Subject Motivation and Psychophysical Thresholds

[Andy Sabin <a-sabin@xxxxxxxxxxxxxxxx>]

Has anyone attempted to determine if a subject's basic discrimination 
thresholds can be manipulated by their level of motivation?

I think many people assume, for good reason, that lack of motivation 
will lead to worse thresholds. Subjects who fall asleep during sessions 
are often excluded from analysis. I am interested in the effect of 
positive motivation. Does anyone have an empirical reason to think that 
thresholds for a given [awake] individual would be lower if an 
additional monetary reward was offered for good performance?

Thanks
Andy Sabin