Abstracts

RATIONALE: Surgery for epilepsy with a technique where the patient is awake for a portion of the operation under local anesthesia provides a unique opportunity to investigate the neurobiology of human cognition. With microelectrode recording during those operations, we have previously shown that 50-70% of neurons in lateral temporal cortex change activity with recent explicit memory, with regional differences in neurons changing activity with different aspects of memory (Brain 1988, 111:1383; J. Neurosci 1999 19:5674; Nature Neuroscience 5:64, 2002). The objective of this study is to further understand the temporal lobe organization of verbal memory by investigating the location of neurons with activity differentiating correct from incorrect identification or memory performance.
METHODS: Activity significantly differentiating correct from incorrect responses was identified in extracellular microelectrode recordings from 114 neurons at 62 sites in lateral or medial temporal lobe of 26 patients undergoing awake neurosurgery for epilepsy, during a measure that assessed identification and recent explicit memory for object names, text or auditory words. Timing of the appearance of this significant differentiation was determined in 50msec bins.
RESULTS: Different neurons in different regions differentiate identification or memory performance. The 13 neurons differentiating identification performance were overrepresented in medial-basal recordings. The 9 neurons differentiating memory performance were overrrepresented in superior temporal gyrus recordings. All neurons showed these effects for only one modality. No neuron differentiated both identification and memory. No lateralization of these effects was observed. Within each group there was separation of neurons showing differentiation early during perception and processing from those showing differentiation late, during output, with early differentiating neurons located more superio-laterally. For identification this effect was significant, with lateral temporal neurons showing early differentiation, medial-basal late. For memory, all neurons showed differentiation during encoding, that continued throughout storage and retrieval in most neurons. Early differentiating neurons were in superior temporal gyrus, late in middle temporal recordings.
CONCLUSIONS: These recordings provide further definition of the role of different regions of temporal lobe in recent verbal memory. Superiolateral temporal cortex neurons have a role in retention of perceptual features (early differentiating neurons) and in monitoring of response (late differentiating neurons) for recent explicit verbal memory, further evidence for role of lateral temporal cortex in recent memory. Inferior lateral-basal-medial temporal neurons are more closely related to general semantic memory, also with separation of neurons retaining perceptual features of the specific stimuli from those related to retrieval of response motor programs or response monitoring.
[Supported by: NIH grant NS 36527 and a McDonnell Pew Cognitive Neuroscience grant ]