Prism adaptation of reaching is dependent on the type of visual feedback of hand and target position.

The present study demonstrated that the magnitude of after-effect due to wedge prisms depends on the form of the visual feedback used to represent hand and target position in fast, targeted, transverse reaches. Trained human subjects made reaches with and without prisms in three visuomotor representations (VR): (1) the subject's actual hand and targets (Direct), (2) a real-time video broadcast of hand and targets (Video), or (3) abstract, computer-generated targets and a cursor representing hand position (Cursor). A significant after-effect occurred in each VR. However, the magnitude of the after-effect was significantly different among VRs: the magnitude was greatest in Direct, smaller in Video and smallest in Cursor. A significant after-effect (carryover) also occurred when a subject prism-adapted reaches in one VR and then removed the prisms and made initial reaches in another VR. Our data showed that when reaches were prism-adapted in Direct and then prisms were removed, there was a large carryover to initial reaches in Video or Cursor (D-->V and D-->C). In contrast, when prisms were worn in Video and removed for reaches in Direct (V-->D), there was a significantly smaller carryover than from both D-->V and D-->C. Finally, when prisms were worn in Cursor and removed for reaches in Direct (C-->D), there was very little detectable carryover. Our results suggest that adaptation is context-dependent and that the magnitude of carryover is dependent on the VR in which adaptation occurred. Interpretations of adaptations made in abstract training and experimental conditions may be greatly affected by this finding.

Throwing accuracy in the vertical direction during prism adaptation: not simply timing of ball release.

In a previous study, others have hypothesized that the variance in vertical errors that occurs while throwing at visual targets is caused by changes in any of three throw parameters: hand location in space, hand translational velocity, and hand orientation. From an analysis of skilled throwers, those authors concluded that vertical error is best correlated with variance in hand orientation, which in turn is related to the timing of ball release. We used a vertical prism adaptation paradigm to investigate which of these throwing parameters subjects use when adapting to external perturbation. Our subjects showed no correlation between hand position or hand translational velocity and ball impact height in normal, over-practiced throwing. However, video-based motion analysis showed that modifications both of position and speed of the hand play an important role when subjects are forced to compensate for a vertically shifting prism perturbation during a dart-like throw (these factors contribute approximately 30% of the adaptation). We concluded that, during adaptation, more degrees of freedom and more sources of potential error are modified to achieve the gaze-throw recalibration required to hit the target than are employed in this type of throw during normal conditions.

Occipital cortico-pyramidal projection in hypothyroid rats.

It is well established that the progressive disappearance of a transient occipito-spinal projection in neonatal rats involves the selective elimination of axonal collaterals. We studied whether the development of the occipito-spinal pathway was affected by hypothyroidism induced by treatment with the goitrogen 6n-propyl-2-thiouracil (PTU) beginning prenatally. Using both anterograde (biocytin and Dil) and retrograde (horseradish peroxidase and Fast Blue) tracing techniques in adult hypothyroid rats, we found that many cells with projections into the pyramidal tract are present in regions of visual cortex that are devoid of such cells in normal adult rats. Our results suggest that hypothyroidism induced by PTU treatment leads to the maintenance of occipito-spinal projections that are normally transient.