Perceiving three-dimensional object motion while moving through the world is hard: not only must optic flow be segmented and parallax resolved into shape and motion, but also observer motion needs to be taken into account in order to perceive absolute, rather than observer-relative motion. In order to simplify the last step, it has recently been suggested that if the visual background is stationary, then foreground object motion, computed relative to the background, directly yields absolute motion. A series of studies with immobile observers and optic flow simulating observer movement have provided evidence that observers actually utilize this so-called "flow parsing" strategy (Rushton & Warren, 2005). We test this hypothesis by using mobile observers (as well as immobile ones) who judge the motion in depth of a foreground object in the presence of a stationary or moving background. We find that background movement does influence motion perception but not as much as predicted by the flow-parsing hypothesis. Thus, we find evidence that, in order to perceive absolute motion, observers partly use flow-parsing but also compensate egocentric motion by a global self-motion estimate.

Although motor actions can profoundly affect the perceptual interpretation of sensory inputs, it is not known whether the combination of sensory and movement signals occurs only for sensory surfaces undergoing movement or whether it is a more general phenomenon. In the haptic modality, the independent movement of multiple sensory surfaces poses a challenge to the nervous system when combining the tactile and kinesthetic signals into a coherent percept. When exploring a stationary object, the tactile and kinesthetic signals come from the same hand. Here we probe the internal structure of haptic combination by directing the two signal streams to separate hands: one hand moves but receives no tactile stimulation, while the other hand feels the consequences of the first hand's movement but remains still. We find that both discrete and continuous tactile and kinesthetic signals are combined as if they came from the same hand. This combination proceeds by direct coupling or transfer of the kinesthetic signal from the moving to the feeling hand, rather than assuming the displacement of a mediating object. The combination of signals is due to perception rather than inference, because a small temporal offset between the signals significantly degrades performance. These results suggest that the brain simplifies the complex coordinate transformation task of remapping sensory inputs to take into account the movements of multiple body parts in haptic perception, and they show that the effects of action are not limited to moving sensors.touch | haptics | perception | sensorimotor integration | kinesthesis M otor and kinesthetic signals arising from the movement of the eyes in the head, and translation of the eyes and ears in space due to head and body movements, have been shown to play an important role in visual (1-3) and auditory (4-6) perception. However, because of the small number of sensory surfaces in these modalities, and the rigid constraints on their movement, the number of kinesthetic degrees of freedom is limited. In active touch or the haptic modality (7-13), the large number of sensory surfaces, and the nearly unlimited ways these surfaces can move, lead to the question of how movement can be represented and associated with the cutaneous or tactile signals. To study how tactile and kinesthetic cues are combined to haptically perceive object shape and size, we created a novel haptic stimulus in which these cues were completely dissociated. This stimulus consisted of simulated triangles felt through a narrow slit, as in anorthoscopic perception in vision (14-16) or haptic perception (17), and as illustrated in Fig.

Impairments in attentional, working memory and sensorimotor processing have been consistently reported in schizophrenia. However, the interaction between cognitive and sensorimotor impairments and the underlying neural mechanisms remains largely uncharted. We hypothesized that altered attentional processing in patients with schizophrenia, probed through saccadic inhibition, would partly explain impaired sensorimotor control and would be reflected as altered task-dependent modulation of cortical excitability and inhibition. Twenty-five stabilized patients with schizophrenia, 17 unaffected siblings and 25 healthy control subjects were recruited. Subjects performed visuomotor grip force-tracking alone (single-task condition) and with increased cognitive load (dual-task condition). In the dual-task condition, two types of trials were randomly presented: trials with visual distractors (requiring inhibition of saccades) or trials with addition of numbers (requiring saccades and addition). Both dual-task trial types required divided visual attention to the force-tracking target and to the distractor or number. Gaze was measured during force-tracking tasks, and task-dependent modulation of cortical excitability and inhibition were assessed using transcranial magnetic stimulation. In the single-task, patients with schizophrenia showed increased force-tracking error. In dual-task distraction trials, force-tracking error increased further in patients, but not in the other two groups. Patients inhibited fewer saccades to distractors, and the capacity to inhibit saccades explained group differences in force-tracking performance. Cortical excitability at rest was not different between groups and increased for all groups during single-task force-tracking, although, to a greater extent in patients (80%) compared to controls (40%). Compared to single-task force-tracking, the dual-task increased cortical excitability in control subjects, whereas patients showed decreased excitability. Again, the group differences in cortical excitability were no longer significant when failure to inhibit saccades was included as a covariate. Cortical inhibition was reduced in patients in all conditions, and only healthy controls increased inhibition in the dual-task. Siblings had similar force-tracking and gaze performance as controls but showed altered task-related modulation of cortical excitability and inhibition in dual-task conditions. In patients, neuropsychological scores of attention correlated with visuomotor performance and with task-dependant modulation of cortical excitability. Disorganization symptoms were greatest in patients with weakest task-dependent modulation of cortical excitability. This study provides insights into neurobiological mechanisms of impaired sensorimotor control in schizophrenia showing that deficient divided visual attention contributes to impaired visuomotor performance and is reflected in impaired modulation of cortical excitability and inhibition. In siblings, altered modulation of cortical excitability and inhibition is consistent with a genetic risk for cortical abnormality.