In our experiment, the performance obtained for the dark condition with linear perspective was as accurate as that obtained in the full cue condition (for example, see Figure 7 ). This indicates that any information our observers used to judge object size was fully available in the dark with a linear perspective condition. Under the specific conditions of our experiment, it was impossible for observers to use the horizon relation discussed in the introduction1. All of our stimulus squares were elevated (above the floor of the miniature corridor) by the amount necessary for seated observers to look at the approximate center of the squares; the bases connecting the stimulus squares to the miniature corridor surface were not visible in the dark. According to Sedgwick1 (p. 21), “using the horizon to determine the height of an object presupposes that the base of the object is in contact with the ground. If it is not, then the horizon relation alone does not provide information about the height of the object; this is so because the portion of the object below the horizon is no longer equal to the height of the point of observation nor has any regular relation to it”.
If our observers did not use the ratio of the horizon ratio1 to perceive the size of the object, what optical information did they use? Here is a likely explanation. Remember that any information used by observers to make their judgments was available in the dark with a linear perspective condition. The linear perspective of our experiment was created by extending three rows of LEDs along both sides of the miniature corridor (see Figures 1, 3). It is important to note that our observers used binocular viewing. They were therefore able to determine which corridor LEDs were closest to the stimulus square on each trial (by finding which corridor LEDs had the smallest binocular disparity to the stimulus LEDs). Observers then know which section of the 7-m corridor is at the same distance as the stimulus. To determine the size at this distance, observers can simply compare the apparent size of the stimulus square itself (eg, visible height) with the adjacent vertical spacing between neighboring rows of LEDs in the corridor (for an explanation similar, see Figure 1). . 12 in Sedgwick10). Since the physically parallel rows of LEDs in the corridor extend almost to the plane of the observers’ heads, it would then be possible to calibrate the size of the perceived object at a distance into accurately judged physical sizes. Our observers’ judgments in the dark under a linear perspective condition were indeed quite accurate (for example, see the results for the filled circle symbol in Figure 7 ).
Our observers’ performance in the full-cue and dark with linear perspective conditions was similar and not significantly different from each other (e.g., see Figure 7 ). Note also the huge difference between the performance of observers in the dark (open circles) and in dark conditions with added linear perspective (filled circles). As discussed above, linear perspective supports not only the perception of distance8,11 but also the perception of object size (present results; see also Sedgwick10). Thus, our results support those of previous research that also found linear perspective to be a valuable source of perceptual information9.
The present results demonstrate that older adults can visually judge object size with as much precision and accuracy as younger adults. Several previous studies have found that older adults can show superior performance for distance judgments5,6. This age-related superiority did not occur in the current experiment. However, it is important to note that although aging produces large impairments for a variety of other visual tasks (such as those involving motion or the perceived 3D shape of motion12,13,14,15, 16,17,18,19,20). ,21,22), aging does not impair the ability to perceive object size. Our current findings demonstrate that while older adults’ judgments are somewhat different (e.g., in the full cue condition, they underestimate the sizes of larger objects, whereas younger adults overestimate the sizes of larger objects, compare the full cue results obtained for younger and older adults in Figure 1). 7), however, are as accurate and precise as people who are 50 years younger. Given the negative stereotypes that exist about the abilities of the elderly23, the current results and others5,6,24,25 are important because they show that a number of fundamental perceptual abilities are well preserved, at least until the age of 80 years.
While it is true that the judgments of the younger and older observers in our experiment were equally accurate overall (Fig. 7), there were nevertheless some significant differences in the observers’ functions relating physical size to the perceived size (Figs. 5, 6). ). The functions for younger adults had significantly higher slopes and were significantly more curved. Perhaps the age-related difference in slopes could be due to differences in how younger and older adults perceived the separation between neighboring rows of lateral LEDs when calibrating their judgments of object size at a distance ( see discussion above); this calibration should occur in the full-cue condition as well as in the dark with the linear perspective condition. The age-related difference in the curvature of the functions related to physical and perceived size is more mysterious. Given that older adults’ functions were more linear, their performance would seem to be superior in this respect (there is no obvious benefit for younger observers to have more accelerating functions in the full-cue and dark with conditions of linear perspective and by having more deceleration functions in dark conditions). Older adults have already been observed to demonstrate superior performance for distance judgments under some circumstances5,6. Therefore, it might not be surprising if older adults also show some analogous superiority in the visual perception of object size.
Our current findings of comparable performance (for object size estimation for physical objects viewed at real depth) for young and older adults are also analogous to those of Norman, Holmin, and Bartholomew26, who assessed aging and size/length. discrimination using computer generated stimuli. In their study, younger and older observers discriminated the length (i.e., size) of relatively short line segments (e.g., the standard length was 9.0 cm) displayed on the monitor d a computer. The performance of older adults in this study was consistently as good as that of younger adults: for example, when discriminating line length using the single-stimulus method (without feedback), the difference thresholds between younger and older observers were 4.74 and 4.71 percent of the standard, respectively.
As previously stated12,13,14,15,16,17,18,19,20,21,22, increasing age is accompanied by substantial deficits in the visual perception of movement speed, the direction of movement and the ability to perceive. 3D shape from movement. These behavioral abilities depend on the proper functionality of neurons within movement-sensitive cortical areas such as MT. In fact, an increase in the variability of the neuronal response to MT has been found for old monkeys27; these researchers concluded by saying (p. 24) “we hypothesize that a degradation of the GABA system in V1 and MT may be an important reason for both the increased response variability and the decreased response-to-noise ratio in old monkeys.” Aging also affects the direction selectivity of motion-sensitive MT neurons. 28 These authors found (p. 869) that “functional degradation occurs in both MT and V1 areas during aging normal, and this MT area is more severely affected by aging than the striate cortex.” Again, the cause of this degradation of neuronal functionality was (p. 871) “decreased GABAergic inhibition” . In this context, it is quite interesting to find in the current study that there was no age-related degradation in either the accuracy or precision of our observers’ judgments of object size. Given that the visual perception of object size depends on areas such as the lateral occipital cortex (LO)29, posterior parietal cortex30, intraparietal sulcus, and lateral prefrontal cortex31, our current behavioral findings suggest little or no degradation of intracortical inhibitory functionality in the interior these areas. It will be an important task for future neurophysiological research to discover why aging negatively affects cortical functioning in some areas but not others.