Abstract
BackgroundMuch previous work on how normal aging affects visual enumeration has been focused on the response time required to enumerate, with unlimited stimulus duration. There is a fundamental question, not yet addressed, of how many visual items the aging visual system can enumerate in a “single glance”, without the confounding influence of eye movements.Methodology/Principal FindingsWe recruited 104 observers with normal vision across the age span (age 21–85). They were briefly (200 ms) presented with a number of well- separated black dots against a gray background on a monitor screen, and were asked to judge the number of dots. By limiting the stimulus presentation time, we can determine the maximum number of visual items an observer can correctly enumerate at a criterion level of performance (counting threshold, defined as the number of visual items at which ≈63% correct rate on a psychometric curve), without confounding by eye movements. Our findings reveal a 30% decrease in the mean counting threshold of the oldest group (age 61–85: ∼5 dots) when compared with the youngest groups (age 21–40: 7 dots). Surprisingly, despite decreased counting threshold, on average counting accuracy function (defined as the mean number of dots reported for each number tested) is largely unaffected by age, reflecting that the threshold loss can be primarily attributed to increased random errors. We further expanded this interesting finding to show that both young and old adults tend to over-count small numbers, but older observers over-count more.Conclusion/SignificanceHere we show that age reduces the ability to correctly enumerate in a glance, but the accuracy (veridicality), on average, remains unchanged with advancing age. Control experiments indicate that the degraded performance cannot be explained by optical, retinal or other perceptual factors, but is cortical in origin.
Highlights
Jevons [1] addressed the question of how many objects the mind can ‘‘embrace at once’’, by enumerating the number of beans that fell into a box, in a single glance
When the stimulus duration is limited to 200 ms, approximately the latency of saccadic eye movements [35,36], observers do not to have enough time to initiate refixational eye movements to examine each dot in the display
We found that a 30% decrease of the threshold for the oldest age group
Summary
Jevons [1] addressed the question of how many objects the mind can ‘‘embrace at once’’, by enumerating the number of beans that fell into a box, in a single glance. Ignored until recently is Jevons observation that, beyond 4, numerical enumeration even for large numbers of beans, is on average, quite veridical, the errors being about 0.12 times the number (i.e., it obeys Weber’s Law). Subsequent studies involving response time measurement suggested that enumeration may be characterized by two distinct components [2,3,4,5,6]. Once the numerosity is beyond the subitizing range, i.e. 4 items or more, the process of enumeration requires more effort and becomes more error-prone. There is a fundamental question, not yet addressed, of how many visual items the aging visual system can enumerate in a ‘‘single glance’’, without the confounding influence of eye movements
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