PERCEPTIONS OF EFFORT AND HEAVINESS DURING FATIGUE AND DURING THE SIZE-WEIGHT ILLUSION

Previous work has shown that force perception and the sense of motor e ffort are different attributes of sensorimotor function. This study ex plores the hypothesis that one reason force and effort perceptions are distinct is to inform an individual of impaired motor function when m uscular force lags effort. This hypothesis predicts that effort and fo rce perceptions will dissociate when motor function is impaired by fat igue but not during the size-weight illusion. All subjects reported a distinct increase in effort when lifting a standard test weight as fat igue developed. When fatigue was sufficiently marked so that they coul d barely lift the test weight, they rated their effort as similar to t hat required to lift a maximal weight in the unfatigued state. The per ceived heaviness of the test weight also increased as fatigue develope d, but this fatigue-weight illusion was smaller than the increase in e ffort for all subjects and displayed greater variability. Zn contrast, both the perceived weight of a small object and the effort required t o lift it increased in parallel when small and large objects were lift ed sequentially. The size-weight and size-effort illusions appear to b e examples of a common phenomenon in which perceptual experience is re scaled to maintain acuity under different working conditions. The fati gue-weight illusion also has the effect of increasing perceptual acuit y as the subject's weight lifting range decreases due to fatigue.