Pivots for pointing: Visually-monitored pointing has higher arm elevations than pointing blindfolded.

Abstract

Observers pointing to a target viewed directly may elevate their fingertip close to the line of sight. However, pointing blindfolded, after viewing the target, they may pivot lower, from the shoulder, aligning the arm with the target as if reaching to the target. Indeed, in Experiment 1 participants elevated their arms more in visually monitored than blindfolded pointing. In Experiment 2, pointing to a visible target they elevated a short pointer more than a long one, raising its tip to the line of sight. In Experiment 3, the Experimenter aligned the participant's arm with the target. Participants judged they were pointing below a visually monitored target. In Experiment 4, participants viewing another person pointing, eyes-open or eyes-closed, judged the target was aligned with the pointing arm. In Experiment 5, participants viewed their arm and the target via a mirror and posed their arm so that it was aligned with the target. Arm elevation was higher in pointing directly.

Different damping responses explain vertical endpoint error differences between visual conditions

“…Eliminating vision not only produced greater endpoint variability (Fig. 3b), as often observed in the literature Hondzinski and Cui 2006;Heath and Binsted 2007;Hondzinski and Kwon 2009;Elliott and Hansen 2010;Elliott et al 2014), it also accompanied a greater tendency to vertically undershoot remember target locations (Soechting and Flanders 1989b;Smetanin and Popov 1997;Henriques et al 1998;Crawford et al 2000;Henriques and Crawford 2002;Admiraal et al 2003;Wnuczko and Kennedy 2011;Elliott et al 2014) and endpoints achieved in the LIGHT condition (Soechting and Flanders 1989b;Flanders et al 1999;Hondzinski and Cui 2006;Wnuczko and Kennedy 2011;Hondzinski and Soebbing 2015). Undershooting in the dark occurred for each body orientation and starting arm position and corroborates evidence that movement excursions decrease for larger movement amplitudes when the arm is not visible (Bock and Eckmiller 1986) and that differences in endpoint precision between visual conditions occurs for targets requiring relatively large hand displacements (Henriques et al 2003;Hondzinski and Cui 2006).…”

“…Reaching lower along the earth-fixed vertical in darkness occurs whether the person sits with closed eyes (Berkinblit et al 1995;Adamovich et al 1998), wearing opaque goggles (Elliott et al 2014), or in a dark room (Brown et al 1948;Henriques et al 1998;Crawford et al 2000;Henriques and Crawford 2000), stands with eyes closed (Smetanin and Popov 1997), while blindfolded (Wnuczko and Kennedy 2011), or in a dark room (Admiraal et al 2003), or takes a step with eyes closed (Flanders et al 1999) or in a dark room (Admiraal et al 2004;Hondzinski and Cui 2006). Most recently, subjects lying supine also ended 3D reaches lower when lights were off (Hondzinski and Soebbing 2015).…”

“…Some researchers blame the difference in endpoint accuracy between lighting conditions on the use of different egocentric reference points used for control. When performing straight arm pointing movements toward remembered target locations on the floor in an illuminated environment, subjects placed their fingertip along the line connecting the eyes and target (Wnuczko and Kennedy 2011). In that study, the same subjects ended the pointing movement with their fingertip or pointing device closer to the line connecting the shoulder and target line when blindfolded.…”

Mirror image arm used in monocular, binocular, and blindfolded pointing

“…4). The apparatus is a variation of one used by Wnuczko and Kennedy (2011) to measure arm elevation. The apparatus allowed the outstretched arm to move up and down (± 180°) and right and left (± 60°).…”

“…For example, Wnuczko and Kennedy (2011) tested onlookers judging other people pointing at real targets and participants actually pointing at targets selected by the experimenter. Onlookers judged the target to be in the direction of an imaginary line extending from the pointing person's arm and finger.…”