The ratio of CBD to THC varies massively across different strains of cannabis. Both THC and CBD have quite differing and at times, opposite medical properties and side effects depending on the concentrations and ratios of the cannabinoids in the specific strain. Recent studies  suggest that CBD has no negative effects of cognition on healthy individuals and can actually improve attention and working memory in patients, whereas THC will negatively affect memory in a concentration dependent manner.
A study recently published in the Journal of Cannabis  investigated the effect of varying CBD:THC ratios on cognitive function. The study involved 32 regular cannabis users who consumed cannabis with varying THC and CBD concentrations and ratios. Assessment was made via bloodwork and a verbal recognition memory test both pre and post consumption. As expected, participants that used primarily THC based strains showed decreased memory function as THC levels in blood samples increased. Conversely, participants that used strains containing both CBD and THC showed no differences in cognitive memory function before and after consumption, regardless of the levels of THC levels in the cannabis they consumed.
These results suggest that using CBD in conjunction with THC can reduce some of the negative cognitive side effects that are associated with THC. A much larger clinical trial is needed to confirm the validity of these results and to assess the impact of using these two cannabinoids in conjunction with each other may have on other medical benefits.
 Osborne, A. L., Solowij, N., & Weston-Green, K. (2017). A systematic review of the effect of cannabidiol on cognitive function: Relevance to schizophrenia. Neuroscience & Biobehavioral Reviews, 72, 310–324. doi:10.1016/j.neubiorev.2016.11.012
 Curran, T., Devillez, H., YorkWilliams, S.L. et al. Acute effects of naturalistic THC vs. CBD use on recognition memory: a preliminary study. J Cannabis Res 2, 28 (2020). https://doi.org/10.1186/s42238-020-00034-0