Experiment 2: Effects of Subliminal Bidirectional Modulation of Numerical Magnitude Perception on Prosocial Decision Making

In experiment 2, we proceeded to examine the effects of subliminally altering a participant’s perception of numerical magnitude on their formulation of prosocial choices during both the dictator game task and an altruism questionnaire.

We have recently demonstrated that certain combined visuo-vestibular stimuli can selectively induce interhemispheric conflict. For example, the combination of binocular rivalry that is predominantly processed by the right hemisphere (Knapen et al. 2011; Lumer et al. 1998) and a vestibular stimulus that is predominantly processed in the left hemisphere (either right sided cold (RIGHTCOLD) or left sided warm (LEFTWARM) water caloric irrigations) (Arshad 2014; Bronstein et al. 2015; Dieterich et al. 2003; Nigmatullina et al. 2016; Suzuki et al. 2001) will result in competition between the two hemispheres leading to unihemispheric inhibition.

The net result of interhemispheric conflict and subsequent unihemispheric inhibition is a systematic bias in numerical magnitude allocation. Namely, when combining binocular rivalry with a left-sided warm irrigation (LEFTWARM + RIV), there is a predominant left hemisphere response, subsequently biasing judgements toward larger numbers, whereas during the combination of rivalry with right-sided cold irrigations (RIGHTCOLD + RIV), there is left hemisphere inhibition and subsequent biasing of numerical judgements toward smaller numbers (Arshad et al. 2016b). It is important to note that no interhemispheric conflict occurs when rivalry is combined with caloric irrigations that are predominantly processed in the right hemisphere (i.e., LEFTCOLD or RIGHTWARM), providing an internal experimental control for any nonspecific effects associated with combined stimulation such as dizziness, visuo-vestibular mis-match, generalized arousal, attention, and eye movements (Arshad et al. 2016b).

Following combined stimulation, the numerical biasing observed is in line with previous findings from lesion studies that have demonstrated that right hemisphere front-parietal lesions induce a pathological bias during number pair bisection toward larger numerical magnitudes attributable to a left hemisphere predominant response (Aiello et al. 2012; Doricchi et al. 2005; Zorzi et al. 2002). One account for this biasing is that the lesion induces a rightward spatial attentional bias leading to a rightward shift on the mental number line toward larger numerical magnitudes (Dehaene et al. 1993; Zorzi et al. 2002). However, more recent research has challenged this account by demonstrating a double dissociation between numerical and spatial biases following right hemisphere lesions (Aiello et al. 2012; Doricchi et al. 2005). Moreover, our recent findings in healthy subjects to induce bidirectional numerical biases during combined visuo-vestibular stimulation do not simultaneously induce a spatial bias as assessed by straight ahead pointing (Arshad et al. 2016b). Thus it is proposed that there is context-dependent hemispheric allocation of numerical magnitude that is dissociated from spatial mechanisms. That is, the right hemisphere preferentially encodes and allocates smaller numerical magnitudes whereas the left hemisphere is preferentially involved in the encoding and allocation of larger numerical magnitudes (Arshad et al. 2016b). Thus RIGHTCOLD + RIV biases judgments toward smaller numbers, whereas LEFTWARM + RIV biases judgements toward larger magnitudes (Arshad et al. 2016b).