Bimanual thumb-index finger indications of noncorresponding extents

Klaus Landwehr

doi:10.3758/s13414-021-02360-8

Abstract

Two experiments tested a prediction derived from the recent finding that the Oppel-Kundt illusion – the overestimation of a filled extent relative to an empty one – was much attenuated when the empty part of a bipartite row of dots was vertical and the filled part horizontal, suggesting that the Horizontal-vertical illusion – the overestimation of vertical extents relative to horizontal ones – only acted on the empty part of an Oppel-Kundt figure. Observers had to bimanually indicate the sizes of the two parts of an Oppel-Kundt figure, which were arranged one above the other with one part vertical and the other part tilted -45°, 0°, or 45°. Results conformed to the prediction but response bias was greater when observers had been instructed to point to the extents’ endpoints than when instructed to estimate the extents’ lengths, suggesting that different concepts and motor programs had been activated.

Highlights

(2) Using digital indications as an alternative method to measure observers’ performance, to provide a replication of the recent finding, obtained with verbal judgments, that the overestimation of vertical extents relative to horizontal ones only seems to hold for empty extents as opposed to subdivided ones (Landwehr, 2021)
(3) To provide a comparison of digital indications and verbal judgments considered as psychophysical research methods
It was important to analyze data separately for the two types of stimuli with either the filled part at a fixed vertical orientation or the empty part in this role

Summary

Introduction

The purpose of the present paper is threefold: (1) To provide empirical evidence that humans are able to simultaneously indicate noncorresponding extents by spreading thumb and index finger of their hands – as postulated by Gibson (1966, pp. 119-120). (2) Using digital indications as an alternative method to measure observers’ performance, to provide a replication of the recent finding, obtained with verbal judgments, that the overestimation of vertical extents relative to horizontal ones only seems to hold for empty extents as opposed to subdivided ones (Landwehr, 2021). (3) To provide a comparison of digital indications and verbal judgments considered as psychophysical research methods. (2) Using digital indications as an alternative method to measure observers’ performance, to provide a replication of the recent finding, obtained with verbal judgments, that the overestimation of vertical extents relative to horizontal ones only seems to hold for empty extents as opposed to subdivided ones (Landwehr, 2021). (3) To provide a comparison of digital indications and verbal judgments considered as psychophysical research methods. In the wake of Aglioti et al.’s (1995) influential publication “Size-contrast illusions deceive the eye but not the hand”, manual-size indications have often been used as a “perceptual” measure of observers’ susceptibility to visual illusions, and been compared to manifest grasping (e.g., Daprati & Gentilucci, 1997; Franz, 2003; Haffenden & Goodale, 1998; Westwood et al, 2000). I discuss different methods of analyzing data, and show that comparisons to other forms of motor behavior (notably, grasping) and to verbal judgments are severely limited

Objectives

Methods

Results