Dick J. Bierman & Jenneke van Ditzhuijzen
University of Amsterdam
Thirty-two subjects participated in a 128 trial slot machine task. The task was initiated by the subjects. With intervals of one second the three windows of the slot machine froze. There were three types of events: three subsequent different fruits (XYZ), two equal fruits followed by a different one (XXY) and three equal fruits (XXX). The events were selected randomly with replacement from a limited pool of possible events. The subject had to pay 0.5 euro (real money) for each trial and received 7 euro for winning (XXX) events. The a priori probability for an XXX-event was 12.5% throughout the experiment. The subject could not know nor learn what the next fruit to be displayed would be. The subjects kept the money they won at the end but never had to pay when they eventually lost money. Following brain research with slot machines we analyzed the pooled medio-frontal signals from the Fz, Cz and Pz lead, using pre-processing parameters specified in the literature. There was a significant difference between the slow wave preceding a "win" and preceding a loss (XYZ). This difference can be explained by the fact that after the second fruit has been "frozen" the subject is aware that in the XYZ condition the possibility for a win has vanished. However the difference was observed to develop before the second fruit froze i.e. before there was any visible difference between the conditions. This anomaly was confirmed by a comparison of the XXY and the XXX condition where, for the relevant period from 1 to 2 seconds, there was no visible difference for the subject and nonetheless the brain signals differed by about 1.9 microvolt on average (t= 2.35, df=31, p=0.026). These anomalous results were not significantly associated with "perceived luckiness" although the 15 subjects who perceived themselves as "lucky" did have a much larger effect of ~ 2.9 microvolt compared with the other subjects (~0.6 microvolt). Exploratory analyses showed some suggestive evidence for the effect of sustained attention and of the belief to be able to "influence" the slot machine.