Recent studies from the growing literature on the neuroscience of gambling have focused on the "near miss" effect in slot machine gambling. Briefly, "near miss" is a term used to describe an effect resembling 'almost winning' on a slot machine when, for instance, two of three matching symbols appear on the payout line. This outcome is still a loss though (a win would be three matching symbols).
It has been speculated that near misses serve to maintain persistent gambling. There are several strands of data that support this assertion. For instance, gamblers and non-gamblers initiate trials following a trial with a near miss faster than trials with either wins or losses (Dixon & Schrieber, 2004). Animals also show similar effects: Weatherly and Derenne (2007) and Peters et al. (2010) developed an animal model of slot machine gambling and found that the pause durations of rats was shortest after losses and longest after large "wins".
Gamblers also rate their chances of winning on the next trial far higher than on trials following other outcomes. These effects serve to suggest that gamblers ignore the "independence of turns" rule and act in way that implies that outcomes of the next trial will, in some way, be related to the outcome of the previous trial. They are not. Slot machines operate according to complex random ratio schedules, where each response is independent of the last. This is what makes gambling, generally, and the near miss effect, in particular, so interesting to study.
The proportion and physical form of the near misses that a gambler experiences has also been shown to effect persistence. A well-cited experiment by Kassinove and Schare (2001) found that a proportion of 30% of near miss trials resulted in greatest persistence (as measured by the number of trials played in the absence of feedback), while Ghezzi and colleagues (2006) showed that near misses on three-reel slot machines of the form X-X-Y were preferred over near misses of the form X-Y-X or Y-Y-X. The findings on the effects of proportion of near misses suggest that they do not resemble direct losses to the gambler. They still lose on these trials. The fact that the form of the near misses exerts an influence highlights how seemingly irrelevant, structural features of the stimuli involved in slot machine gambling may come to exert an influence over the behaviour of seemingly "free", language-able humans.
B.F. Skinner argued that near misses may function as conditioned reinforcers:
Gambling devices make an effective use of conditioned reinforcers which are set up by pairing certain stimuli with the economic reinforcers which occasionally appear. For example, the standard slot machine reinforces the player when certain arrangements of three pictures appear in a window on the front of the machine. By paying off very generously—with the jack pot—for "three bars," the device
eventually makes two bars plus any other figure strongly reinforcing. Almost hitting the jack pot" increases the probability that the individual will play the machine, although this reinforcer costs the owner of the device nothing. (1953, p. 397).
Recent neuroscience evidence supports this view (see the excellent Skepticon posting, for instance). fMRI studies by Chase and Clark (2010) and Habib and Dixon (2010) have each shown that near-misses activated the same brain regions as wins for gamblers and regions related to losses in non-gamblers. This suggests that near misses have conditioned reinforcing functions for gamblers (i.e., they behave "as if" they are wins). Interestingly, the extent of brain activation in mid-brain regions declines as a function of gambling severity. The more severe the gambling problem, the less activation. This implies that a degree of tolerance has occurred: repeated experience with near-misses leads to habituation-like effects on activation, and suggests that gambling persistence must be, at least in part, maintained in other ways. There is an awful lot more work to be on the behavioural and neural correlates of gambling, but the handful of studies just described have already made quite an impact on our understanding of this fascinating behaviour.
Chase, H., & Clark, L. (2010). Gambling severity predicts midbrain response to near-miss outcomes Journal of Neuroscience, 30 (18), 6180-6187 DOI: 10.1523/JNEUROSCI.5758-09.2010
Dixon, M. R., & Schreiber, J. (2004). Near-miss effects on response latencies and win estimations of slot machine players. The Psychological Record, 54, 335–348.
Ghezzi, P. M., Wilson, G. R., & Porter, J. C. K. (2006). The near-miss effect in simulated slot machine play. In P. M. Ghezzi, C. A. Lyons, M. R. Dixon, & G. R. Wilson (Eds.), Gambling: Behavior theory, research and application (pp. 155-170). Reno, NV: Context Press.
Habib, R. & Dixon, M.R. (2010). Neurobehavioral evidence for the “near-miss” effect in pathological gamblers. Journal of the Experimental Analysis of Behavior, 93 (3), 313-328. DOI: 10.1901/jeab.2010.93-313
Kassinove, J. I., & Schare, M. L. (2001). Effects of the ‘near miss’ and the ‘big win’ on persistence at slot machine gambling. Psychology of Addictive Behaviors, 15, 155–158.
Peters, H., Hunt, M., & Harper, D. (2010). An animal model of slot machine gambling: The effect of structural characteristics on response latency and persistence. Journal of Gambling Studies, 26, 521-531.
Weatherly, J.N., & Derenne, A. (2007). Rats playing a slot machine: A preliminary attempt at an animal gambling model. Analysis of Gambling Behavior, 2, 79-89.