Defining filled and empty space: reassessing the filled space illusion for active touch and vision

Abstract

In the filled space illusion, an extent filled with gratings is estimated as longer than an equivalent extent that is apparently empty. However, researchers do not seem to have carefully considered the terms filled and empty when describing this illusion. Specifically, for active touch, smooth, solid surfaces have typically been used to represent empty space. Thus, it is not known whether comparing gratings to truly empty space (air) during active exploration by touch elicits the same illusionary effect. In Experiments 1 and 2, gratings were estimated as longer if they were compared to smooth, solid surfaces rather than being compared to truly empty space. Consistent with this, Experiment 3 showed that empty space was perceived as longer than solid surfaces when the two were compared directly. Together these results are consistent with the hypothesis that, for touch, the standard filled space illusion only occurs if gratings are compared to smooth, solid surfaces and that it may reverse if gratings are compared to empty space. Finally, Experiment 4 showed that gratings were estimated as longer than both solid and empty extents in vision, so the direction of the filled space illusion in vision was not affected by the nature of the comparator. These results are discussed in relation to the dual nature of active touch.

Keywords: Cutaneous; Filled space illusion; Kinaesthetic; Length; Touch; Vision.

Fig. 1

Typical configuration of the visual filled space illusion, where distance AB, filled with gratings, is perceived as longer than the unfilled, comparator distance BC. In fact, distances AB and BC are equal

Fig. 2

Setup for Experiments 1–3. The diagram shows two example trials using the four types of 12 cm stimuli: solid extents followed by 4-mm gratings (top) and 8-mm gratings followed by empty extents (bottom). The distance of each stimulus from the startpoint markers was varied by the experimenter so that the two successive stimuli on each trial were always placed in different positions

Fig. 3

Results of Experiment 1: a Cumulative Gaussian curve for one participant in the solid-gratings group (top) and one participant in the empty-gratings group (bottom) b Mean Cumulative Gaussian curves for each group. c Mean bias in cm for each group. Positive biases indicate that gratings were perceived as longer than the comparator (standard filled space illusion) whereas negative biases indicate that the comparator was perceived as longer than the gratings (reverse filled space illusion). All error bars show ± one standard error of the mean

Fig. 4

Mean biases in each condition in Experiment 2. Positive biases indicate show that gratings were perceived as longer than the comparator (standard filled space illusion), whereas negative biases indicate that the comparator was perceived as longer than the gratings (reverse filled space illusion). Error bars represent ± standard errors of the mean

Fig. 5

Reasoning behind Experiment 3. Though the results did not reach statistical significance, the results of Experiments 1 and 2 showed a trend where a gratings were perceived as longer than solid surfaces (standard filled space illusion), and b empty space was perceived as longer than gratings (reverse filled space illusion). Thus, in Experiment 3 we predicted that c empty space would be perceived as longer than solid surfaces

Fig. 6

Mean biases in each condition in Experiment 3. Positive biases indicate that empty space was perceived as longer than solid surfaces. Error bars represent ± standard errors of the mean

Fig. 7

a View of the box in which stimuli were presented in Experiment 4. b An illustration of the time course of two example trials in Experiment 4 using the 12 cm extents: top a solid then 4-mm gratings trial; bottom an 8-mm gratings then empty trial

Fig. 8

Mean biases in each condition of Experiment 4: Positive biases indicate that gratings were perceived as longer than the comparator (standard filled space illusion). Error bars represent ± one standard error of the mean

Fig. 9

Diagram summarising our interpretation of the results of Experiments 1–3. The increased cutaneous input from the low density gratings means that they are perceived as longer than solid surfaces. Feeling high density gratings provides even more cutaneous input so these are perceived as still longer. However, when cutaneous input is minimal, for the empty extents, participants may switch strategies for length estimation and rely only on kinaesthetic information. This may lead to empty space being perceived as longer than the other kinds of stimuli