risk and ranged from 9% overestimation at low falling risks to 47% at high falling risks. This is nearly the exact range found for the corresponding situations in vertical surfaces (Jackson & Cormack, 2008). Environments (and individuals) exhibiting even greater falling risk or avoidance are the exact situations in which overestimation magnitude exceeds the range found here (Jackson, 2009; Jackson & Cormack, 2007).
Dissociating two aspects of human 3D spatial perception by studying fighter pilots
2023, Scientific Reports