45 research outputs found
Processing 3D form and 3D motion: Respective contributions of attention-based and stimulus-driven activity
International audienceThis study aims at segregating the neural substrate for the 3D-form and 3D-motion attributes in structure-from-motion perception, and at disentangling the stimulus-driven and endogenous-attention-driven processing of these attributes. Attention and stimulus were manipulated independently: participants had to detect the transitions of one attribute âform, 3D motion or colourâ while the visual stimulus underwent successive transitions of all attributes. We compared the BOLD activity related to form and 3D motion in three conditions: stimulus-driven processing (unattended transitions), endogenous attentional selection (task) or both stimulus-driven processing and attentional selection (attended transitions). In all conditions, the form versus 3D-motion contrasts revealed a clear dorsal/ventral segregation. However, while the form-related activity is consistent with previously described shape-selective areas, the activity related to 3D motion does not encompass the usual âvisual motionâ areas, but rather corresponds to a high-level motion system, including IPL and STS areas. Second, we found a dissociation between the neural processing of unattended attributes and that involved in endogenous attentional selection. Areas selective for 3D-motion and form showed either increased activity at transitions of these respective attributes or decreased activity when subjectsâ attention was directed to a competing attribute. We propose that both facilitatory and suppressive mechanisms of attribute selection are involved depending on the conditions driving this selection. Therefore, attentional selection is not limited to an increased activity in areas processing stimulus properties, and may unveil different functional localization from stimulus modulation
Carrying heavy asymmetrical loads increases postural sway during quiet standing in older adults
Effects of external loads on postural sway during quiet stance in adults aged 20â80 years
The purpose of this study was to investigate the effects of holding external loads on postural sway during upright stance across age decades. Sixty-five healthy adults (females, n = 35), aged 18â80 years were assessed in four conditions; (1) standing without holding a load, holding a load corresponding to 5% body mass in the (2) left hand, (3) right hand and (4) both hands. The centre of pressure (COP) path length and anteroposterior and mediolateral COP displacement were used to indirectly assess postural sway. External loading elicited reductions in COP measures of postural sway in older age groups only (P 0.05). Holding external loads during standing is relevant to many activities of daily living (i.e. holding groceries). The reduction in postural sway may suggest this type of loading has a stabilising effect during quiet standing among older adults
Contribution of extraretinal signals to the scaling of object distance during self-motion
Perception of three-dimensional shape from ego- and object-motion: Comparison between small- and large-field stimuli
Contains fulltext :
20826___.PDF (publisher's version ) (Open Access
The visual perception of plane tilt from motion in small field and large field: Psychophysics and theory
10.1016/j.visres.2006.04.003Vision Research46203494-3513VISR
Lines and dots: characteristics of the motion integration process
Local motion detectors can only provide the velocity component perpendicular to a moving line that crosses their receptive field, leading to an ambiguity known as the âaperture problemâ. This problem is solved exactly for rigid objects translating in the screen plane via the intersection of constraints (IOC). In natural scenes, however, object motions are not restricted to fronto-parallel translations, and several objects with distinct motions may be present in the visual space. Under these conditions the usual IOC construction is no longer valid, which raises questions as its use as a basis for spatial integration and selection of motion signals in uniform and non-uniform velocity fields. The influence of the motion of random dots on the perceived direction of a horizontal line grating was measured, when dots and lines are seen through different apertures. The random dots were mapped on a plane that translates in a fronto-parallel plane (uniform 2D translation) or in depth (3D, corresponding to a non-uniform projected velocity field, either expanding or contracting). The grating was either moving rigidly with the dots or in the opposite direction. Subjects â responses show that the direction of line grating movement was reliably influenced only in conditions consistent with rigid motion; where there was a reliable influence, the perceived direction was consistent with the dot motion pattern. This finding points to the existence of a motion-based selection mechanism that operates prior to the disambiguation of the line movement direction. Disambiguation could occur for both uniform and non-uniform velocity fields, even though in the last case none of the individual dots indicated the proper direction in 2D velocity space. Finally, the captur