Biomedical subjects
Publications and source records attributed to J E Desmedt.
The inability to identify objects by manipulation with eyes closed (astereognosia) results from a focal cortical lesion of the second order parietal area 2, which integrates inputs from finger joints with tactile information.
Brain mechanisms involved in selective attention in humans can be studied by measures of regional blood flow and metabolism (by positron emission tomography) which help identify the various locations with enhanced activities over a period of time of seconds. The physiological measures provided by scalp-recorded brain electrical potentials have a better resolution (milliseconds) and can reveal the actual sequences of distinct neural events and their precise timing. We studied selective attention to sensory inputs from fingers because the brain somatic representations are deployed over the brain convexity under the scalp thereby making it possible to assess distinct stages of cortical processing and representation through their characteristic scalp topographies. In the electrical response to a finger input attended by the subject, the well-known P300 manifests a widespread inhibitory mechanism which is released after a target stimulus has been identified. P300 is preceded by distinct cognitive electrogeneses such as P40, P100 and N140 which can be differentiated from the control (obligatory) profile by superimposition or electronic subtraction. The first cortical response N20 is stable across conditions, suggesting that the first afferent thalamocortical volley is not affected by selective attention. At the next stage of modality-specific cortex in which the sensory features are processed and represented, responses were enhanced (cognitive P40) only a very few milliseconds after arrival of the afferent volley at the cortex, thus documenting a remarkable precocity of attention gain control in the somatic modality. The physiology of selective attention also provides useful cues in relation to non-target inputs which the subject must differentiate in order to perform the task. When having to tell fingers apart, the brain strategy for non-target fingers is not to inhibit or filter them out, but rather to submit their input to several processing operations that are actually enhanced when the discrimination from targets becomes more difficult. While resolving a number of such issues, averaged data cannot disclose the flexibility of brain mechanisms nor the detailed features of cognitive electrogeneses because response variations along time have been ironed out by the bulk treatment. We attempted to address the remarkable versatility of humans in dealing with their sensory environment under ecological conditions by studying single non-averaged responses. We identified distinct cognitive P40, P100, N140 and P300 electrogeneses in spite of the noise by numerically assessing their characteristic scalp topography signatures. Single-trial data suggest reconsiderations of current psychophysiological issues. The study of non-averaged responses can clarify issues raised by averaging studies as illustrated by our recent study of cognitive brain potentials for finger stimuli which remain outside the subject's awareness. This has to do with the physiological basis of the 'cognitive unconscious', that is, current mental processes lying on the fringe or outside of phenomenal awareness and voluntary control, but which can influence ongoing behaviour. Averaged data suggest that, in selective auditory attention, the subject may not notice mild concomitant finger inputs. The study of non-averaged responses documents the optional and independent occurrence of the cognitive P40, P100 and N140 (but not P300) electrogeneses while the finger inputs remain outside phenomenal awareness. These results suggest that the subject unconsciously assigns limited cognitive resources to distinct somatic cortical areas thereby submitting finger inputs to an intermittent curtailed surveillance which can remain on the fringe or outside consciousness. The study of cognitive electrogeneses in single non-averaged responses is making possible a neurophysiology of cognition in real time.
Non-averaged scalp-recorded human brain potentials were analyzed during selective attention to somatic (finger) sensory stimuli. Distinct cognitive electrogeneses were identified by numerical assessment of their scalp topographical congruities with appropriate templates. The P300 electrogenesis and the 40 Hz neuronal oscillations disclosed, at different scalp sites, variations in timing from trial to trial. However, in any given single trial the transient 40 Hz phase-locking between the somatic prefrontal and parietal subareas was found to be disrupted at P300 onset. The results suggest that the P300 cognitive electrogenesis manifests an inhibition of the neurons assemblies involved in the perceptual processing of the attended sensory input, thereby achieving a 'closure' of the cognitive operations dealing with the currently attended sensory input. P300 can thus be viewed as one of the physiological mechanisms which can control the kinetics of the 40 Hz binding process.
1. Non-averaged scalp-recorded brain potentials were studied in humans during selective attention to randomly intermixed series of stimuli to fingers. Physiological tests were use for validating the presence or absence of the short-latency cognition-related P40 electrogeneses in parietal cortex in the response to a single-target stimulus (P40 signifies a positive polarity of about 40 ms peak latency). 2. To minimize interference from the electroencephalogram and noise we mapped single brain responses over the scalp and identified P40 topographies by an updated form of the numerical estimator Z for assessment of recorded potentials over time. We found that Z should exceed 0.96 for at least 15 ms for validation of the topographical congruity between the single P40 and an averaged P40 template. 3. Individual responses to 145 target finger stimuli correctly identified by the subject were analysed. P40 occurred only intermittently (34.5%) in a series of targets, but its voltage was unexpectedly large, exceeding the P40 voltage in averaged responses by a factor of about 10. 4. The usual assumption in the averaging method that the single brain responses combined in the average are stable but merely contaminated by unrelated noise was shown to be false for the cognition-related P40, which was considerably underestimated because of its intermittency in the averaged single trials. 5. The reaction time of the subject was on average 19% shorter in the trials in which a P40 was present, thus suggesting that P40 can influence subsequent perceptual processing by the brain in the same trial. 6. The feasibility of identifying specific cognition-related electrogeneses in single brain responses opens up the study of momentary shifts in brain processing strategies thereby allowing the neurophysiology of cognition to be based in real time.
Somatosensory evoked potentials (SEPs) to median nerve stimulation were recorded in 3 patients with a brain-stem or medullary lesion documented by clinical and CT or MRI evidence. The positive P14 and negative N18 scalp far-fields were preserved. The results suggest that P14 reflects the spike volley in caudal medial lemniscus, and that the N18 neural generators are located in the medulla, probably in the dorsal column nuclei and/or the accessory inferior olives.
Consciousness offers a major challenge to the neurosciences. Even though consciousness is by definition subjective and private to the organism concerned, we consider it to be an intrinsic feature of biological processes in the brain. As such, it should be viewed in the Darwinian perspective of natural selection which implies that the conscious brain function does have survival value and cannot be a mere epiphenomenon. We attempted a neurophysiological approach by assessing perceptual processing of simple somatic sensory inputs in humans. We found that short-latency cortical potentials evoked by a target finger stimulus attended by the subject are strongly enhanced, thus manifesting a remarkable potentiation of the cognitive representations in primary parietal cortex. About 80 msec later, the dorsolateral prefrontal cortex discloses enhanced electrogeneses which we believe to reflect activation of somatic representations in 'working memory'. A functional 'binding' between these critical areas has been revealed by the transient and selective synchrony of 40 Hz oscillations recorded in the cortical areas of the parietal and prefrontal cortices. We consider these re-entrant interactions at 40 Hz to be an essential part of the conscious brain mechanisms that achieve the identification of an object (in this example, a finger) and the decision to release a motor behavioral response.
Studies of scalp-recorded brain event-related potentials in humans currently depend on the electronic averaging of many responses to the stimulus. In non-averaged single responses, it is sometimes possible to see late components such as the so-called P300, but not the shorter latency components that are much smaller and masked in background noise. We tried to identify short-latency cognitive potentials evoked by finger stimulation by comparing single trial responses that are concomitantly recorded at the contralateral and ipsilateral parietal scalp respectively. We developed a single trial topographic mapping method that proved important for assessing whether any left-right difference at short latency indeed reflected genuine cognitive electrogeneses. These results make it possible to analyze on a trial-by-trial basis the short latency cognitive processing in somatic perception.
Electrical potential oscillations in the range of 35-45 Hz (gamma waves) have recently been shown to occur rather ubiquitously in the brain of awake humans. During selective somatic attention, we demonstrate a transient phase-locking of the gamma waves generated in the contralateral prefrontal and parietal cortical areas that we had previously shown to be involved in such selective attention tasks. In line with other microphysiological evidence obtained on mammalian visual cortex, this selective functional synchronization between critical human brain areas (as far as about 9 cm apart) is proposed to reflect the transient 'binding' of discrete cognitive features that are processed in distributed neuronal assemblies of the brain whereby the conscious perception of an object or event can be achieved. On this basis we emphasize that the conscious function of the brain is neither epiphenomenal nor delayed, but operates transiently to integrate relevant perceptual features at the time of target object identification and of conscious behavioural decision.
Four patients with a unilateral focal vascular accident involving the internal capsule (but not the cortex) were studied electrophysiologically. Averaged somatosensory evoked potentials (SEPs) to electrical stimulation of the median nerve on the left or the right side were analyzed. In the 3 patients with hemiparesis and normal somatic sensation, the precentral P22 and N30 SEP components were lost, whereas the parietal components were preserved. In another patient with clinical somatosensory loss unaccompanied by any central motor impairment, the precentral SEP components were preserved, whereas the parietal SEP components were lost. Thus, a small capsular lesion can eliminate distinct cortical SEP components by selectively involving either the axons of the thalamic VPLc nucleus going to parietal receiving cortex or the axons of thalamic VPLo going to motor area 4. These findings extend to subcortical lesions the diagnostic value of SEPs in patients with dissociated clinical motor and sensory signs.
Different methods for estimating reaction times (RTs) from either finger flexion or finger extension responses have been evaluated. The onset of finger movement was recorded with a photoelectric method and the results are compared with RT measures based on microswitch closure or onset of electromyographic (EMG) activity in the prime move muscle. EMG analysis showed the voluntary motor commands to present a characteristic ballistic pattern in RTs. However, this was not true for a number of trials with unusually long RTs which involved ramp or double burst EMG patterns that were interpreted as reflecting errors in the force calibration of motor commands. RTs based on photoelectric recording of onset of finger extension were consistently related to the RTs estimated from EMG onset in the prime mover muscle. It is concluded that the EMG onset or the finger lift photoelectric method is best suited for reliable RT recording.
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The average reference introduces ghost potential fields at the latencies for which the integral of scalp-recorded potentials differs from zero. These spurious effects occur because the average reference is computed from a limited number of (scalp) electrodes which do not survey the bottom half of the head. By arbitrarily re-setting the zero at each latency in the maps to be compared, it can also obliterate or even reverse topographical differences in the case of focal brain potentials enhancements thereby defeating the purpose of brain mapping.
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Four patients were studied electrophysiologically 8 to 24 years after surgical removal of one cerebral hemisphere without damage to the striatum or diencephalon. Somatosensory evoked potentials (SEPs) to electrical stimulation of the median nerve on the left or right side were averaged and mapped out over the scalp. Stimulation on the side opposite to the missing hemisphere evoked brief P9 and P14 farfields and a slow N18 negative potential of 15- to 25-msec duration bilaterally. No additional focal response was detected over the remaining (ipsilateral) hemisphere for 60 msec after the stimulus. Because long-standing hemispherectomy entails massive retrograde degeneration of thalamocortical neurons, the preserved P14 and N18 responses must reflect neural activities generated below the thalamus that are volume conducted to the scalp bilaterally. The data clarify several current issues in the evaluation of SEP components.
In 30 patients with a thalamic vascular lesion and clinical somatosensory disturbances in the opposite hemibody without hemiplegia, four nosological groups were identified: group 1 had no central pain but complete hemianesthesia and loss of cortical somatosensory evoked potentials (SEPs) on the affected side (analgic thalamic syndrome). Group 2 had central pain, severe hypoesthesia, and loss of cortical SEPs. Group 3 had central pain and hypoesthesia, with cortical SEPs present, although reduced or delayed on the affected side. Group 4 had central pain with preserved touch and joint sensations and normal SEPs (pure algetic thalamic syndrome). Clinical signs and SEP titration of the actual involvement of lemniscal pathways in these four groups of patients with thalamic syndrome are discussed in relation to current pathophysiology of central pain.
Bit-mapped color imaging of scalp potential fields evoked by sensory stimulation in humans disclosed significant features not identified by mere inspection of multichannel traces. Methodological problems are considered in detail for early cortical SEPs which include several components with sharp rise times occurring at spatially distinct scalp locations. A manageable yet efficient imaging system requires recording electrodes in adequate number and scalp locations, bandpass fidelity to resolve slow and fast components, consistency of bioelectric input data, optimal interpolation and mapping algorithms, and consistent color scaling. Critical steps in these procedures were investigated in conjunction with new evidence on the scalp topography and neural generators of the N20, P20, P22, P27 and N30 SEP components. It is concluded that N20-P20 reflect a tangential equivalent dipole in parietal area 3b while P22 reflects a radial equivalent dipole in motor area 4.
The physiology of somatic sensation can be investigated noninvasively in man by recording the electric activity of peripheral nerves, spinal cord and brain. Since these responses have a small voltage, it is necessary to use electronic averaging methods for improving the signal-to-noise ratio. These methods are described and discussed, as well as principles of interpretation of somatosensory evoked potentials. It is agreed that the traces thus obtained involve a series of components (extracellular potentials) which reflect distinct neural generators. These generators have been identified and localized at different levels of the subcortical somatosensory pathway and in different cortical areas. Several components reflect generators located under the recording electrodes (nearfield potentials), while other reflect extracellular potentials diffusing at a distance in the volume conductor of the neck and head (farfield potentials). The analysis of these components provides a wealth of new data for the physiology and pathophysiology of the somatic sensory system in man. Besides so-called "obligatory" components that are present irrespective of the attention of the subject, the studies have uncovered "cognitive" components which reflect neural mechanisms involved in the intellectual processus of perception and decision.
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