Biomedical subjects

Andres Posada

Publications and source records attributed to Andres Posada.

7 recordsLinked to original sources

PubMed2005

Event-related potentials during rule processing in schizophrenia.

Several studies have shown that schizophrenia is characterized by impaired frontal lobe functions, functions that are responsible, for example, for the management of rules, strategic reasoning, and selective attention. Using event-related potentials (ERP), we assessed the brain's electrical activity in a group of patients with schizophrenia (n=11) and a healthy control group (n=14) during a reaction time task requiring the use of a rule. ERP waves were compared with those elicited in a similar task based on a direct sensory association. In the control group, ERP analyses showed a negative wave moving from the posterior to the anterior regions of the scalp in a latency range of 250-400 ms. Then, the negativity remained at the frontal scalp region in a latency range of 400-800 ms. In this group, the amplitude was higher during the rule operation than during the sensory association task. In schizophrenic patients, the anteroposterior component of the negative wave was totally absent in both tasks, and we did not find a modulation of the ERP by the task. Frontal scalp negativity was observed, but its latency was longer and its amplitude lower than in the control group. We discuss these findings in terms of the frontoposterior disconnection hypothesis.

Adult↗

PubMed2005

Modulations in the degree of synchronization during ongoing oscillatory activity in the human brain.

When a subject is asked to respond as quickly as possible to a stimulus he/she responds much faster if this stimulus is preceded by a warning cue. This cue enables the subject to anticipate the forthcoming stimulus, initiating neural processes subserving the future perception and processing of the target stimulus and the motor preparation of the associated response action. It has recently been suggested that neuronal activity before such an anticipated target stimulus could be associated with modulations in neuronal synchronization and oscillatory activity. Here we recorded electrical brain activity whilst subjects performed a choice reaction time task, in which one of the stimuli could be predicted with 90% certainty. We show that the prediction of a forthcoming stimulus was associated with an increase in gamma oscillations overlying occipital areas and a decrease in beta oscillations overlying sensorimotor cortex before the stimulus was presented. We suggest that these regionally specific modulations in oscillatory activity reflect the establishment of neural networks that are 'primed' for the future processing of the forthcoming predictable visual stimulus.

Adult↗

PubMed2003

Altered awareness of voluntary action after damage to the parietal cortex.

A central question in the study of human behavior is the origin of willed action. EEG recordings of surface brain activity from human subjects performing a self-initiated movement show that the subjective experience of wanting to move follows, rather than precedes, the 'readiness potential'--an electrophysiological mark of motor preparation. This raises the issue of how conscious experience of willed action is generated. Here we show that patients with parietal lesions can report when they started moving, but not when they first became aware of their intention to move. This stands in contrast with the performance of cerebellar patients who behaved as normal subjects. We thus propose that when a movement is planned, activity in the parietal cortex, as part of a cortico-cortical sensorimotor processing loop, generates a predictive internal model of the upcoming movement. This model might form the neural correlate of motor awareness.

Adult↗

PubMed2003

Stimulus and response ERP analyses of a two-level reaction time task.

Higher cognitive processes include the ability to reliably transform sensory or mnemonic information. These processes either occur automatically or they are consciously controlled. To compare these two types of information processing, we developed a reaction time task that requires either a rule operation or else a direct sensory association. We were interested in evaluating the brain's electrical activity corresponding to both tasks, using event-related potentials (ERPs). In order to gain complete insight into the electrical activity of a stimulus-response segment, we analyzed the ERPs corresponding to the processing of the stimulus and the ERPs corresponding to the preparation of the response. To complete the analysis, we also evaluated the lateralized readiness potential (LRP) matched to the stimulus and to the response onset. Compared with the sensory association task, rule operation generated a higher negative potential field at frontocentral scalp areas in a latency range of 312-512 ms after the stimulus. In contrast, the LRP showed a negative component in the sensory association task which was absent during the rule operation; the latency of the difference was in the range 374-532 ms after the stimulus. The ERP component obtained by the response onset analysis was more negative in the rule condition up to a latency of -214 ms before the generation of the movement; the effect was localized at frontal and central scalp regions. We failed to find any significant difference in the LRP matched to the response onset. These results suggest that the brain computation of the rule operation takes place approximately in the middle of the stimulus-response time interval and that it is an additive process to the sensory association response.

Adult↗

PubMed2003

Characterizing the time course of an implicature: an evoked potentials study.

This work employs Evoked Potential techniques as 19 participants are confronted with sentences that have the potential to produce scalar implicatures, like in Some elephants have trunks. Such an Underinformative utterance is of interest to pragmatists because it can be considered to have two different truth values. It can be considered true when taken at face value but false if one were to treat Some with the implicature Not All. Two accounts of implicature production are compared. The neo-Gricean approach (e.g., Levinson, 2000) assumes that implicatures intrude automatically on the semantics of a term like Some. Relevance Theory (Sperber & Wilson, 1985/1996) assumes that implicatures are effortful and not automatic. In this experiment, the participants are presented with 25 Underinformative sentences along with 25 sentences that are Patently True (e.g. Some houses have bricks) and 25 that are Patently False (e.g. Some crows have radios). As reported in an earlier study (Noveck, 2001), Underinformative sentences prompt strong individual differences. Seven participants here responded true to all (or nearly all) of the Underinformative sentences and the remaining 12 responded false to all (or nearly all) of them. The present study showed that those who responded false to the Underinformative sentences took significantly longer to do so that those who responded true. The ERP data indicate that: (a) the Patently True and Patently False sentences prompt steeper N400's--indicating greater semantic integration--than the Underinformative sentences and that (b) regardless of one's ultimate response to the Underinformative sentences, the N400's were remarkably flat, indicating no particular reaction to these sentences. Collectively, the data are taken to show that implicatures are part of a late-arriving, effort-demanding decision process.

Adolescent↗

PubMed2003

Augmentation of induced visual gamma activity by increased task complexity.

Recently the study of induced gamma band oscillations has focused on their modulation by top-down processes, mainly attention. Numerous studies have observed an increase in induced gamma band energy with increases in covert selective attention and visual perception. The current study investigated the modulation of visually induced gamma band oscillations by top-down processes associated with task complexity. Fourteen human subjects performed a reaction time task under two experimental conditions that differed in task complexity. In one, subjects simply had to press one of four buttons that corresponded to a colour stimulus shown to the subject. In the second, the stimulus response mapping was altered by the implementation of a rule, thus increasing task complexity. Cortical electrical activity was recorded using a 65 electrode whole scalp electroencephalographic (EEG) net. The EEG activity was analysed using Morlet wavelets to produce time-frequency maps. Although induced gamma band activity was observed in both conditions, there was significantly greater energy during the rule-operation condition at approximately 276 ms after the appearance of the stimulus. This increase was localized to electrodes overlying the right-central parietal scalp. The results of this study show that top-down processes modulate the level of induced gamma band activity. We discuss these findings in terms of the role of gamma oscillations in the construction of a sensory representation useful for a correct motor response.

Adolescent↗

PubMed2002

Reduced P300 amplitude in a visual recognition task in patients with schizophrenia.

We studied top-down visual processes in schizophrenia by analyzing visual event-related potentials (ERPs) during a gestalt recognition task, after subjects (patients with schizophrenia, n = 10; controls, n = 14) were trained to perceive three different geometrical shapes. Recognition performance of patients was poorer under both the figure and the nonfigure conditions then that of normal controls. ERPs analysis indicated that P300 amplitudes of the patients were significantly smaller than those of controls during correct figure detection trials. Moreover, topographical analysis of the differences in ERPs during the figure vs the nonfigure condition showed an earlier, more positive and more widely distributed P300 in controls than in patients with schizophrenia. Our study supports the misconnection hypothesis of schizophrenia and highlights the difficulty of the patients to refer to previous experience in order to filter incoming information. In a visual recognition task, this misconnection syndrome might induce a failure to integrate information stored in the frontal and prefrontal sites with incoming stimuli.

Adult↗