Correspondence between brain ERP and behavioral asymmetries in a dichotic complex tone test

Electrophysiologic correlates of perceptual asymmetry for dichotic pitch discrimination were investigated in 20 normal subjects. Brain event-related potentials (ERPs) elicited by dichotic pairs and binaural probe tones in the Complex Tone Test (Sidtis, 1981) were recorded from homologous scalp locations over left and right hemispheres (F3, F4; C3, C4; P3, P4; O1, O2). Baseline-to-peak amplitudes were measured for N100, P200, and a late positive complex consisting of P350, P550, and slow wave. A left ear advantage (LEA) was evident in 70% of the subjects, and hemispheric asymmetries related to this behavioral asymmetry were found for P350 and P550 amplitudes to probe stimuli. Subjects with a strong LEA had greater amplitudes over the right hemisphere than the left, whereas subjects with little or no LEA showed a nonsignificant trend toward the opposite hemispheric asymmetry. Hemispheric asymmetry of these late ERPs at parietal and occipital sites was highly correlated with behavioral asymmetry. These findings suggest the utility of electrophysiological measures in assessing hemispheric asymmetries for processing complex pitch information.     

Differences in dichotic listening asymmetries in depression according to symptomatology

Subjects suffering from major depressive disorder were compared to normal controls on two verbal dichotic listening tasks. Although there were no significant differences between the groups on a task primarily of language perception, significant differences were obtained on a task with an attentional component. Overall performance was lower for the depressed group and ear asymmetry was reduced. Within the depressed group ear asymmetry varied according to symptomatology; withdrawal-retardation was associated with a lack of asymmetry and anxiety with a normal right ear advantage. The results were interpreted in terms of an interaction between affect and attention, and possible underlying mechanisms of cerebral hemisphere function were discussed.     

Brain event-related potentials (ERPs) in schizophrenia during a word recognition memory task.

Impairments of recognition memory for words and attenuation of the ERP 'old-new' effect have been found in patients with left medial temporal lobe damage. If left temporal lobe dysfunction in schizophrenia involves medial structures (e.g. hippocampus), then schizophrenic patients might show similar abnormalities of verbal recognition memory. This study recorded ERPs from 30 electrode sites while subjects were engaged in a continuous word recognition memory task. Results are reported for 24 patients having a diagnosis of schizophrenia (n = 16) or schizoaffective disorder (n = 8) and 19 age-matched healthy controls. Both patients and controls showed the expected 'old-new' effect, with greater late positivity to correctly recognized old words at posterior sites, and there was also no significant difference between groups in P3 amplitude. However, accuracy of word recognition memory was poorer in patients than controls, and patients showed markedly smaller N2 amplitude. Reduced amplitudes of N2 and N2-P3 were associated with poorer performance, with highest correlations over the left inferior parietal (N2) and left medial parietal (N2-P3) region. Moreover, patients failed to show significantly greater left than right hemisphere amplitude of N2-P3 at posterior sites, which was seen for healthy controls. These findings suggest that impaired word recognition in schizophrenia may arise from a left lateralized deficit at an early stage of processing, beginning at 200-300 ms after word onset.     

Chronic partial sleep loss increases the facilitatory role of a masked prime in a word recognition task

Neurobehavioural performance deficits associated with sleep loss have been extensively studied, in particular, the effects on psychomotor performance. However, there is no consensus as to which, if any, cognitive functions are impaired by sleep loss. To examine how sleep loss might affect cognition, the automatic processes supporting word recognition were examined using the masked priming paradigm in participants who had been exposed to two consecutive days of sleep restriction. Twelve healthy volunteers (mean age 24.5 years) were recruited. Nocturnal sleep duration was restricted to 60% of each participant's habitual sleep duration for two consecutive nights by delaying scheduled time of sleep onset and advancing time of awakening. In controlled laboratory conditions, participants completed the Psychomotor Vigilance Task and Karolinska Sleepiness Scale and a masked priming word recognition task. As expected, significant increases in subjective sleepiness and impaired psychomotor performance were observed after sleep loss. In contrast, response times and error rate on the masked priming task were not significantly affected. However, the magnitude of the masked priming effect, which can be taken as an index of automaticity of lexical processing, increased following sleep loss. These findings suggest that while no evidence of impairment to lexical access was observed after sleep loss, an increase in automatic processing may occur as a consequence of compensatory mechanisms.     

Implicit memory within a word recognition task: an event-related potential study in human subjects.

First, we recorded brain potentials from 15 healthy young subjects during the performance of a word/non-word discrimination task. During continuous visual presentation, some of the meaningful words were repeated after 86-94 s. We found a significant decrease of response time associated with the classification of repeated words which is an index for priming, an unconscious brain process. However, event-related potentials (ERPs) did not differ significantly between first and second presentations. Second, we recorded brain potentials during a following recognition test. Some of the meaningful words which were presented only once during the semantic discrimination task were repeated and had to be discriminated from randomly interspersed new words. We compared ERPs produced by incorrectly classified repeated words (misses) with ERPs produced by correctly classified new words (correct rejections). We found early ERP differences between 250 and 400 ms and later differences starting at about 500 ms after the stimulus onset. The early effect occurred over parietal scalp locations and the later effect over frontal, parietal and occipital scalp locations. This is evidence for unconscious brain activity related to the processing of missed repeated words. We suggest that the later frontal effect we found is due to an enhanced effort of the retrieval of item representations during word recognition and that the earlier parietal effect reflects partial recognition.     

Neural correlates of evidence accumulation in a perceptual decision task

Sequential sampling models provide a useful framework for understanding human decision making. A key component of these models is an evidence accumulation process in which information is accrued over time to a threshold, at which point a choice is made. Previous neurophysiological studies on perceptual decision making have suggested accumulation occurs only in sensorimotor areas involved in making the action for the choice. Here we investigated the neural correlates of evidence accumulation in the human brain using functional magnetic resonance imaging (fMRI) while manipulating the quality of sensory evidence, the response modality, and the foreknowledge of the response modality. We trained subjects to perform a random dot motion direction discrimination task by either moving their eyes or pressing buttons to make their responses. In addition, they were cued about the response modality either in advance of the stimulus or after a delay. We isolated fMRI responses for perceptual decisions in both independently defined sensorimotor areas and task-defined nonsensorimotor areas. We found neural signatures of evidence accumulation, a higher fMRI response on low coherence trials than high coherence trials, primarily in saccade-related sensorimotor areas (frontal eye field and intraparietal sulcus) and nonsensorimotor areas in anterior insula and inferior frontal sulcus. Critically, such neural signatures did not depend on response modality or foreknowledge. These results help establish human brain areas involved in evidence accumulation and suggest that the neural mechanism for evidence accumulation is not specific to effectors. Instead, the neural system might accumulate evidence for particular stimulus features relevant to a perceptual task.     

P300 hemispheric amplitude asymmetries from a visual oddball task

The P3(00) event-related potential (ERP) was elicited in 80 normal, right-handed male subjects using a simple visual discrimination task, with electroencephalographic (EEG) activity recorded at 19 electrodes. P3 amplitude was larger over the right than over the left hemisphere electrode sites primarily at anteromedial locations (F3/4, C3/4) for target, novel, and standard stimuli. The N1, P2, and N2 components also demonstrated hemispheric asymmetries. The strongest P3 hemispheric asymmetries for all stimuli were observed at anterior locations, suggesting a frontal right hemisphere localization for initial stimulus processing, although target stimuli produced larger P3 amplitudes at parietal locations than did novel stimuli. The relationships of hemispheric asymmetries to anatomical variables, background EEG activity, and neurocognitive factors are discussed.     

Dynamics of brain activation during an explicit word and image recognition task: an electrophysiological study

Recent brain imaging studies suggest that semantic processing of words and images may share a common neural network, although modality-specific activation can also be observed. Other studies using event-related potentials (ERPs) report that brain responses to words and images may already differ at approximately 150 ms following stimulus presentation. The question thus remains, which differences are due to perceptual categorization processes and which differences are due to the semantic ones? Using ERP recordings and spatio-temporal source localization analysis, we investigated the dynamics of brain activation during a recognition task. The stimuli consisted of a randomized set of verbal (words vs. non-words) and pictorial items (line drawings of objects vs. scrambled drawings). After each stimulus, subjects had to decide whether it corresponds to a recognizable word or objects. ERP map series were first analyzed in terms of segments of quasi-stable map topography using a cluster analysis. This showed that verbal and pictorial stimuli elicited different field patterns in two time segments between approximately 190-400 ms. Before and after this period, map patterns were similar between verbal and pictorial conditions indicating that the same brain structures were engaged during the early and late steps of processing. Source localization analysis of map segments corresponding to the P100 and the N150 components first showed activation of posterior bilateral regions and then of left temporo-posterior areas. During the period differentiating conditions, other patterns of activation, involving mainly left anterior and posterior regions for words and bilateral posterior regions for images, were observed. These findings suggest that, while sharing an initial common network, recognition of verbal and pictorial stimuli subsequently engage different brain regions during time periods generally allocated to the semantic processing of stimuli.     

EEG asymmetries in recognition of faces: comparison with a tachistoscopic technique

Twelve field-dependent and twelve field-independent women, who had previously shown opposite superiorities in a tachistoscopic face recognition task, returned to the laboratory for a session in which FEG asymmetry was measured during two facial and two verbal recognition tasks. Although task-related EEG asymmetries were observed, there was no effect of cognitive style on either direction or amount of asymmetry. These results suggest a lack of comparability among different methods of assessing individual differences in lateral functions.     

Differences in the magnitude and direction of forces during a submaximal matching task in hemiparetic subjects

Stroke patients often exhibit large imbalances in strength between the two upper limbs (ULs) and between the different muscle groups of the paretic UL. The aim of the study was to compare the ability of hemiparetic and normal subjects to produce symmetrical forces with both ULs and to determine whether the differences between force vectors can be predicted by a model accounting for the weakness of the different muscle groups or from clinical characteristics. Sixteen hemiparetic and 16 control subjects were assessed using static dynamometers in which both ULs were fixed. They were asked to produce symmetrical sub-maximal forces (in terms of direction and magnitude) with both ULs in four directions in the sagittal plane, which covers all possible combinations of flexion and extension at the shoulder and the elbow. Fifteen trials were performed, and the subjects were asked to gradually increase the forces produced from one trial to another. In addition, the maximal voluntary force (MVF) was measured unilaterally on both sides under two conditions: single-joint MVF (isolated flexion or extension at the elbow or the shoulder) and multi-joint MVF (same directions as the bilateral task). During the bilateral task, the stroke subjects generally showed larger errors between limbs than the control subjects, although they were able to produce sufficient force in the required directions during multi-joint MVF. The difference between groups was statistically significant in the four target directions for the errors in the magnitude, and in two target directions for the errors in direction (p<0.01). Differences predicted by a model based on the relative single-joint MVFs show a moderate association with errors observed for the magnitude (p=0.001–0.11, R²=0.17–0.54), but not for the direction of forces (p>0.1, R²<0.16). Several clinical features were examined as potential predictors using stepwise multiple regressions. The mean multi-joint MVF and proprioceptive impairments were the best predictors of mean errors in the magnitude of force (p<0.001, R²=0.82) whereas the mean angular errors were best explained by proprioceptive impairments (p=0.01, R²=0.38). Inaccurate internal representations of the paretic UL might explain why asymmetries were observed even though motor capacities were sufficient to perform successfully.     

Glucose enhancement of a facial recognition task in young adults

Numerous studies have reported that glucose administration enhances memory processes in both elderly and young adult subjects. Although these studies have utilized a variety of procedures and paradigms, investigations of both young and elderly subjects have typically used verbal tasks (word list recall, paragraph recall, etc.). In the present study, the effect of glucose consumption on a nonverbal, facial recognition task in young adults was examined. Lemonade sweetened with either glucose (50 g) or saccharin (23.7 mg) was consumed by college students (mean age of 21.1 years) 15 min prior to a facial recognition task. The task consisted of a familiarization phase in which subjects were presented with "target" faces, followed immediately by a recognition phase in which subjects had to identify the targets among a random array of familiar target and novel "distractor" faces. Statistical analysis indicated that there were no differences on hit rate (target identification) for subjects who consumed either saccharin or glucose prior to the test. However, further analyses revealed that subjects who consumed glucose committed significantly fewer false alarms and had (marginally) higher d-prime scores (a signal detection measure) compared to subjects who consumed saccharin prior to the test. These results parallel a previous report demonstrating glucose enhancement of a facial recognition task in probable Alzheimer's patients; however, this is believed to be the first demonstration of glucose enhancement for a facial recognition task in healthy, young adults.     

Chance guessing in a forced-choice recognition task and the detection of malingering

Guessing occurs on forced-choice (FC) tests for which responses cannot be based on relevant knowledge. Its importance is in inverse relation to the level of knowledge being measured, so that it becomes an increasing component of test scores as the level of knowledge decreases. It is also used as a benchmark to detect simulated impairment. This investigation examined the role of guessing in a 2-alternative FC face recognition test. Chance groups shown only the test items were asked to guess which were the targets, to measure the variation in scores likely to be found with pure guessing. Controls performed normally, and two simulation groups tried to fake amnesia. Results suggested that simple guessing in the chance group produced variable scores that overlapped both low genuine and "malingering" performance. Low control scores were hidden by the guessing "chance bonus," which ameliorated the apparent level of decline in memory. Simulators told of their role at the outset (before presentation of the target items) produced more convincing "amnesic" scores than those told only just before the test, who produced the expected below-chance level of score. It is suggested that guessing variability should be taken into account in interpreting FC scores.     

Disentangling perceptual from motor implicit sequence learning with a serial color-matching task

This paper contributes to the domain of implicit sequence learning by presenting a new version of the serial reaction time (SRT) task that allows unambiguously separating perceptual from motor learning. Participants matched the colors of three small squares with the color of a subsequently presented large target square. An identical sequential structure was tied to the colors of the target square (perceptual version, Experiment 1) or to the manual responses (motor version, Experiment 2). Short blocks of sequenced and randomized trials alternated and hence provided a continuous monitoring of the learning process. Reaction time measurements demonstrated clear evidence of independently learning perceptual and motor serial information, though revealed different time courses between both learning processes. No explicit awareness of the serial structure was needed for either of the two types of learning to occur. The paradigm introduced in this paper evidenced that perceptual learning can occur with SRT measurements and opens important perspectives for future imaging studies to answer the ongoing question, which brain areas are involved in the implicit learning of modality specific (motor vs. perceptual) or general serial order.     

P200 and phonological processing in Chinese word recognition

The present study examined the relationship between P200 and phonological processing in Chinese word recognition. Participants did a semantic judgment task on pairs of words. The critical pairs were all semantically unrelated in one of three conditions: homophonic, rhyme, or phonologically unrelated. Noting the possibility that P200 may be affected by phonological similarity and orthographic similarity and that literature studies may not have assessed such effects separately, the present study used visually dissimilar word pairs sharing no phonetic radicals. Relative to the control pairs, both the homophonic and rhyme pairs elicited a significantly larger P200 with a scalp distribution centering at the centro-parietal areas. The results present strong evidence that P200 can be modulated by lexical phonology alone, independent of sub-lexical phonology, or lexical or sub-lexical orthography. P200 effects were comparable in amplitude and topography between the homophonic and the rhyme conditions, suggesting that P200 is sensitive to phonology at both the syllabic and the sub-syllabic levels.     

Interhemispheric asymmetries in the visual evoked response: Effects of stimulus lateralisation and task

VER's were elicited by briefly presented lateralised letters in two experiments. In experiment 1 the subjects engaged in verbal processing of the letters and in experiment 2 they engaged in visuospatial processing. In both experiments the latencies of the first two components of the VER's were consistent with previous findings and with the anatomy of the retino-cortical visual pathways.The pattern of hemisphere asymmetries of the amplitudes of the middle and late components differed in the two experiments in that (i) in the second experiment the amplitude of the middle component from the left hemisphere did not vary with field of stimulus presentation although this was the case in the first experiment and (ii) a hemisphere asymmetry (left greater than right) was observed in the late component only in the second experiment.The results are interpreted as evidence of differences in the cerebral processing of the stimuli in respect of the task demands of the two experiments.