Andres Posada

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.

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.

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.

Neural Dynamics of the Intention to Speak

When we talk we communicate our intentions. Although the origin of intentional action is debated in cognitive neuroscience, the question of how the brain generates the intention in speech remains still open. Using magnetoencephalography, we investigated the cortical dynamics engaged when healthy subjects attended to either their intention to speak or their actual speech. We found that activity in the right and left parietal cortex increased before subjects became aware of intending to speak. Within the time window of parietal activation, we also observed a transient left frontal activity in Brocas area, a crucial region for inner speech. During attention to speech, neural activity was detected in left prefrontal and temporal areas and in the temporoparietal junction. In agreement with previous results, our findings suggest that the parietal cortex plays a multimodal role in monitoring intentional mechanisms in both action and language. The coactivation of parietal regions and Brocas area may constitute the cortical circuit specific for controlling intentional processes during speech.

Use and automation of a rule in schizophrenia

Schizophrenia is associated with cognitive deficits and information-processing anomalies. Several studies have shown impairments in frontal lobe functions such as discovering rules, adopting strategies and working memory. We have evaluated the performances of schizophrenic patients on three different tasks of information processing. The first was a simple reaction time task, the second consisted of discriminating the color of the stimuli, and the third required the use of a rule of permutation. The schizophrenic patients (n=17) showed a task-dependent increase in time for responses in comparison with the control group (n=17). This increase was particularly important in the last task, which requires the use of the rule. These results confirm and expand the previously described impairment of patients with schizophrenia in the ability to use rules. They are discussed in terms of deficit in executive functions probably due to frontal lobe malfunctioning.

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 brains 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.