Slow potential topography

Slow cortical potentials capture decision processes during temporal discounting

Various neuroimaging studies have detected brain regions involved in discounting the value of temporally delayed rewards. This study used slow cortical potentials (SCPs) to elaborate the time course of cognitive processing during temporal discounting. Depending on their strength of discounting, subjects were categorised as low and high impulsive. Low impulsives, but not high impulsives, showed faster reaction times for making decisions when the delayed reward was of high amount than when it was of low amount. Both low impulsives and high impulsives chose the delayed reward more often when its amount was high than when it was low, but this behavior was more pronounced for low impulsives. Moreover, only low impulsives showed more negative SCPs for low than for high amounts. All three measures indicated that only low impulsives experienced extended conflict for delayed low amounts than for high amounts. Additionally, the SCPs of low impulsives were more sensitive to the delay of the delayed reward than those of high impulsives, extending seconds after the response. This indicates that they continued evaluating their choices even after the decision. Altogether, the present study demonstrated that SCPs are sensitive to decision-related resource allocation during inter-temporal decision-making. Resource allocation depended both on the choice situation and on impulsivity. Furthermore, the time course of SCPs suggested that decision-related processes occurred both prior to and after the response.

Arousal and attention: self-chosen stimulation optimizes cortical excitability and minimizes compensatory effort

Cortical excitability is assumed to depend on cortical arousal level in an inverted U-shaped fashion: Largest (optimal) excitability is usually associated with medium levels of arousal. It has been proposed that under conditions of low arousal, compensatory effort is exerted if attentional demands persist. People tend to avoid this resource-consuming top-down compensation by creating or selecting environmental conditions that provide sufficient bottom-up stimulation. These assumptions were tested in an attention-demanding dual-task situation: We combined a simulated driving task to induce three different arousal levels by varying stimulation (high vs. low vs. self-chosen) with a visual two-stimulus paradigm to assess cortical excitability by the initial contingent negative variation (iCNV) component of the event-related potential. Additionally, we analyzed the oscillatory power of the beta2 band of the electroencephalogram at anterior frontal sites, which is assumed to reflect low-arousal compensatory activity. The iCNV amplitude differed in all three arousal conditions as expected: It was highest in the condition of self-chosen stimulation and lowest in the low- and high-arousal conditions. Additionally, in the low-arousal condition, anterior frontal beta2 power was found to be significantly higher than in the other two conditions and correlated positively with subjective strain. This pattern of results suggests that subjects select medium levels of stimulation which optimize cortical excitability under attentional demand conditions. The elevated fronto-central beta2 power in the low-stimulation condition may indicate the involvement of the anterior cingulate cortex in compensating for reduced arousal by top-down stimulation of the noradrenergic arousal system.

Neural correlates of monocular and binocular depth cues based on natural images: A LORETA analysis

Functional imaging studies investigating perception of depth rely solely on one type of depth cue based on non-natural stimulus material. To overcome these limitations and to provide a more realistic and complete set of depth cues natural stereoscopic images were used in this study. Using slow cortical potentials and source localization we aimed to identify the neural correlates of monocular and binocular depth cues. This study confirms and extends functional imaging studies, showing that natural images provide a good, reliable, and more realistic alternative to artificial stimuli, and demonstrates the possibility to separate the processing of different depth cues.

Impact of background noise on reaction time and brain DC potential changes of VDT-based spatial attention

Background noise is often discussed in terms of mental costs. In this study the effect of background noise on brain activity as reflected by the direct coupled (DC) potential was investigated by a within design in ten participants. During two successive blocks of 7 min each, participants performed 156 trials of a visual display terminal (VDT)-based visual-spatial attention task without noise and two blocks with a mixture of the environmental noises of barking dogs, traffic noise and irrelevant speech of 60 dBA. Brain DC potentials were recorded along the midline and analysed for change by time on task. For noise conditions, reaction time was prolonged and the DC-potential shifted towards positivity, contrary to control condition, independent of block and location. Results suggest reduced cortical resources by widespread inhibitory activation through background noise. It can be concluded that even low intensity background noise is associated with energy consumption and with impaired performance in spatial attention.

Using ICA for removal of ocular artifacts in EEG recorded from blind subjects

One of the standard applications of Independent Component Analysis (ICA) to EEG is removal of artifacts due to movements of the eye bulbs. Short blinks as well as slower saccadic movements are removed by subtracting respective independent components (ICs). EEG recorded from blind subjects poses special problems, since it shows a higher quantity of eye movements, which are also more irregular and very different across subjects. It is demonstrated that ICA can still be of use by comparing results from four blind subjects with results from one subject without eye bulbs who therefore does not show eye movement artifacts at all.

Functional neuroanatomy of learned helplessness

In the experiments reported here, female subjects were presented with reasoning tasks that changed from solvable to unsolvable, evoking "learned helplessness" or "loss of control" reactions in some subjects. Significant differences in slow cortical potential (SCP) changes were found between emotionally highly and lowly reactive subjects (grouped according to responses in postexperimental questionnaires) when processing unsolvable tasks. Cortical LORETA of SCP topographies and subsequent statistical nonparametric mapping (SnPM; analysis indicate clear reduction of anterior cingulate activity only with emotionally highly reactive subjects. In these subjects a region of the brain that is indispensable for goal-directed handling of tasks was switched off, whereas regions that are primarily engaged in processing the task stimuli were even more active during loss of control, although not at a statistically significant level. According to the anterior cingulate monitors the conflicts among brain regions and issues calls for further processing to the PFC that then guides behavior toward a goal. Learned helplessness might then be seen a state in which the function of the anterior cingulate is no longer maintained, perhaps due to the inhibitory influence of the amygdala possibly mediated via the brainstem dopaminergic ventral tegmental area.

Millivolt-scale DC shifts in the human scalp EEG: evidence for a nonneuronal generator

Slow shifts in the human scalp-recorded EEG, including those related to changes in brain CO(2) levels, have been generally assumed to result from changes in the level of tonic excitation of apical dendrites of cortical pyramidal neurons. We readdressed this issue using DC-EEG shifts elicited in healthy adult subjects by hypo- or hypercapnia. A 3-min period of hyperventilation resulted in a prompt negative shift with a rate of up to 10 microV/s at the vertex (Cz) and an extremely steep dependence (up to 100 microV/mmHg) on the end-tidal Pco(2). This shift had a maximum of up to -2 mV at Cz versus the temporal derivations (T3/T4). Hyperventilation-like breathing of 5% CO(2)-95% O(2), which does not lead to a significant hypocapnia, resulted in a near-complete block of the negative DC shift at Cz. Hypoventilation, or breathing 5% CO(2) in air at normal respiratory rate, induced a positive shift. The high amplitude of the voltage gradients on the scalp induced by hyperventilation is not consistent with a neuronal origin. Instead, the present data suggest that they are generated by extracortical volume currents driven by a Pco(2)-dependent potential difference across epithelia separating the cerebrospinal fluid and blood. Since changes in respiratory patterns and, hence, in the level of brain Pco(2), are likely to occur under a number of experimental conditions in which slow EEG responses have been reported (e.g., attention shifts, preparatory states, epileptic seizures, and hypoxic episodes), the present results call for a thorough reexamination of the mechanisms underlying scalp-recorded DC-EEG responses.

Evidence for premotor cortex activity during dynamic visuospatial imagery from single-trial functional magnetic resonance imaging and event-related slow cortical potentials

A strong correspondence has been repeatedly observed between actually performed and mentally imagined object rotation. This suggests an overlap in the brain regions involved in these processes. Functional neuroimaging studies have consistently revealed parietal and occipital cortex activity during dynamic visuospatial imagery. However, results concerning the involvement of higher-order cortical motor areas have been less consistent. We investigated if and when premotor structures are active during processing of a three-dimensional cube comparison task that requires dynamic visuospatial imagery. In order to achieve a good temporal and spatial resolution, single-trial functional magnetic resonance imaging (fMRI) and scalp-recorded event-related slow cortical potentials (SCPs) were recorded from the same subjects in two separate measurement sessions. In order to reduce inter-subject variability in brain activity due to individual differences, only male subjects (n = 13) with high task-specific ability were investigated. Functional MRI revealed consistent bilateral activity in the occipital (Brodmann area BA18/19) and parietal cortex (BA7), in lateral and medial premotor areas (BA6), the dorsolateral prefrontal cortex (BA9), and the anterior insular cortex. The time-course of SCPs indicated that task-related activity in these areas commenced approximately 550-650 ms after stimulus presentation and persisted until task completion. These results provide strong and consistent evidence that the human premotor cortex is involved in dynamic visuospatial imagery.

Brain DC potential changes of computerized tasks and paper/pencil tasks

Analysis of slow cortical potentials and their topography is currently discussed as an indication of cortical activity associated with cognitive operations/performance. In this paper, changes of the EEG DC potential were analyzed in two computerized tasks (correcting typing errors, performing Excel) and two paper/pencil tasks (correcting typing errors, a cognitive test) to assess mental load related to ergonomical and task characteristics. DC recordings were analyzed for the mean values of baseline and the first and the second 4 min of each task from 24 persons. A 2 (computer usage experience low vs. high)x4 (Task)x3 (Time: baseline, first half of task, second half)x6 (Lead) MANOVA of DC potential changes (DCPCs) showed at F3, F4 and C3 positive DCPCs for paper/pencil tasks and negative DCPCs for computerized tasks. Ratings of task difficulty were related to high vs. low task demands, whereas DCPCs were related to task medium, time on task and lead. Highly experienced persons showed a pronounced left-right difference at parietal locations and at frontal and central locations related to task medium by trend. Results were interpreted as higher cortical activation associated with mental load caused by additional attentional/controlling demands of computerized tasks.

Direct current auditory evoked potentials during wakefulness, anesthesia, and emergence from anesthesia

Direct current auditory evoked potentials (DC-AEPs) are a sensitive indicator of depth of anesthesia in animals. However, they have never been investigated in humans. To assess the potential usefulness of DC-AEPs as an indicator of anesthesia in humans, we performed an explorative study in which DC-AEPs were recorded during propofol and methohexital anesthesia in humans. DC-AEPs were recorded via 22 scalp electrodes in 19 volunteers randomly assigned to receive either propofol or methohexital. DC-AEPs were evoked by binaurally presented 2-s, 60-dB, 800-Hz tones; measurements were taken during awake baseline, anesthesia, and emergence. Statistical analysis included analysis of variance and discriminant analysis of data acquired during these three conditions. About 500 ms after stimulus presentation, DC-AEPs could be observed. These potentials were present only during baseline and emergence-not during anesthesia. Statistically significant differences were found between baseline and anesthesia and between anesthesia and emergence. In conclusion, similar effects, as reported in animal studies of anesthetics on the DC-AEPs, could be observed in anesthetized humans. These results demonstrate that DC-AEPs are potentially useful in the assessment of cortical function during anesthesia and might qualify the method for monitoring anesthesia in humans.

Restriction of task processing time affects cortical activity during processing of a cognitive task: an event-related slow cortical potential study

As is known from psychometrics, restriction of task processing time by the instruction to respond as quickly and accurately as possible leads to task-unspecific cognitive processing. Since this task processing mode is used in most functional neuroimaging studies of human cognition, this may evoke cortical activity that is functionally not essential for the particular task under investigation. Using topographic recordings of event-related slow cortical potentials, two experiments have been performed to investigate whether cortical activity during processing of a visuo-spatial imagery task is substantially influenced by the time provided to process the task. Furthermore, it was investigated whether this effect is additionally modulated by a subject's task-specific ability. The instruction to respond as quickly and accurately as possible led to increased negative slow cortical potential amplitudes over parietal and frontal regions and significantly interacted with task-specific ability. While cortical activity recorded over parietal and frontal regions was different between subjects with low and high spatial ability when processing time was unrestricted, no such differences were found between ability groups when subjects were instructed to answer both quickly and accurately. These results suggest that restricting processing time has considerable effects on the amount and the pattern of brain activity during cognitive processing and should be taken into account more explicitly in the experimental design and interpretation of neuroimaging studies of cognition.

Loss of control and negative emotions: a cortical slow potential topography study

This study investigated cortical steady potential changes in 18 subjects while processing a series of solvable arithmetic items (induction of control) that became unsolvable (withdrawal of control). Two different phases of induction and withdrawal of control (early and late) were dealt with separately in the analyses. The DC EEG was recorded from 20 locations. In all experimental conditions the overall slow potential topographical pattern did not change. However, negative-going DC shifts at occipito-parietal and left posterior-frontal regions were observed during induction of control whereas a generalized positive-going DC shift developed during phases of withdrawal of control. This positive-going shift persisted for the duration of the item presentation, resulting in pronounced positive values at temporal sites. The authors assume that temporal lobe activity (inferior and/or ventral surface) correlated to emotional/motivational processes that was picked up via the linked mastoid reference locations contributed essentially to these observed phenomena.

Differences in the ability to process a visuo-spatial task are reflected in event-related slow cortical potentials of human subjects

Recent positron emission tomography (PET) and electroencephalographic (EEG) studies suggest that higher ability in a cognitive task is associated with a more efficient neuronal processing of this task. However, the validity and generalizability of these studies is limited for several reasons. We investigated 20 male and 18 female human subjects with good vs. poor spatial ability performing a visuo-spatial task (cube test). Processing-related slow event-related potentials were recorded via 22 electrodes, evenly distributed over the scalp. Significant differences between good and poor performers were found in both sexes: poor subjects showed higher activity in the parietal region, and their topography was more extended into fronto-central regions. Since the amount and topography of brain activity may vary considerably depending on subjects' ability, we conclude that careful (experimental) control of task-specific ability of subjects is mandatory for cognitive neuroscience studies.

Eye movement control of computer functions

The control of computer functions by eye movements was demonstrated in 14 normal volunteers. Electrical potentials recorded by horizontal and vertical electrooculography (EOG) were transformed into a cursor that represented a moving fixation point on a computer display. Subjects were able to spell words and sentences by using eye movements to place the cursor on target letters in the display of an alphabet matrix. The successful demonstration of computer-controlled syntactic construction by eye movements offers a potentially useful technique for computer-assisted communication in special groups, such as developmentally-disabled individuals who have motor paralysis and who cannot speak.