Raised Dot Enumeration Via Haptic Exploration

In two experiments we investigated blindfolded, sighted participants’ capacity to extract the number of raised dots from arrays of braille cells that they scanned once via active touch. The arrays could contain between one and 12 raised dots and estimates were based on scanning with one or more fingers on one or both hands (Experiment 1), or when the dots were as maximally or minimally spaced as the braille code permits (Experiment 2). We sought evidence of discontinuities in performance that reflect more than one mode of enumeration. We found that participants’ estimates of numerosity increased in a linear fashion with actual numerosity, but were increasingly underestimated beyond numerosity of six, and confidence in the judgment declined linearly with increasing numerosity. Finger combinations made no difference to accuracy, errors, or confidence. Increasing dot density had the effect of diminishing perceptual accuracy, exaggerating underestimation and reducing confidence. While perceptual accuracy was generally high up to six raised dots, patterns of confusions and scaling analyses suggest that numerosities of four or less are perceptually unique. In this article, we discuss these data in terms of enumeration in touch and other modalities, and consider whether this discontinuity in enumeration signifies a subitize-to-count or a count-to-estimate transition.