Examining the Role of Object Size in Judgments of Lateral Separation

Examining the Role of Object Size in Judgments of Lateral Separation Robert Thomson (rthomson@connect.carleton.ca) Guy Lacroix (guy_lacroix@carleton.ca) Psychology Department, Carleton University 1125 Colonel By Drive, Ottawa, ON K1S5B6 Canada asked to judge how far away an object feels to them (a Abstract judgment of apparent distance), their answers tend to ignore Research on depth judgments has found a small but familiar size cues and correspond more to the object’s significant effect of object size on perceived depth (Gogel & Da Silva, 1987). Research on judgments of separation (e.g., angular size. If instead observers are asked to judge how far Levin & Haber, 1993), however, has found that visual angle away an object would be if measured with a meter stick (its is the predominant determiner of exocentric distance objective distance), then their answers tend to be influenced judgments. The goal of the present research was to examine by familiar size cues. No process specified in the size- the influence of object size on judgments of lateral separation distance invariance hypothesis captures the effect of task using a one-shot change detection paradigm. Experiment 1 instruction on size and distance judgments. used a forced-choice response, finding a significant influence of object size on distance judgments. Experiment 2 replicated In addition, visual illusions have also provided evidence these results using a distance reproduction task. These results not captured by the SDIH. For instance, the zenith moon are discussed in terms of Gogel and Da Silva’s (1987) Theory appears both smaller and farther away than the horizon of Off-Sized Perceptions. moon, a finding that is inconsistent with the size-distance Keywords: distance estimation; perception; spatial cognition; invariance hypothesis (Kaufman & Rock, 1962). The size- change detection; distance invariance hypothesis instead predicts that when the moon appears smaller at its zenith, it should appear Introduction closer than when along the horizon, not farther away. This The ability to recognize objects and judge distances is incongruence is called the size-distance paradox. essential for navigating through the environment. During Dissociating Perceived Size and Cognitive Size object recognition, an observer will automatically determine To account for the differential effect of task instruction and its angular size and, in many cases, then perceive the object illusions such as the size-distance paradox, the SDIH was as having an expected size or range of acceptable sizes (i.e., modified to represent a relation between apparent size, its familiar size; Haber & Levin, 2001). For instance, once a apparent distance, and (arguably) apparent visual angle student recognizes that an object on a table is a beer bottle, (McCready, 1985). This single-process model of size and the student has access to its familiar size (around 20 cm distance perception presupposed that size judgments could tall). This metrical size judgment is also referred to as the not dissociate linear size determined primarily by angular linear size of an object. In addition to size perception, there size from linear size influenced by familiar size and other are two types of distance perceptions: egocentric and “cognitive” cues. In other words, perceived linear size and exocentric. Egocentric distance perception is the perception cognitive size judgments could not be disentangled and were of an object in depth, that is, a judgment made of the thus part of an encapsulated process. distance between one’s self and an object. Using a football To test the single process model, Gogel (1976) developed analogy, it is the kind of perception that the quarterback an indirect head-motion tracking technique to measure uses when he decides how far to throw the ball downfield to perceived egocentric distance. He found only a negligible reach his receiver. Exocentric distance perception is the effect of familiar size on perceived distance. Observers perception of inter-object distance, that is, a judgment made made both verbal and head-motion judgments of the of the distance between two objects irrespective of observer distance of similar-sized transparencies of three familiar position. Returning to the football analogy, exocentric objects (a key, sunglasses, and a guitar) whose sizes distance perception arises, for instance, when the simulated distances of 63 cm, 185 cm, and 1236 cm quarterback estimates the distance between his wide respectively. The actual distance of the transparencies was receiver and the opposing team's defensive back. 133 cm. Familiar size cues influenced verbal reports of Size-Distance Invariance distance and, to a lesser extent, head-motion responses. The The size-distance invariance hypothesis (SDIH) was an actual size of a guitar is 19.6 times the size of the key. early attempt to capture the relationship between size and Nonetheless, the transparency of the guitar was reported to distance perception (Filpatrick & Ittelson, 1953). It states be only 10 times the size of the transparency of the key for that observers expect that an object at a relatively farther verbal reports and 1.4 times the size of the key in head- distance will project a smaller retinal image size than the motion responses. This result is noteworthy considering that same object at a relatively closer distance. While the the study used apparent instructions, which should have majority of size and distance judgments are consistent with reduced the effectiveness of familiar size cues. While both this hypothesis, task instructions and certain visual illusions verbal reports and head-motion responses exhibited familiar have generated apparently contradictory evidence (Epstein, size effects, they were much more pronounced in the verbal 1963 McCready; 1985). For instance, when observers are reports. This evidence suggested that the cognitive size