Right‐Left Discrimination and Finger Localization: Development and Pathology

The study investigated how hearing depends on the whole body, head and trunk orientation relative to a sound source. In normal hearing humans we examined auditory thresholds and their ability to recognize logatomes (bi-syllabic non-sense words) at different whole body, head and trunk rotation relative to a sound source. We found that auditory threshold was increased and logatome recognition was impaired when the body or the trunk were rotated 40° away from a sound source compared to when the body or the trunk was oriented towards the sound source. Conversely, no effects were seen when only the head was rotated. Further, an increase of thresholds and impairment of logatome recognition were also observed after unilateral vibration of dorsal neck muscles that induces, per se, long-lasting illusory trunk displacement relative to the head. Thus, our findings support the idea that processing of acoustic signals depends on where a sound is located within a reference system defined by the subject’s trunk coordinates.

Auditory perception is influenced by the orientation of the trunk relative to a sound source

Egocentric representations allow us to describe the external world as experienced from an individual’s bodily location. We recently developed a novel method of quantifying the weight given to different body parts in egocentric judgments (the Misalignment Paradigm). We found that both head and torso contribute to simple alter-egocentric spatial judgments. We hypothesised that artificial stimulation of the vestibular system would provide a head-related signal, which might affect the weighting given to the head in egocentric spatial judgments. Bipolar Galvanic Vestibular Stimulation (GVS) was applied during the Misalignment Paradigm. A Sham stimulation condition was also included to control for non-specific effects. Our data show that the weight given to the head was increased during left anodal and right cathodal GVS, compared to the opposite GVS polarity (right anodal and left cathodal GVS) and Sham stimulation. That is, the polarity of GVS, which preferentially activates vestibular areas in the right cerebral hemisphere, influenced the relative weightings of head and torso in egocentric spatial judgments.

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The vestibular system modulates the contributions of head and torso to egocentric spatial judgements

The usual visual experiences possess a perspectival phenomenology as they seem to present objects from a certain perspective. Nevertheless, it is not obvious how to characterise experiential content determining such phenomenology. In particular, while there are many works investigating perspectival properties of experienced objects, a question regarding how subject is represented in visual perspectival experiences attracted less attention. In order to address this problem, I consider four popular phenomenal intuitions regarding perspectival experiences and argue that the major theories of perspectival experiences do not account for all of them. Relying on these observations, I show how a theory which accommodates all these intuitions can be developed by (a) recognising that visual perspectival experiences are, in fact, multimodal visuo-bodily experiences and (b) distinguishing between egocentric and structural contents. 

Sampath S. Rajapakse

Papers

Shared contributions of the head and torso to spatial reference frames across spatial judgments