Scopolamine impairs memory performance and reduces frontal but not parietal visual P3 amplitude.

It has been suggested that the P3 event-related potential (ERP) may mark the operation of certain working or long-term memory processes. It has also been reported that cholinergic blockade by scopolamine induces significant memory impairment and is associated with an increased latency, as well as amplitude reduction or abolition of the auditory P3, thus supporting hypothesised links between P3 and long-term memory function. An intriguing anomaly is that, while visual P3 latency is also increased by scopolamine, amplitude is not changed. The aim of this study was to make a more detailed assessment of the effects of scopolamine on the visual P3 at a drug dose known to induce memory impairment. After drug administration, memory performance was significantly impaired and visual P3 latency was significantly increased. There was little evidence of parietal P3 amplitude reduction, but frontal P3 amplitude was significantly reduced in both target and non-target conditions. These findings, when considered in the light of a more recent study of the effects of scopolamine on auditory P3, suggest that cholinergic blockade produces a common effect in both visual and auditory modalities of significant frontal P3 amplitude reduction, but no significant parietal P3 amplitude reduction. These results are consistent with the view that there are modality-independent generators of the parietal and frontal P3. The finding of drug-induced memory impairment and modulations of frontal ERP deflections is also consistent with recent evidence of a significant role for regions of the frontal lobe in encoding and retrieval of long-term memories.

Cognitive brain potentials in a three-stimulus auditory "oddball" task after closed head injury.

Event-related potentials (ERPs) were recorded in a three stimulus oddball task from 16 patients who had sustained a severe closed head injury at least 6 months before testing, and from 16 control subjects. The stimuli comprised a random sequence of frequent non-target tones (P = 0.70), rare target tones (P = 0.15), and rare novel sounds (P = 0.15). The task requirement was to respond promptly to each target tone. From a latency of 200 msec onwards, the ERPs evoked by frequent non-targets were substantially more negative-going in the head-injured than in the control group. When this difference in the ERPs to the frequent tones was taken into account, there was no evidence to suggest that either the latency or the amplitude of the target-evoked N2 and P3b components differed between the groups. The novel stimuli evoked a prominent P3a component. The amplitude and scalp distribution of this component differed little between the groups, but its peak latency was reliably longer in the head-injured subjects. The findings in respect of the N2 and P3b components suggest that impairments in early processing of task-relevant stimuli are not an invariant feature of closed head injury. The findings regarding P3a suggest that, in the majority of patients, head injury has only a limited effect on the neural systems underlying involuntary shifts of attention.

The effects of scopolamine on event-related potentials in a continuous recognition memory task.

Event-related potentials (ERPs) were recorded during a task requiring continuous recognition memory for visually-presented words. Twelve subjects each performed the task twice, once following the administration of scopolamine, and once after receiving a saline placebo. In the placebo condition, correctly detected "old" words (i.e., words that had been presented once before during the task) evoked more positive-going ERPs than did "new" words. Scopolamine caused a substantial impairment in task performance, but did not reduce the size of these old-word/new-word ERP differences. It is concluded that old/new ERP effects are unlikely to reflect cholinergically-mediated neural activity underlying normal recognition memory.

Event-related potentials related to recognition memory. Effects of unilateral temporal lobectomy and temporal lobe epilepsy.

Event-related potentials (ERPs) were recorded during a continuous recognition memory task for visually presented words, with a 6 item lag between the first and second presentation of each word. The subjects consisted of: (i) a control group of patients with primary generalized epilepsy; (ii) patients who had undergone either a left or a right anterior temporal lobectomy; (iii) unoperated patients with either left or right unilateral temporal lobe epilepsy. In the controls, ERPs to detected 'old' words were reliably more positive-going in the interval 300 to 600 ms post-stimulus than were ERPs to 'new' items. In both left and right lobectomy patients, 'old/new' ERP differences in the same latency range were significantly smaller than in the controls, and did not differ significantly from zero. At midline electrodes, old/new effects in the temporal lobe epilepsy patients were of similar magnitude to those of the controls. In contrast to the control data, the old/new effects in the temporal lobe epilepsy patients were asymmetric, in that they were smaller over the hemisphere ipsilateral to the seizure focus than over the contralateral hemisphere. No relationship was found across subjects between the magnitude of old/new ERP effects and verbal memory performance. In a second task, occasional non-words had to be discriminated against a background of sequentially presented words, some of which were repetitions of the immediately preceding item. ERPs evoked by repeated words were more positive-going than were those to first presentations; this effect was reliable, and of equivalent size, in all patient groups. It is concluded that in the recognition task, old/new ERP effects are dependent on temporal lobe functioning, but that the anterior temporal lobe is not the principal locus of the generators of these effects. The cognitive processes reflected by these effects do not appear to be strongly lateralized to one hemisphere, and neither do they seem to be necessary for normal verbal memory.

Normal P300 following extensive damage to the left medial temporal lobe.

Event-related potentials (ERPs) were recorded during auditory and visual "oddball" tasks from a patient with a severe verbal memory deficit due to a low grade infiltrating glioma which involved the full extent of the left medial temporal lobe. In both sensory modalities, the patient's oddball-evoked P300s were symmetrical and of normal amplitude. These findings are difficult to reconcile with the hypothesis that the hippocampus, or any other medial temporal structure, makes a substantial contribution to the scalp P300.

Endogenous event-related potentials from sphenoidal electrodes.

Event-related potentials (ERPs) were recorded from left and right sphenoidal electrodes during 2 cognitive tasks (visual oddball and a word repetition procedure) known to evoke endogenous ERP activity in the medial temporal lobe. Both tasks gave rise to large, reliable modulations of scalp-recorded ERPs. In the oddball task, no consistent task-related ERP activity could be recorded from the sphenoidal electrodes concurrently with the scalp-recorded P3 component, although in the latency region following the peak of P3 these electrodes exhibited an enhanced late negative wave to target stimuli. In the word repetition task, no consistent repetition-related ERP effects of any kind were observed from the sphenoidal electrodes. Sphenoidal electrodes do not appear to detect the endogenous ERP components that are generated in the medial temporal lobe concurrently with scalp-recorded components.