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22 abril 2015 3 22 /04 /abril /2015 16:33

Cannabis consumers show greater susceptibility to false ...

www.eurekalert.org/pub...04/uadb-ccs042115.php

 

Cannabis Consumers Show Greater Susceptibility to False Memories

La imagen muestra el patrón de activación cerebral que permite descartar un estímulo como falsa memoria. En el grupo control, las activaciones son mucho más intensas y extensas que en el grupo de los consumidores de cannabis.

 

Abusar del cannabis puede provocar, en aquellos que lo consumen, una distorsión acentuada de los recuerdos, haciéndoles creer que han pasado por situaciones que distan mucho de parecerse a las reales; esto es, el consumo excesivo y crónico de esta sustancia psicoactiva genera falsos recuerdos, según revela el último estudio llevado a cabo por el Instituto de Investigación Biomédica del Hospital de Sant Pau y la Universidad Autónoma de Barcelona (España) y que recoge la revista Molecular Psychiatry.

Los problemas de memoria asociados al consumo de cannabis no son nuevos, pero esta investigación va más lejos, concluyendo que los consumidores de esta sustancia tienen también mayor predisposición a experimentar recuerdos ficticios.

“Un fenómeno habitual es tener falsos recuerdos de situaciones vividas en la infancia y que creemos recordar porque las personas de nuestro entorno nos las han contando muchas veces”, explican los autores.

Para su experimento, los científicos contaron con la participación de un grupo de consumidores crónicos de cannabis y un grupo de control. Todos ellos fueron sometidos a una prueba de memoria con palabras que tenían que aprender. Tras unos minutos memorizándolas, los voluntarios vieron las palabras antiguas junto a palabras nuevas tanto relacionadas semánticamente con las primeras como sin ninguna relación. Los participantes tenían que recordar qué palabras pertenecían al primer grupo, las que habían memorizado anteriormente.

Los resultados, que fueron contrastados con pruebas de imagen por resonancia magnética mientras realizaban el experimento, revelaron que los consumidores de cannabis creían haber visto las palabras que estaban relacionadas semánticamente con las de la primera lista con mucha mayor frecuencia que el grupo de control. Además, los escáneres cerebrales mostraron una menor activación en los cerebros de los consumidores de cannabis en la zona asociada al procesamiento de los recuerdos y al control de los recursos cognitivos.

La investigación demuestra que los consumidores de cannabis sufren las consecuencias de esta droga en los mecanismos cerebrales que nos facilitan la distinción entre todo aquello que es real y lo que no lo es. Este negativo efecto perdura aún semanas después de haber cesado el consumo, según evidencia el estudio.

 

Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

Behavioral data. The graphs show performance results in the memory task. Cannabis users performed significantly worse than controls, showing increased false recognition and decreased false memory rejection. Error bars denote one s.d. of mean.

 

Figure 2.

Figure 2 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

Rendering of fMRI results for each participant group. The statistical maps show the results of the voxel-wise comparison ‘false memory rejection’>‘correct rejection of new word’. For depiction purposes results are shown at P=0.01.

Full figure and legend (176K)Download PowerPoint slide (358 KB)

 

Figure 3.

Figure 3 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

Group differences between controls and cannabis users. The images show the results of the voxel-wise independent-samples t-test controls>cannabis users for the contrast ‘false memory rejection’>‘correct rejection of new word’. The brain regions depicted showed significantly higher activation in the control group as compared with the cannabis using group at P=0.001 uncorrected. No significant results were obtained for the contrast cannabis users>controls. For depiction purposes results are shown at P=0.005.

Full figure and legend (146K)Download PowerPoint slide (346 KB)



Article: J Riba, M Valle, F Sampedro, A Rodríguez-Pujadas, S Martínez-Horta, J Kulisevsky and A Rodríguez-Fornells. Telling true from false: cannabis users show increased susceptibility to false memories. Molecular Psychiatry; doi: 10.1038/mp.2015.36

 

 

Our results show that cannabis users had a higher susceptibility to memory illusions, as observed in certain neurologic and psychiatric populations,21 and elderly individuals.23 They further identify the functional substrate of this deficit in the hypoactivation of a series of spatially distributed brain regions participating in the network involved in semantic30 and episodic31 retrieval. The network identified fits nicely with previous studies that have shown that compared with new items, recognition of false stimuli leads to greater activation of the hippocampus and the parahippocampal gyrus, and also of the left parietal and left dorsolateral prefrontal cortices in healthy subjects.26 Although activation of MTL structures in these tasks can be directly associated with memory,32 the parietal cortex can be linked to attentional processes and the dorsolateral prefrontal cortex to monitoring issues in this context.33 It has been shown that the effective rejection of lures leads to greater activation of the dorsolateral prefrontal cortex34 and lesions at this level lead to increased false recognition in neurological patients.35 Thus, rather than a compromise of memory structures per se (that is, the hippocampus), our results point to a more diffuse impairment, which leads to a reduced capacity to deal with the retrieval and monitoring demands needed to differentiate between illusory and real events.

From a theoretical perspective, two main accounts have been put forward to explain the false memory phenomenon: the fuzzy trace theory and the activation-monitoring account. The fuzzy trace theory postulates that stimuli are encoded into two types of memory traces: a ‘verbatim’ trace containing specific details and features associated with the stimulus, and a ‘gist’ trace that contains more general aspects of the encoding event. False memories occur when new stimuli share certain features with past events and elicit the retrieval of the gist trace, but not the verbatim trace.36 In contrast, the activation-monitoring account37 postulates that cognitive control mechanisms need to be engaged to correctly identify and reject the highly activated lures. According to this view, false memories occur when monitoring mechanisms fail to identify the non-studied but semantically related lures.

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Our findings can be interpreted in the light of the two accounts described above. The between-groups comparison of fMRI activation maps showed activity not only in distributed brain areas participating in semantic30 and episodic31 retrieval, but also in cognitive control, as suggested by the significant dorsolateral prefrontal clusters identified.38, 39 The greater activation found for the control group in the medial and lateral temporal cortices suggests access to both the semantic (lateral) and episodic (medial) features of the studied stimuli. Using the terminology of the fuzzy trace account, controls would take advantage of both the verbatim and gist traces when deciding to reject a false memory. On the contrary, the inverse correlation found between lifetime cannabis use and the BOLD response in the MTL suggests that chronic exposure to cannabis may be especially detrimental to the brain structure providing the episodic or gist features to stored information. Cannabis users may have been left more dependent on the verbatim features of stimuli to decide whether a given word was a legitimate memory or not. Paradoxically, the greater activation of gist-related information in the control group compared with the cannabis group might have made them more vulnerable to false memories. Concurrent retrieval of item-based (verbatim) and context-based (gist) information in the control group might elicit conflict and require the engagement of cognitive control mechanisms, explaining the increased frontal activation observed in the controls. Thus, a more efficient conflict- or activation-monitoring, as signaled by increased dorsolateral prefrontal activity, may have led to the final outcome of better performance in the control group.

Further evidence of MTL and prefrontal impairment by cannabis is provided by magnetic resonance spectroscopy studies. Using this technique, researchers have found detrimental neurometabolic changes in these brain areas. For instance, Silveri and colleagues have reported decreased myo-inositol/creatine levels in the MTL and thalamus of users.40, 41 Hermann et al.42 have found reduced N-acetyl-aspartate/total creatine ratios in the dorsolateral prefrontal of recreational users, and Cowan et al.43 have found analogous decreases in Brodmann area 45 in the inferior frontal gyrus. Considering that analogous neurometabolic changes can be observed in older individuals44 and that reality monitoring deficits increase with age,45 we speculate that chronic cannabis use could aggravate the memory deficits associated with the normal ageing process.

Our findings extend previous knowledge on the impact of cannabis use on memory12 and executive function.8 Although there are contradictory results regarding the normalization of memory in the long term,9, 10, 16 impairment has been associated with the intensity of cannabis use, with heavy users showing deficits in various memory functions.46 Interestingly, many neuroimaging studies implementing simple memory tasks have failed to find differences in performance between heavy cannabis users and controls.12 Our findings suggest that impairment may be more subtle and affect more complex cognitive processes, like those involved in the Deese-Roediger-McDermott paradigm.

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A limitation of our study is the potential presence of residual THC levels in the brain in the absence of detectable levels in other biological matrices (in our case, urine). Whereas most studies in humans consider that cognitive testing after a 4-week period will assess the long-term effects of cannabis rather than its residual effects,8 a longer persistence of THC in the brain has also been observed.47 Thus, although unlikely, the presence of small amounts of THC in the body cannot be entirely ruled out.

Taken together, the present results indicate that long-term heavy cannabis users are at an increased risk of experiencing memory errors even when abstinent and drug-free. These deficits show a neural basis and suggest a subtle compromise of brain mechanisms involved in reality monitoring. Though subtle, the deficits found bear similarities with alterations observed in psychiatric and neurologic conditions and also with age-related cognitive decline. This lingering diminished ability to tell true from false may have medical, and legal implications. Future studies should address these issues and assess whether the deficits found here extend to other forms of memory distortion and reality monitoring beyond the false memory phenomenon.

 

 

 

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