Raised dot number perception (subitizing?) via haptic exploration*

In two experiments we investigated blindfolded, sighted participants’ capacity to extract the number of raised dots from arrays of braille cells 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 spatially compact or as spatially separated as the braille code permits (Experiment 2). We found participants’ estimates of numerosity increased in a strongly linear fashion with actual numerosity, and confidence in the judgment declined linearly with increasing numerosity. Finger combinations made no difference to accuracy, errors, or confidence. Spatially compacting the configuration of dots had the effect of diminishing perceptual accuracy, exaggerating underestimation and reducing confidence. We found partial evidence that perceptual accuracy was particularly high with up to six raised dots but beyond six, accuracy and confidence both diminished and variance increased. We interpret the results in terms of haptic information processing demands in space and time.