Biomedical subjects

W Pieter Medendorp

Publications and source records attributed to W Pieter Medendorp.

10 recordsLinked to original sources

PubMed2006

Behavioral reference frames for planning human reaching movements.

At some stage in the process of a sensorimotor transformation for a reaching movement, information about the current position of the hand and information about the location of the target must be encoded in the same frame of reference to compute the hand-to-target difference vector. Two main hypotheses have been proposed regarding this reference frame: an eye-centered and a body-centered frame. Here we evaluated these hypotheses using the pointing errors that subjects made when planning and executing arm movements to memorized targets starting from various initial hand positions while keeping gaze fixed in various directions. One group of subjects (n = 10) was tested without visual information about hand position during movement planning (unseen-hand condition); another group (n = 8) was tested with hand and target position simultaneously visible before movement onset (seen-hand condition). We found that both initial hand position and gaze fixation direction had a significant effect on the magnitude and direction of the pointing error. Errors were significantly smaller in the seen-hand condition. For both conditions, though, a reference frame analysis showed that the errors arose at an eye- or hand-centered stage or both, but not at a body-centered stage. As a common reference frame is required to specify a movement vector, these results suggest that an eye-centered mechanism is involved in integrating target and hand position in programming reaching movements. We discuss how simple gain elements modulating the eye-centered target and hand-position signals can account for these results.

Adult↗

PubMed2005

Directional selectivity of BOLD activity in human posterior parietal cortex for memory-guided double-step saccades.

We used functional magnetic resonance imaging (fMRI) to investigate the role of the human posterior parietal cortex (PPC) in storing target locations for delayed double-step saccades. To do so, we exploited the laterality of a subregion of PPC that preferentially responds to the memory of a target location presented in the contralateral visual field. Using an event-related design, we tracked fMRI signal changes in this region while subjects remembered the locations of two sequentially flashed targets, presented in either the same or different visual hemifields, and then saccaded to them in sequence. After presentation of the first target, the fMRI signal was always related to the side of the visual field in which it had been presented. When the second target was added, the cortical activity depended on the respective locations of both targets but was still significantly selective for the target of the first saccade. We conclude that this region within the human posterior parietal cortex not only acts as spatial storage center by retaining target locations for subsequent saccades but is also involved in selecting the target for the first intended saccade.

Adult↗

PubMed2005

Remapping the remembered target location for anti-saccades in human posterior parietal cortex.

We used functional magnetic resonance imaging (fMRI) to investigate the role of the human posterior parietal cortex (PPC) in anti-saccades. To do so, we exploited the laterality of a subregion of the PPC for remembered target location. Using an event-related design, we tracked fMRI signal changes in this region while subjects remembered the location of a flashed target, then were instructed to plan either an anti- or pro-saccade to that location, and finally were instructed to execute the movement. At first, the region responded preferentially to the memory of a target location presented in the contralateral visual field. However, when an anti-cue specified a saccadic response into the opposite visual field, we observed a dynamic shift in cortical activity from one hemisphere to the other. This shows that this region within the human posterior parietal cortex codes the target location for an upcoming saccade, rather than the location of the remembered visual stimulus in an anti-saccade task.

Adult↗

PubMed2004

Frames of reference for eye-head gaze commands in primate supplementary eye fields.

The supplementary eye field (SEF) is a region within medial frontal cortex that integrates complex visuospatial information and controls eye-head gaze shifts. Here, we test if the SEF encodes desired gaze directions in a simple retinal (eye-centered) frame, such as the superior colliculus, or in some other, more complex frame. We electrically stimulated 55 SEF sites in two head-unrestrained monkeys to evoke 3D eye-head gaze shifts and then mathematically rotated these trajectories into various reference frames. Each stimulation site specified a specific spatial goal when plotted in its intrinsic frame. These intrinsic frames varied site by site, in a continuum from eye-, to head-, to space/body-centered coding schemes. This variety of coding schemes provides the SEF with a unique potential for implementing arbitrary reference frame transformations.

Animals↗

PubMed2004

Integration of target and effector information in human posterior parietal cortex for the planning of action.

Recently, using event-related functional MRI (fMRI), we located a bilateral region in the human posterior parietal cortex (retIPS) that topographically represents and updates targets for saccades and pointing movements in eye-centered coordinates. To generate movements, this spatial information must be integrated with the selected effector. We now tested whether the activation in retIPS is dependent on the hand selected. Using 4-T fMRI, we compared the activation produced by movements, using either eyes or the left or right hand, to targets presented either leftward or rightward of central fixation. The majority of the regions activated during saccades were also activated during pointing movements, including occipital, posterior parietal, and premotor cortex. The topographic retIPS region was activated more strongly for saccades than for pointing. The activation associated with pointing was significantly greater when pointing with the unseen hand to targets ipsilateral to the hand. For example, although there was activation in the left retIPS when pointing to targets on the right with the left hand, the activation was significantly greater when using the right hand. The mirror symmetric effect was observed in the right retIPS. Similar hand preferences were observed in a nearby anterior occipital region. This effector specificity is consistent with previous clinical and behavioral studies showing that each hand is more effective in directing movements to targets in ipsilateral visual space. We conclude that not only do these regions code target location, but they also appear to integrate target selection with effector selection.

Adult↗

PubMed2003

Motion parallax is computed in the updating of human spatial memory.

As we move through space, stationary objects around us show motion parallax: their directions relative to us change at different rates, depending on their distance. Does the brain incorporate parallax when it updates its stored representations of space? We had subjects fixate a distant target and then we flashed lights, at different distances, onto the retinal periphery. Subjects translated sideways while keeping their gaze on the distant target, and then they looked to the remembered location of the flash. Their responses corrected almost perfectly for parallax: they turned their eyes farther for nearer targets, in the predicted nonlinear patterns. Computer simulations suggest a neural mechanism in which feedback about self-motion updates remembered locations of objects within an internal map of three-dimensional visual space.

Adult↗

PubMed2003

Gaze-centered updating of visual space in human parietal cortex.

Single-unit recordings have identified a region in the posterior parietal cortex (PPC) of the monkey that represents and updates visual space in a gaze-centered frame. Here, using event-related functional magnetic resonance imaging, we identified an analogous bilateral region in the human PPC that shows contralateral topography for memory-guided eye movements and arm movements. Furthermore, when eye movements reversed the remembered horizontal target location relative to the gaze fixation point, this PPC region exchanged activity across the two cortical lobules. This shows that the human PPC dynamically updates the spatial goals for action in a gaze-centered frame.

Adult↗

PubMed2003

Neural control of 3-D gaze shifts in the primate.

The neural mechanisms that specify target locations for gaze shifts and then convert these into desired patterns of coordinated eye and head movements are complex. Much of this complexity is only revealed when one takes a realistic three-dimensional (3-D) view of these processes, where fundamental computational problems such as kinematic redundancy, reference-frame transformations, and non-commutativity emerge. Here we review the underlying mechanisms and solutions for these problems, starting with a consideration of the kinematics of 3-D gaze shifts in human and non-human primates. We then consider the neural mechanisms, including cortical representation of gaze targets, the nature of the gaze motor command used by the superior colliculus, and how these gaze commands are decomposed into brainstem motor commands for the eyes and head. A general conclusion is that fairly simple coding mechanisms may be used to represent gaze at the cortical and collicular level, but this then necessitates complexity for the spatial updating of these representations and in the brainstem sensorimotor transformations that convert these signals into eye and head movements.

Animals↗

PubMed2002

Visuospatial updating of reaching targets in near and far space.

The brain constructs multiple representations of near and far space but it is unclear which spatial mechanism guides reaching across eye movements in near space. Retinocentric reaching representations are known to exist in parietal cortex, but must be updated during eye movements, in order to remain accurate. In contrast, non-retinal (e.g. muscle-centered) reaching plans in motor cortex do not require updating, and so may provide a more stable encoding mechanism. To test between these, we employed a behavioral test. Subjects briefly foveated a target (located at various depths in near and far space) looked peripherally, then reached toward its remembered location. Surprisingly, subjects did not use the stable non-retinal reaching plan (compared to controls without eye movements). Instead, the intervening eye movements induced a systematic pattern of reaching errors for targets at all depths consistent with updating in a retinal frame. We conclude that a common eye-centered updating mechanism prevails in programming arm movements in both near and far space.

Adult↗

PubMed2002

Rotational remapping in human spatial memory during eye and head motion.

The brain uses vision and other senses to compute the locations of objects relative to the body, and then must update these locations when the body moves. How geometrically sophisticated is this internal updating? It has been suggested that updating simply shifts the stored locations of all objects uniformly, by a common vector, when the eye or head turns. For horizontal and vertical turns, a uniform shift would often approximate the real changes in location of objects in front of the subject. But for torsional rotations, a shift would be inadequate: accurate updating would call for a more geometrically exact remapping, not shifting but rotating the stored locations through the inverse of the rotation of the eye in space. Here we asked human subjects to make eye saccades to remembered targets after torsional head rotations. Their accuracy showed that spatial updating works in the torsional dimension and operates by rotation rather than shifting.

Adult↗