Complex sentence: Longer train durations produce more vigorous responding up to a point, after which rate of responding varies inversely with train length because animals begin to lever-press for additional stimulation before the previously earned train has finished.
Split into simpler sentences: Longer train durations produce more vigorous responding up to a point, after which rate of responding varies inversely with train length. This is due to lever-pressing for additional stimulation before the previously earned train has finished.