Contribution of Bodily and Gravitational Orientation Cues to Face and Letter Recognition

Sensory information provided by the vestibular system is crucial in cognitive processes such as the ability to recognize objects. The orientation at which objects are most easily recognized--the perceptual upright (PU)--is influenced by body orientation with respect to gravity as detected from the somatosensory and vestibular systems. To date, the influence of these sensory cues on the PU has been measured using a letter recognition task. Here we assessed whether gravitational influences on letter recognition also extend to human face recognition. 13 right-handed observers were positioned in four body orientations (upright, left-side-down, right-side-down, supine) and visually discriminated ambiguous characters ('p'-from-'d'; 'i'-from-'!') and ambiguous faces used in popular visual illusions ('young woman'-from-'old woman'; 'grinning man'-from-'frowning man') in a forced-choice paradigm. The two transition points (e.g., 'p-to-d' and 'd-to-p'; 'young woman-to-old woman' and 'old woman-to-young woman') were fit with a sigmoidal psychometric function and the average of these transitions was taken as the PU for each stimulus category. The results show that both faces and letters are more influenced by body orientation than gravity. However, faces are more optimally recognized when closer in alignment with body orientation than letters--which are more influenced by gravity. Our results indicate that the brain does not utilize a common representation of upright that governs recognition of all object categories. Distinct areas of ventro-temporal cortex that represent faces and letters may weight bodily and gravitational cues differently--possibly to facilitate the specific demands of face and letter recognition.