We adopted a reaching task in which pasta is presented in one of two orientations, vertical or horizontal, to test whether rats preshape their paws to fit the orientation of their target. We also tested whether the ability to preshape the paw improved with training by comparing animals at the beginning and end of a 10-week training period. Finally, we asked whether lesion of the CST at the medullary pyramid abrogates preshaping that had already been acquired, and we continued to train rats to test their ability to reacquire preshaping after injury.
Representative reaches to horizontal and vertical pasta are depicted in . After removing the barrier in front of the target, rats position themselves with their nose at the aperture and quickly initiate a reach. Rats lift their nose and extend the reaching paw through the aperture. At the −2 time point, the rat has just extended the reaching paw through the aperture of the reaching box. The rat further extends the forelimb at the −1 time point and begins to rotate the paw to match the target orientation. Importantly, we saw no interaction of the long guard hairs of the paw or the hairs of the forelimb with the pasta before contact(Hermer-Vazquez et al., 2007
). At the point of contact, which is any interaction of the paw and the pasta, the hairless portion of the paw touches the pasta first. While reaching for the vertical pasta the second digit makes initial contact, whereas the fifth digit initially contacts the horizontal pasta (arrows in , Contact). By time point +1, most of the digits contact the target. Rats do not fully grasp the pasta until several frames after contact (usually +3 or more).
Representative reaches for pasta oriented either vertically (VERT) or horizontally (HORIZ) shown in consecutive 16.7msec video frames. Contact is defined as the time when the paw first touches the pasta (white arrows).
To determine the degree to which rats preshape their paws, we made two comparisons for each animal. First, we compared the paw angles between reaches to vertical pasta with those to horizontal pasta (e.g. , “preshaping between”). Our prediction was that paw angles would diverge significantly while reaching to targets of different orientation. Second, we measured the change in paw angle within a reach, at each successive time point or over all time points (e.g. , “preshaping within”). We predicted that the paw angle would change within the reach to approximate the angle of the target.
depicts the paw angles over time while reaching for vertically oriented pasta (black) or horizontally oriented (gray) pasta. Animals included in this analysis (n=8) were tested both before (A) and after (B; described below) CST injury. Before injury there was a highly significant difference in paw angles when reaching for pasta of different orientations (preshaping between, F1,42=182.0, p<0.0001, repeated measures ANOVA). At the −2 time point, the average paw angle for vertical reaches was not significantly different than the angle for horizontal reaches (p>0.05). The angles did differ at the −1 time point (p<0.01, Bonferroni post hoc test). Thus, even before contact of the paw with the target, the paw angles were different when reaching for objects of different orientations, indicating feed-forward control. The paw angles diverged further at the point of contact (p<.001) and at the +1 time point (p<.001). Importantly, paw angles did not differ whether the previous reach had been to the same orientation or not (data not shown). This demonstrates feed-forward control of paw angle based on sensory information about the target acquired before tactile feedback.
Figure 3 Preshaping in the rat is abrogated by CST injury. Paw angle is measured at successive 16.7msec time points while reaching for pasta oriented vertically (vert, 90°, black) or horizontally (horiz, 0°, gray). The schematic relation shown (more ...)
In addition to comparing paw angles when reaching for targets of different orientations, the amount of preshaping can also be measured by angle changes at successive time points within each reach (preshaping within). If rats display feed-forward control of their paw, then the paw angle should become progressively closer to the angle of the target with each successive measurement (i.e. vertical paw angles trend towards 90° and horizontal paw angles trend towards 0°). Indeed, paw angles changed significantly between time points for both horizontal (ANOVA, F3,26=7.13, p=0.001) and vertical (F3,26=10.57, p=0.0001) reaches. For horizontal reaches, there was a paw angle decrease of 5° (denoted by the negative sign, “−5°”) between time point −2 and −1 (p>0.05). Vertical reaches increased by 1° between time point −2 and −1 (p>0.05). However, from the −1 time point to the time of contact, there was a significant change in paw angle towards the angle of the target for both horizontal (−5°, p=0.001, Bonferroni post hoc test) and vertical reaches (7°, p=0.009). From the time of contact to time point +1 there was also a highly significant change in paw angle for horizontally and vertically oriented pasta pieces (−5°, p=0.008 and 13°, p=0.0003, respectively). Thus, rats demonstrate preshaping of their paws, both by comparing the paw angles between horizontal and vertical reaches (preshaping between), and by measuring paw angle change over time within each reach (preshaping within).
Preshaping increases with training
To test if preshaping improved with training, we trained a cohort of rats (n=6; “long-trained;” see Methods) to the point at which each was consistently reaching for pasta, an average of 2 weeks. We then continued to train the rats daily for an additional 8 weeks. We compared preshaping and reaching efficiency at the beginning of training with performance at the end of 10 weeks of training. plots the amount of preshaping according to the length of training. Preshaping is determined as the difference in paw angle between vertical and horizontal reaches (e.g. , preshaping between) at the point of contact. The long-trained rats showed significant increases in preshaping with training ( week 2 (gray) versus week 10 (black); repeated measures ANOVA, F1,33=11.54, p=0.008). In addition, we tested whether the efficiency of pasta retrieval increased during this period. As demonstrated in , the success rate for retrieval improved for all of the long-trained rats (t-test, t11=3.11, p=0.01). These results indicate that both preshaping and reaching efficiency improved with training.
Figure 4 Preshaping (A) and reaching efficiency (B) increased with training. Long-trained rats (n=6) had paw angles and retrieval rates measured at 2 weeks and at the end of training, 10 weeks. A. Pictured is the difference between paw angles for vertical reaches (more ...)
CST lesion abrogates preshaping
To test whether preshaping relies on the CST, we subjected rats to complete unilateral lesion of the medullary pyramid (), which severs all of the CST axons emanating from one hemisphere and causes deficits in the reaching paw opposite to the lesion. We asked whether the loss of the CST would diminish the amount of forepaw preshaping during reaching. demonstrates the forepaw angle progression in rats (n=8; same rats as ) that underwent unilateral pyramidotomy. Two changes from baseline angle progression can be appreciated. First, and most obvious, CST lesion abolishes vertical preshaping (ANOVA of paw angles at each time point after injury, F2,21=0.09, p=0.91). In addition to there being no significant difference in paw angles at each time point after CST injury, paw angles were significantly diminished by injury (black lines, Fig. A versus Fig. B; repeated measures ANOVA, F1,42=5.07, p=0.03). Second, the change in horizontal angle begins later in the reach after injury (gray lines, ; repeated measures ANOVA, F1,42=4.56, p=0.04).
To further demonstrate the effect of injury on paw orientation change, we plotted the preshaping within vertical and horizontal reaches before and after CST injury in . Preshaping within is the difference in paw angle from time point −2 to time point +1 (). For vertical reaches, the effect of injury is a large and highly significant decrease in preshaping (t-test, t15=5.88, p<0.0001). For horizontal reaches there was no significant difference in preshaping with injury (t15=0.90, p=0.38). So, while there was a significant delay in horizontal angle change with injury, the predominant effect of CST injury on preshaping was the complete loss of vertical preshaping. Thus, CST lesion abrogates preshaping primarily through its effect on vertical reaches.
Figure 5 CST injury diminishes vertical preshaping. We compared the amount of preshaping within a reach, which is the change in paw orientation between time points −2 and +1 (as indicated in ), before and after selective CST injury. Vertical preshaping (more ...)
Effects of CST injury persist
We measured paw angle during reaching at several times after CST lesion to determine if preshaping recovered. Rats (n=8) were first tested an average of 14 days after pyramidotomy and at least weekly thereafter to an average of 50 days after injury. They continued to receive 2–3 reaching sessions a week during this period, similar to the initial training period. To determine if preshaping recovered over time, we compared the difference in paw angles when reaching for vertically or horizontally oriented pasta soon after injury and at the last testing time. For each time point, there were no significant differences (−1 time point: 5° at day 14 versus 4° at day 50; contact: 17° versus 11°; +1 time point: 26° versus 20°; repeated measures ANOVA, F2,44=1.93, p=0.17). Neither horizontal nor vertical reaches demonstrated improvement (data not shown). Thus, preshaping does not recover after CST lesion.
Preshaping lost after injury correlates with the amount of baseline preshaping
From the data presented thus far, preshaping is a motor skill that depends, in part, on the CST. We reason that if learning to preshape depends on the CST, the more a rat preshapes its paw to fit its target, the more preshaping will be lost by severing the CST. Thus, we expected that there would be a strong relationship between amount of baseline preshaping and the amount of preshaping lost due to injury. This relationship is demonstrated in for values at the point of contact. The abscissa plots the angle difference between horizontal and vertical reaches before injury (preshaping between). The ordinate represents the effect of CST lesion on preshaping between, measured as the difference in preshaping before injury and preshaping after CST lesion. The graph demonstrates two important phenomena. First, the hypothesized relationship holds true: the amount of preshaping lost due to CST lesion is strongly and positively correlated with the amount of preshaping (R7=0.88, p=0.004). There were also strong correlations at the −1 time point (p=0.0005) and +1 time point (p=0.04) as well (data not shown). Second, demonstrates a clustering of the long-trained rats (open squares) and the short-trained rats (closed squares). Long-trained rats demonstrated greater preshaping before injury. Thus, not only do the long-trained rats acquire more preshaping before injury, but they also lose more preshaping after CST injury.
Figure 6 Amount of preshaping before injury predicts the effect of injury on preshaping. Preshaping improves with practice (e.g. ) and worsens with CST injury (e.g. ). We reasoned that if preshaping depends on the CST for learning, then the (more ...)
Loss of preshaping after CST injury lowers reaching success
To determine whether loss of preshaping was associated with reduced capacity to retrieve the pasta, we measured retrieval rates before and after CST lesion. shows the success rate for pasta retrieval before and after injury. At baseline, rats (n=8) succeeded in retrieving a piece of pasta on 47% of attempts. After CST lesion, the success rate fell to 30%, a significant reduction (t-test, t15=2.60, p=0.02). This loss of skill persisted. The success rate did not change between early times after CST lesion (average 12 days; 32%) and late times (average 26 days; 27%; t-test, t15=0.54, p=0.60).
Figure 7 Injury reduces reaching efficiency, an effect that correlates with loss of preshaping. A. The success rate for pasta retrieval declined significantly after injury (pooled from all times tested, t-test, t15=2.60, p=0.02). B. Decline in success rate correlated (more ...)
We asked whether the loss of reaching skill correlated with loss of preshaping (). If this were true, it would suggest that loss of preshaping has a functional consequence. To do this we created a ratio for each animal of performance before CST lesion and the performance after for both retrieval efficiency (abscissa; retrieval ratio) and preshaping between (ordinate; preshaping ratio). The relationship between these measures is very strong (R7=.82, p<0.01). Rats that had substantial loss of preshaping also had substantial loss of retrieval efficiency, whereas rats that had little change in preshaping also had no loss of retrieval efficiency. This indicates that loss of feed-forward planning of wrist orientation, one component of a complex reaching and grasping movement, has an important functional consequence.