Perceiving affordances for fitting through apertures

Abstract

Affordances--possibilities for action--are constrained by the match between actors and their environments. For motor decisions to be adaptive, affordances must be detected accurately. Three experiments examined the correspondence between motor decisions and affordances as participants reached through apertures of varying size. A psychophysical procedure was used to estimate an affordance threshold for each participant (smallest aperture they could fit their hand through on 50% of trials), and motor decisions were assessed relative to affordance thresholds. Experiment 1 showed that participants scale motor decisions to hand size, and motor decisions and affordance thresholds are reliable over two blocked protocols. Experiment 2 examined the effects of habitual practice: Motor decisions were equally accurate when reaching with the more practiced dominant hand and less practiced nondominant hand. Experiment 3 showed that participants recalibrate motor decisions to take changing body dimensions into account: Motor decisions while wearing a hand-enlarging prosthesis were similar to motor decisions without the prosthesis when data were normalized to affordance thresholds. Across experiments, errors in decisions to reach through too-small apertures were likely due to low penalty for error.

Figure 1

Adjustable aperture apparatus. Participants sat on a swivel chair and reached through the aperture for a target. Between trials, participants turned away from the aperture while an experimenter adjusted the size of the aperture by pulling the handle.

Figure 2

(A) Proportion of successful reaches (S/(S+F)) for S#8 in one condition plotted by absolute aperture size, overlaid with the affordance function fitted to the success rate. The horizontal dashed line denotes 50% success and the vertical dashed line denotes the aperture size at the 50% success rate (affordance threshold). (B) Proportion of attempted reaches for S#8 plotted by absolute aperture size and normalized to the affordance threshold (lower axis). Solid vertical line denotes the affordance threshold.

Figure 3

Individual affordance thresholds in (A) Experiment 1 for the first (open circles) and second conditions (closed circles), (B) Experiment 2 for dominant (open circles) and non-dominant hands (filled circles), and (C) Experiment 3 for normal hand (open circles) and big hand conditions (closed circles).

Figure 4

Mean proportion of attempts at each aperture group for (A) Experiment 1, (B) Experiment 2 and (C) Experiment 3. Attempts are normalized to each participant’s affordance threshold, denoted by the solid vertical line at 0 cm, for each condition. On the x-axis, passable apertures are represented by positive numbers to the left of the affordance threshold, and impassable apertures are represented by negative numbers to the right of the affordance threshold. Error bars denote standard errors.

Figure 5

The distribution of hand orientations within each aperture group for Experiment 1 (top row), Experiment 2 (middle row), and Experiment 3 (bottom row). Vertical stripes indicate palm facing up. Diagonal stripes indicate palm sideways and thumb down. Solid black indicates palm sideways and thumb facing up. Solid white indicates palm facing down. Note, the number of participants contributing data to each aperture group is given above each bar.

Figure 6

The distribution of participants’ exploratory behaviors on refusal trials within each aperture group for Experiment 1 (top row), Experiment 2 (middle row), and Experiment 3 (bottom row). Solid white bars indicate participants saying “No” without moving their hands. Diagonal stripes indicate lifting the hand to the aperture but withdrawing the arm without attempting to reach. Vertical stripes indicate inserting 1 or 2 fingertips into the aperture before retracting the arm. Solid black indicates inserting all fingertips into the aperture before retracting the arm. Note, the number of participants contributing data to each aperture group is given above each bar.

Figure 7

Neoprene prostheses worn in Experiment 3. Velcro straps across the palm and wrist secured the prosthesis to the hand. (A) Unpadded prosthesis for normal hand condition. (B) Padded prosthesis for big hand condition.

Figure 8

Overall average proportion of attempts by absolute aperture size for the normal and big hand conditions for Experiment 3.

Figure 9

Overall average proportion of attempts at the normal and big hand thresholds in the normal and big hand conditions in Experiment 3.