Sensory gating of auditory evoked and induced gamma band activity in intracranial recordings.

Oscillatory activity in the gamma band range (30-50 Hz) and its functional relation to auditory evoked potentials (AEPs) is yet poorly understood. In the current study, we capitalized on the advantage of intracranial recordings and studied gamma band activity (GBA) in an auditory sensory gating experiment. Recordings were obtained from the lateral surface of the temporal lobe in 34 epileptic patients undergoing presurgical evaluation. Two kinds of activity were differentiated: evoked (phase locked) and induced (not phase locked) GBA. In 18 patients, an intracranial P50 was observed. At electrodes with maximal P50, evoked GBA occurred with a similar peak latency as the P50. However, the intensities of P50 and evoked GBA were only modestly correlated, suggesting that the intracranial P50 does not represent a subset of evoked GBA. The peak frequency of the intracranial evoked GBA was on average relatively low (approximately 25 Hz) and is, therefore, probably not equivalent to extracranially recorded GBA which has normally a peak frequency of approximately 40 Hz. Induced GBA was detected in 10 subjects, nearly exclusively in the region of the superior temporal lobe. The induced GBA was increased after stimulation for several hundred milliseconds and encompassed frequencies up to 200 Hz. Single-trial analysis revealed that induced GBA occurred in relatively short bursts (mostly <<100 ms), indicating that the duration of the induced GBA in the averages originates from summation effects. Both types of gamma band activity showed a clear attenuation with stimulus repetition.

Sensory gating in the human hippocampal and rhinal regions.

OBJECTIVE: The objective of this work was to ascertain if sensory gating can be demonstrated within the human medial temporal lobe. METHODS: Eight patients with intractable epilepsy with depth electrodes implanted in the medial temporal lobe for pre-surgery evaluation underwent evoked response recording to auditory paired-stimuli (S1-S2). Each of the eight subjects had a diagnosis of left medial temporal lobe epilepsy (MTLE). RESULTS: Data from the non-focal right hippocampi revealed a large negative response on S1 (starting at about 190 ms and lasting for approximately 300 ms from stimulus onset). Rhinal region recordings revealed a positive response (starting at about 240 ms with a rapid incline, followed by a long-lasting decline). A significant attenuation of both responses to S2 stimuli was observed. CONCLUSIONS: Data are suggestive of an involvement of the human medial temporal lobe in the processing of simple auditory information which occurs in a time frame later than the neocortical auditory evoked components. The exact role of these anatomical structures in the sensory gating process remains to be defined. SIGNIFICANCE: This study provides the first evidence of an activation of the rhinal cortex after simple auditory stimulation and provides new evidence that the activation of the medial temporal lobe structures occurs at a later stage than that of the neocortex.

Electrophysiological evidence of enhanced distractibility in ADHD children.

Abnormal involuntary attention leading to enhanced distractibility may account for different behavioral and cognitive problems in children with attention deficit hyperactivity disorder (ADHD). This was investigated in the present experiment by recording event-related brain potentials (ERPs) to distracting novel sounds during performance of a visual discrimination task. The overall performance in the visual task was less accurate in the ADHD children than in the control children, and the ADHD children had a higher number of omitted responses following novel sounds. In both groups, the distracting novel sounds elicited a biphasic P3a ERP component and a subsequent frontal Late Negativity (LN). The early phase of P3a (180-240 ms) had significantly smaller amplitudes over the fronto-central left-hemisphere recording sites in the ADHD children than in the control group presumably due to an overlapping enhanced left-hemisphere dominant negative ERP component elicited in the ADHD group. Moreover, the late phase of P3a (300-350 ms) was significantly larger over the left parietal scalp areas in the ADHD children than in the controls. The LN had a smaller amplitude and shorter latency over the frontal scalp in the ADHD group than in the controls. In conclusion, the ERP and behavioral effects caused by the novel sounds reveal deficient control of involuntary attention in ADHD children that may underlie their abnormal distractibility.

Brain activity index of distractibility in normal school-age children.

Children's attention is easily diverted from a current activity to a new event in the environment. This was indexed in school-age children by diminished performance speed and accuracy in a visual discrimination task caused by task-irrelevant novel sounds. Event-related brain potentials (ERPs) elicited by these distracting sounds showed a prominent positive deflection that was generated by brain processes associated with involuntary switching of attention to novel sounds. Recordings of the magnetoencephalographic (MEG) counterpart of this brain activity revealed a major bilateral generator source in the superior temporal cortex. However, ERP scalp distributions indicated also overlapping brain activity generated in other brain areas involved in involuntary attention switching. Moreover, differences in ERP amplitudes and in their correlations with the reaction times between younger (7-10 years) and older (11-13 years) children indicated developmental changes in attentional brain functions.

Effects of acoustic gradient noise from functional magnetic resonance imaging on auditory processing as reflected by event-related brain potentials.

The processing of sound changes and involuntary attention to them has been widely studied with event-related brain potentials (ERPs). Recently, functional magnetic resonance imaging (fMRI) has been applied to determine the neural mechanisms of involuntary attention and the sources of the corresponding ERP components. The gradient-coil switching noise from the MRI scanner, however, is a challenge to any experimental design using auditory stimuli. In the present study, the effects of MRI noise on ERPs associated with preattentive processing of sound changes and involuntary switching of attention to them were investigated. Auditory stimuli consisted of frequently presented "standard" sounds, infrequent, slightly higher "deviant" sounds, and infrequent natural "novel" sounds. The standard and deviant sounds were either sinusoidal tones or musical chords, in separate stimulus sequences. The mismatch negativity (MMN) ERP associated with preattentive sound change detection was elicited by the deviant and novel sounds and was not affected by the prerecorded background MRI noise (in comparison with the condition with no background noise). The succeeding positive P3a ERP responses associated with involuntary attention switching elicited by novel sounds were also not affected by the MRI noise. However, in ERPs to standard tones and chords, the P1, N1, and P2 peak latencies were significantly prolonged by the MRI noise. Moreover, the amplitude of the subsequent "exogenous" N2 to the standard sounds was significantly attenuated by the presence of MRI noise. In conclusion, the present results suggest that in fMRI the background noise does not interfere with the imaging of auditory processing related to involuntary attention.

Electromagnetic responses of the human auditory cortex generated by sensory-memory based processing of tone-frequency changes.

Event-related brain potentials (ERPs) and magnetoencephalographic (MEG) responses to infrequent ('deviant') tones occurring among frequent ('standard') tones of different pitch were compared with responses to rare tones presented alone. The subjects were to ignore the tones. Deviant tones elicited the mismatch negativity (MMN) and its MEG counterpart (MMNm), while the rare tones delivered alone elicited a larger N1 and its MEG counterpart (N1m) than did standard tones. Source modeling of MEG responses indicated a difference in auditory-cortex source locations between the MMNm to deviant tones and the enhanced N1m to the rare tones presented alone. Thus, the MMN/MMNm is elicited by infrequent sounds only when they occur among frequent sounds. This supports the idea that a sensory-memory trace formed in the auditory cortex by preceding repetitive sounds is a necessary precondition for MMN/MMNm elicitation.