Left-handers show no self-advantage in detecting a delay in visual feedback concerning an active movement.

Right-handed people show an advantage in detecting a delay in visual feedback concerning an active movement of their right hand when it is viewed in a natural perspective compared to when it is seen as if viewing another person's hand (Hoover and Harris in Exp Brain Res 233:1053-1060, 2012. doi: 10.1007/s00221-014-4181-9 ; Exp Brain Res 222:389-397, 2015a. doi: 10.1007/s00221-012-3224-3 ). This self-advantage is unique to their dominant hand and may reflect an enhanced sense of ownership which contributes to how right-handed people relate to the world. Here we asked whether left-handers show the same pattern of performance for their dominant hand. We measured the minimum delay that could be detected by 29 left-handers when viewing either their dominant or non-dominant hand from 'self' or 'other' perspectives and compared their thresholds to an age-matched sample of 22 right-handers. Right-handers showed a significant signature self-advantage of 19 ms when viewing their dominant hand in an expected 'self' perspective compared to 'other' perspectives. Left-handers, however, showed no such advantage for either their dominant or non-dominant hand. This lack of self-advantage in detecting delayed visual feedback might indicate a less secure sense of body ownership amongst left-handers.

Inducing ownership over an 'other' perspective with a visuo-tactile manipulation.

Seeing our body from a 'self' perspective while performing a movement improves our ability to detect asynchrony between the visual and proprioceptive information concerning that movement: a signature of enhanced body ownership referred to as the 'self-advantage'. We consequently experience no self-advantage when seeing our body from an 'other' perspective. Here we ask whether introducing visuo-tactile stimulation (VTS), similar to that used in the rubber hand illusion to invoke ownership over a dummy hand, would produce a self-advantage when viewing the body from a typically 'other' perspective. Prior to the experiment, participants watched a live video of their own back using a camera mounted behind them while their back was tapped with a rod for 2 min. The video was either synchronous (sVTS) or asynchronous (aVTS) with the tapping. Participants then raised their hands and made a stereotyped finger movement that they watched from the same camera either in the original, natural perspective or upside down. Participants indicated which of two periods (one with minimum delay and one with an added delay of 33-264 ms) appeared delayed. Sensitivity was calculated using psychometric functions. The sVTS group showed a self-advantage of about 45 ms in the natural visual condition compared to the upside down condition, whereas the aVTS group showed no difference between the two conditions. Synchronous visuo-tactile experience increased the feeling of ownership over a typically 'other' perspective in a quantifiable way indicating the multisensory and malleable nature of body representation.

The role of the viewpoint on body ownership.

People are more sensitive to detecting asynchrony between a self-generated movement of the hand and delayed visual feedback when what they see matches the expected "self" perspective rather than an "other" perspective (Hoover and Harris in Exp Brain Res 222:389-397, 2012). We take this as corresponding to the ability to distinguish self from others and call it the "self-advantage": a measure of body ownership. What about views of the body that cannot be seen directly? Here, we assessed the effect of familiarity of the view of the body on the self-advantage. Participants performed self-generated hand and head movements viewed directly, in a mirror, and from behind with a variable delay added to the visual feedback. Each view was shown either in the natural perspective or flipped about the vertical or horizontal axes to provide a view from another perspective. Thresholds for detecting a delay in visual feedback were calculated. Dependency of the self-advantage on perspective was most evident for views of the body that are seen most often. Results support the importance of correlating visual feedback with movement information in creating the sense of body ownership.

Detecting delay in visual feedback of an action as a monitor of self recognition.

How do we distinguish "self" from "other"? The correlation between willing an action and seeing it occur is an important cue. We exploited the fact that this correlation needs to occur within a restricted temporal window in order to obtain a quantitative assessment of when a body part is identified as "self". We measured the threshold and sensitivity (d') for detecting a delay between movements of the finger (of both the dominant and non-dominant hands) and visual feedback as seen from four visual perspectives (the natural view, and mirror-reversed and/or inverted views). Each trial consisted of one presentation with minimum delay and another with a delay of between 33 and 150 ms. Participants indicated which presentation contained the delayed view. We varied the amount of efference copy available for this task by comparing performances for discrete movements and continuous movements. Discrete movements are associated with a stronger efference copy. Sensitivity to detect asynchrony between visual and proprioceptive information was significantly higher when movements were viewed from a "plausible" self perspective compared with when the view was reversed or inverted. Further, we found differences in performance between dominant and non-dominant hand finger movements across the continuous and single movements. Performance varied with the viewpoint from which the visual feedback was presented and on the efferent component such that optimal performance was obtained when the presentation was in the normal natural orientation and clear efferent information was available. Variations in sensitivity to visual/non-visual temporal incongruence with the viewpoint in which a movement is seen may help determine the arrangement of the underlying visual representation of the body.

Sensory compensation in sound localization in people with one eye.

Some blind people are better at locating sounds than people with normal vision indicating cross-modal plasticity. People who have lost one eye have a unique form of visual deprivation that reduces visual afferent signals by half and can potentially also lead to cross-modal (as well as intra-modal) plasticity. To look for evidence of auditory-visual cross-modal compensation, we measured binaural and monaural sound localization in one-eyed people and compared them with normally sighted controls. One-eyed people showed significantly better binaural sound localization than controls in the central region of space (±78° from straight ahead), but they mislocalized sounds in the far periphery (on both the blind and intact side) by up to 15° towards the centre. One-eyed people showed significantly better monaural sound localization compared with controls. Controls' performance became asymmetric when they had one eye patched. Patching improved accuracy in the viewing field but decreased accuracy in the occluded field. These results are discussed in terms of cross-modal sensory compensation and the possible contribution of visual depth to interpreting sound localization cues.

Short and long-term visual deprivation leads to adapted use of audiovisual information for face-voice recognition.

Person identification is essential for everyday social interactions. We quickly identify people from cues such as a person's face or the sound of their voice. A change in sensory input, such as losing one's vision, can alter how one uses sensory information. We asked how people with only one eye, who have had reduced visual input during postnatal maturation of the visual system, use faces and voices for person identity recognition. We used an old/new paradigm to investigate unimodal (visual or auditory) and bimodal (audiovisual) identity recognition of people (face, voice and face-voice) and a control category, objects (car, horn and car-horn). Participants learned the identity of 10 pairs of faces and voices (Experiment 1) and 10 cars and horns (Experiment 2) and were asked to identify the learned face/voice or car/horn among 20 distractors. People with one eye were more sensitive to voice identification compared to controls viewing binocularly or with an eye-patch. However, both people with one eye and eye-patched viewing controls use combined audiovisual information for person identification more equally than binocular viewing controls, who favour vision. People with one eye were no different from controls at object identification. The observed visual dominance for binocular controls is larger for person compared to object identification, indicating that faces (vision) play a larger role in person identification and that person identity processing is unique from that for objects. People with long-term visual deprivation from the loss of one eye may have adaptive strategies, such as placing less reliance on vision to achieve intact performance, particularly for face processing.

Disrupting Vestibular Activity Disrupts Body Ownership.

People are more sensitive at detecting asynchrony between a self-generated movement and visual feedback concerning that movement when the movement is viewed from a first-person perspective. We call this the 'self-advantage' and interpret it as an objective measure of self. Here we ask if disruption of the vestibular system in healthy individuals affects the self-advantage. Participants performed finger movements while viewing their hand in a first-person ('self') or third-person ('other') perspective and indicated which of two periods (one with minimum delay and the other with an added delay of 33-264 ms) was delayed. Their sensitivity to the delay was calculated from the psychometric functions obtained. During the testing, disruptive galvanic vestibular stimulation (GVS) was applied in five-minute blocks interleaved with five minutes of no stimulation for a total of 40 min. We confirmed the self-advantage under no stimulation (31 ms). In the presence of disruptive GVS this advantage disappeared and there was no longer a difference in performance between perspectives. The threshold delay for the 'other' perspective was not affected by the GVS. These results suggest that an intact vestibular signal is required to distinguish 'self' from 'other' and to maintain a sense of body ownership.

How our body influences our perception of the world.

Incorporating the fact that the senses are embodied is necessary for an organism to interpret sensory information. Before a unified perception of the world can be formed, sensory signals must be processed with reference to body representation. The various attributes of the body such as shape, proportion, posture, and movement can be both derived from the various sensory systems and can affect perception of the world (including the body itself). In this review we examine the relationships between sensory and motor information, body representations, and perceptions of the world and the body. We provide several examples of how the body affects perception (including but not limited to body perception). First we show that body orientation effects visual distance perception and object orientation. Also, visual-auditory crossmodal-correspondences depend on the orientation of the body: audio "high" frequencies correspond to a visual "up" defined by both gravity and body coordinates. Next, we show that perceived locations of touch is affected by the orientation of the head and eyes on the body, suggesting a visual component to coding body locations. Additionally, the reference-frame used for coding touch locations seems to depend on whether gaze is static or moved relative to the body during the tactile task. The perceived attributes of the body such as body size, affect tactile perception even at the level of detection thresholds and two-point discrimination. Next, long-range tactile masking provides clues to the posture of the body in a canonical body schema. Finally, ownership of seen body parts depends on the orientation and perspective of the body part in view. Together, all of these findings demonstrate how sensory and motor information, body representations, and perceptions (of the body and the world) are interdependent.

Superior voice recognition in a patient with acquired prosopagnosia and object agnosia.

Anecdotally, it has been reported that individuals with acquired prosopagnosia compensate for their inability to recognize faces by using other person identity cues such as hair, gait or the voice. Are they therefore superior at the use of non-face cues, specifically voices, to person identity? Here, we empirically measure person and object identity recognition in a patient with acquired prosopagnosia and object agnosia. We quantify person identity (face and voice) and object identity (car and horn) recognition for visual, auditory, and bimodal (visual and auditory) stimuli. The patient is unable to recognize faces or cars, consistent with his prosopagnosia and object agnosia, respectively. He is perfectly able to recognize people's voices and car horns and bimodal stimuli. These data show a reverse shift in the typical weighting of visual over auditory information for audiovisual stimuli in a compromised visual recognition system. Moreover, the patient shows selectively superior voice recognition compared to the controls revealing that two different stimulus domains, persons and objects, may not be equally affected by sensory adaptation effects. This also implies that person and object identity recognition are processed in separate pathways. These data demonstrate that an individual with acquired prosopagnosia and object agnosia can compensate for the visual impairment and become quite skilled at using spared aspects of sensory processing. In the case of acquired prosopagnosia it is advantageous to develop a superior use of voices for person identity recognition in everyday life.