Because alcohol was a common co-substance for nearly all of the people in the polysubstance use group, it was difficult to tell exactly which substances were affecting certain cognitive functions. The researchers found that the difference in the two groups performance levels on executive function, or higher-level cognitive processing tasks were consistently showing that the polysubstance group scored lower than the control group. In general, this meant that multiple substances negatively affected the polysubstance groups cognitive functioning. More specifically, the researchers found that the amount of cannabis and cocaine affected the verbal part of working memory, the reasoning task, and decision making, while cocaine and heroin had a similar negative effect on visual and spatial tasks, but cannabis particularly affected visual and spatial working memory. These results suggest that the combined use of cannabis, cocaine, and heroin impair more cognitive functions more severely than if used separately.Alcohols negative effects on learning, spatial abilities and memory has been shown in many studies. This raises a question: does using alcohol in combination with other substances impair cognitive functioning even more? One study decided to try to determine if people with polysubstance use who also recreationally use alcohol would display poorer performance on a verbal learning and memory test in comparison to those who consumed excessive amounts of alcohol specifically. The California Verbal Learning Test (CVLT) was used due to its ability to "quantify small changes in verbal learning and memory" by evaluating errors made during the test and the strategies used to make those errors.
A study involving 63 polysubstance dependent women and 46 controls (participants who were not using substances) used the Benton Visual Retention Test (BVRT) and the California Verbal Learning Test (CVLT) to look at visual memory and verbal ability. This study showed that in polysubstance dependent women, verbal learning ability was significantly decreased, though visual memory was not affected. In addition, alcohol and cocaine use led to more severe issues with verbal learning, recall, and recognition. Memory, reasoning and decision making
Sometimes studies about specific groups in the general population can be informative. One study decided to test the cognitive abilities of participants in rave parties who used multiple substances. To do this, they compared 25 rave party attenders with 27 control participants who were not using substances. The results of this study indicated that in general, the rave attender group did not perform as well on tasks that tested speed of information processing, working memory, knowledge of similarities between words, ability to attend to a task with interference in the background, and decision making. Certain substances were associated with particular mental functions, but the researchers suggested that the impairments for working memory and reasoning were caused by the misuse of multiple substances.Another study that tried to find differences between the effects of particular substances focused on people with polysubstance use who were seeking treatment for addictions to cannabis, cocaine, and heroin. They studied a group of people with polysubstance use and a group that was not dependent on any substances.