Grasping tiny objects

Abstract

In grasping studies, maximum grip aperture (MGA) is commonly used as an indicator of the object size representation within the visuomotor system. However, a number of additional factors, such as movement safety, comfort, and efficiency, might affect the scaling of MGA with object size and potentially mask perceptual effects on actions. While unimanual grasping has been investigated for a wide range of object sizes, so far very small objects (<5 mm) have not been included. Investigating grasping of these tiny objects is particularly interesting because it allows us to evaluate the three most prominent explanatory accounts of grasping (the perception-action model, the digits-in-space hypothesis, and the biomechanical account) by comparing the predictions that they make for these small objects. In the first experiment, participants ( $N=26$ ) grasped and manually estimated the height of square cuboids with heights from 0.5 to 5 mm. In the second experiment, a different sample of participants ( $N=24$ ) performed the same tasks with square cuboids with heights from 5 to 20 mm. We determined MGAs, manual estimation apertures (MEA), and the corresponding just-noticeable differences (JND). In both experiments, MEAs scaled with object height and adhered to Weber's law. MGAs for grasping scaled with object height in the second experiment but not consistently in the first experiment. JNDs for grasping never scaled with object height. We argue that the digits-in-space hypothesis provides the most plausible account of the data. Furthermore, the findings highlight that the reliability of MGA as an indicator of object size is strongly task-dependent.

Fig. 1

Predictions of three explanatory accounts of grasping—perception-action model (1st column), digits-in-space hypothesis (2nd column), biomechanical account (3rd column)—for the slopes of MGAs and JNDs for grasping for Experiments A (top row) and B (bottom row). In Experiment A, object heights ranged from 0.5 to 5 mm and in Experiment B from 5 to 20 mm. Lines indicate whether MGAs/JNDs linearly increase with object height (lines with slope > 0) or not (lines with slope = 0). See text for details

Fig. 2

Setup with example stimulus. At the beginning of each trial, the participants’ index finger and thumb rested on the front button of the button box. After an auditory signal, they reached up to grasp the stimulus held in place by a clothes-peg. They placed the stimulus on the table and then placed their finger and thumb back on the front button

Fig. 3

Stimuli used in Experiment A (0.5–5 mm, magenta outline) and Experiment B (5–20 mm, green outline). The width and depth of all stimuli was 5 $\times$ 5 cm

Fig. 4

MGAs (a) and MEAs (b) for Experiment A (black data points) and Experiment B (blue data points). Error bars show ±1 SEM (between-subjects). Red lines are regression lines based on the averaged slopes and intercepts resulting from fits to individual participants’ MGAs/MEAs (also see Figures S1–S4 in the Supplemental material). The cyan line is the regression line fitted to the MGAs for object heights from 0.5 to 4 mm, i.e., excluding the highest object (5 mm). Note that MGAs/MEAs include the height of index finger and thumb and of the two markers sitting on top of the digits

Fig. 5

JNDs for grasping (a) and manual estimation (b) for Experiment A (black data points) and Experiment B (blue data points). JNDs were computed as the mean of the standard deviations of individual participants’ MGAs/MEAs. Error bars show ±1 SEM (between-subjects). Red lines are regression lines based on the averaged regression parameters resulting from fits to individual participants’ JNDs