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C Tomberg

Publications and source records attributed to C Tomberg.

At least 19 recordsLinked to original sources
PubMed1999

Finger kinaesthesia: cognitive electrogeneses to attended joint input.

Study of brain mechanisms subserving perception of passive finger movements revealed an unexpected contrast between cutaneous and deep inputs from fingers. Selective attention to tactile inputs from finger tips did not change the first response of primary area 3b, but elicited a cognitive P40 in second order postcentral cortex. For finger joint inputs, attention enhanced the very first cortical response elicited by thalamo-cortical input in postcentral area 2 whereby finger kinaesthesia information was integrated with the cutaneous features information received from primary somatic areas via corticocortical connections. Cognitive electrogeneses P40, P100, N140 and P300 manifested serial sensory features processing and prefrontal working memory matching whereby manipulated objects can be identified from their shape and texture in active touch.

Adult↗
PubMed1999

The challenge of non-invasive cognitive physiology of the human brain: how to negotiate the irrelevant background noise without spoiling the recorded data through electronic averaging.

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.

Attention↗
PubMed1999

Cognitive N140 electrogenesis and concomitant 40 Hz synchronization in mid-dorsolateral prefrontal cortex (area 46) identified in non-averaged human brain potentials.

The mid-dorsolateral prefrontal area 46 has working memory functions for putting current cognitive processing into context and for updating relevant information on a trial-by-trial basis. Using non-averaged human brain responses to a target finger stimulus attended by the subject, we identified the cognitive prefrontal N140 electrogenesis with the Z method which numerically assesses the detailed consistency between scalp topographies of any single response and a grand average template. The cognitive N140 was present between 100 and 180 ms at the scalp contralateral to the attended target finger stimulus. Control responses to physically identical finger stimuli that were ignored by the subject did not elicit a prefrontal N140. In brain mapping the N140 focus was located 30-80 mm from scalp midline, and 50-60 mm in front of the vertex (Cz) coronal plane of the head. The data were consistent with recently disclosed anatomical locations of area 46 and they further document its interindividual variations in brain-to-scalp relationship. N140, is thought to manifest cortico-cortical activation from the posterior parietal area 7b which generates the cognitive P100 electrogenesis that precedes N140 in the target response. At the prefrontal site, the non-averaged EEG showed desynchronized 40 Hz oscillations, but these became transiently phase-locked in conjunction with the cognitive N140 excitatory effect. This seems in line with the Traub model suggesting that excitation of certain cortical inhibitory interneurons generates doublet firings which will pace the spiking of pyramidal neurons so as to promote synchronization at about 40 Hz. These results complement our recent finding that P300 inhibition conversely disrupts on-going 40 Hz phase-locking ('binding'). Considering the marked trial-to-trial variations in timing of the cortical cognitive processes, such physiological relationships between target response components and 40 Hz dynamics could only be uncovered by studying non-averaged single responses.

Adult↗
PubMed1999

Focal enhancement of chaotic strange attractor dimension in the left semantic (Wernicke) human cortex during reading without concomitant change in vigilance level.

Non-averaged scalp-recorded brain potentials were analyzed in four right-handed humans while they were either silently reading a book, or in idle alert conditions. The dimensionality of chaotic strange attractors was estimated from the EEG at each of 14 scalp electrodes, using the point correlation dimension PD2 algorithm. When the subject was fully alert but not engaged in any specific task, PD2 was between 5.2 and 5.9. When the subject was silently reading a book, PD2 bifurcated to significantly higher values (6.6-6.9) in the left lateral anterior extrasylvian temporal cortex, but did not change in the adjacent left perisylvian cortex nor over the right side of the brain. The PD2 increases occurred without any significant change in vigilance level as assessed from the unchanged mean voltage of concomitant gamma (40 Hz) EEG oscillations. A small barely significant PD2 increase was only noticed in the left lateral precentral region. The increase of chaotic dimensionality in the left lateral extrasylvian temporal cortex during reading reflects enhanced processing operations in the semantic (Wernicke) areas and is in line with recent PET imaging data. These results provide for the first time evidence of a focal chaotic fractal dimension increase that is related to specific cognitive brain processing operations.

Adult↗
PubMed1999

Unconscious attention manifested in non-averaged human brain potentials by optional short-latency cognitive electrogeneses without subsequent P300.

The cognitive unconscious is distinct from the psychoanalytic unconscious and is defined in experimental psychology as including mental processes which can influence behavior while remaining outside phenomenal awareness. We analyzed its physiological basis in scalp-recorded averaged and non-averaged human brain potentials evoked by near-threshold somatic (finger) sensory stimuli while the subject attended a non-somatic task. The early N20 electrogenesis in parietal area 3b, which was present and did not change across conditions, is thought to reflect automatic procedural processing of stimulus features. Short-latency cognitive P40, P100 and N140 electrogeneses were identified in non-averaged responses by assessing their characteristic scalp topographies with the Z method. They were found to be present intermittently in only 4-16% of the trials, respectively. These results establish that the unattended finger inputs are occasionally submitted to incomplete optional cognitive processing in the parietal cortex. There was no associated P300 electrogenesis which is taken to substantiate the view that these sensory inputs remained outside the subject's awareness. On this basis, we propose to relate the psychological phenomena of 'unconscious attention' and implicit perception to characteristic physiological mechanisms in the human brain, namely the incomplete parallel cognitive processing of sensory inputs which remain on the fringe of the consciously attended objects or events.

Adult↗
PubMed1998

Human perceptual processing: inhibition of transient prefrontal-parietal 40 Hz binding at P300 onset documented in non-averaged cognitive brain potentials.

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.

Adult↗
PubMed1996

Non-averaged human brain potentials in somatic attention: the short-latency cognition-related P40 component.

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.

Adult↗
PubMed1995

Transcutaneous magnetic stimulation of descending tracts in the cervical spinal cord in humans.

Percutaneous magnetic stimulation in humans allows non-invasive stimulation of deeply situated nervous structures with little, if any, discomfort and has proven its utility for brain stimulation. At the level of the vertebral canal, magnetic stimulation readily elicits muscle responses through activation of the motor spinal roots, but there has been no evidence for direct stimulation of the spinal cord itself. The present results document the feasibility of directly stimulating descending systems of the cervical spinal cord which synaptically activate motoneurones. A working hypothesis is that the structure thus stimulated may be cortico-motoneuronal pyramidal axons or the propriospinal system.

Adult↗
PubMed1995

Differential voluntary programming of fingers extensor commands revealed by non-invasive transcranial magnetic stimulation of the human brain.

Slightly suprathreshold magnetic stimuli were delivered over the left scalp while a normal human subject attended a sensory signal which called for a motor response of right fingers in a reaction time paradigm. The cortico-motoneuronal systems of the two muscles producing finger extension disclosed a remarkable physiological differentiation. Brain magnetic stimulation delivered just before the subject's motor response revealed cortico-motoneuronal facilitation for one extensor muscle and concomitant inhibition for the other, depending on the verbal instructions that had been given to the subject before the experimental trial. Thus, during the planning of specific finger manipulatory activities, the motor brain systems can achieve significant contrast in the membrane excitatory states of the motor cortical pyramidal neurones, even for synergic muscles.

Adult↗
PubMed1995

New application of the Z estimator to identify the cognitive P300 in non-averaged human brain potentials.

Electronic averaging is currently used for displaying event-related potentials such as P300 from the electroencephalogram (EEG). However, there is a growing need for upgraded methods allowing cognitive components to be identified in single trial brain responses. The Z estimation method has been adapted for the topographic testing of non-averaged scalp recordings. Z values approximating +1 help validate a genuine P300 while failure to pass the Z test may suggest the spurious nature of a late positivity that mimicks a P300. Z testing can also identify interference from transient EEG alpha activity by showing alternations between +1 and -1 values, as expected from an oscillating alpha generator. In contrast with previous methods based on EEG recordings from a single scalp site, our topographic Z method takes into account 28 scalp sites for single trials testing.

Adult↗
PubMed1995

Consciousness.

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.

Brain↗
PubMed1994

A method for identifying short-latency human cognitive potentials in single trials by scalp mapping.

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.

Adult↗
PubMed1994

Transient phase-locking of 40 Hz electrical oscillations in prefrontal and parietal human cortex reflects the process of conscious 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.

Adult↗
PubMed1991

Reaction times recording methods: reliability and EMG analysis of patterns of motor commands.

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.

Electricity↗
PubMed1991

Prime mover muscle in finger lift or finger flexion reaction times: identification with transcranial magnetic stimulation.

When recording the onset of the electromyographic (EMG) voluntary response in reaction time (RT) studies, the electrodes should be placed on the muscle which is first and foremost involved in executing the response. It is thus necessary to identify which is the prime mover muscle among active synergic muscles. This has been investigated for index finger lift or flexion RTs by delivering a magnetic stimulus to motor cortical areas prior to the subject's voluntary response. The EMG responses to the magnetic stimulus were selectively facilitated either in the extensor indicis proprius muscle (in index lift RTs) or in the first dorsal interosseous muscle (in index flexion RTs). These effects are robust and provide a method for identifying the prime mover muscle in voluntary movements.

Adult↗