Biomedical subjects

J B de Graaf

Publications and source records attributed to J B de Graaf.

7 recordsLinked to original sources

PubMed2001

Neuromagnetic source localization of auditory evoked fields and intracerebral evoked potentials: a comparison of data in the same patients.

OBJECTIVE: To compare the localizations of different neural sources (a) obtained from intracerebral evoked responses and (b) calculated from surface auditory evoked field responses recorded in the same subjects. Our aim was to evaluate the resolving power of a source localization method currently used in our laboratory, which is based on a recent spatio-temporal algorithm used in magneto-encephalography (MEG). METHODS: Auditory evoked responses were studied in 4 patients with medically intractable epilepsy. These responses were recorded from depth electrodes implanted in the auditory cortex for pre-surgical evaluation (stereo-electro-encephalography (SEEG)), as well as from surface captors (for MEG) placed on the scalp after removal of the depth electrodes. Auditory stimuli were clicks and short tone bursts with different frequencies. RESULTS: All middle-latency components (from 13 to 70 ms post-stimulus onset) were recorded and localized (via SEEG) along Heschl's gyrus (HG). MEG reliably localized Pam and P1m in the same area of HG that intracerebral recordings localized them in. No significant delay between SEEG and MEG latencies was observed. Both methods suggest that N1 is generated from different sources in the intermediate and lateral parts of the HG and in the planum temporale (PT). The source of P2 (PT and/or Area 22) remains unclear and was in one case, localized in different regions according to the method used. This latter component may therefore also be generated by different sources. CONCLUSIONS: The results suggest that both techniques are useful and may be used together in a complementary fashion. Intracerebral recordings allow the researcher to validate and interpret surface recordings.

Acoustic Stimulation↗

PubMed1996

Vector coding in slow goal-directed arm movements.

It has been found that the estimate of relative target direction is consistently biased. Relative target direction refers to the direction in which a target is located relative to another location in space (e.g., a starting position in the case of goal-directed movements). In this study, we have tested two models that could underlie this biased estimate. The first proposed model is based on a distorted internal representation of locations (i.e., we perceive a target at the "wrong" location). We call this the distorted location model. The second model is based on the idea that the derivation of target direction from spatial information about starting and target position is biased. We call this the biased direction model. These two models lead to different predictions of the deviations that occur when the distance between the starting position and the target position is increased. Since we know from previous studies that the initial direction of slow arm movements reflects the target direction estimate, we tested the two models by analyzing the initial direction of slow arm movements. The results show that the biased direction model can account for the biases we find in the target direction estimate for various target distances, whereas the distorted location model cannot. In two additional experiments, we explored this model further. The results show that the biases depend only on the orientation of the line through starting position and target position relative to the plane through longitudinal head or body axis and starting position. We conclude that the initial part of (slow) goal-directed arm movements is planned on the basis of a (biased) target direction estimate and not on the basis of a wrong internal representation of target location. This supports the hypothesis that we code displacements of our limbs in space as a vector.

Adult↗

PubMed1995

Modifications in end positions of arm movements following short-term saccadic adaptation.

We investigated whether short-term saccadic adaptation modifies hand pointing. Subjects were presented with double-step targets, the second target jump occurring during the saccade to the first one and bringing the target back to 66% of the first target eccentricity, in order to reduce the gain of their gaze saccades. Before and after this adaptation phase, they pointed with their hand to single step targets while keeping their gaze straight ahead. The results show that the hand movements terminated at positions that were significantly less eccentric following the adaptation phase, resembling the adaptive modification seen in the gaze movements. These results suggest that the motor systems controlling gaze and hand use common information about target position.

Adaptation, Physiological↗

PubMed1994

Misdirections in slow, goal-directed arm movements are not primarily visually based.

In a previous study we found that the initial direction of slow, goal-directed arm movements deviates consistently from the direction of the actual straight line between the starting position and the target position. We now investigate whether these deviations are caused by imperfections or peculiarities in the processing of vision-related spatial information, such as retinal information, and eye- and head-position information. This could lead to incorrect localization of the target relative to the starting position. Subjects were seated in front of a horizontal surface and had to move their arm slowly and accurately in the direction of target positions. We varied the amount of vision-related spatial information. In experiment 1, subjects were presented with visual targets and could see their moving arm. In experiment 2, the subjects were again presented with visual targets, but now they could not see their moving arm. In experiment 3, the subjects were blindfolded and had to move their arm towards tactile targets. In all three experiments we found comparable consistent deviations in the initial movement direction. We also instructed congenitally and early-blind subjects to move their arm towards tactile targets. Their performance showed deviations congruous with those found in the sighted subjects, and possibly somewhat larger. We conclude that the deviations in the initial movement direction of slow, goal-directed arm movements are not primarily visually based. The deviations are generated after all spatial information has been integrated.

Adult↗

PubMed1991

Misdirections in slow goal-directed arm movements and pointer-setting tasks.

Information about the direction of the virtual line between two positions in space (directional information) is used in many decision-making and motor tasks. We investigated how accurately directional information is processed by the brain. Subjects performed two types of task. In both tasks they sat at a table. In the first task they had to move their hand slowly and accurately from an initial position 40 cm in from of them to visually presented targets at a distance of 30 cm from the initial position (movement task). We analysed the initial movement direction. In the second task subjects had to position pointers in the direction of the targets as accurately as they could (perceptive task). We found that in the movement task the subjects started the movements to most targets in a direction that deviated consistently from the direction of the straight line between initial position and target position. The maximum deviation ranged from 5-10 degrees for the various subjects. The mean standard deviation was 4 degrees. In the perceptive task the subjects positioned the pointer in similarly deviating directions. Furthermore, we found that the maximum deviation in the pointer direction depended on the length of the pointer: the smaller the pointer, the larger the consistent deviations in the pointer direction. The shortest pointer showed deviations comparable to the deviations found in the movement task. These findings suggest that the deviations in the two tasks stem from the same source.

Arm↗

PubMed1988

A simulation of rat edl force output based on intrinsic muscle properties.

Force-velocity and force-length relations were obtained for the edl of four Wistar rats in order to characterise the contractile properties (CE) of these muscle-tendon complexes. Compliances of the undamped part of the series components (SE) were measured in quick length decreases. Force-extension relations of SEs were obtained by integration of compliance to force. A muscle model consisting of CE, SE and a visco-elastic element was used to simulate the force output of the muscle tendon complex in response to a changing muscle length lOI as input. This simulated force was compared with the experimental force of the same muscle measured in response to the same lOI as input. Tetanic contractions were used in all experiments. The results show that this muscle model can predict the experimental force within a mean maximal error not larger than approximately 14% of the force amplitude. However the comparison of simulated force with experimental force and a few additional experiments show that the muscles do not have a unique instantaneous force-velocity characteristic. As shown by several other studies, force seems to be influenced by many other variables (time, history etc.) than CE length and velocity.

Animals↗

PubMedDate not supplied

Specialization of left auditory cortex for speech perception in man depends on temporal coding.

Speech perception requires cortical mechanisms capable of analysing and encoding successive spectral (frequency) changes in the acoustic signal. To study temporal speech processing in the human auditory cortex, we recorded intracerebral evoked potentials to syllables in right and left human auditory cortices including Heschl's gyrus (HG), planum temporale (PT) and the posterior part of superior temporal gyrus (area 22). Natural voiced /ba/, /da/, /ga/) and voiceless (/pa/, /ta/, /ka/) syllables, spoken by a native French speaker, were used to study the processing of a specific temporally based acoustico-phonetic feature, the voice onset time (VOT). This acoustic feature is present in nearly all languages, and it is the VOT that provides the basis for the perceptual distinction between voiced and voiceless consonants. The present results show a lateralized processing of acoustic elements of syllables. First, processing of voiced and voiceless syllables is distinct in the left, but not in the right HG and PT. Second, only the evoked potentials in the left HG, and to a lesser extent in PT, reflect a sequential processing of the different components of the syllables. Third, we show that this acoustic temporal processing is not limited to speech sounds but applies also to non-verbal sounds mimicking the temporal structure of the syllable. Fourth, there was no difference between responses to voiced and voiceless syllables in either left or right areas 22. Our data suggest that a single mechanism in the auditory cortex, involved in general (not only speech-specific) temporal processing, may underlie the further processing of verbal (and non-verbal) stimuli. This coding, bilaterally localized in auditory cortex in animals, takes place specifically in the left HG in man. A defect of this mechanism could account for hearing discrimination impairments associated with language disorders.

Acoustic Stimulation↗