Serotonin: Modulator of a drive to withdraw

Serotonin is a fundamental neuromodulator in both vertebrate and invertebrate nervous systems, with a suspected role in many human mental disorders. Yet, because of the complexity of serotonergic function, researchers have been unable to agree on a general theory. One function suggested for serotonin systems is the avoidance of threat. We propose and review evidence for an alternative hypothesis, that a phylogenetically primitive of function of serotonin is to oppose the activating neuromodulators (particularly noradrenalin and dopamine). The functional effect of this opposition can be seen as applying a drive to withdraw from dangerous, aversive or high stimulation environments. Proposing that serotonin is involved in a drive to withdraw and seek contentment, instead of a drive to avoid, may be compatible with several lines of evidence on serotonin function and may facilitate a better understanding of serotonergic neuromodulation in human psychopathology.     

Does familiarity or conflict account for performance in the word-stem completion task? Evidence from behavioural and event-related-potential data

The conflict monitoring theory (CMT) assumes that word-stems associated with several completions should lead to crosstalk and conflict due to underdetermined responding situation (Botvinick et al. in Psychol Rev 108(3):624–652, 2001). In contrast, the Multiple-Read-Out-Model (MROM) of Jacobs and Grainger (J Exp Psychol 20(6): 1311–1334, 1994) predicts a high level of general lexical activity (GLA) for word-stems with many completions, indicating a higher stimulus familiarity because these stems are more probable to be read. We compared word-stems with several completions against word-stems with one possible completion while measuring response times and electrophysiological recordings. Slowest response times and a distinct FN400 component, which has previously been related to the concept of familiarity (Curran in Memory Cogn 28(6):923–938, 2000), were apparent for word-stems that could only be associated with a single response. These findings support the claims of the MROM. Furthermore, the lack of the N2-component for word-stems with several completions continues to challenge the EEG-extension of the CMT (Yeung et al. in Psychol Rev 111(4):2004).     

Modality effects in the coding reproduction of rhythms

The temporal coding assumption is that time of presentation is coded more accurately for auditory events than for visual events. This assumption has been used to explain the modality effect, in which recall of recent auditory events is superior to recall of recent visual events. We tested the temporal coding assumption by examining the coding and reproduction of quintessentially temporal stimuli-rhythms. The rhythms were produced by sequences of short and long auditory stimuli or short and long visual stimuli; in either case, the task was to reproduce the temporal sequence. The results from four experiments demonstrated reproduction of auditory rhythms superior to that of visual rhythms. We conclude that speech-based explanations of modality effects cannot accommodate these findings, whereas the findings are consistent with explanations based on the temporal coding assumption.     

Frontal brain asymmetry in depression with comorbid anxiety: a neuropsychological investigation

The approach-withdrawal model posits that depression and anxiety are associated with a relative right asymmetry in frontal brain activity. Most studies have tested this model using measures of cortical brain activity such as electroencephalography. However, neuropsychological tasks that differentially use left versus right frontal cortical regions can also be used to test hypotheses from the model. In two independent samples (Study 1 and 2), the present study investigated the performance of currently depressed individuals with or without a comorbid anxiety disorder and healthy controls on neuropsychological tasks tapping primarily left (verbal fluency) or right (design fluency) frontal brain regions. Across both samples, results indicated that comorbid participants performed more poorly than depressed only and control participants on design fluency, while all groups showed equivalent performance on verbal fluency. Moreover, comorbid participants showed "asymmetrical" performance on these two tasks (i.e., poorer design [right frontal] relative to verbal [left frontal] fluency), whereas depressed only and control participants showed approximately symmetrical profiles of performance. Results from these two samples suggest an abnormal frontal asymmetry in neurocognitive performance driven primarily by right frontal dysfunction among anxious-depressed individuals and highlight the importance of considering comorbid anxiety when examining frontal brain functioning in depression.     

The sensorimotor contributions to implicit memory, familiarity, and recollection

The sensorimotor contributions to memory for prior occurrence were investigated. Previous research has shown that both implicit memory and familiarity draw on gains in stimulus-related processing fluency for old, compared with novel, stimuli, but recollection does not. Recently, it has been demonstrated that processing fluency itself resides in stimulus-specific motor simulations or reenactment (e.g., covert pronouncing simulations for words as stimuli). Combining these lines of evidence, it was predicted that stimulus-specific motor interference preventing simulations should impair both implicit memory and familiarity but leave recollection unaffected. This was tested for words as verbal stimuli associated to pronouncing simulations in the oral muscle system (but also for tunes as vocal stimuli and their associated vocal system, Experiment 2). It was found that oral (e.g., chewing gum), compared with manual (kneading a ball), motor interference prevented mere exposure effects (Experiments 1-2), substantially reduced repetition priming in word fragment completion (Experiment 3), reduced the familiarity estimates in a remember-know task (Experiment 5) and in receiver-operating characteristics (Experiment 6), and completely neutralized familiarity measured by self-reports (Experiment 4) and skin conductance responses (Experiment 7), while leaving recollection and free recall unaffected (across Experiments 1-7). This pattern establishes a rare memory dissociation in healthy participants, that is, explicit without implicit memory or recognizing without feeling familiar. Implications for embodied memory and neuropsychology are discussed.