Normal P 300 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. Wellcome Brain Research Group, Department of Psychology, University of St Andrews, St Andrews MD Rugg C D Pickles D D Potter Department of Medicine, University of Dundee and Department of Neurology, Dundee Royal Infirmary, Dundee, UK R C Roberts Correspondence to: M D Rugg, Wellcome Brain Research Group, Department of Psychology, University of St Andrews, St Andrews, Fife KY16 9JU, UK. Received 13 March 1990 and in revised form 30 May 1990. Accepted 15 June 1990 The P300, an endogenous component* of the scalp-recorded event-related potential (ERP), takes the form of a positive deflection with an amplitude of up to 30uV and a peak latency of between approximately 300 and 600 ms. It exhibits a parietal-maximum scalp distribution, and a similar range of functional properties, when evoked by stimuli of any sensory modality (for a recent review see Verleger'). It is frequently used to study cognitive dysfunction in neurological disease.2 The P300 is commonly elicited with simple stimuli by use of the "oddball" task. In its simplest form, this task consists of an unpredictable sequence of two classes of stimulus. The subject must detect one class (the oddballs or targets), while ignoring the other more probable class (the non-targets). In comparison with the ERPs evoked by nontargets, target ERPs contain an enhanced P300, the amplitude of which is inversely related to target probability.3 Intracerebral recordings from the medial temporal lobe (MTL) have demonstrated the presence of a sizeable oddball-sensitive ERP wave, which appears to be locally generated. '7 This wave is roughly contemporaneous with the P300 recorded from the scalp, and shares many functional properties with the scalp component.5 Analysis of its intracerebral voltage distribution suggests that it is generated in the hippocampus,5 a conclusion consistent with reports that the wave is frequently abnormal when recorded from an MTL containing a sclerotic hippocampus. >'0 Footnote *The term endogenous components refers to features ofthe ERP that are largely under the influence of psychological variables such as stimulus probability, attention, etc. These can be contrasted with exogenous components, which are influenced mainly by physical characteristics of the evoking stimulus, such as sensory modality or intensity. It is uncertain whether volume-conduction of the P300-like ERP component generated in the hippocampus contributes significantly to the scalp P300. Currently, the most substantial body of relevant evidence arises from studies of the scalp P300 in patients who have had unilateral anterior temporal lobectomy for the relief of epilepsy. If the P300 reflects to any substantial extent the activity of bilateral MTL generators, damage to one such generator would be expected to result in a reduction in amplitude and a shift in the lateral distribution of this component. Such findings have not been obtained; the P300s of temporal lobectomy patients are neither grossly asymmetric nor reduced in amplitude. "'1-4 Two problems exist in making the inference from these findings that MTL structures do not contribute to the scalp P300. First, lobectomy patients have unilateral skull defects. The effects that these might have on the amplitude and distribution of the scalp P300 are unknown (although, as noted by Knight15 and Stapleton et al,13 the likelihood that any effects exactly cancel those of a lobectomy seems very remote). Second, Knight'5 points out that temporal lobectomy for the relief of epilepsy usually spares a substantial proportion of the posterior hippocampus,16 a region of the MTL which typically gives rise to high amplitude oddball-evoked ERP activity.5 The lobectomy data are therefore consistent with the hypothesis that it is the posterior rather than the anterior region of the MTL that makes a significant contribution to the scalp P300. We describe a case in which non-surgical damage has occurred along the entire extent of the MTL of one hemisphere, but in whom the amplitude and lateral distribution of the parietal P300 are normal. This allows further evaluation of the role of MTL structures in the generation of the scalp P300. Method We describe a teacher aged 44 years. She presented as an emergency in November 1987, having had a first generalised tonic clonic seizure preceded by nausea, a sense of doom and seeing coloured lights. She was admitted to hospital and had two further generalised tonic clonic seizures within 24 hours. Physical examination was normal as were haematological and biochemical investigations. A CT scan of the brain showed a low attenuation area in the left posterior temporal region, with no mass effect and no enhance217 group.bmj. com on June 15, 2017 Published by http://jnnp.bmj. com/ Downloaded from