Influence of biases in numerical magnitude allocation on human prosocial decision making.

Over the past decade neuroscientific research has attempted to probe the neurobiological underpinnings of human prosocial decision making. Such research has almost ubiquitously employed tasks such as the dictator game or similar variations (i.e., ultimatum game). Considering the explicit numerical nature of such tasks, it is surprising that the influence of numerical cognition on decision making during task performance remains unknown. While performing these tasks, participants typically tend to anchor on a 50:50 split that necessitates an explicit numerical judgement (i.e., number-pair bisection). Accordingly, we hypothesize that the decision-making process during the dictator game recruits overlapping cognitive processes to those known to be engaged during number-pair bisection. We observed that biases in numerical magnitude allocation correlated with the formulation of decisions during the dictator game. That is, intrinsic biases toward smaller numerical magnitudes were associated with the formulation of less favorable decisions, whereas biases toward larger magnitudes were associated with more favorable choices. We proceeded to corroborate this relationship by subliminally and systematically inducing biases in numerical magnitude toward either higher or lower numbers using a visuo-vestibular stimulation paradigm. Such subliminal alterations in numerical magnitude allocation led to proportional and corresponding changes to an individual's decision making during the dictator game. Critically, no relationship was observed between neither intrinsic nor induced biases in numerical magnitude on decision making when assessed using a nonnumerical-based prosocial questionnaire. Our findings demonstrate numerical influences on decisions formulated during the dictator game and highlight the necessity to control for confounds associated with numerical cognition in human decision-making paradigms.NEW & NOTEWORTHY We demonstrate that intrinsic biases in numerical magnitude can directly predict the amount of money donated by an individual to an anonymous stranger during the dictator game. Furthermore, subliminally inducing perceptual biases in numerical-magnitude allocation can actively drive prosocial choices in the corresponding direction. Our findings provide evidence for numerical influences on decision making during performance of the dictator game. Accordingly, without the implementation of an adequate control for numerical influences, the dictator game and other tasks with an inherent numerical component (i.e., ultimatum game) should be employed with caution in the assessment of human behavior.

Temporoparietal encoding of space and time during vestibular-guided orientation.

When we walk in our environment, we readily determine our travelled distance and location using visual cues. In the dark, estimating travelled distance uses a combination of somatosensory and vestibular (i.e., inertial) cues. The observed inability of patients with complete peripheral vestibular failure to update their angular travelled distance during active or passive turns in the dark implies a privileged role for vestibular cues during human angular orientation. As vestibular signals only provide inertial cues of self-motion (e.g., velocity, °/s), the brain must convert motion information to distance information (a process called 'path integration') to maintain our spatial orientation during self-motion in the dark. It is unknown, however, what brain areas are involved in converting vestibular-motion signals to those that enable such vestibular-spatial orientation. Hence, using voxel-based lesion-symptom mapping techniques, we explored the effect of acute right hemisphere lesions in 18 patients on perceived angular position, velocity and motion duration during whole-body angular rotations in the dark. First, compared to healthy controls' spatial orientation performance, we found that of the 18 acute stroke patients tested, only the four patients with damage to the temporoparietal junction showed impaired spatial orientation performance for leftward (contralesional) compared to rightward (ipsilesional) rotations. Second, only patients with temporoparietal junction damage showed a congruent underestimation in both their travelled distance (perceived as shorter) and motion duration (perceived as briefer) for leftward compared to rightward rotations. All 18 lesion patients tested showed normal self-motion perception. These data suggest that the cerebral cortical regions mediating vestibular-motion ('am I moving?') and vestibular-spatial perception ('where am I?') are distinct. Furthermore, the congruent contralesional deficit in time (motion duration) and position perception, seen only in temporoparietal junction patients, may reflect a common neural substrate in the temporoparietal junction that mediates the encoding of motion duration and travelled distance during vestibular-guided navigation. Alternatively, the deficits in timing and spatial orientation with temporoparietal junction lesions could be functionally linked, implying that the temporoparietal junction may act as a cortical temporal integrator, combining estimates of self-motion velocity over time to derive an estimate of travelled distance. This intriguing possibility predicts that timing abnormalities could lead to spatial disorientation.

Bidirectional Modulation of Numerical Magnitude.

Numerical cognition is critical for modern life; however, the precise neural mechanisms underpinning numerical magnitude allocation in humans remain obscure. Based upon previous reports demonstrating the close behavioral and neuro-anatomical relationship between number allocation and spatial attention, we hypothesized that these systems would be subject to similar control mechanisms, namely dynamic interhemispheric competition. We employed a physiological paradigm, combining visual and vestibular stimulation, to induce interhemispheric conflict and subsequent unihemispheric inhibition, as confirmed by transcranial direct current stimulation (tDCS). This allowed us to demonstrate the first systematic bidirectional modulation of numerical magnitude toward either higher or lower numbers, independently of either eye movements or spatial attention mediated biases. We incorporated both our findings and those from the most widely accepted theoretical framework for numerical cognition to present a novel unifying computational model that describes how numerical magnitude allocation is subject to dynamic interhemispheric competition. That is, numerical allocation is continually updated in a contextual manner based upon relative magnitude, with the right hemisphere responsible for smaller magnitudes and the left hemisphere for larger magnitudes.

The neuroanatomical correlates of training-related perceptuo-reflex uncoupling in dancers.

Sensory input evokes low-order reflexes and higher-order perceptual responses. Vestibular stimulation elicits vestibular-ocular reflex (VOR) and self-motion perception (e.g., vertigo) whose response durations are normally equal. Adaptation to repeated whole-body rotations, for example, ballet training, is known to reduce vestibular responses. We investigated the neuroanatomical correlates of vestibular perceptuo-reflex adaptation in ballet dancers and controls. Dancers' vestibular-reflex and perceptual responses to whole-body yaw-plane step rotations were: (1) Briefer and (2) uncorrelated (controls' reflex and perception were correlated). Voxel-based morphometry showed a selective gray matter (GM) reduction in dancers' vestibular cerebellum correlating with ballet experience. Dancers' vestibular cerebellar GM density reduction was related to shorter perceptual responses (i.e. positively correlated) but longer VOR duration (negatively correlated). Contrastingly, controls' vestibular cerebellar GM density negatively correlated with perception and VOR. Diffusion-tensor imaging showed that cerebral cortex white matter (WM) microstructure correlated with vestibular perception but only in controls. In summary, dancers display vestibular perceptuo-reflex dissociation with the neuronatomical correlate localized to the vestibular cerebellum. Controls' robust vestibular perception correlated with a cortical WM network conspicuously absent in dancers. Since primary vestibular afferents synapse in the vestibular cerebellum, we speculate that a cerebellar gating of perceptual signals to cortical regions mediates the training-related attenuation of vestibular perception and perceptuo-reflex uncoupling.

Handedness-related cortical modulation of the vestibular-ocular reflex.

Multisensory visuo-vestibular cortical areas are important for spatial orientation and facilitate the control of the brainstem-mediated vestibular ocular reflex (VOR). Despite reports of visual input and cognitive tasks modulating the VOR through cortical control, it is unknown whether higher-order visual stimuli such as bistable perception and attention tasks involving visual imagery have an effect on the VOR. This is a possibility since such stimuli recruit cortical areas overlapping with those engaged during vestibular activation. Here we used a novel paradigm in which human subjects view bistable perceptual stimuli or perform complex attention tasks during concurrent vestibular stimulation. Bistable perceptual phenomena and attention tasks asymmetrically modulated the VOR but only if they involved a visuospatial component (e.g., binocular motion rivalry but not color rivalry). Strikingly, the lateralization effect was dependent upon the subjects' handedness, making this report the first behavioral demonstration that vestibular cortical processing is strongly lateralized to the non-dominant hemisphere. Furthermore, we show that perceptual transitions can modulate the dynamics of the vestibular system contingent upon the presence of a spatial component in the perceptual transition stimuli. Both perceptual transitions and attentional tasks are thought to invoke a redirection of spatial attention. We infer that such redirection of spatial attention engages multisensory vestibular cortical areas that modulate low-level vestibular function which, in turn, may contribute to spatial orientation.

Left cathodal trans-cranial direct current stimulation of the parietal cortex leads to an asymmetrical modulation of the vestibular-ocular reflex.

Multi-sensory visuo-vestibular cortical areas within the parietal lobe are important for spatial orientation and possibly for descending modulation of the vestibular-ocular reflex (VOR). Functional imaging and lesion studies suggest that vestibular cortical processing is localized primarily in the non-dominant parietal lobe. However, the role of inter-hemispheric parietal balance in vestibular processing is poorly understood. Therefore, we tested whether experimentally induced asymmetries in right versus left parietal excitability would modulate vestibular function. VOR function was assessed in right-handed normal subjects during caloric ear irrigation (30 °C), before and after trans-cranial direct current stimulation (tDCS) was applied bilaterally over the parietal cortex. Bilateral tDCS with the anode over the right and the cathode over the left parietal region resulted in significant asymmetrical modulation of the VOR, with highly suppressed responses during the right caloric irrigation (i.e. rightward slow phase nystagmus). In contrast, we observed no VOR modulation during either cathodal stimulation of the right parietal cortex or SHAM tDCS conditions. Application of unilateral tDCS revealed that the left cathodal stimulation was critical in inducing the observed modulation of the VOR. We show that disruption of parietal inter-hemispheric balance can induce asymmetries in vestibular function. This is the first report using neuromodulation to show right hemisphere dominance for vestibular cortical processing.