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Controversy exists as to the cause of disturbed prepulse inhibition (PPI) in patients with schizophrenia. This study aimed to clarify the nature of PPI in schizophrenia using improved methodology. Startle and PPI were measured in 44 patients with schizophrenia and 32 controls across a range of startling stimulus intensities under two conditions, one while participants were attending to the auditory stimuli (ATTEND condition) and one while participants completed a visual task in order to ensure they were ignoring the auditory stimuli (IGNORE condition). Patients showed reduced PPI of R(MAX) (reflex capacity) and increased PPI of Hillslope (reflex efficacy) only under the INGORE condition, and failed to show the same pattern of attentional modulation of the reflex parameters as controls. In conclusion, disturbed PPI in schizophrenia appears to result from deficits in selective attention, rather than from preattentive dysfunction.

Schizophrenia is commonly associated with impairments in pre-attentive change detection, as represented by reduced mismatch negativity (MMN). While the neurochemical basis of MMN has been linked to N-methyl-d-aspartic acid (NMDA) receptor function, the roles of the dopaminergic and/or the serotonergic systems are not fully explored in humans. The aim of the present study was to investigate the effects of acutely depleting dopamine (DA) and serotonin (5-hydroxytryptamine, 5-HT) alone or simultaneously by depleting their amino acid precursors on MMN in healthy participants. Sixteen healthy male subjects participated in a double-blind, placebo-controlled, cross-over design in which each subject’s duration MMN was assessed under four acute treatment conditions separated by a 5-day washout period: balanced amino acid control (no depletion), tyrosine/phenylalanine depletion (to reduce DA neurotransmission), tryptophan depletion (to reduce 5-HT neurotransmission) and tryptophan/tyrosine/phenylalanine depletion (to reduce DA and 5-HT neurotransmission simultaneously). Acute depletion of either DA and 5-HT alone or simultaneously had no effect on MMN. These findings suggest that modulation of the dopaminergic and serotonergic systems acutely does not lead to changes in MMN.

BackgroundThe effects of cannabis use on neuropsychological indices that show characteristic disturbances in schizophrenia are unclear. The effect of cannabis use on these cognitive functions is of particular interest given the hypothesized association between cannabis use and schizophrenia. Therefore, this study aimed to examine the effects of cannabis use on attentional control, working memory and executive functioning, in both healthy individuals and patients with schizophrenia.MethodNeuropsychological performance was assessed in 36 cannabis users who were otherwise healthy, 35 healthy non-users, 22 cannabis-using patients with schizophrenia, and 49 non-using patients with schizophrenia. Participants were administered the Stroop task, the letter–number sequencing and spatial span subtests of the Wechsler Memory Scale, and the Wisconsin Card Sorting Test (WCST).ResultsPatients with schizophrenia (both cannabis users and non-users) showed significantly poorer performance across all neuropsychological tasks, relative to controls; however, there were no significant differences between schizophrenic cannabis users and schizophrenic non-users on any measures, with the exception of increased non-perseverative errors on the WCST in cannabis-using patients. Similarly, healthy cannabis users showed no significant differences from healthy non-users in any of the cognitive domains, with the exception of a schizophrenic-like increase in perseveration on the WCST.ConclusionsAmongst both healthy individuals and patients with schizophrenia there appears to be little difference in cognitive performance between cannabis users and non-users, suggesting that cannabis use has only subtle effects on the neurocognitive performance indices assessed here, which have been well established to be disturbed in schizophrenia.

Given the hypothesised association between cannabis use and schizophrenia, and the well documented alterations in pre-pulse inhibition (PPI) that are observed in schizophrenia, it is of interest to examine the effects of cannabis use on PPI.The objective of the study was to use novel methodology for the measurement and characterisation of attentional modulation of PPI, in order to examine the nature of PPI in chronic cannabis users.PPI was measured in 34 chronic cannabis users (who were otherwise healthy) and 32 healthy controls, across a range of startling stimulus intensities, during two attention set conditions, one in which they were instructed to attend to the auditory stimuli and one in which they were instructed to ignore the auditory stimuli and focus on a visual task. Curves of best fit were fitted to the startle magnitudes, across the stimulus intensities. A number of reflex parameters were extracted from these logistic functions, each of which reflects a different characteristic of the startle response.Cannabis users failed to show attentional modulation of any of the reflex parameters and showed altered PPI, relative to controls, but only when they were instructed to sustain attention to the auditory stimuli.Cannabis users showed an attention-dependant alteration in PPI, which appeared to reflect a deficit in sustain attention, and which was different to that which has been observed in schizophrenia using the same methodology.

Prepulse inhibition (PPI) is suggested to reflect a basic inhibitory mechanism which regulates sensory input to the brain, preventing sensory overload and cognitive fragmentation. However, studies directly investigating the relationship between PPI and cognition have produced inconsistent findings; this is likely to be due to the use of uninstructed PPI tasks, and limitations with the methods for measurement of PPI and startle. Therefore, the current study examined the relationship between cognitive performance and attentional modulation of PPI, using novel methods for the measurement of PPI. PPI was measured in 44 patients with schizophrenia and 32 healthy controls, across a range of startling stimulus intensities, under two attention set conditions. A range of neuropsychological tasks was also administered. Curves of best fit were fitted to the startle magnitudes, across the stimulus intensities, and a number of parameters were extracted from these curves, each of which reflects a different characteristic of the startle response. Correlations and quartile splits of the sample (highest versus lowest PPI) revealed that more PPI of response measures and less PPI of stimulus measures under the IGNORE condition was associated with superior performance in the colour-word subtest of the Stroop task. Further, more PPI of stimulus measures and less PPI of response measures under the ATTEND condition was associated with better performance on a memory task. These relationships appear to be mediated by common attentional processes active within both PPI and cognitive tasks, rather than by common underlying neurophysiological inhibitory processes.

Background The loudness dependence of the auditory evoked potential (LDAEP) has been suggested as a possible in vivo measure of central serotonin function. However, more recent studies suggest that the LDAEP may be modulated by multiple neuromodulatory systems in addition to the serotonergic system. Accordingly we further examined the effects of selective serotonin, dopamine and simultaneous serotonin and dopamine depletion on the LDAEP in healthy subjects. Methods The study employed a placebo-controlled, double-blind, cross over design. Fourteen subjects were tested under four acute treatment conditions: placebo (balanced amino acid drink), tryptophan (serotonin) depletion (ATD), tyrosine/phenylalanine (dopamine) depletion (ATPD) and combined tryptophan/tyrosine/phenylalanine (serotonin and dopamine) depletion (CMD). Testing was conducted 5.5 h post-depletion and changes in the amplitude of the N1/P2 at varying intensities (60, 70, 80, 90, 100 dB) were examined at CZ. Results Greater than 80% plasma precursor depletion was achieved across all conditions. Despite significant depletion of monoamine precursors, ATD, (p = 0.318), ATPD (p = 0.061) and CMD (p = 0.104) had no effects on the LDAEP (60–100 dB). Conclusion Acute serotonin and dopamine depletion did not modulate the LDAEP. This finding adds support to growing evidence that the LDAEP is insensitive to acute changes in serotonin and dopamine neurotransmission. Copyright © 2008 John Wiley & Sons, Ltd.

Schizophrenia is associated with impairments of sensorimotor and sensory gating as measured by prepulse inhibition (PPI) of the acoustic startle response and P50 suppression of the auditory event-related potential respectively. While serotonin and dopamine play an important role in the pathophysiology and treatment of schizophrenia, their role in modulating PPI and P50 suppression in humans is yet to be fully clarified. To further explore the role of serotonin and dopamine in PPI and P50 suppression, we examined the effects of acute tryptophan depletion (to decrease serotonin) and acute tyrosine/phenylalanine depletion (to decrease dopamine) on PPI and P50 suppression in healthy human participants. In addition, we also examined for the first time, the effects of simultaneous serotonin and dopamine depletion (ie combined monoamine depletion) on PPI and P50 suppression. The study was a double-blind, placebo-controlled cross-over design in which 16 healthy male participants completed the PPI and P50 paradigms under four acute treatment conditions: (a) balanced/placebo control, (b) acute tryptophan depletion, (c) acute tyrosine/phenylalanine depletion, and (d) acute tyrosine/phenylalanine/tryptophan depletion (combined monoamine depletion). Selective depletion of dopamine had no significant effect on either PPI or P50 suppression, whereas selective serotonin depletion significantly disrupted PPI, but not P50 suppression. Finally, the simultaneous depletion of both serotonin and dopamine resulted in significant reduction of both PPI and P50 suppression. We suggest these results can be explained by theories relating to optimal levels of monoaminergic neurotransmission and synergistic interactions between serotonergic and dopaminergic systems for normal ‘gating’ function. These findings suggest that a dysfunction in both serotonin and dopamine neurotransmission may, in part, be responsible for the gating deficits observed in schizophrenia, and their normalization following administration of atypical antipsychotic drugs.

Schizophrenia is associated with impairments of attentional control on classic experimental paradigms such as the Stroop task. However, at a basic level the neurochemical mechanisms that may be responsible for such impairments are poorly understood. In this study, we sought to investigate the influence of brain monoamine function on Stroop task performance in healthy participants using the established methods of acute dietary serotonin, dopamine, and combined monoamine depletion. The study was a double-blind placebo controlled design in which 12 healthy male participants completed the Stroop task under four acute treatment conditions: (a) balanced/placebo control, (b) acute tryptophan depletion, (c) acute tyrosine/phenylalanine depletion, and (d) acute tyrosine/phenylalanine/tryptophan depletion (combined monoamine depletion). Decreased Stroop interference indicating improved attentional control was observed after both tryptophan depletion and tyrosine/phenylalanine depletion, while there was no significant change in interference after combined monoamine depletion. Findings suggest that reduced tonic dopamine or serotonin activity within specific neural circuits (such as the striatum, anterior cingulate, or prefrontal cortex) may play a critical role in attentional control, possibly by improving gating of information via reducing noise in monoaminergic systems. These findings enhance our understanding of the neurochemical basis of attentional control and the possible cause of attentional control deficits in schizophrenia.