Biomedical subjects

A J Mendonça

Publications and source records attributed to A J Mendonça.

5 recordsLinked to original sources

PubMed1995

Field potential oscillatory bursts in parietal cortex before and during reach.

Local field potentials were recorded in parietal cortex, areas 5, 7a and 7b, of a macaque monkey to determine if oscillatory bursts occurred in an observable relationship to behavioral events. The monkey performed a visually-guided reaching task to targets displayed on a touch-sensitive video monitor. The task was pre-cued with a 1.6 s preparatory period. Intracortical recordings were made with a microelectrode or epidural recordings with silver ball electrodes. Compared to the relaxed state, task performance was distinguished by a drop in power for frequencies below 20 Hz (most prominent in area 7), and an increase for frequencies above 20 Hz. For the beta frequency band 20-25 Hz, maximal power occurred during the preparatory periods, and minimal power during reach performance. Above 30 Hz, reach preparation and performance episodes did not differ significantly in spectral power, except in parts of area 5 where 40 Hz activity was observed to increase during movement. The spatial extent of the beta preparatory activity was monitored using an array of 15 epidural electrodes, positioned in 2 rows stretching from the arcuate sulcus to the lunate sulcus. During each preparation, premotor cortex was found to be the major focus of increased power at 20 Hz, whereas posterior parietal cortex was the dominant focus of increased power in the 21-29 Hz band. Although beta frequencies were most prevalent during early stages of motor preparation, the oscillatory bursts were not tightly time-locked to the visual signals. beta Frequencies may be associated with an internally-triggered process to prepare the upcoming movement.

Animals↗

PubMed1994

Spatially modulated touch responses in parietal cortex.

Cortical neurons with low-threshold, cutaneous receptive fields on the fingers were recorded in areas 5 and 7b of the parietal lobe in two awake monkeys, trained in a visually guided reach task. 72% (81/113) of the cells responded when targets displayed on a videomonitor were actively touched. Of these, 20 neurons discharged preferentially when target contact was made on one side of the screen compared with the other. This spatial modulation of the cutaneous modality may have originated in neighboring joint-related neurons which were directionally selective.

Animals↗

PubMed1994

EEG rhythms of the sensorimotor region during hand movements.

The aim of this study was to determine what motor behaviors or conditions were associated with an increased occurrence of beta activity in the sensorimotor region of human subjects. EEG recordings were obtained from 8 electrodes symmetrically arranged around C3, with 3 cm interelectrode spacing. The electrode montage allowed calculation of the Laplacian operator at two positions, C3r and C3c, overlying the hand area of the motor cortex and of the somatosensory cortex, respectively. A variety of tasks involving right-hand movements of different levels of complexity, attention and preparation were performed. The corresponding EEG power spectra were subsequently computed for frequencies between 7 and 50 Hz. Repetitive hand movements alone (either drawing circles or writing one's signature) did not result in significantly increased beta activity in the sensorimotor region compared to relaxed conditions. However, both motor preparations and focused attention, whether movements were performed or not, were associated with an increase of high frequency beta activity (30-50 Hz) in the sensorimotor region. Therefore, the facilitatory effect of attention and motor preparation and not the functional activation of the sensorimotor cortex by hand movements was associated with an increase in synchronized fast beta activity.

Adolescent↗

PubMed1992

Visual responses to reward-related cues in inferior parietal lobule.

A sample of 263 neurones was recorded in area 7a of the parietal lobe, in a monkey performing a reach task to visual targets displayed on a touch-sensitive videomonitor. The task had been operantly conditioned on food or juice rewards, and 78 (30%) of the units showed activity changes linked in some way to the reward. For most of these cells, the response was to the approach of the trainer's hand with the food reward. This specific visual response was similar irrespective of the direction of approach. Six cells increased discharge as soon as the task was completed in apparent anticipation of the reward. Another two neurones responded to missing a reward: they fired vigorously if the videoscreen was blanked in mid-trial because a target was not correctly touched. In many cases (40/78) the same cells responding to some aspect of the reward also responded to visual cues given during the task, especially the presentation of the target location. Reward-related activity in area 7a probably results from an integration of the visual and limbic inputs to this region, such that visual information which foretells behaviourally important events is emphasized.

Animals↗

PubMed1990

Reaching in the dark: enhanced responses in posterior parietal cortex.

In order to assess the relative importance of visual input to area 7 reach-related neuronal activity, a monkey was trained to reach to visual targets displayed on a video-monitor, both with and without visual feedback. Visual feedback was removed by having the monkey reach in darkness to a previously illuminated target. Of 19 reach-related cells recorded in area 7 both in the light and the dark, ten showed an enhancement of discharge in the dark. These included area 7b cells sensitive to screen contact and area 7a cells active during reach. Dark enhancement of active somatic responsiveness may partially compensate for the loss of visual guidance.

Animals↗