Visuospatial updating of reaching targets in near and far space

Abstract

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 brie£y 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. NeuroReport13:633^636

Specificity of Human Parietal Saccade and Reach Regions during Transcranial Magnetic Stimulation

“…This bias effect in accuracy most likely reflects an influence of the experimental configuration, in which the targets were aligned horizontally with the fixation location. As observed in previous studies (Bock, 1986;Henriques et al, 1998;Henriques and Crawford, 2000;Medendorp and Crawford, 2002;Pouget et al, 2002;Poljac and Van Den Berg, 2003), baseline horizontal errors for reaches to peripheral targets are biased relative to gaze-position angle; therefore, any rTMS-induced errors reported here are relative to these baseline errors (supplemental Fig. S1, available at www.jneurosci.org as supplemental material).…”

Visual motion due to eye movements helps guide the hand

“…If targets were coded in body-centered coordinates, then retinal motion information would not be informative about how the position of the hand has changed relative to the target and would thus not be useful for guiding action (it would be detrimental, if anything). Recent behavioral and neurophysiological studies, however, have demonstrated that targets are actually coded in eye-centered coordinates (Henriques et al 1998;Batista et al 1999;Andersen and Buneo 2002;Buneo et al 2002;Medendorp and Crawford 2002;Pouget et al 2002). Coding the target and hand in an eye-centered coordinate frame means that every time the body, head, or eye moves, the representation of the hand's position relative to the target must be updated.…”

“…In the direct model, the pursuit underestimation could be due to a systematic encoding error or to delays in coordinate transformations. For example, to reach to an object, the target and hand must be represented in a common coordinate frame, which is likely to be eye-centered (Henriques et al 1998;Batista et al 1999;Andersen and Buneo 2002;Buneo et al 2002;Medendorp and Crawford 2002;Pouget et al 2002). A coordinate transformation is therefore required, which involves necessary delays.…”

“…If targets are coded in eye-centered coordinates, as is increasingly believed (Henriques et al 1998;Batista et al 1999;Andersen and Buneo 2002;Buneo et al 2002;Medendorp and Crawford 2002;Pouget et al 2002), then the visuomotor system could adaptively incorporate visual motion information when assigning or updating the positions of objects. This explains why, when a physically stationary background was present in our experiment (providing retinal motion opposite to the direction of pursuit) reaching movements were more accurate than when there was no retinal motion.…”

Posterior parietal cortex and visuospatial control in near and far space

“…An example of the investigation of the dynamic process of space coding in humans required subjects to either point or reach, after an intervening eye movement, towards a remembered location of an initially foveally viewed target [54] which showed that the retinocentric reaching representations must be updated during eye movements in order to remain accurate [54,55]. patients.…”