Question: 

Picking a raspberry requires increasing pull force appropriately until the fruit is plucked. In the "raspberry task" healthy subjects show both increasing pull forces and associative learning of the temporal sequence of the motor behaviour. Here we provide evidence for the influence of the parietal cortex in force production and motor sequence learning in a patient with a left parietal cortex lesion.

Methodology: 

The raspberry task consists of increasing pull force until the end of the pull phase and then rapidly reducing it. Associative learning of the timing of this alteration of motor behaviour (unconditioned stimulus, US) was tested using the method of classical conditioning (CS, 1kHz tone, 465 ms before US).The slope of the pull force / time relationship typically showed two phases, thus allowing each trial to be divided into two intervals: I1 (before the slope change) and I2 (after). Trials were classified as with or without conditioned response (wCR / woCR). Pull-force slopes of wCR and woCR trials were studied in I1 and I2 and compared between the lesioned and non-lesioned side (LS and non-LS, respectively).

Result: 

Pull-force slope-changes from I1 to I2 were similar on the non-LS independent of trial classification (DwCR 9.8 N/s,DwoCR 9.4 N/s). In contrast, on the LS the slope changes were significantly smoother in woCR than in wCR trials (6.4 N/s vs. 8.9 N/s). Detailed analysis revealed that slopes of woCR trials on the LS were less steep for both intervals with respect to the non-LS, indicating a strong effort to generate linearly increasing pull forces on the LS. In wCR trials the slopes in I1 were similar for both sides, whereas in I2 the slopes were less steep on the LS compared with the non-LS. This result implies a lack of effort on the LS in generating increasing pull forces allowing the subject to react to the CS.

Conclusion: 

On the non-LS the subject produced a similar motor behaviour during both intervals, independently of whether the patient produced a CR. In contrast, on the LS, the patient switched his attention between two strategies: linearly increasing pull-force slope or producing a CR. These results suggest that the parietal cortex is involved in establishing a temporal sequence of motor behaviour.

(Supported by the "Else-Kröner-Fresenius Stiftung" (A12/07)