New York, Aug 20 (IANS): Although individuals who use marijuana plus tobacco have smaller a hippocampus, their memory function is greater, reveals a new study.

"Hippocampal size of non-users reflects a direct relationship to memory function -- the smaller the hippocampus, the poorer the memory function," said principal investigator Francesca Filbey from the University of Texas at Dallas.

"However, individuals who use marijuana plus tobacco show an inverse relationship -- the smaller the hippocampus size, the greater memory the function," Fibley said.

The study found that the size of the hippocampus -- an area of the brain associated with memory and learning -- is significantly smaller in both the marijuana group and marijuana plus tobacco group compared to non-using controls and individuals who use tobacco exclusively.

Also, the number of nicotine cigarettes smoked per day in the marijuana and nicotine-using group appeared to be related to the severity of hippocampal shrinkage.

The greater the number of cigarettes smoked per day, the smaller the hippocampal volume and the greater the memory performance.

There were no significant associations between hippocampal size and memory performance in individuals who only use tobacco or only use marijuana.

Approximately 70 percent of individuals who use marijuana also use tobacco, the study noted.

"Our findings confirm that the interaction between marijuana and nicotine is indeed much more complicated due to the different mechanisms at play," Filbey said.

The study appeared online in the journal Behavioural Brain Research.