Chronic cannabis consumption has been associated with poor psychosocial functioning that could be associated to cerebellar dysfunction. The cerebellum has a relevant role in adaptation processes and has a high density of cannabinoid 1 receptor (CB1R). Implicit motor learning is a cerebellum dependent function that can be measured with a visuomotor rotation task (VRT).

The project aims to identify a sensitive and specific biomarker of cerebellum dysfunction in chronic cannabis users. The investigators would like to demonstrate that the visuomotor rotation paradigm is valid to measure and quantify such a dysfunction.

A longitudinal prospective study with a 3 month follow-up is proposed. 3 groups will be included: 1) chronic cannabis users; 2) individuals with an alcohol use disorder; and 3) healthy controls. All groups will be matched by sex and age. Forty individuals will be included in each group. Individuals will be assessed at baseline, at first month and at 3-months of follow-up. Sociodemographic and clinical data will be recorded. Information on cannabis consumption will be registered using an App.

Participants will do the visuomotor rotation task and answer three questionnaires: the Intrinsic Motivation Inventory, the Scale for the assessment and rating of ataxia (SARA) and the Harris tests for lateral dominance.

The biomarker developed by this project will facilitate the detection of cerebellar alterations in chronic cannabis users, and will permit to quantify and monitor such alteration over time. The team's intention is to patent the proposed model and disseminate it in order to use it in clinical practice at both primary and specialized health centres.

The subjects have to realize movements towards a target, at a digitizing tablet, holding a digitizing pencil. Visual feedback is projected on a screen surface in front of the subject, which hides the sight of actual movements from the latter. In the experimental condition of rotation, the experimenter introduces a rotation of 45 grades between the movement realized by the subject and the visual feedback that is provided on the screen. Then, the subject is provided with a strategy in order to overcome this perturbation: that is, to make the movement aiming 45 grades to the opposite direction from the introduced rotation. This explicit strategy leads to immediate correction of the error, but as the time passes subjects tend to commit more and more errors due to implicit motor adaptation. This happens because the motor control system tends to correct the perceived perturbation between the anticipated and the actual location of the hand in an automatic and unconscious way. The conflict between the implicit and explicit strategies has the surprising consequence of a persistent and accumulative deterioration of performance that can only be resolved when the participants abandon the explicit strategy. Subjects with cerebellar damage show deficits in implicit motor learning and for this reason they do not show this progressive deterioration of performance. Given that chronic cannabis abuse can lead to cerebellar damage, the investigators hypothesize that subjects from the experimental group 1 (cannabis use) will have a significant smaller directional error on the rotation condition than controls. A second hypothesis is that this effect will be cannabis-specific, due to the high CB1R concentration on the cerebellum, so the alcohol dependence group is expected to perform similarly to control group

Inclusion Criteria:

• 18-50 years old
• Right-handed
• Daily consumption of cannabis for at least 2 years (cannabis group) or diagnosed Alcohol Use Disorder (alcohol group)

Exclusion Criteria:

• Non-corrected visual deficits
• Regular consumption of other drugs except nicotine
• Other Axis I or neurological diagnosis

The experimental group 1 (cannabis use) will consist of the patients of the Addiction Unit of Hospital Clinic that are seeking or already are in treatment for cannabis use or participants that fulfil the eligibility criteria and contact the responsible for this study investigators , after the diffusion of the study. The experimental group 2 (alcohol use) will consist of patients of the Addiction Unit of Hospital Clinic that are in treatment for alcohol dependency. The control subjects will be recruited through the public diffusion of the study.