***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: Rationale Serotonin is shown to regulate the activity of primary auditory cortex , but little is known about serotonin modulation of other sensory cortices . Methods We investigated somatosensory evoked magnetic fields ( SEF ) to left median nerve stimulation in eight healthy subjects in a double-blind , controlled , cross-over design study after acute tryptophan depletion ( ATD ) and  control mixture .  SEFs were recorded with the whole-head magnetoencephalography 6 h after ingestion of mixtures . The SEF sources and strength were estimated by a least-squares fit of a single equivalent current dipole . Results ATD decreased the total and free TPR levels by 75 and 48 % and control mixture increased them by 98 % and 44 % . ATD had no effect on the amplitudes or latencies of SEF components . The source locations of the responses were not significantly affected by ATD . ConclusionS erotonin does not affect stimuli processing in the primary somatosensory cortex Abstract Serotonin ( 5-hydroxytryptamine ; 5-HT ) circuits may play a role in cognitive performance , particularly in learning and memory . Cognitive impairment is often seen in depressed patients , in whom 5-HT turnover in the brain is thought to be lowered . A possible human pharmacological model to study the involvement of the serotonergic system in cognitive impairment is to reduce central 5-HT synthesis through L-tryptophan depletion in healthy subjects . In this study , the cognitive effects of tryptophan depletion were assessed and whether genetically or developmentally determined vulnerability factors were predictive of the cognitive impairment induced by tryptophan depletion . Sixteen healthy volunteers with a positive family history of depression and 11 without were given 100 g of an amino acid mixture with or without tryptophan , according to a double-blind , cross-over design . Tryptophan depletion specifically impaired long-term memory performance in all subjects : delayed recall performance , recognition sensitivity , and recognition reaction times were significantly impaired after tryptophan depletion relative to placebo . Short-term memory and perceptual and psychomotor functions were unchanged . There were no differences between groups with a positive and a negative family history for depression . On the basis of these results , it is concluded that tryptophan depletion specifically impairs long-term memory formation , presumably as a result of an acute decrease in 5-HT turnover in the brain Rationale and objective In animal and human studies , the neurotransmitter serotonin ( 5-hydroxytryptamine ; 5-HT ) has been implicated in mediating impulsiveness and aggression . To test the hypothesis that 5-HT modulates neuro-cognitive brain activation during inhibitory control , we examined the effect of acute tryptophan depletion ( ATD ) , a dietary challenge , which has been shown to decrease 5-HT synthesis in the brain , on functional brain activation during a go/no-go task . Methods Nine healthy , right-h and ed volunteers performed a rapid , event-related go/no-go task in two functional magnetic resonance imaging ( fMRI ) scanning sessions , 5 h after either a tryptophan-free or a balanced amino acid drink in a double-blind , sham depletion-controlled , counterbalanced , crossover design . The task required subjects to selectively execute or inhibit a motor response . Tryptophan depletion significantly lowered total plasma tryptophan concentration by 80 % , but did not significantly alter inhibitory performance or mood ratings . Results ATD significantly reduced right orbito-inferior prefrontal activation during the no-go condition , and increased activation in superior and medial temporal cortices . Conclusions These findings provide neuro-functional evidence of a serotonergic modulation of right inferior prefrontal during inhibitory motor control . The increased engagement of temporal brain regions may reflect compensatory mechanisms BACKGROUND The amygdala has a central role in processing emotions , particularly fear . During functional magnetic resonance imaging ( fMRI ) amygdala activation has been demonstrated outside of conscious awareness using masked emotional faces . METHODS We applied the masked faces paradigm to patients with major depression ( n = 11 ) and matched control subjects ( n = 11 ) during  fMRI to compare amygdala activation in response to masked emotional faces before and after antidepressant treatment . Data were analyzed using left and right amygdala a priori regions of interest , in an analysis of variance block analysis and r and om effects model . RESULTS Depressed patients had exaggerated left amygdala activation to all faces , greater for fearful faces . Right amygdala did not differ from control subjects . Following treatment , patients had bilateral reduced amygdala activation to masked fearful faces and bilateral reduced amygdala activation to all faces . Control subjects had no differences between the two scanning sessions . CONCLUSIONS Depressed patients have left amygdala hyperarousal , even when processing stimuli outside conscious awareness . Increased amygdala activation normalizes with antidepressant treatment Involuntary attention shifting , i.e. , detecting and orienting to unexpected stimulus changes , may be altered at low brain serotonin ( 5-hydroxytryptamine ; 5-HT ) levels . This was studied in 13 healthy subjects ( 21 - 30 years old ; 6 females ) by using a dietary challenge , acute tryptophan depletion ( ATD ) , which decreases 5-HT synthesis in the brain . Five hours after ingestion of either ATD or control mixture ( r and omized , double-blinded , crossover design ) , brain responses indexing involuntary attention were measured with simultaneous 64-channel electroencephalography ( EEG ) and 122-channel magnetoencephalography ( MEG ) . During the measurement , the subjects were instructed to discriminate equiprobable 200- and 400-ms tones by pressing one of two buttons rapidly . Occasionally , the frequency of the tones changed ( 10 % increase/decrease ) , causing involuntary attention shifting . ATD significantly lowered plasma tryptophan concentrations ( total tryptophan decreased by 75 % , free tryptophan decreased by 35 % ) . As compared to the control condition , ATD reduced the amplitude of the deviant-tone N2 wave , including the overlapping mismatch negativity ( MMN ) and N2b subcomponents , which are suggested to reflect change detection in the brain . The EEG results were accompanied by a significant increase in the peak latency of the magnetic counterpart of MMN . However , no ATD effects were observed in P3 to task-irrelevant frequency change . Reaction time ( RT ) to deviants per se was not significantly affected , but RT in trials succeeding the deviant-frequency tones was increased by ATD , which suggested impaired reorienting to the task-relevant activity . In conclusion , the results suggest that decreased level of central 5-HT function after ATD may decrease involuntary attention shifting to task-irrelevant sound changes and thus modulate re source allocation to the task-relevant activity To eluci date serotonin modulation of selective attention , 13 volunteers ( 21 - 30 years ) were studied in two sessions , 5 h after either acute tryptophan depletion ( ATD ) that decreases brain serotonin synthesis , or control-mixture ingestion ( r and omized , double-blind , cross-over design ) .  Simultaneous electroencephalogram and magnetoencephalogram were measured during dichotic listening of two concurrent trains of st and ard and deviant tones . Subjects counted the deviants presented to one ear and ignored those presented to the other ear . ATD lowered plasma total tryptophan by 75 % and free tryptophan by 39 % . ATD suppressed the amplitude enhancement of P50 and N1 to selectively attended tones , but did not affect the later aspects of processing negativity . The P50 latencies were increased after ATD , irrespective of attention . In conclusion , serotonin may regulate attentional modulation of early cortical stimulus processing Several lines of evidence suggest that the auditory evoked potential ( AEP ) augmenting/reducing slope may serve as a biological marker of central serotonergic activity . According to Hegerl and Juckel ( Biol . Psychiatry , 33 , 1993 , 173 ) , reduced serotonergic activity is hypothesized to increase the slope of the AEP amplitude stimulus intensity function ( ASF-slope ) . Hints for this hypothesis were investigated by employing the acute tryptophan depletion paradigm in 18 healthy females . A within-subject , placebo controlled double-blind cross over design was used for that purpose . Subjects ingested both a 50 g amino-acid drink with ( placebo condition ) and without tryptophan ( depletion condition ) . With respect to the N1/P2-slope , test-retest reliability of a 1 week interval ranged between r=0.56 and 0.58 for the pre-ingestion baseline recording sessions . Affect was not altered by tryptophan depletion and not related to the ASF-slope . The comparison between placebo and depletion conditions did not reveal significant alterations of the ASF-slope , neither after 5 nor 6 h post-ingestion . Thus , the results do not support the assumption of the ASF-slope reflecting central serotonergic function Abstract Rationale . Intensity dependence of the N1/P2 components may be regulated by serotonergic neurons in the primary auditory cortex , where low activity leads to a high intensity dependence and vice versa . Depletion of tryptophan ( TRP ) , a precursor for serotonin has been described to reduce serotonin content in brain of animals and humans . Objective . We investigated the intensity dependence of magnetic and electric N1/P2 components in ten subjects in a double-blind , controlled , cross-over design study after oral mixture of amino-acids leading to acute tryptophan depletion ( ATD ) and control . Methods .  Auditory evoked magnetic fields ( AEF ) and potentials ( AEP )  were recorded with 122-channel magnetoencephalography simultaneously with 64-channel EEG 5 h after ingestion of mixtures . The AEF sources and strength were estimated by a least-squares fit of a single equivalent current dipole . The amplitudes and latencies of N1 and P2 recorded with EEG were analyzed at frontal electrode site . Results . TRP depletion decreased the total and free TRP levels by 76 and 45 % and control mixture increased it by 48 and 28 % . ANOVA showed that ATD had a significant main effect on the N1m/P2 m dipole moments at the contralateral ( P=0.02 ) , but failed significantly to influence the ipsilateral responses . A significant mixture ingestion-by-stimulus intensity interaction was observed on the N1m/P2 m dipole moments at the contralateral hemisphere ( P=0.01 ) . The N1/P2 slope for intensity dependence function was decreased following ATD compared with the control experiment ( P=0.01 ) at the contralateral hemisphere . For EEG , a significant mixture ingestion-by-stimulus intensity interaction on the N1 latencies at the Fz electrode position was observed ( P=0.01 ) . Conclusion . ATD decreased the intensity dependence of N1m/P2 m source dipole moments in the primary auditory cortex at the hemisphere contralateral to the ear stimulated . These results suggest that serotonin participates in the regulation of intensity of auditory stimulation We studied the effects of acute  tryptophan depletion ( ATD ) on early cortical auditory processing . Middle-latency auditory evoked fields ( MAEF ) were investigated in 14 healthy subjects after 5 h of ATD or control mixture ingestion  in a r and omized , double-blinded , controlled cross-over design study . MAEFs to monaural click stimuli ( 0.1-ms duration ) were recorded with a 122-channel neuromagnetometer . Total plasma tryptophan ( Trp ) , free Trp , and large neutral amino acid ( LNAA ) concentrations were determined by using high-performance liquid chromatography . ATD lowered the total plasma Trp levels by 75 % , free Trp level by 47 % , and the ratio Trp/ΣLNAA by 92 % . The control mixture increased total Trp level by 45 % and free Trp by 32 % , and decreased the ratio Trp/ΣLNAA by 35 % . The ratio tyrosine/ΣLNAA did not differ between ATD and control experiment . ATD result ed in a significant main effect on Pam latencies and a near-significant main effect on Pam amplitudes . A significant Mixture ingestion X Sex interaction on Nbm amplitude and a significant Mixture ingestion X Sex X Hemisphere interaction on Pam latency were observed . ATD did not affect the MAEF source dipoles . The Pam latencies in both hemispheres had a significant negative relationship with the extent of ATD . The results suggest that the neurotransmitter serotonin is involved in early auditory cortical processing . Further , the serotonin modulation may be different in males and females Event-related functional magnetic resonance imaging was used to measure blood oxygenation level-dependent responses in 13 young healthy human volunteers during performance of a probabilistic reversal-learning task .  The task allowed the separate investigation of the relearning of stimulus – reward associations and the reception of negative feedback . Significant signal change in the right ventrolateral prefrontal cortex was demonstrated on trials when subjects stopped responding to the previously relevant stimulus and shifted responding to the newly relevant stimulus . Significant signal change in the region of the ventral striatum was also observed on such reversal errors , from a region of interest analysis . The ventrolateral prefrontal cortex and ventral striatum were not significantly activated by the other , preceding reversal errors , or when subjects received negative feedback for correct responses . Moreover , the response on the final reversal error , before shifting , was not modulated by the number of preceding reversal errors , indicating that error-related activity does not simply accumulate in this network . The signal change in this ventral fr 
    Output:
    
Although tryptophan depletion reduced plasma serotonin levels in all studies , significant effects on mood were only observed in studies with recovered depressed patients .
In functional neuroimaging studies ATD was consistently found to modulate cortical activity in prefrontal areas implicated in mnemonic and executive functions and in orbitofrontal , cingulate , and subcortical regions associated with emotional processing .
Electrophysiological studies indicated that ATD has a significant effect on both “ selective ” and “ involuntary ” attention .
Conclusions The combination of ATD with modern brain imaging techniques allows the investigation of the neurophysiological effects of reduced 5-HT synthesis on global brain activity , executive functions , memory , attention , and affect