Biomedical subjects
F Richer
Publications and source records attributed to F Richer.
Frontal lesions increase post-target interference in rapid stimulus streams.
We examined the effects of frontal lesions on the attentional processes surrounding the discrimination of target stimuli by comparing patients with frontal excisions, patients with temporal excisions and controls on target-letter identification in rapid visual streams. Subjects were asked to look at streams of 18-26 letters presented centrally at rates of 6, 8, or 10 letters/sec and to name the two white target letters (T1 and T2) embedded among black letters in each stream. The two target letters were separated by either 0, 2,4, or 6 black letters. Normals and temporals correctly reported T1 at all rates, they showed the expected T2 identification errors peaking 300 msec after T1 at high rate and little T2 interference at lower rates. However, frontals showed T2 interference at the two lower rates and were unable to identify T1 at high rate. The effects observed suggest that an inertia of target discrimination processes contributes to the frontal attention deficits.
Inter-response interference contributes to the sequencing deficit in frontal lobe lesions.
In this study we investigated the contribution of inter-response interference to the sequencing deficit in frontal lobe lesions. We examined inter-response interference in choice sequences through the reduction in inter-response interval produced by stimulus preview when compared with sequences performed without preview. If frontal lobe lesions result in a stronger inter-response interference, the facilitative effect of preview on inter-response interval should be attenuated. We compared nine patients with a frontal excision with nine patients with a temporal excision and nine controls in a task requiring rapid keypress responses to each of five letters in a sequence. In the no-preview condition, the five letters were presented one at a time, immediately following the previous response. In the preview condition, the five letters were presented simultaneously before the response sequence. Patients with a frontal lesion showed slower response times than the other groups. In normal subjects and patients with a temporal lesion, stimulus preview produced the expected reduction of inter-response time and the slowing of sequence initiation. In frontal lesions, however, preview did not reduce inter-response time and exacerbated the slowing of sequence initiation. The results indicate that patients with a frontal lobe lesion show increased interference between adjacent responses as well as a sequence initiation problem.
Outcome of severe congenital hypothyroidism: closing the developmental gap with early high dose levothyroxine treatment.
We have previously reported that despite neonatal screening, children with severe congenital hypothyroidism treated at 5 weeks of age with 6 micrograms/kg.day levothyroxine have clinically significant intellectual impairment, whereas those with the moderate form of the disease are indistinguishable from controls. The developmental outcome of children with severe congenital hypothyroidism treated earlier with higher initial doses of levothyroxine remained to be determined. In the present study, 45 infants with permanent congenital hypothyroidism detected by neonatal screening are described. For the group, the median age at starting treatment was 14 days, and the median initial dose of levothyroxine was 11.6 micrograms/kg.day. Based on the area of their knee epiphyses at diagnosis, the patients were divided into 2 subgroups: severe (< 0.05 cm2; n = 10) and moderate (> or = 0.05 cm2; n = 35). The psychomotor development of 8 patients in each subgroup, matched for the socioeducational level of their families, was assessed at 18 months. Mean plasma free T4 levels were supraphysiological during the first few months of life, but mean plasma T3 levels remained within the normal range, and there were no signs or symptoms of hyperthyroidism. The mean plasma TSH concentration was less than 4.5 mIU/L 4 weeks after starting treatment. Bone maturation remained delayed at 12 months in the severe cases and was not unduly advanced in the moderate cases. The mean (+/- SD) developmental quotients at 18 months were similar in severe and moderate cases (107 +/- 10 and 110 +/- 5, respectively). We conclude that with earlier treatment and a higher initial dose of levothyroxine, the early developmental outcome of infants with severe congenital hypothyroidism is now indistinguishable from that of infants with the moderate form of the disease who were used as controls.
Response selection deficits in frontal excisions.
We compared the performance of patients with frontal excisions, patients with temporal excisions and controls in tasks involving speeded choice responses in which a number of variables were manipulated including: perceptual difficulty, stimulus and response set-size, associative complexity, and spatial stimulus-response compatibility. Response times were sensitive to all manipulations but did not show any group differences. The error rates of the three groups were equally affected by perceptual difficulty and response set-size but frontals were preferentially affected by spatial S-R compatibility, associative complexity, and the number of stimuli per response. The results are consistent with a basic deficit in response selection processes which could underly many problems produced by frontal lesions.
Target detection deficits in frontal lobectomy.
We examined the hypothesis of dorsomedial frontal lobe involvement in target detection through the effects of distractor interference and multiple target interference on unilateral lobectomy patients. Seven patients who underwent a unilateral frontal lobectomy for epilepsy involving dorsomedial cortex and variable amounts of lateral cortex were compared to 10 patients with a unilateral temporal lobectomy and to 10 normal adults on a visual character cancellation task. The task involved detecting occurrences of target characters embedded in rows of characters under three conditions: detection of one target character in the absence of distractors, detection of one target character among distractors, and detection of three targets among distractors. Visual detection performance was compared to that in the Stroop reading interference task. Frontals were predictably slower than the other groups in the baseline conditions of the character cancellation task and the Stroop task. After partialing out baseline detection performance in the character cancellation task, frontals showed an almost normal detection in the presence of distractors but were distinctly slower and made more errors than the other groups in multiple target detection. Frontals were also slower on the Stroop even after partialing out baseline naming performance. Temporals were normal on all tasks. Results suggest that frontal damage can affect selectivity in target detection as well as the Stroop and that this deficit is independent of the general psychomotor slowing observed in these patients.
Stimulation of human somatosensory cortex: tactile and body displacement perceptions in medial regions.
We examined the somatosensory perceptions evoked by stimulation of rolandic and parietal brain regions in 40 epileptic patients undergoing a presurgical investigation with intracerebral electrodes. Bipolar stimulation trains were delivered in an incremental sequence at medial and/or lateral contact pairs of stereotaxically implanted, multi-contact electrodes, while monitoring stimulus after discharge propagation with electrodes in frontal and temporal lobes. Rolandic stimulation evoked contralateral sensations, whereas sensations from either side were evoked in the opercular region. Stimulation of lateral posterior parietal cortex evoked only few sensations and these were restricted to the supramarginal gyrus. Contralateral sensations were evoked in the posterior cingulate gyrus, whereas ipsilateral sensations were evoked in the vicinity of the cingulate sulcus. Complex proprioceptive sensations in the form of bilateral feelings of levitation were elicited in a medial parietal region around the subparietal sulcus and not at any other site. These perceptions were not accompanied by any movement, tactile, or vestibular sensations, suggesting that the human sub-parietal sulcus region is linked to the proprioceptive processing system.
Persistent neuropsychological deficits and vigilance impairment in sleep apnea syndrome after treatment with continuous positive airways pressure (CPAP).
The obstructive sleep apnea syndrome is characterized by nocturnal sleep disturbance, excessive daytime sleepiness and neuropsychological deficits in the areas of memory, attention, and executive tasks. In the present study, these clinical manifestations were assessed in apneic patients before and 6 months after treatment with nasally applied continuous positive airway pressure (CPAP). CPAP treatment was found to restore normal respiration during sleep and to normalize sleep organization. Daytime vigilance greatly improved with treatment but some degree of somnolence as compared to normal controls persisted. Similarly, most neuropsychological deficits normalized with treatment. The exception was for planning abilities and manual dexterity, two neuropsychological deficits that have been found to be highly correlated with the severity of nocturnal hypoxemia. These results raise the possibility that anoxic brain damage is a pathogenic factor in severe obstructive sleep apnea syndrome.
Perceptual context and the selective attention effect on auditory event-related brain potentials.
Two experiments examined the effect of the perceptual context established through tonal grouping on neuroelectric responses during selective listening. Subjects monitored one of the extreme pitches in four-pitch tone sequences and detected rare longer tones of the designated pitch. In the first study, tonal grouping was manipulated by changing the tonal separation between the extreme pitches and their nearest neighbor, keeping the extreme pitches constant. Grouping increased the negativity of the attended-unattended difference potential. A second study examined the effect of grouping on the attention-related negativity when it opposed the effect of physical similarity. The proximity of extreme pitches varied, keeping the middle pitches constant. The effect of grouping on event-related potentials (ERPs) for the middle pitches was varied according to the tone being attended. ERPs for the middle tone adjacent to the attended tone became more negative with grouping, whereas ERPs for the distant middle tone became less negative with grouping. These effects suggest that the attention-related negativity is sensitive to contextual information.
Neurobehavioral manifestations in obstructive sleep apnea syndrome before and after treatment with continuous positive airway pressure.
Obstructive sleep apnea syndrome (OSAS) is characterized by recurrent apneas during sleep, resulting in repetitive hypoxemic episodes and a constant interruption of the normal sleep pattern. Vigilance impairment and neuropsychological deficits are among the main symptoms seen in this condition. One of the major questions in this field concerns the reciprocal interactions between nocturnal hypoxemia, sleep disruption, excessive daytime sleepiness and cognitive deficits. Results of this study suggest that vigilance impairment is attributable mostly to nocturnal hypoxemia. However, in cognitive deficits, hypoxemia seems to play a major role in executive and psychomotor tasks, whereas attention and memory functions appear to be related to vigilance impairment. After treatment, hypoxemia-related deficits and some degree of sleepiness persist. These results raise the possibility of an irreversible anoxic central nervous system (CNS) damage in severe OSAS.
[Research and surgical treatment of epilepsy].
The currently available surgical procedures for the treatment of epilepsy, from fundamental data to therapeutic results, including various means of investigation are reported. The work is based on a review of the literature and on the cases studied by the two teams from the Universities of Montreal and Bordeaux who share the same concept of epilepsy surgery. The patient groups of the two teams include 316 S.E.E.G., 214 cortectomies, 39 callosotomies and 2 multiple sub-pial transsections. In the first part, the authors attempt to demonstrate that the epileptic focus corresponds to the region where the seizures arise, that this focus is not directly comparable to the region where inter-ictal spikes are recorded and sometimes becomes autonomous from the causal lesion. The epileptic phenomenon has a definite harmful effect on cerebral functions and a probable self-aggravating potential. The second chapter summarizes the clinical data on which the indications and contraindications are based. These obviously depend on whether the intervention is intended to be curative or palliative. Various non-invasive and invasive investigations are then reviewed, according to their relative importance and the experience of each team. The main points developed are: the electroclinical correlations during seizures, the symptomatological data for differentiating between temporal and frontal lobe seizures, the contribution of M.R.I. in demonstrating the epileptogenic and epileptic lesions, the electrophysiological information suggesting that S.E.E.G. remains the most informative mean of investigation. The various methods of investigation of assessing electrical, functional (cerebral blood flow, metabolism) and morphological aspects of epilepsy, supply non-redondant findings about the localisation of the epileptic focus. The chapter on surgical techniques mainly discusses the various modes of implantation of subdural and intracerebral electrodes and reports the same rate of morbidity in both cases. Orthogonal teleradiography is still perfectly suited to the implantation of intracerebral electrodes. S.E.E.G. is still the most anatomically precise technique. However, in certain conditions, extraoperative E.Co.G. is more adequate. New surgical modalities have recently appeared such as the multiple subpial transsections which allow treatment of epileptic foci unapproachable by cortectomy and such as modified techniques of hemispherectomy, which by decreasing morbidity, renew interest in them. In the chapter on surgical results, the authors emphasize the methodological problems of evaluation that partly account for their wide variability. The results obtained with the various surgical modalities are reviewed. The outcome in cortectomies is discussed at length in terms of the data from the literature as well as the results reported by both teams.(ABSTRACT TRUNCATED AT 400 WORDS)
Visual motion perception from stimulation of the human medial parieto-occipital cortex.
Visual phenomena evoked by direct electrical stimulation of extrastriate cortex were observed in 30 epileptic patients as part of a presurgical investigation. An incremental sequence of low-level bipolar stimulation trains was delivered at medial and lateral pairs of contacts of stereotaxically-implanted multilead intracerebral electrodes in parietal, occipital and posterior temporal regions. Diffusion of stimulus afterdischarges was monitored by electrodes in temporal and frontal lobes and by the non-stimulated contacts of the stimulated electrode. Localized stimulations evoked few visual phenomena. The strongest anatomo-perceptual correlation was found for stimulation in the medial parieto-occipital fissure which evoked visual motion phenomena in all three patients stimulated in that region. The evoked motion perceptions were not associated with eye movements or any particular localization of the epileptic focus. These perceptions were only evoked once outside of the medial PO region at the 61 sites examined. The results suggest that the medial parieto-occipital region is closely linked to the human visual motion processing system.
Methodological considerations for the evaluation of spatio-temporal source models.
The effects of physiological noise and modelling procedure on spatio-temporal source modelling (STSM) solutions were examined by adding EEG noise (3-4% of total energy) to synthetic signals from 3 dipoles producing topographies that partially cancelled at the surface of an homogeneous sphere. Three patterns of source activation profiles were each associated with two EEG records. The STSM solutions were subjected to statistical tests that detect signal left in the residuals. All models substantially accounted for noise. Essentially correct models left no consistent signal in the residuals, but other statistically acceptable models were also occasionally found. Unambiguous correct solutions were found for moderately correlated source activations. Statistically adequate 2-source and 3-source models were found for the 4 data sets incorporating nearly parallel activations of 2 sources. When all 3 sources had comparable amplitudes, the 2-source models represented compromises containing gross mislocalizations but correct 3-source models were found. When one of the parallel sources was attenuated by 75%, only 2-source models satisfactorily approximated the 2 main sources. The localization errors in the "correct" models ranged from 2.5% to 13% of the sphere radius. Thus, even with a perfect signal propagation model, STSM could at best claim approximate localizations from data containing structured noise.
Relative localizing value of common tests used in the preoperative investigation of epileptic patients.
We have compared the relative localizing value of common tests in the presurgical evaluation of epilepsy in 73 patients with depth electrode (SEEG) confirmed epileptogenic foci and excellent surgical outcome. We found the tests of abnormality (CT, amytal, neuropsychology) to be frequently non-informative and often discordant with SEEG. The EEG was concordant with the SEEG in 38% of cases and provided lateralization in 78%. EEG was less localizing than ictal behavior in frontals but not in temporals. The need for SEEG is rather evident in these results but could become attenuated by the development of the localizing power of scalp EEG.
Obstructive sleep apnea syndrome: pathogenesis of neuropsychological deficits.
Neuropsychological deficits have been documented in patients with obstructive sleep apnea syndrome (OSAS). Both nocturnal hypoxemia and impairement of daytime vigilance have been suggested as the pathogenesis of these deficits, yet it remains difficult to find good correlations between cognitive deficits and either of these physiological parameters. In the present study, 10 normal controls were compared to 10 moderately and 10 severely apneic patients, all recorded in a sleep laboratory for two consecutive nights, with a vigilance and neuropsychological assessment made during the intervening day. Relative to the controls, moderate and severe OSAS showed differences in many cognitive functions, although the severely affected showed the greater differences. Moreover, severe apneics were also worse than moderate apneics on tests that were found to be normal in the latter group. This suggests a discontinuity in the appearance of neuropsychological deficits as OSAS progresses. Further analyses revealed that reductions in general intellectual measures, as well as in executive and psychomotor tasks were all attributable to the severity of hypoxemia, while other attention and memory deficits were related to vigiance impairment. Therefore, both vigilance impairment and nocturnal hypoxemia may differentially contribute to the cognitive dysfunctions found in OSAS.
Nocturnal hypoxemia as a determinant of vigilance impairment in sleep apnea syndrome.
In sleep apnea syndrome (SAS), vigilance impairment is typically associated with highly disrupted sleep, but recently, nocturnal hypoxemia has also been identified as a second pathogenetic factor in patients with a high degree of desaturation. However, although sleep disruption has been demonstrated to play a role in both the propensity to fall asleep and the capacity to stay awake, the role of nocturnal hypoxemia has been implicated only in the latter. In the present study, both sleep disruption and nocturnal hypoxemia were assessed in 20 moderately to severely apneic patients. During the day, vigilance was assessed both by the multiple sleep latency test (MSLT), as a measure of the propensity to fall asleep, and by the four-choice reaction time test (FCRTT), as a measure of the capacity to stay awake in a performance task. Severity of nocturnal hypoxemia was found to predict performance on the MSLT, as well as on the FCRTT, but sleep disruption was found to predict performance only on the FCRTT. These results suggest that in moderately to severely affected SAS patients, nocturnal hypoxemia may play a primary role in the pathogenesis of vigilance impairment.
Human intracerebral potentials associated with target, novel, and omitted auditory stimuli.
We recorded late auditory potentials from lateral and medial regions in the frontal, temporal and parietal lobes of patients with temporal lobe epilepsy implanted with horizontal depth electrodes. Tone sequences were presented in three tasks: 1) auditory target detection in a tone sequence, 2) target detection with interspersed novel stimuli, and 3) detection of stimulus omissions. At frontal sites, potentials to targets showed a triphasic response with peak latencies around 200, 270 and 350 ms. At temporal sites, potentials consisted of a generally positive 285 ms peak which was sometimes accompanied by a negative peak at 200 ms or at 400 ms. At parietal sites, potentials were generally triphasic with latencies of about 230, 300, and 370 ms. At most sites, potentials evoked by novel stimuli had shorter latencies than those evoked by targets. The frontal and parietal potentials were either absent or strongly attenuated during stimulus omissions. The results lend further support to the multiple generator hypothesis of late potentials and suggest that some of the cerebral sources of the late potentials are stimulus dependent while others are not.
Methods for separating temporally overlapping sources of neuroelectric data.
The localization of intracranial sources of EEG or MEG signals can be misled by the combined effect of several sources, as illustrated by simulated MEG data in which two of the three dipolar sources have slightly out of phase activity and partly complementary scalp topographies. These data were analysed by three different source localization methods. Fitting a single source to each sequential topography worked perfectly when only one source was active; this could also account for as much as 95% of the spatial variance of topographies resulting from two overlapping sources, although the solution was then far from any source. A principal component analysis approach followed by an oblique rotation (fitting one source to the spatial aspect of each component) correctly localized two of the sources but severely mislocated the source that was never active alone. Spatio-temporal source modeling (simultaneously fitting a set of sources to all consecutive topographies) correctly localized all three sources, provided that the parameter optimization method could escape sub-optimal local minima of the error function. Temporally overlapping sources can thus be separated and correctly identified if the mathematical model is adequate and the optimization procedure is well adapted.