By contrast, rms velocity contributed equally to the frequency discrimination when vibrations were on the same finger or opposite fingers.
The subjects' discrimination thresholds were higher when comparing vibrations on opposite fingers than when comparing vibrations on the same finger (Figure 3; F 1,19 = 61.08, p<.001), confirming previous studies [6], [13].
Therefore, if information about velocity is held by neurons in cortical regions beyond SI, as suggested by Experiment 2, then subjects should depend on velocity when discriminating between vibrations presented to opposite fingers.
Therefore, for Experiments 3, 5 and 6 in which vibrations were presented on opposite fingers, stimulators were built using nickel bimorph wafers (38×19×0.5 mm, length×width×thickness; Morgan Matroc, Bedford, OH, USA).
That is, the increase in frequency threshold that occurs when subjects compare vibrations with matched velocity should be equivalent for same-finger and opposite-finger comparisons.
They compared the frequency of two vibrations separated by 500 msec, this being within the interval range at which differences between same-finger and opposite-finger comparisons are evident [6], [13].
Moreover, this effect was observed for both same-finger and opposite-finger comparisons, indicating that rms velocity is represented by neurons with bilateral receptive fields, pointing to the involvement of regions beyond SI.
Paired t-tests confirmed that there was a significant difference between equal-amplitude and matched-velocity vibrations for both same-finger and opposite-finger comparisons (t 19 = 3.03 and 2.47, p = .007 and.023).023
As is evident in Figure 6, this interaction reflects the fact that noise had a greater effect on discrimination thresholds for same-finger comparisons than for opposite-finger comparisons.
Because evidence for a velocity code was obtained with both single-finger and opposite-finger comparisons, we conclude that this velocity is coded by neurons with bilateral receptive fields, such as in SII.
In Experiment 5 (see Figure 5) there was a significant difference between the thresholds obtained for same-finger and opposite-finger comparisons (F 1,9 = 5.51, p = .044), and a significant difference between the thresholds obtained with regular versus noisy vibrations (F 1,9 = 26.51, p<.001), confirming the findings of Experiments 3 and 4.
