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Title: Hyper-priming in cannabis users: a naturalistic study of cannabis use and
semantic memory function
Running Head: Hyper-priming in cannabis users
Morgan, C.J.A. Rothwell, E., Atkinson, H., Curran H.V.
Address correspondence to:
Celia Morgan
Clinical Psychopharmacology Unit
Sub-Department of Clinical Health Psychology
University College London
Gower St
London
WC1E 6BT
FAX: +44-207-916-1989
Tel: +44-207-679-1932
Email: c.morgan@ucl.ac.uk
Abstract
Introduction : Cannabis is the world’s most popular illicit drug. Episodic memory in
long-term users has been frequently investigated but little research has examined the
impact of cannabis use on semantic memory. Psychotic symptoms such as thought
disorder have also been theoretically linked to semantic memory impairments in
patients with schizophrenia. In addition to investigating the long-term effects of
cannabis on semantic memory, this study also aimed to investigate if any
schizophrenia-like symptoms observed in this population were related to semantic
memory abnormalities. Method: In a naturalistic study (n=74) using an acute-on-
chronic and chronic design, 36 cannabis users were compared with 38 controls whilst
under the influence of cannabis and then three to five days later when participants
were not intoxicated. Drug use was verified by urinanalysis. Semantic memory was
assessed using a semantic priming task with both a long and short stimulus onset
asynchrony (SOA) to assess automatic and controlled semantic processing. State and
trait schizotypy were also indexed. Results: Under the influence of cannabis, users
showed increased automatic semnatic priming and schizotypal symptoms than
controls. Three to five days later following abstinence from cannabis, users exhibited
hyper-priming at long SOAs on a direct priming task with a trend for reduced priming
at the short SOA. Cannabis users did not differ in trait schizotypy and were similar to
controls on state schizotypy when not intoxicated. Conclusions: Acutely, cannabis
increases semantic priming which may be related to some of the psychotomimetic
effects of the drug. Regular cannabis use produces semantic abnormalities when not
under the influence of the drug. The pattern of long-term semantic abnormalities
differs markedly from those observed in patients with schizophrenia and looks similar
to that observed in individuals scoring highly on a schizotypy scale.
Introduction
Cannabis is the most widely used illegal recreational drug: amongst all recreational
drugs its use is second only to that of alcohol. The British Crime Survey reported that
among young people (aged 16-24), 30% of men and 18% of women said they had
used cannabis during 2004/5 (British Crime Survey, 2006). There have been growing
concerns recently over the consequences of long-term cannabis use for cognition and
mental health, in particular in light of claims that the drug may induce psychosis in
some individuals (e.g. DeLisi et al., 2007). Acute cannabis intoxication is known to
impair memory (see Ranganthan & D’Souza, 2006 for a review). There are many
chemical compound constituents of the cannabis plant, the most psychoactively potent
of which has been identified as Δ-9-tetrahydrocannabinol (THC). The memory
impairing effects of an acute dose of cannabis are thought to relate to the action of
THC at the Cannibinoid 1 (CB1) receptor, which is found at particularly high
densities in the hippocampus, cerebellum and basal ganglia. Evidence of memory
deficits following long-term cannabis use is less convincing, with verbal memory
deficits found in some studies (e.g. Block & Ghoneim, 1993; Dafters et al., 2004;
Messinis et al., 2006) but not others (Pope et al., 2001; Verdejo-Garcia et al., 2004,
Fried et al., 2005). Reasons that have been suggested to account for these
discrepancies are time since abstinence, age of onset and extent of use (Di Forti et al.,
2007).
Despite numerous investigations of the affect of acute and chronic cannabis
administration on what is termed episodic memory, or ‘time-locked’ memory for
events and occurrences, another area of memory has been relatively neglected:
semantic memory. Semantic memory refers to the memory for facts and general
knowledge in the world, including memory for meanings and language. Semantic
memory in cannabis users is particularly interesting as semantic memory impairments
are suggested to underlie some of the symptoms observed in psychosis such as
thought disorder (e.g. Spitzer et al., 1999). A recent study found no impairment in
speed of semantic processing in cannabis users (Wadsworth et al., 2007), however
some previous studies have found evidence for an impairment in category fluency
-where participants are required to generate as many category exemplars as they can
within a given interval- (e.g. Messinis et al., 2006) which is suggestive of a semantic
memory impairment.
A more valid method of assessing semantic processing than category fluency is the
semantic priming paradigm, a lexical-decision task where participants must respond
to a target word that is preceded by a prime word which is either related to the target
or not (Meyer & Schevaneveldt, 1971). Semantic priming is facilitated responding to
concepts that are semantically related. When a prime word is related to the target
word (e.g. chair, table) people generally respond faster than when they are unrelated
(e.g. chair, fish), and this decrease in reaction time is the ‘priming effect’. The
mechanistic explanation for this describes the semantic system as organised in a map-
like network of nodes, representing concepts (Neely et al., 1991). When one node is
activated, spreading of activation occurs so that connected nodes are also activated.
This spreading of activation facilitates faster responding to related nodes in the
network.
Semantic priming is a popular methodology for investigating semantic memory as the
standard priming task can be manipulated to investigate different semantic
mechanisms. Stimulus onset asynchrony (SOA, the time between the onset of the
prime and the target) manipulations are used to assess effects on either automatic or
controlled processes. SOAs shorter than 250msec are considered to tap into
automatic, pre-attentional processing, whereas controlled processing, which is slower
and thought to involve attention, is tapped by SOAs longer than 700msec.
Recent claims have related cannabis use and psychosis, with implications that there
may be a causal link between the two (e.g. Fergusson et al, 2005; Ferdinand et al.,
2005). Disturbance in the organisation and processing of knowledge has long been
considered a central feature of schizophrenia and more broadly, psychosis (Bleuler,
1911/1950). As a result of this semantic processing tasks are at the forefront of much
research on cognitive deficits associated with psychosis and schizophrenia. Whilst
normal semantic priming is associated with normal language processing, it is thought
that overactive priming (faster and further spreading of activation), may relate to
schizophrenic thought disorder and language disruptions. A faster spreading of
activation in semantic networks in schizophrenia may result in spurious nodes
becoming activated and interfering with the normal train of thoughts. In patients with
schizophrenia, research has supported this hypothesis with increased priming (or
hyper-priming) found in some studies (e.g. Moritz et al., 2001a; 2001b). However the
results of research on semantic priming in schizophrenia have been far from
consistent. A recent review concluded that hyper-priming was most robustly found in
conditions of ‘indirect’ priming (Rossell & Stefanovic, 2007). Indirect priming refers
to priming where, unlike ‘direct’ priming where the words are directly semantically
related (e.g. lemon-sour), the two words presented have a mediator word that is
related to both of them but not presented (e.g. lemon- sweet, not presented mediator =
sour). As this produces the most robust results in schizophrenia research, we included
an indirect priming condition in the current study which set out to examine the effects
of cannabis, both acutely and chronically, on semantic memory.
The aim of the present research was to compare a naturalistic sample of cannabis-
users with controls on semantic processing and schizophrenia-like symptoms, both
under the influence of the drug and then again when not intoxicated. We set out to
investigate semantic priming with a computer-based, lexical decision task, with an
indirect component and SOA manipulations. State schizotypy was assessed using the
new, state schizotypy questionnaire (Mason, in prep). We hypothesised that
intoxicated cannabis-users will show a similar pattern of semantic priming that has
been shown in thought-disordered schizophrenic patients, i.e. hyper-priming and that
this would be related to schizophrenia-like symptoms. As little research has
investigated the long-term effects of cannabis on semantic memory, we tentatively
hypothesised changes in semantic priming when abstinent from the drug and this
should again be related to levels of schizophrenia-like symptoms when drug-free.
Method
Design and participants
An independent groups, repeated measures design compared a sample of 38 non-
cannabis-using controls with 36 recreational cannabis users on two test occasions 3-5
days apart. Inclusion criteria required that participants were at least 18 years old, had
English as a native language, were non-dyslexic and had normal or corrected-to-
normal vision. The cannabis-using group were required to be recreational cannabis
users (at least once a month for at least a year). They were recruited using a snowball
sampling method (Solowij et al., 1992). All participants gave written, witnessed,
informed consent on both occasions. This study was approved by the University
College London Graduate School ethics committee and the U.K. Home Office.
Procedure
All individuals were tested on two separate occasions, with the first session of
cannabis-users occurring when the participants under the influence of the drug (day 0)
and the second 3-5 days later when drug-free for 24 hours (day 3-5). The non-
cannabis group were drug free both times. Urine tests were used to confirm the
consumption or abstinence from cannabis and to screen for other drug use (THC,
opiates, cocaine, amphetamine, benzodiazepines and other related compounds).
Detailed drug use histories were also collected from participants using a structured
interview. Participants then completed the test battery below.
Assessments
Semantic priming
(See Figure 1 for a diagrammatic representation of the task)
A computer-based, lexical decision-making task was used to assess semantic priming
with relatedness and stimulus onset asynchrony (SOA) manipulations. Participants
had to decide whether a target ‘word’ was a real, English word or a pseudo-word.
Each target was preceded by a prime word, which varied in its semantic relatedness to
the target: directly related, indirectly related, unrelated, or the target was a pseudo-
word. In addition, the SOA (time between the onset of presentation of the prime and
presentation of the target) varied so that half the word pairs were separated by 250ms
(short SOA) and half by 750ms (long SOA). Each word in the pair was presented for
200ms. Participants could respond for 2000ms after presentation of each prime.
Between each word-pair trial there was 2500ms gap.
<Figure 1 about here>
The stimuli were 450 concrete nouns and 150 pseudo words arranged in four word-
pair conditions: directly related (50 word pairs) indirectly related (50 word pairs),
unrelated (50 word pairs) and word-prime, pseudo-target (150 word pairs). The
related pairs were co-exemplars of a given category, formed using category norms
(Battig & Montague 1969). Pseudo-words were legally spelled and pronounceable
letter strings selected from the ARC non-word database (Rastle, Harrington, &
Coltheart, 2002). All letters were presented in lower case Times New Roman, 44-
point in white in the centre of a black screen on a laptop using DMDX software
(Forster, 1999).
Two matched versions of the word-pair list were created randomly, with the
constraint that no condition could appear more than 3 times consecutively. 10 practice
trials preceded 3 blocks of 100 test trials. Participants received a different list on each
occasion, which was counterbalanced across group and day. They were asked to read
the first word to themselves and then respond to the second word as quickly and
accurately as they could, using a labelled (‘word’, ‘non-word’) two-button press.
Reaction time (RT) and accuracy data were recorded.
Subjective Measures
O-NOW (Mason et al., in prep): A 48-item questionnaire designed to assess current
(state) schizotypal symptomology (Mason, in prep). Participants rate statements that
describe their current experience from 0 (not at all) to 3 (strongly).
Schizotypal Personality Questionnaire (SPQ): A standard questionnaire assessing trait
schizotypy (Raine, 1991). Subjective Effects Scale (SES): A scale comprising 11
subjective effects of cannabis rated on 10 cm visual analogue scales (adapted from
Curran et al., 2002) and is sensitive to state change. Hospital Anxiety and Depression
Scale (HADS): This brief assessment was used to tap trait anxiety and depression
(Zigand & Snaith, 1983).
Statistical analyses
For the semantic priming task RTs faster than 250ms and slower than 1500ms were
excluded. Participants were also excluded from analysis if their mean RTs were
consistently more than 2 standard deviations away from their group mean (1 control )
and if they made over 20% errors (1 cannabis-user). All data is reported without these
participants. For the semantic priming task, a 2x2x4x2 repeated measures ANOVA
with 1 between-subjects factor: Group (cannabis user, control), and three within-
subjects factors: Day (0, 3-5), Relatedness (directly, indirectly, unrelated, pseudo),
SOA (short, long) was performed on both the RT and error data. Results were
checked for a main effect of relatedness in RT data, indicating that priming has
occurred, and then for parsimony priming effects (by subtracting the RT for related or
indirectly from the RT to unrelated pairs, see Spitzer et al., 1999) were computed and
direct and indirect priming were analysed separately using repeated measures
ANOVA with 1 between subjects factor of Group and two within-subjects factors of
Day and SOA. When significant interactions occurred, post-hoc simple effects tests
were conducted using Bonferroni correction.
Correlations between priming and other variables were conducted using Pearson’s
correlation coefficient.
Results
Demographics
One-way ANOVAs showed no significant group differences in age, trait schizotypy
(SPQ), anxiety or depression (Table 1), and Χ2 analysis showed no significant group
differences of gender [Control group: 22 males, 15 females; Cannabis group 21
males, 14 females]. There were no group differences in the highest educational level
achieved.
<Table 1 about here>
Drug Use
All cannabis-users smoked cannabis on day 0. All controls gave THC-negative urine
samples. Three cannabis users also tested positive for other drugs ( 2 MDMA and 1
amphetamine) however none of the subjects reported significant use of other drugs.
Table 2 details cannabis and alcohol use across the groups. There were no significant
group differences in years of alcohol use, days used per month or units per session.
Seventeen controls reported having tried cannabis but no more than 5 times.
<Table 2 about here>
Five cannabis users and three controls reported having used alcohol on day 0 (max 2
units). One of these cannabis users and two of the controls also consumed alcohol on
day 3-5. In the interim (including the evening of day 0) nine cannabis users reported
having consumed cannabis. 13 cannabis users and 22 controls reported having
consumed alcohol during the interim. None of the participants reported cannabis use
on the day of follow-up testing.
Semantic Priming
Reaction time data (Table 3)
A 2 x 2 x 3 x 2 (Group x Day x Relatedness x SOA) RMANOVA analysis indicated a
significant main effect of Relatedness [F (1,140)=72.54, p<0.001) with shorter RTs
for related word-pairs, indicating priming had occurred therefore direct and indirect
priming effects were calculated (direct RT – unrelated RT / indirect RT – unrelated
RT) for both day and SOA conditions and analysed separately (see Spitzer et al.,
1998).
Indirect priming
A RMANOVA showed no significant main effects of group or interactions for the
indirect priming data, only a main effect of day (F(1,69)=5.41, p=0.023). Participants
generally demonstrated less priming on day 3-5 than on day 0.
<Table 3 about here>
Direct priming
A RMANOVA revealed a significant 3-way Day x SOA x Group interaction
[F(1,70)=6.71, p=0.012] in addition to a Day x SOA interaction [F(1,70)= 5.55,
p=0.021] and a main effect of SOA [F(1,70)= 7.43, p=0.008].
<Insert Figure 2 about here>
Post-hoc tests demonstrated a significant difference between cannabis users priming
on day 0 and day 3 at the short SOA [ t(34)= 3.12, p=0.004] reflecting greater priming
in the cannabis users on day 0 than day 3. No other significant differences emerged.
[To further explore the chronic effects of cannabis, group differences on day 3 were
examined. A significant difference between controls and cannabis users at the long
SOA emerged [F(1,70) =5.62, p=0.02] with a trend for greater priming in controls
compared to cannabis users at the short SOA [F(1,70) = 2.80, p=0.099].
Errors
ANOVA analysis (group x day x relatedness x SOA) showed a 2-way Relatedness x
SOA interaction (F(2,140)=5.13, p=0.007) and main effects of Relatedness
(F(2,140)= 110.55, p<0.001) and SOA (F1,70)= 10.06, p=0.002). There was no main
effect of group or group-wise interaction (See Table 4).
<Insert Table 4 about here>
State Schizotypy
A 2 x 2 RMANOVA of total state schizotypy scores found a significant Day x Group
iteraction [ F (1,70) = 15.07, p<0.001] and significant main effects of Day [ F(1,70)=
29.99, p<0.001] and Group [ F (1,70) = 15.84, p<0.001]. As can be seen from Figure
3, the cannabis group scored significantly higher on schizotypy on the night of drug
use (p<0.001) but 3-5 days later there were no differences in levels of schizotypal
symptoms.
<Insert Figure 3 about here>
Subjective Effects
Seven scales showed significant day x group interactions (stoned, tipsy, dizzy, dry
mouth, impaired memory, loss of concentration, increased heart rate). All of these
scales showed marked group differences, with both cannabis groups scoring higher
than controls on day 0 and no differences on day 3-5, with controls remaining about
the same on both days.
Correlations
No significant correlations occurred between cannabis use, change in schizotypy and
change in priming were observed.
Discussion
This study investigated semantic priming in cannabis users both under the influence
of the drug and when they had abstained for between 3 and 5 days. The main finding
was of an increase in priming, or ‘hyper-priming’ in cannabis users at the short SOA
when they were under the influence of the drug compared to when they were drug
free. In addition, un-intoxicated cannabis users showed a pattern of greater priming at
the long SOA and less priming at the short SOA compared to non-cannabis using
controls. Although groups did not differ in trait schizotypy, on the night of drug use
there was a marked increase in schizotypal symptoms in cannabis users but no group
differences following 3-5 days of abstinence.
As far as we are aware, this is the first study to examine semantic priming in cannabis
users. Our results imply that cannabis use is associated with semantic abnormalities.
Acutely, cannabis use was associated with an increase in automatic semantic priming,
albeit from a decreased baseline automatic priming. The classic theories of semantic
priming dysfunction and its relationship to schizophrenic symptoms suggest that it is
an increase in automatic spreading of activation (i.e. manifest as hyper-priming at a
short SOA) that accounts for symptoms such as thought disorder. This is supported by
some research showing hyper-priming at a short SOA in patients with thought
disorder (Moritz et al., 2001; Spitzer et al., 1993; Manschreck et al., 1988). In the
current study, the increase in automatic semantic priming was not correlated with any
of the schizophrenic symptoms reported under the influence of cannabis. However,
the similarity between the effects observed here and those observed in some studies
with patients with schizophrenia suggest that increased automatic semantic priming
may yet reflect the cognitive basis of some of the psychotomimetic effects of
cannabis. These findings are in accordance with the acute effects of cannabis (e.g.
Curran et al., 2002) and chronic effects from some (Messinis et al., 2006) but not all
(Pope et al., 2001) previous studies with more primitive semantic tasks, such as
category fluency.
The mechanism underpinning this pattern of semantic abnormalities in cannabis users
is as yet unclear. Psychologically, as cannabis, acutely, causes an increase in
automatic semantic processing it is possible that following long-term use, an adaptive
mechanism may result in the reduction of automatic semantic processing and a
compensatory increase in controlled processes. Neurochemically, the semantic
processing changes observed in the current study could be mediated by a variety of
mechanisms. Acutely, cannabis has been shown to increase dopaminergic neuronal
firing and increase the release of dopamine at terminal fields in the striatum and
prefrontal cortex (Lupica & Riegel, 2005). A previous study that examined semantic
priming following an acute dose of the dopamine agonist levo-dopa found an increase
in automatic semantic priming (Kishka et al., 1996). Thus it is possible that the acute
increase in semantic priming observed in this study was dopaminergically mediated.
At follow up testing when users were unintoxicated the reduction in automatic
priming may relate to decreases in dopmaine levels following cannabis self-
administration, as occurs in stimulant users (Volkow et al., 2004). Recent research has
demonstrated reduced semantic priming at a short SOA in Parkinson disease (PD:
Angwin et al. 2006). It has been suggested that the reductions in semantic priming in
PD are as a result of reduced dopaminergic modulation of the signal-to-noise ratio of
semantic networks. Another way in which cannabis might evoke long term changes in
semantic priming is via CB1 receptors found on excitatory terminals in areas
important in memory such as the hippocampus. It has been suggested that CB1
receptors on excitatory terminals may act as a ‘safety mechanism’ preventing network
excitability ( DiForti et al., 2007).Changes in network excitability could theoretically
cause increases in semantic priming acutely, which may explain the increases in
priming we observed in cannabis users under the influence of the drug. Upregulation
of CB1 receptors, following prolonged cannabis use, could result in a higher threshold
for excitability which may be manifest in decreases in automatic semantic priming.
Again compensatory mechanisms may account for the increases in priming seen at the
long SOA in this study.
There were no group differences in the error data, indicating that cannabis-users were
not different from controls in their ability to perform the task, their concentration or
motivation. Controls were very homogenous in their priming scores on both days and
at both SOA manipulations, whereas both cannabis groups had wide variation in
priming scores. With larger sample sizes it is possible that sub-groups with different
semantic profiles would emerge. There were also no differences on the indirect
priming task. This was somewhat surprising as hyper-priming has been suggested to
be the most robustly elicited by indirect priming tasks. However this may stem from
including the indirect and direct conditions in the same task. It has been suggested
that including indirectly and directly related word pairs in the same task induces a bias
to classify the indirectly related words as unrelated (McNamara & Altaribba, 1988).
Although the current data showed no correlation between cannabis use and semantic
priming, they did show an increase in schizophrenia-like symptoms during cannabis
use. This suggests that cannabis users do encounter a number of unusual, psychotic-
type experiences compared with non-users, and which may possibly be a risk factor
for individuals prone to psychosis. The absence of higher scores in trait schizotypy
and state schizotypy when drug free in cannabis users is contrary to the majority of
previous studies (e.g. Nunn et al., 2001; Dumas et al., 2002; Schiffman et al., 2004;
Williams et al., 1996). This may be a power issue as all of the above studies had large
sample sizes. However, it may also be the use of a state schizotypy questionnaire in
the current study. It is possible that, when answering trait questionnaires, cannabis
users find it difficult to disentangle their experiences on cannabis from those they
experience in their ‘drug-free’ life. As the first acquaintance of participants in this
study with such a measure was a state schizotypy questionnaire under the influence of
cannabis, it may be that when they come to answer the same (or similar trait
schizotypy) questionnaires whilst not under the influence of the drug it is easier for
them to separate the two experiences.
The finding of hyper-priming at a long SOA in abstinent cannabis users is intriguing
from the perspective of psychosis. The pattern of semantic priming when acutely
intoxicated is similar to that observed in patients schizophrenia but when un-
intoxicated it is very different. It does, however, look similar to the pattern of results
observed in a study of semantic priming in high schizotypes, or people scoring highly
on a trait schizotypy scale (Morgan et al., 2006). In this study, high scorers on a
schizotypy scale showed reduced priming at the short SOA and increased priming at
the long SOA. It is possible that cannabis users are actually high in schizotypy but
have not scored highly on questionnaires in this study for the reasons given above.
The semantic abnormalities observed in the current study may be not necessarily have
negative implications. The link between creativity and semantic priming has been
alluded to in one previous study (Spitzer et al., 1996). Future research should address
whether the enhanced controlled semantic priming observed in regular cannabis users,
rather than a pathological effect, is in fact producing beneficial effects and enhanced
creative processes.
This was a naturalistic study and therefore subject to limitations that could be
circumvented with a residential laboratory study, for example knowing the precise
dose and purity of cannabis and accurately verifying abstinence. Although urine
screens allowed us to objectively assess drug use, the long half life of cannabis made
confirming abstinence problematic. Despite this, a strength of this research is its
ecologicaly validity - individuals in the current study were taking cannabis in the
quantity that they would normally taking it in a usual setting. A further problem with
this cross-sectional study is that it is impossible to say that semantic abnormalities
observed when unintoxicated were as a result of cannabis use and not pre-existing
differences. Prospective studies are the only way to address this issue.
In summary, cannabis users showed increased state schizotypy and an increase in
automatic semantic priming when under the influence of the drug. Controls and
cannabis users did not differ in trait or state schizotypy when drug-free but hyper-
priming was observed at the long SOA in cannabis users when drug free, with a
suggestion of reduced priming at the short SOA. The acute increase in priming may
relate to the psychotomimetic effects of cannabis and long-term changes may be a
compensatory mechanism that has evolved following repeated cannabis use.
Acknoledgements
We would like to thank Chief Inspector Alan Macfarlane, Home Office UK, for his
support in this research.The research was funded in by the ESRC (UK).
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Figure 1: Schematic of the semantic priming task
0
10
20
30
40
50
60
Day 0
short
Day 0
long
Day 3-5
short
Day 3-5
long
Priming effect, secs
Control
Cannabis
Figure 2: Direct priming effects across Day, SOA (short or long) and group.
0
10
20
30
40
50
60
Day 0
short
Day 3-5
short
Day 0
long
Day 3-5
long
Priming effect, secs
Control
Cannabis
Which graph do you reckon
is best Val? I tend to think this one.
**
0
5
10
15
20
25
30
35
40
Day 0 Day 3-5
Schizotypy s core
Control
Cannabis
Figure 3 : State schizotypy scores across day and group
**
Table 1: Group means (sd) of age, trait schizotypy, anxiety and depression.
Cannabis users
(N=35)
Control
(N=37)
Age 26.37 (9.63) 26.24 (10.73)
SPQ 16.74 (11.97) 13.03 (8.35)
HADS
Anxiety
5.74 (3.75) 5.14 (3.11)
HADS
Depression
2.57 (2.65) 2.41 (2.48)
Table 2: Group means (sd) of cannabis and alcohol use.
Cannabis users Controls
Cannabis Years used 7.6 ( 5.8) -
Days/month 12.8 (5.1) -
Joints/session 2.3 (1.3) -
Last used (days) 2.2 (1.1) 414.1 (495.1)
Alcohol Years used 11.3 (9.2) 9.9 (9.9)
Days/month 12.0 (8.5) 10.1 (7.0)
Units/session 7.5 (5.8) 6.2 (5.0)
Last used (days) 5.5 (11.6) 10.7 (32.7)
Table 3: Mean reaction times (sd) across group, day, word-pair type and condition
Cannabis Controls
Relation SOA Day 0 Day 3-5 Day 0 Day 3-5
Direct Short 657
(113)
623 (112) 612 (105) 577
(65)
Long 680
(115)
612
(96)
628 (82) 592
(78)
Indirect Short 659
(116)
615
(110)
606 (100) 582
(72)
Long 685
(127)
632 (128) 622 (82) 593
(73)
Unrelated Short 692
(125)
630 (106) 639 (104) 600
(71)
Long 719
(121)
661
(118)
660 (89) 618
(70)
Pseudo Short 771
(121)
725 (113) 724 (88) 681
(74)
Long 767
(139)
703 (108) 708 (85) 660
(72)
Table 4: Mean percentage errors (sd) across group, day, word-pair type and condition
Cannabis Controls
Relation SOA Day 0 Day 3-5 Day 0 Day 3-5
Direct Short 3.09 (3.55) 1.50 (2.26) 1.55 (2.56) 1.50 (2.26)
Long 3.2 (3.64) 1.93 (4.08) 1.63 (2.73) 1.89 (2.56)
Indirect Short 3.22 (4.26) 1.76 (3.44) 3.08 (4.34) 2.57 (2.84)
Long 1.99 (3.00) 1.15 (2.46) 1.39 (2.64) 1.77 (3.02)
Unrelated Short 6.61 (3.30) 6.65 (3.73) 6.45 (3.35) 6.44 (3.45)
Long 5.32 (6.22) 5.19 (5.33) 4.95 (5.42) 4.19 (4.44)
Pseudo Short 11.45
(8.18)
9.23 (7.17) 12.42
(15.88)
9.74 (8.93)
Long 8.61 (7.19) 6.36 (5.97) 8.31 (6.38) 6.97 (6.16)