Non-invasive detection of neuronal population activity in human hippocampus.

Medial temporal brain structures and hippocampus are associated with memory encoding and activate during registration and transfer of information within cortico-hippocampal circuits. Hippocampal dysfunction may cause learning and memory deficits as well as epileptogenesis. We monitored neuronal population activity of the human hippocampal formation with a whole-head magnetoencephalographic array. Attention-dependent hippocampal responses were elicited by auditory oddball stimuli. The most prominent responses emerged 200-500 ms after attended oddballs in good agreement with earlier intracranial recordings. Some activation of hippocampal source areas was seen at 60-100 ms. This completely non-invasive method permits access to cortico-hippocampal neural networks on the millisecond time scale during normal and abnormal brain activation and thus provides a new tool for evaluation of cognitive processes and pathological conditions involving hippocampal formation.

Effects of intensity variation on human auditory evoked magnetic fields.

We recorded auditory evoked magnetic fields from 6 healthy subjects with a 122-channel whole-head neuromagnetometer. The stimuli were 200-ms 1-kHz tones delivered at 4 different intensities (40, 50, 60, and 65 dB HL). The tones were given once every second, binaurally in the first session, and monaurally to each ear in the second one. The four intensities were presented randomly and equiprobably within a single sequence. In both stimulus conditions, the 100-ms response (N100m) decreased in latency and increased in amplitude as a function of intensity in both hemispheres. No systematic dependence was found between stimulus intensity and the N100m source location in the auditory cortex. Our study illustrates a noninvasive method to examine the functional properties of human auditory cortex, allowing simultaneous comparison between signals arising from both hemispheres.