Abstract
The intra-arterial amobarbital procedure (IAP or Wada test) is used to determine language lateralization and contralateral memory functioning in patients eligible for neurosurgery because of pharmaco-resistant epilepsy. During unilateral sedation, functioning of the contralateral hemisphere is assessed by means of neuropsychological tests. We use the IAP as a reversible model for the effect of lesions on brain network topology. Three artifact-free epochs (4096 samples) were selected from each electroencephalogram record before and after amobarbital injection. Functional connectivity was assessed by means of the synchronization likelihood. The resulting functional connectivity matrices were constructed for all six epochs per patient in four frequency bands, and weighted network analysis was performed. The clustering coefficient, average path length, small-world index, and edge weight correlation were calculated. Recordings of 33 patients were available. Network topology changed significantly after amobarbital injection: clustering decreased in all frequency bands, while path length decreased in the theta and lower alpha band, indicating a shift toward a more random network topology. Likewise, the edge weight correlation decreased after injection of amobarbital in the theta and beta bands. Network characteristics after injection of amobarbital were correlated with memory score: higher theta band small-world index and increased upper alpha path length were related to better memory score. The whole-brain network topology in patients eligible for epilepsy surgery becomes more random and less optimally organized after selective sedation of one hemisphere, as has been reported in studies with brain tumor patients. Furthermore, memory functioning after injection seems related to network topology, indicating that functional performance is related to topological network properties of the brain.
Highlights
The Wada test is a commonly used test to determine language dominance and memory capacity in surgery candidates with temporal lobe epilepsy (TLE; Wada and Rasmussen, 1960; Rosenow and Luders, 2001)
Our results indicate that the topology of whole-brain functional networks become more random after sedation of one hemisphere by means of the intra-arterial amobarbital procedure, marked by a decrease in the Normalized values of both local clustering and average path length
Network properties are related to function to some extent: both higher theta band small-world index and longer path length in the upper alpha band are associated with better memory performance of the presumably non-affected hemisphere
Summary
The Wada test (intra-arterial amobarbital procedure, IAP) is a commonly used test to determine language dominance and memory capacity in surgery candidates with temporal lobe epilepsy (TLE; Wada and Rasmussen, 1960; Rosenow and Luders, 2001). The IAP has for long been a clinically essential tool in determining patients’ eligibility for epilepsy surgery, since it determines language dominance and whether the non-sedated hemisphere has sufficient reserve capacity to sustain memory functions after resection of (parts of) the affected temporal lobe. Changes in connectivity were found in the injected and contralateral hemispheres themselves, and in the interaction between both hemispheres These results suggest that connectivity throughout the whole brain immediately reacts to changes in activity level in one part of the brain
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