Biomedical subjects

M A Bobes

Publications and source records attributed to M A Bobes.

14 recordsLinked to original sources

PubMed2004

Random field-union intersection tests for EEG/MEG imaging.

Electrophysiological (EEG/MEG) imaging challenges statistics by providing two views of the same spatiotemporal data: topographic and tomographic. Until now, statistical tests for these two situations have developed separately. This work introduces statistical tests for assessing simultaneously the significance of spatiotemporal event-related potential/event-related field (ERP/ERF) components and that of their sources. The test for detecting a component at a given time instant is provided by a Hotelling's T(2) statistic. This statistic is constructed in such a manner to be invariant to any choice of reference and is based upon a generalized version of the average reference transform of the data. As a consequence, the proposed test is a generalization of the well-known Global Field Power statistic. Consideration of tests at all time instants leads to a multiple comparison problem addressed by the use of Random Field Theory (RFT). The Union-Intersection (UI) principle is the basis for testing hypotheses about the topographic and tomographic distributions of such ERP/ERF components. The performance of the method is illustrated with actual EEG recordings obtained from a visual experiment of pattern reversal stimuli.

Algorithms↗

PubMed2000

Different scalp topography of brain potentials related to expression and identity matching of faces.

Event-related potentials (ERPs) and behavioral data were recorded while subjects performed two tasks on the same set of faces (presented in pairs). One task was identity matching and the other expression matching. Two groups of subjects participated, one familiar and the other unfamiliar with the faces. Subjects were less accurate in matching expressions than identity. Familiarity facilitated identity but not expression matching. ERPs to mismatches in both tasks elicited a negativity around 400 ms, which was similar in latency and amplitude in the two tasks, but differed in scalp topography. Whereas the mismatch negativity had the same landscape over the left hemisphere for both tasks, the component related to expression had larger amplitudes over the right-temporal regions. Familiarity had no effect on these negativities, although it affected a late positivity (LP). These results support the idea of distinct neural systems subserving face processing, and agree with a role of the right hemisphere for the processing of emotional expressions.

Adult↗

PubMed2000

Making features relevant: learning faces and event-related potentials recording using an analytic procedure.

We present a procedure designed for the learning of faces (represented by realistic drawings) and a task for the subsequent recording of Event-Related Potentials (ERPs), with the aim of investigating the psychobiological mechanisms involved in the recognition of familiar (or known) faces. The learning procedure consisted in a series of six sessions in which subjects familiarized themselves with the faces through a forced-choice features task: after the study of a set of 40 faces, each of these was presented at random in an incomplete way (without the eyes/eyebrows fragment), together with two sets of eyes and eyebrows for the subject to decide which of them corresponded to each face. This procedure is thought to facilitate for subjects the acquisition of the information related to the structural description of the faces (that is, without associated verbal/semantic information) using preferentially analytic processing strategies. The parameter d' of the Theory of Signal Detection allowed us to evaluate in the course of the learning sessions the degree of familiarization achieved with the studied faces. Subsequently, an ERPs' recording session was carried out, during which subjects executed a face-feature matching task. In this task, similar in structure to that carried out in the learning sessions, the subject had to decide whether the automatic completion of the faces that was made was correct or incorrect. ERPs' effects related to mismatching features were obtained, which indicate the existence of specific cerebral mechanisms involved in the recognition of faces.

Adult↗

PubMed1999

Visual evoked potentials related to motion-onset are modulated by attention.

The effects of attention on visual evoked potentials triggered by motion-onset were examined in four experiments. A set of randomly oriented bars was used as stimuli. The first experiment showed that responses to motion-onset following pattern-onset by less than 300 ms were suppressed. In the other three experiments, the amplitude of N170 was reduced when attention was drawn away from the moving elements and towards spatially interspersed bars that remained static. The superposition of the two sets made spatial selection unlikely. These results support the existence of an attentional 'motion filter' (separating stationary from moving elements) that can operate at early stages of visual processing.

Adolescent↗

PubMed1998

Cognitive brain activity in Alzheimer's disease: electrophysiological response during picture semantic categorization.

Semantic memory deterioration is a major component of the cognitive decline seen in patients with dementia of the Alzheimer's type (DAT); however, the exact nature of this deficit remains unclear. Some research data support a procedural deficit where there is an inability to access or retrieve the contents of semantic memory, while other data point to a degraded semantic store where the actual content of semantic memory is degraded. Additional information about semantic processing in DAT can be obtained through the use of an event-related potential (ERP) component known as N400. In the present study, ERPs were recorded from 10 young control participants, 10 elderly control participants, and 10 DAT patients in a picture-semantic matching task. Stimuli were presented sequentially as prime-target pairs, with one-half of the targets matching the primes via semantic relationships (e.g., piano-violin) and the other half mismatching the prime (e.g., helmet-violin). The task was to discriminate between semantically related and unrelated pairs of pictures. In the young and elderly control groups, ERPs generated a larger N400 for unrelated than related target pictures, with a maximum amplitude around 380 ms in the young group and around 480 ms in the elderly group. The amplitude of the N400 was significantly reduced in the DAT patients. However, a separate analysis of congruent and incongruent ERPs trials revealed significant differences only with the incongruent trials. The amplitude of incongruent recordings was larger for the elderly control group than for the DAT patients, while the amplitude for congruent recordings was similar in both groups. These findings are consistent with the neuropathological evidence that Alzheimer's disease is a neocortical disconnection syndrome in which there is a loss of structural and functional integrity of long corticocortical tracts. The semantic activation created by the context is not used efficiently in processing stimuli, which affects access to specific concepts and gradually leads to a breakdown in the structure and organization of semantic memory.

Adult↗

PubMed1998

Switching attention without shifting the spotlight object-based attentional modulation of brain potentials.

Although psychophysical evidence for object-based attention has been reported, corresponding studies with event-related potentials (ERPs) are scarce. Here subjects were presented with perceptual fields containing two superimposed objects (transparent surfaces generated by two sets of dots in rigid rotation around fixation, each set of a different color and direction of motion) or only one object (the same dots but either at rest or all rotating in the same direction). Brief (150-msec) rectilinear displacements affected either of the sets at random ISIs of 350 to 550 msec. Attention was directed to one set of dots, guided by color, in order to discriminate the direction of their displacement. Motion-onset ERPs elicited by these displacements were compared for attended and unattended dots. When the perceptual field consisted of two objects, strong suppression of P1 and N1 was obtained in the ERPs associated with the unattended object. No suppression was found with the field containing a single object, although an enhanced selection negativity was found in ERPs associated with attended dots (selected by color). Since the two objects occupied the same region of visual space, the suppression of P1/N1 cannot be explained by the space-based mechanisms but is consistent with object-based attentional selection at early stages of vision. The results highlight the role of perceptual organizations in enabling alternative attentional mechanisms.

Adult↗

PubMed1997

ERP assessment of semantic memory in Alzheimer's disease.

This study used the technique of Event-Related Potentials (ERPs), specifically the component N400, to evaluate semantic functions in Alzheimer's disease (AD). ERPs of 10 patients with probable AD and 10 asymptomatic elderly control subjects were recorded during a semantic categorization task using pictures as stimuli, 50% of the stimuli were congruent and the other 50% were incongruent. Significant differences for the incongruent ERPs, with a diminished N400 component, was observed in the patients group whereas the amplitude of the congruent ERPs was similar in both groups. The characteristics of the N400 in the patients with probable AD suggest that there is a lack of efficiency in accessing information and shows that there are deficient associative connections within the semantic network.

Aged↗

PubMed1996

Semantic matching of pictures in schizophrenia: a cross-cultural ERP study.

In this study the N400 of schizophrenics was compared with that of control subjects in a picture semantic-matching task. The comparison of N400 difference waveforms (subtraction of event-related potentials of congruent from those of incongruent trials) between control and patients was supplemented by separate analysis for congruent and incongruent trials. The N400 latency was delayed in patients. Also, the amplitude of N400 in the difference waveform was reduced in schizophrenics; however only congruent trials were different for patients (more negative) with respect to controls. This result is consistent with the hypothesis that schizophrenics use context poorly, but inconsistent with simple versions of the idea that associations are generally disinhibited in schizophrenia. Since the amplitudes of N400 and an auditory P300 were not correlated, a general processing deficit does not explain the results. Finally, by using picture matching, a cross-cultural comparison of N400 in schizophrenics from Cuba and China was possible, which indicated that the N400 abnormalities were similar in both groups.

Adult↗

PubMed1994

Brain potentials and the availability of semantic and phonological codes over time.

ERPs were recorded from subjects performing semantic and rhyme matching tasks using either spoken words, printed words or pictures as stimuli. Mismatches enhanced N400 (in the semantic task) and N450 (in the rhyme task). Onset and peak latencies were shorter for N450 than for N400 with spoken words; this relationship was inverted for pictures. Thus these latencies could index availability of semantic and phonological codes. For printed words, the latencies were shorter for N400 than N450, a result that supports direct-access modes of reading with late phonological code activation. The longer latencies found for N400 and N450 to pictures could suggest longer initial decoding for pictures with respect to words.

Adolescent↗

PubMed1994

An ERP study of expectancy violation in face perception.

Expectancies about face-structure can be induced by viewing parts of faces, which generates constraints due to two types of knowledge: feature-content and configuration. In a first experiment ERPs were recorded when parts of familiar faces were completed with incongruent features (from another face), as opposed to congruent features (from the same face). All features were in the correct configuration. An enhanced negativity was found for incongruent completions (N374) that was larger over the right side of the scalp. This replicates the results of Valdes-Sosa and Bobes (1990). In another two experiments, ERPs were recorded when parts of familiar faces were completed by congruent features, but sometimes placed in an incorrect position. In one experiment the features were jumbled; in the other, the features were slightly displaced. These configuration distortions were associated to late positive components, with a maxima at the centro-parietal region, of equal amplitude for both kinds of configuration anomalies. The N374 component seems to be a non-linguistic analog of the N400. The different ERP signatures for expectancy violations of configuration and feature-content suggest that these types of information are processed separately at some stages.

Adult↗

PubMed1994

Associative ERP effects with memories of artificial faces.

A previous study (Valdes-Sosa and Bobes, 1990) described a negative ERP component evoked by mis-matches in a face-feature matching task, using photographs of real faces. This component could be N400 (or an analogue), elicited by associative priming within a non-linguistic domain: that of face structure. To confirm this it is necessary to demonstrate that semantic/linguistic recoding was not a necessary condition in triggering the negativity. This means falsifying what we call the 'priming by proxy' hypothesis, and locating the triggering mis-match within face structure. In this paper subjects studied artificial schematic faces over several sessions, and 1 week later were presented with a face-feature matching task with simultaneous ERP recording. Since no semantic information or verbal labels were available, eliciting a mis-match negativity with these faces contradicts the 'priming by proxy' hypothesis. In a first experiment, in which the subjects learning was controlled through a face familiarity decision task, no significant mis-match negativity was found. However, in a second experiment in which learning was controlled through a forced-choice face-feature match, a significant mis-match negativity was found in the subsequent recording session. This result supports the idea that a component similar to N400 can be elicited by an associative mis-match restricted to the face-structural domain.

Adolescent↗

PubMed1993

Brain potentials in a phonological matching task using Chinese characters.

In readers of English, involved in a rhyme judgement task, mismatch trials are associated with an enhanced N450 component of the Event Related Potentials (ERPs). It has been suggested that N450 is related to orthographic or phonological priming. In this paper ERPs were recorded during a phonological matching task, using pairs of logographically dissimilar Chinese characters. A pair was considered to match if they sounded alike with identical phoneme sequences. The subjects (native Chinese speakers) were instructed to ignore vowel-inflections, which in Chinese have lexical status. Since sublexical assembly of phonology is not used in reading Chinese characters, and the members of each pair were logographically dissimilar, match and mismatch trials did not suffer in the amount of orthographic or sublexical phonological priming. An enhanced negative component (latency near 400 msec), was observed in ERPs elicited by the second character in non-matching pairs. The negativity could be similar to N450. If this were so, then N450 could not be associated with orthographic priming, nor with sublexical phonology, but would probably be associated with postlexical processing. Also, in both readers of Chinese and English, the negativity enhanced in non-match trials is larger over the right side of the scalp, suggesting a similar brain lateralization of the underlying processes.

Adolescent↗

PubMed1988

Different functional properties of on and off components in auditory brainstem responses to tone bursts.

Auditory brainstem responses to tone bursts of constant rise and fall time and variable plateau were obtained in 7 normal hearing adults with a vertex to mastoid electrode configuration. In all records, two vertex-positive components (A, B) were present. Peak A is probably an onset response. Peak B latency increased linearly with plateau duration (r = 0.93) and seems to be an off response. White and notched acoustic noise masking had a different effect on the two components. A greater latency shift was observed for peak A than for peak B, thus reducing the interpeak interval in the masked response. When using high-pass noise, as we lowered the cut-off points from 4 to 0.5 kHz, there was also a greater latency increment for peak A than for peak B. These results suggest a more apical cochlear origin for the off response.

Adult↗

PubMed1987

Comparison of auditory-evoked potential detection methods using signal detection theory.

The performance of statistical evoked-potential detection methods was compared with that of human observers and among themselves by means of receiver operating characteristics (ROC) curves. The test material was a collection of brain stem auditory-evoked responses obtained from 98 infants with 60 and 30 dB nHL clicks. The observers and the statistical methods had to discriminate these responses from control recordings obtained without acoustic stimulation. Although the observers' criteria on different days varied considerably, the discrimination capacity was more stable. The discrimination capacity depended on the observers' experience. The statistical methods tested were the correlation coefficient (CCR), the standard deviation ratio (SDR) and a new method named T2R. The most efficient detection method was T2R. For false-alarm rates of 0.01 the statistical methods were more efficient than the human observers. Signal detection theory is useful for the evaluation of evoked-potential analysis methods.

Brain Stem↗