Causal involvement of medial inferior frontal gyrus of non-dominant hemisphere in higher order auditory perception: A single case study.

The medial side of the operculum is invisible from the lateral surface of cerebral cortex, and its functions remain largely unexplored using direct evidence. Non-invasive and invasive studies have proved functions on peri-sylvian area including the inferior frontal gyrus (IFG) and superior temporal gyrus within the language-dominant hemisphere for semantic processing during verbal communication. However, within the non-dominant hemisphere, there was less evidence of its functions except for pitch or prosody processing. Here we add direct evidence for the functions of the non-dominant hemisphere, the causal involvement of the medial IFG for subjective auditory perception, which is affected by the context of the condition, regarded as a contribution in higher order auditory perception. The phenomenon was clearly distinguished from absolute and invariant pitch perception which is regarded as lower order auditory perception. Electrical stimulation of the medial surface of pars triangularis of IFG in non-dominant hemisphere via depth electrode in an epilepsy patient rapidly and reproducibly elicited perception of pitch changes of auditory input. Pitches were perceived as either higher or lower than those given without stimulation and there was no selectivity for sound type. The patient perceived sounds as higher when she had greater control over the situation when her eyes were open and there were self-cues, and as lower when her eyes were closed and there were investigator-cues. Time-frequency analysis of electrocorticography signals during auditory naming demonstrated medial IFG activation, characterized by low-gamma band augmentation during her own vocal response. The overall evidence provides a neural substrate for altered perception of other vocal tones according to the condition context.

Different contra-sound effects between noise and music stimuli seen in N1m and psychophysical responses.

Auditory-evoked responses can be affected by the sound presented to the contralateral ear. The different contra-sound effects between noise and music stimuli on N1m responses of auditory-evoked fields and those on psychophysical response were examined in 12 and 15 subjects, respectively. In the magnetoencephalographic study, the stimulus to elicit the N1m response was a tone burst of 500 ms duration at a frequency of 250 Hz, presented at a level of 70 dB, and white noise filtered with high-pass filter at 2000 Hz and music stimuli filtered with high-pass filter at 2000 Hz were used as contralateral noise. The contralateral stimuli (noise or music) were presented in 10 dB steps from 80 dB to 30 dB. Subjects were instructed to focus their attention to the left ear and to press the response button each time they heard burst stimuli presented to the left ear. In the psychophysical study, the effects of contralateral sound presentation on the response time for detection of the probe sound of a 250 Hz tone burst presented at a level of 70 dB were examined for the same contra-noise and contra-music used in the magnetoencephalographic study. The amplitude reduction and latency delay of N1m caused by contra-music stimuli were significantly larger than those by contra-noise stimuli in bilateral hemisphere, even for low level of contra-music near the psychophysical threshold. Moreover, this larger suppressive effect induced by contra-music effects was also observed psychophysically; i.e., the change in response time for detection of the probe sound was significantly longer by adding contralateral music stimuli than by adding contra-noise stimuli. Regarding differences in effect between contra-music and contra-noise, differences in the degree of saliency may be responsible for their different abilities to disturb auditory attention to the probe sound, but further investigation is required to confirm this hypothesis.

N100m latency shortening caused by selective attention.

The N100m response to a specific same-sound stimulus may be altered by the degree of attention paid to the stimulus. When participants selectively pay attention to the stimulus, the N100m amplitude increases; however, minimal effects are observed on the N100m latency. In this study, we examined the effects of selective special attention (motivation) to extract the frequency (or pitch) information from a probe tone on the N100m response to the probe tone. We compared the N100m latencies and amplitudes using magnetoencephalography, with the following three experimental conditions: 1) vocalization task protocol (vocalize in tune with the pitch of the probe tone after the presentation of the probe tone), 2) hearing task protocol (just listen to the probe tone), and 3) imagining (just imagine the vocalization in tune with the probe tone). The results indicated that the N100m latency in response to the probe tone was significantly shortened in the vocalization and imagining tasks compared with the hearing task in the right hemisphere of the brain. The amplitude was significantly increased in the vocalization task compared with the imagining and hearing tasks in the right hemisphere, and in the vocalization task compared with the hearing task in the left hemisphere of the brain; that is, the attention and/or motivation required to extract the information from the stimulus tones may have caused N100m latency shortening. To our knowledge, this study is the first to demonstrate that the N100m latency may be shortened under particular attentional conditions in response to a simple tone.

Impact of Audio-Visual Asynchrony on Lip-Reading Effects -Neuromagnetic and Psychophysical Study.

The effects of asynchrony between audio and visual (A/V) stimuli on the N100m responses of magnetoencephalography in the left hemisphere were compared with those on the psychophysical responses in 11 participants. The latency and amplitude of N100m were significantly shortened and reduced in the left hemisphere by the presentation of visual speech as long as the temporal asynchrony between A/V stimuli was within 100 ms, but were not significantly affected with audio lags of -500 and +500 ms. However, some small effects were still preserved on average with audio lags of 500 ms, suggesting similar asymmetry of the temporal window to that observed in psychophysical measurements, which tended to be more robust (wider) for audio lags; i.e., the pattern of visual-speech effects as a function of A/V lag observed in the N100m in the left hemisphere grossly resembled that in psychophysical measurements on average, although the individual responses were somewhat varied. The present results suggest that the basic configuration of the temporal window of visual effects on auditory-speech perception could be observed from the early auditory processing stage.

Positive auditory cortical responses in patients with absent brainstem response.

OBJECTIVE: To compare the detectability of the different auditory evoked responses in patients with retrocochlear lesion. METHODS: The 40-Hz auditory steady state response (ASSR) and the N1m auditory cortical response were examined by magnetoencephalography in 4 patients with vestibular schwannoma, in whom the auditory brainstem response (ABR) was absent. RESULTS: Apparent N1m responses were observed despite total absence of the ABR or absence except for small wave I in all patients, although the latency of N1m was delayed in most patients. On the other hand, clear ASSFs could be observed only in one patient. Very small 40-Hz ASSFs could be detected in 2 patients (amplitude less than 1fT), but no apparent ASSFs were observed in one patient, in whom maximum speech intelligibility was extremely low and the latency of N1m was most prolonged. CONCLUSION: The N1m response and 40-Hz ASSR could be detected in patients with absent ABR, but the N1m response appeared to be more detectable than the 40-Hz ASSR. SIGNIFICANCE: Combined assessment with several different evoked responses may be useful to evaluate the disease conditions of patients with retrocochlear lesions.

Auditory evoked magnetic fields in patients with absent brainstem responses due to auditory neuropathy with optic atrophy.

OBJECTIVE: To examine whether auditory evoked fields (AEFs) can be used to objectively evaluate hearing in patients with absent auditory brainstem responses (ABRs) due to auditory neuropathy. METHODS: Subjects were 3 patients with auditory neuropathy, 1 male aged 29 years and 2 females aged 18 and 27 years, with absence of click evoked ABRs for bilateral ear stimuli at a level of 105 dB nHL. All patients also had optic atrophy. AEFs were measured with a helmet-shaped magnetoencephalography system for 2.0 kHz tone bursts of 60 ms duration to the unilateral ear. RESULTS: Bihemispherical AEF responses were clearly recorded in all three patients for either left or right ear stimulus. Although the latencies of N100m were severely prolonged and amplitudes were considerably decreased compared to the normal range of N100m responses in our facilities, N100m latency of AEF was shorter in the contralateral hemisphere to the stimulated ear, as usually found in normal subjects, despite the abnormal delay in N100m latency in all conditions. CONCLUSIONS: Presence and abnormality of auditory cortical responses can be evaluated by AEFs in patients with auditory neuropathy even under null responses in ABRs. SIGNIFICANCE: AEFs are useful to evaluate residual hearing in patients with auditory neuropathy.

Primary somatosensory evoked magnetic fields elicited by sacral surface electrical stimulation.

To explore the brain response to sacral surface therapeutic electrical stimulation (SSTES) for the treatment of refractory urinary incontinence and frequent micturition, evoked magnetic fields were measured in six healthy males. Electrical stimuli were applied between bilateral surface electrodes over the second through fourth posterior sacral foramens with intensity just below the pain threshold. Somatosensory evoked magnetic fields (SEFs) for the bilateral median (MN) and posterior tibial nerves (PTN) were also measured for the comparison. Sources of the early SEF peaks were superimposed on individual magnetic resonance images. The first peak latency for sacral stimuli, M30, occurred at 30.2+/-0.8 ms (mean+/-standard deviation, N=6), with shorter latency than those for PTN stimulus (39.3+/-1.4 ms, N=12) and longer latency than those for MN stimulus (21.0+/-0.9 ms, N=12). The second peak latency for sacral stimuli, M50, occurred at 47.2+/-2.9 ms (N=6). Both M30 and M50 peaks showed a single dipole pattern over the vertex in the isofield maps. The equivalent current dipoles of M30 and M50 were both estimated near the medial end of the central sulcus with approximately posterior current direction. These results suggest that the sacral M30 and M50 are responses from the primary somatosensory cortex. The relatively long time lag between the onset and peak of M30 suggests that SSTES directly affects both the cauda equina and cutaneous nerve of the sacral surface.

High-frequency gamma-band activity in the basal temporal cortex during picture-naming and lexical-decision tasks.

Gamma-band activity (GBA) in electroencephalograms (EEGs) has been shown to reflect various cognitive processes. GBA has typically been recorded in the 30-60 Hz range in scalp EEGs. Recently, task-related "high GBA" (HGBA) with frequencies up to 100 Hz has been observed in studies with invasive electrocorticograms (ECoGs). In the present study, we recorded ECoGs from the bilateral basal temporal cortices in a patient with epilepsy and evaluated the task-related HGBA (most prominently in the 80-120 Hz range) accompanying picture-naming and lexical-decision tasks. We examined picture naming using two categories (line drawings of animals and tools). The lexical-decision task was performed using words and pseudowords of two distinct Japanese writing forms, kanji (morphograms) and kana (syllabograms). Task-related HGBA was observed bilaterally during the naming task. Recordings from some electrodes revealed significant differences in HGBA between animal and tool pictures. In contrast to the naming task, there was apparent left dominance in the lexical-decision task. Furthermore, significant differences in HGBA were observed between the Japanese kanji and kana words and between the kanji words and kanji pseudowords. A number of differences in the HGBA observed in the recordings from the basal temporal area were consistent with previous findings from neuroimaging and patient studies and suggest that HGBA is a good correlate of visual cognitive functions.