Age effects on the P300 to novel somatosensory stimuli.

Event-related brain potentials (ERPs) to somatosensory task-relevant targets and task-irrelevant novel (tactile and shock) stimuli were studied in 30 subjects between the ages of 18 and 79. Target and novel P300 latencies increased linearly with age at comparable rates. P300 amplitudes and scalp topographies also changed with age. P300 amplitudes remained constant at frontal sites and decreased at central and parietal sites for both target and novel stimuli with increasing age. The current results extend the age-related novel P300 changes reported in the auditory and visual modalities to the somatosensory system. The age-related amplitude reduction at posterior scalp sites supports independent contributions of frontal and posterior association cortex to P300 generation.     

A model for P300 generation based on responses to near-threshold visual stimuli

Near-threshold and suprathreshold visual ERPs and their frequency components were compared with the aim to obtain further information on the generation mechanism of the P300 wave. Decrease of the stimulus energy from suprathreshold to near-threshold level resulted in an increase of the P300 amplitude specifically in the occipital region. This finding was in contrast with the P300 amplitude decrease in central and frontal regions and its constancy in parietal area. Delta and theta responses showed a similar distribution pattern, whereas alpha responses decreased in all regions as the stimulus energy decreased. We conclude that P300 wave may correspond to a delta oscillation during a widespread, transient interruption of afferent inputs from subcortical structures to the cortical neurons including those in the visual sensory area and simultaneous increase of the cortico-cortical interactions. If visual inputs are of suprathreshold strength, they override this effect specifically in the primary visual area and disrupt the cortico-cortical interactions and the emergence of P300 in the occipital cortex.     

P300 to high intensity stimuli during REM sleep.

OBJECTIVES: The P300 component of the auditory event-related potential (ERP) is a large amplitude positive wave peaking at approximately 300 ms following detection of a rare significant stimulus. Although P300 is typically only evoked when the subject attends to the stimulus, it may also be elicited in an awake, inattentive subject if the stimulus is sufficiently intrusive. We therefore employed an oddball task to determine if high intensity stimuli would elicit the P300 during sleep. METHODS: A loud 90 dB SPL tone pip was presented infrequently (P = 0.05) in a train of lower intensity 70 dB SPL standard stimuli. A multiple channel EEG was recorded from 8 good sleepers during a single night. RESULTS: A large amplitude parieto-central P300, peaking at 321 ms, was apparent in REM sleep to the loud deviant stimulus. In stage two non-REM sleep, a later positive wave, peaking at 446 ms, was apparent even after K-Complexes were removed from the average. This non-REM P450 was however maximum over occipito-parietal areas of the scalp. CONCLUSION: The presence of a P300 in REM sleep following a loud, rare stimulus indicates that sensory discrimination capabilities remain intact during this state. This may be associated with either pre- or conscious processing of relevant stimuli.     

Subliminal visual oddball stimuli evoke a P300 component.

OBJECTIVE: To provide evidence that a P300 component can be elicited by subliminal stimuli in an oddball paradigm. METHODS: The words LEFT and RIGHT were presented in a frequent-rare ratio (80-20%), counterbalanced between subjects. Stimuli were presented at the objective detection threshold (d'=0, via unmasked 1 ms presentations), a stringent measure for detecting any conscious perception. RESULTS: A significantly larger amplitude component was found for rare vs. frequent stimulus presentations across electrodes Fz, Cz, and Pz using both a broad 200-900 ms window (F(1,27)=5.75, P<0.012, eta(2)=0.18; one-tailed), and a more narrow 400-760 ms window defined using principal component analysis (F(1,27)=10.10, P<0.002, eta(2)=0.27; one-tailed). No significant component latency effects were found. An analysis of the conscious perception index (d') and the oddball effect (rare-frequent amplitude difference) revealed a negative relationship, further supporting the contention that conscious perception does not account for the finding, and suggesting that any conscious stimulus detection may inhibit this subliminal effect. CONCLUSIONS: Results provide evidence that an endogenous component can be elicited by undetectable subliminal stimuli in an oddball paradigm. Implications are discussed for comparing conscious and unconscious information processing, unconscious learning, and the measurement of ERPs to subliminal stimuli.     

Dipolar source modeling of the P300 event-related potential after somatosensory stimulation.

The cerebral generators of the P300 potential evoked by somatosensory stimuli were investigated. Event-related potentials elicited by an oddball paradigm were recorded in 15 healthy subjects by 19 scalp electrodes. Nontarget and target electric stimuli were delivered on the anterior surface of the left elbow and of the wrist, respectively. Target traces showed an N140 potential followed by a widely distributed P300 response. Dipolar source modeling of target traces resulted in a six-dipole model. In the earlier latency range (up to 200 ms), one dipole in the contralateral perirolandic region and two dipoles in the parasylvian cortex of both hemispheres were activated. Two dipolar sources located bilaterally in the medial temporal region (MTR) showed their maximal activity at the P300 latency. Finally, a dipole in the contralateral frontal lobe was activated both at the latency of the N140 response and after 200 ms. It was found that two symmetrical MTR sources and a frontal dipole contributed to P300 generation.     

Anterior and posterior association cortex contributions to the somatosensory P300

A P300 (P3)-evoked response is generated in a variety of mammalian species upon detection of significant environmental events. The P3 component has been proposed to index a neural system involved in attention and memory capacity. We investigated the contribution of anterior and posterior association cortex to somatosensory P3 generation. Somatosensory event-related potentials (ERPs) were recorded in controls (n = 10) and patients with unilateral lesions in temporal-parietal junction (n = 8), lateral parietal cortex (n = 8), or dorsolateral frontal cortex (n = 10). Subjects pressed a button to mechanical taps of the fifth finger (targets; p = 0.12), randomly interposed in sequences of taps to the second (standards; p = 0.76) and the third or fourth finger (tactile novels; p = 0.06). Occasional shock stimuli were delivered to the wrist (shock novels; p = 0.06). The scalp-recorded P3 was differentially affected by anterior and posterior association cortex lesions. Subjects with temporal-parietal lesions showed markedly reduced P3s to all types of stimuli at all scalp locations. The reductions were largest at the parietal electrode site over the lesioned hemisphere. Parietal patients had normal P3s for all stimulus types except for contralateral shock novels, which generated reduced P3s. Frontal lesions had reductions of the novelty P3 over frontal sites with minimal changes in the target P3. The data support the existence of multiple intracranial P3 sources. The data further indicate that association cortex in the temporal-parietal junction is critical for generating the scalp-recorded target and novelty P3s, whereas dorsolateral frontal cortex contributes preferentially to novelty P3 generation. The N2 component was reduced by parietal and frontal lesions in patients who had intact target P3s, suggesting that different neural systems underlie N2 and P3 generation.     

Attenuation in detection of somatosensory stimuli by transcranial magnetic stimulation.

Effects of magnetic stimulation (MS) of the scalp and direct cortical electrical stimulation on detection of an electrical stimulus to the index finger (S1) were studied in 7 normal volunteers and a patient with epilepsy. Detection of somatosensory stimuli was attenuated when MS was delivered 200 msec before S1, was blocked when MS was delivered simultaneously to and 20 msec after S1, and was fully recovered when MS was delivered 200 msec after S1. This effect showed topographic specificity, being produced by scalp stimulation of restricted scalp positions contralateral to the finger stimulated, was maximal with low intensities of finger stimulation and high intensities of MS (usually over that required for motor threshold), and could also be produced in the absence of motor evoked responses in a peripheral hand muscle. These results show that a focal cortical stimulus can briefly attenuate detection of somatosensory stimuli before, during, and after cortical arrival of a somatosensory afferent volley. Several different mechanisms probably contribute to this phenomenon.     

The effect of accomplishment and failure on P300 potentials evoked by neutral stimuli

The effects of emotional states of 'being successful' vs. 'being unsuccessful' were studied by measuring the P300 component of event-related potentials (ERPs). Nine subjects were instructed to reduce their P300 amplitude using feedback. Feedback was random but the relative probability of different signals created the situations of 'being successful' or 'being unsuccessful'. The probability of 'small' feedback was 0.7 in 'successful' and 0.15 in 'unsuccessful' trials. ERPs recorded without the feedback were used as a reference. Potentials, evoked by light stimuli in a standard 'odd-ball' procedure, were recorded from Fz, Cz and Pz scalp sites. The amplitudes of P300 components were reduced in 'unsuccessful' trials whereas in 'successful' trials they did not differ significantly from responses recorded without the feedback. There were no significant differences in peak latencies. These findings indicate that tonic emotional states affect the processing of neutral stimuli and that late components of ERPs can be useful indices in the analysis of these alterations. The results also indicate that the effects of positive and negative emotional states are not always reciprocal. Manipulated feedback is suggested as an useful model in the studies of emotions. Data can also facilitate the interpretation of the real feedback effects.     

On how P300 amplitude varies with the utility of the eliciting stimuli.

The bulk of the relevant evidence indicates that stimuli which elicit the P300 component of the human event-related potential (ERP) tend to be unexpected and task relevant. Yet, contradictory evidence has been presented which implied that P300 amplitude is modulated by the intensity difference between two equally relevant, equally probable feedback stimuli (Adams and Benson, 1973). The two experiments reported here were designed to clarify this contradiction. Subjects were instructed to press a button one second after a cue light. A tone at one of two intensities was presented 200 msec later indicating whether the time estimate was 'correct' or 'incorrect'. The magnitude of P300 varied as a function of the difference in intensity between S+ and S-; the smaller the difference, the smaller was P300, regardless of stimulus meaning (S+ or S-). We attribute this effect to the degree to which the subject chooses to utilize the tones in the performance of the time estimation task. In support of this hypothesis, we found that performance of the subjects as time estimators degraded as the difference between S+ and S- decreased. Moreover, a study of choice reaction times taken for the different pairings of S+ and S- showed that the stimulus pairs were not equally discriminable. It appears that as the S+/S- pairs become increasingly difficult to discriminate, their feedback value decreased and this decrease is positively correlated with a decrease in P300 amplitude. This interpretation received support from Exp. 2 in which the stimulus conditions were the same as in Exp. 1, but the subject's task was to count the "s+ stimuli. Here, when subject had no option but to process each individual stimulus, the P300 amplitude remained constant despite variations in S- intensity. Thus, it appears that task relevance must be defined in terms of the subject's behavior rather than by the experimenter's instructions to the subject.     

Differences between paranoid and nonparanoid schizophrenic patients on the somatosensory P300 event-related potential

Event-related potentials were recorded from schizophrenic patients (n = 30) and healthy controls (n = 30) using a somatosensory-reaction-time version of the oddball paradigm, by stimulating the right and the left median nerve. Latency, amplitude, duration and area of the P300 were measured. The patient group was subdivided into a paranoid (n = 16) and a nonparanoid (n = 14) subgroup and each was compared to controls. After stimulation of the right median nerve the nonparanoid group had a significantly prolonged P300 latency and a normal amplitude. The paranoid subgroup had a trend toward reduction of the P300 amplitude; its P300 latency was normal. After stimulation of the left median nerve, a prolongation of the P300 latency was observed in the paranoid subgroup. This subgroup had also a reduced P300 amplitude, while the nonparanoid patients had both values comparable to those of the controls. Duration and area were not significantly different between the two subgroups of patients and controls. Paranoid and nonparanoid patients showed a different behavior on reaction time parameters. No relationship was observed between P300 parameters and clinical ratings, neuroleptic dose and demographic data. The P300 parameters did not correlate with the reaction time measures. These results are discussed in terms of a disturbance of CNS inhibitory mechanisms in cognitive processes of paranoid schizophrenic patients and could be a further indication that different subtypes of schizophrenia may have different biological substrates.     

Subcortical correlates of the P300 potential complex in man to auditory stimuli

Bipolar and referential responses correlated to the surface P300 auditory potential complex (components N2, P3 and N4) were studied in different subcortical structures of parkinsonian and epileptic patients with implanted electrodes used as an electrophysiological procedure for surgical treatment. Bipolar and referential subcortical responses were recorded from a wide subcortical region which included subthalamo-thalamic (Sth-Th), dorsothalamic (DTh), orbitofronto-hippocampal (OFH) and striatal (STR) structures. They were formed by 3 consecutive components C, D and E correlated to components N2, P3 and N4 of the surface responses. Component D correlated to surface P3 was either absent or very small in all subcortical bipolar responses and in referential responses recorded from OFH structures. Polarity of bipolar component C was inverted between DTh and OFH while that of bipolar component E was inverted between Sth-Th and DTh, between OFH and STR and between Sth-Th and STR. Polarity of the referential component C was inverted between STh-Th and DTh and between OFH and STR while that of referential components D and E remained unchanged along all subcortical structures. Referential components C-E and D attained maximal amplitudes and minimal latencies at the subthalamic and at the medial geniculate thalamic nucleus regions respectively. From here, their amplitudes decreased and latencies increased with distance in other structures rostrally and caudally located, with the exception of those at the orbitofrontal (components C-E) and dorsal thalamic and striatal regions (component D) where no amplitude-latency gradients were observed.     

P300 and blink instructions.

OBJECTIVES: The effects of instructions to refrain from blinking on the P300 event-related brain potential (ERP) from auditory and visual stimuli were assessed. METHODS: An oddball paradigm was employed in which young adult subjects (n = 20) silently counted the infrequent target stimuli and were given either no instructions or told "do not blink" in different conditions, with auditory and visual stimulus tasks employed for all subjects. ERPs were recorded from the midline electrodes, with amplitude and latency of the P300 and other components obtained. RESULTS: P300 amplitude for both modalities and target/standard stimulus conditions was smaller and visual peak latency longer in the "do not blink" condition. Blink instructions did not directly affect the other components. CONCLUSIONS: Instructions to refrain from blinking can decrease P300 amplitude and can increase peak latency.     

Frequency, intensity, and duration as determinants of P300 from auditory stimuli

The auditory event-related potential was assessed in two experiments that evaluated the effects of stimulus factors on P300 (P3) component amplitude and latency. The first experiment manipulated the frequency of the target stimuli (500, 1,000, 1,500, and 2,000 Hz) and standard stimuli (250 and 4,000 Hz) in different conditions. Both P3 amplitude and latency were affected by the target and standard tone combinations in specific ways. The second experiment varied the target tone intensity and duration factorially. P3 amplitude was unaffected by these variables, but peak latency was significantly shorter for the loudest and longest target stimuli. The findings indicate that variation in auditory stimulus parameters can produce small but reliable effects for P3 values, which should be considered when results are compared across studies.     

Relationships between attention effects and intensity effects on the cognitive N140 and P300 components of somatosensory ERPs.

OBJECTIVES: This study attempts to elucidate the relative contributions of exogenous and endogenous components to the N140 and P300 potentials elicited by somatosensory stimulation. METHODS: Somatosensory event-related potentials (ERPs) were evoked using an odd-ball paradigm with the frequent (80%) stimuli delivered to the left index finger and the infrequent (20%) stimuli delivered to the right index finger. Both types of stimuli had the same intensity within each experiment. The experiment was repeated using 6 different stimulus intensities ranging from the sensory threshold to 3 times the threshold. Each experiment was done under two conditions. In one, the subjects were asked to count and respond to the infrequent stimuli. In the other, the subjects were instructed to ignore the stimuli whether frequent or infrequent. In addition, the compound sensory potential of the right median nerve was separately recorded from electrodes at the wrist using the same range of stimulus intensities applied to the right index finger. RESULTS: Amplitudes of the N140 and P300 elicited by both attended and unattended infrequent stimuli increased in a parallel fashion as a function of stimulus intensity, so that the amplitude difference between attended and unattended responses was independent of the stimulus intensity. The amplitude of the compound sensory nerve potential at the wrist exhibited a similar slope to those of the N140 and P300. CONCLUSIONS: Thus, it is concluded that the scalp N140 and P300 consist of two components: an endogenous component, which is independent of the stimulus intensity, and an exogenous component, which increases as a function of stimulus intensity. The relative contribution of these components to the N140 and P300 amplitudes is also discussed.     

Anesthesia and temperature effects on somatosensory evoked potentials produced by train stimuli

OBJECTIVE: To determine the effect of anesthesia, temperature, and stimulus characteristics on the response of upper extremity somatosensory evoked responses (SSEP) to repetitive stimulation. METHODS: Pairs and trains of electrical stimuli were used to elicit the upper extremity SSEP, and the amplitudes of the N20-P22, N13, and Erb's point potentials produced by each stimulus were measured. The ratio of the amplitude of the response to each stimulus to that produced by the first stimulus in a given train was computed. Recordings were also made directly from the cortical surface in a single patient. RESULTS: There were only minimal effects of anesthesia, temperature, or stimulus characteristics on the amplitude ratios for the N13 and Erb's point potentials. The N20-P22 amplitude ratio demonstrated facilitation with interstimulus intervals in the 20-100 ms range and was markedly reduced with hypothermia. The degree of facilitation was strongly dependent on isoflurane and nitrous oxide concentrations. In response to stimulation with trains of four stimuli, increased amplitudes of the N20-P22 complex were seen maximally with the second response while the third and fourth responses were of lower amplitude. CONCLUSIONS: There are strong effects of anesthesia and temperature on the cortical responses to repetitive stimulation of the upper extremity. Speculations on the origin of these effects were made.     

In vivo neuronal and astrocytic activation in somatosensory cortex by acupuncture stimuli

Acupuncture is a medical treatment that has been widely practiced in China for over 3000 years, yet the neural mechanisms of acupuncture are not fully understoo...     

Thalamo-cortical processing of near-threshold somatosensory stimuli in humans

Somatosensory stimuli elicit complex cortical responses that are discernible as somatosensory evoked potentials (SEPs) in scalp electroencephalographic recordings. Whereas earlier SEP components, occurring up to 100 ms after stimulus delivery, have been labeled 'preconscious', later responses have been associated with stimulus awareness. To date, how far these processes are primarily cortical or comprise additional subcortical operations remains open. Therefore, we recorded thalamic and scalp SEPs evoked by perceived as well as unperceived median nerve stimulation in neurosurgical patients with electrodes implanted into the ventral intermediate nucleus of the thalamus for deep brain stimulation. At stimulation intensities below perceptual threshold, only thalamic SEP components appeared consistently during the first 75 ms after stimulus delivery. Stimulation that was perceived by the patients elicited cortical as well as thalamic SEPs that lasted longer than 75 ms. These results indicate that the thalamus remains active after the primary propagation of a sensory signal to the cortex, and suggest that the transition from elementary to higher-order somatosensory processing is based on thalamo-cortical interactions.     

Changes in auditory P300 with clinical remission in schizophrenia: Effects of facial-affect stimuli

The auditory P300 is affected by arousal and emotion. We examined the effect of drawings expressing sadness or pleasure on auditory event-related potentials (ERP) during recovery from schizophrenia. Oddball-paradigm auditory ERP were recorded while presenting one of these drawings to nine patients with paranoid schizophrenia and seven patients with other types of schizophrenia during the acute stage and again during remission. P300 parameters (amplitude, area, and latency), reaction time, and symptom scores were evaluated. P300 amplitude and area were significantly greater in the recovery than in the acute phase. Increases were larger when viewing drawings that expressed sadness than when viewing drawings that expressed pleasure. In paranoid-type patients, P300 when viewing pleasurable drawings was similar to that when viewing sad drawings during the acute phase; in remission, P300 was significantly larger when viewing sad drawings than when viewing pleasurable drawings. In non-paranoid patients, P300 while viewing pleasurable drawings was significantly larger than that while viewing sad drawings during the acute phase; this difference disappeared during recovery. P300 latency and reaction time were not influenced by facial-affect stimuli or recovery. A significant negative correlation was noted between amplitude and negative symptom scores. P300 amplitude and area may be useful state markers during recovery in schizophrenic patients. Effects involving attention resources and emotional arousal are also involved.