A standard test of how we perceive local vs global features of visual objects uses Navon figures — large letters made up of smaller ones (see below for an example). As in the Stroop test when colors and color words disagree (RED), the viewer can focus either on the large letter or the smaller ones. When the viewer is faster at seeing the larger letter, they are said to be showing global precedence; when they’re faster at seeing the component letters, they are said to be showing local precedence. Typically, the greater the number of component letters, the easier it is to see the larger letter. This is consistent with the Gestalt principles of proximity and continuity — elements that are close together and form smooth lines will tend to be perceptually grouped together and seen as a unit (the greater the number of component letters, the closer they will be, and the smoother the line).
In previous research, older adults have often demonstrated local precedence rather than global, although the results have been inconsistent. One earlier study found that older adults performed poorly when asked to report in which direction (horizontal or vertical) dots formed smooth lines, suggesting an age-related decline in perceptual grouping. The present study therefore investigated whether this decline was behind the decrease in global precedence.
In the study 20 young men (average age 22) and 20 older men (average age 57) were shown Navon figures and asked whether the target letter formed the large letter or the smaller letters (e.g., “Is the big or the small letter an E?”). The number of component letters was systematically varied across five quantities. Under such circumstances it is expected that at a certain level of letter density everyone will switch to global precedence, but if a person is impaired at perceptual grouping, this will occur at a higher level of density.
The young men were, unsurprisingly, markedly faster than the older men in their responses. They were also significantly faster at responding when the target was the global letter, compared to when it was the local letter (i.e. they showed global precedence). The older adults, on the other hand, had equal reaction times to global and local targets. Moreover, they showed no improvement as the letter-density increased (unlike the young men).
It is noteworthy that the older men, while they failed to show global precedence, also failed to show local precedence (remember that results are based on group averages; this suggests that the group was evenly balanced between those showing local precedence and those showing global precedence). Interestingly, previous research has suggested that women are more likely to show local precedence.
The link between perceptual grouping and global precedence is further supported by individual differences — older men who were insensitive to changes in letter-density were almost exclusively the ones that showed persistent local precedence. Indeed, increases in letter-density were sometimes counter-productive for these men, leading to even slower reaction times for global targets. This may be the result of greater distractor interference, to which older adults are more vulnerable, and to which this sub-group of older men may have been especially susceptible.
Example of a Navon figure: