Katherine R. Henshall

Reduced connectivity of the auditory cortex in patients with auditory hallucinations: a resting state functional magnetic resonance imaging study

BACKGROUND Previous research has reported auditory processing deficits that are specific to schizophrenia patients with a history of auditory hallucinations (AH). One explanation for these findings is that there are abnormalities in the interhemispheric connectivity of auditory cortex pathways in AH patients; as yet this explanation has not been experimentally investigated. We assessed the interhemispheric connectivity of both primary (A1) and secondary (A2) auditory cortices in n=13 AH patients, n=13 schizophrenia patients without auditory hallucinations (non-AH) and n=16 healthy controls using functional connectivity measures from functional magnetic resonance imaging (fMRI) data. METHOD Functional connectivity was estimated from resting state fMRI data using regions of interest defined for each participant based on functional activation maps in response to passive listening to words. Additionally, stimulus-induced responses were regressed out of the stimulus data and the functional connectivity was estimated for the same regions to investigate the reliability of the estimates. RESULTS AH patients had significantly reduced interhemispheric connectivity in both A1 and A2 when compared with non-AH patients and healthy controls. The latter two groups did not show any differences in functional connectivity. Further, this pattern of findings was similar across the two datasets, indicating the reliability of our estimates. CONCLUSIONS These data have identified a trait deficit specific to AH patients. Since this deficit was characterized within both A1 and A2 it is expected to result in the disruption of multiple auditory functions, for example, the integration of basic auditory information between hemispheres (via A1) and higher-order language processing abilities (via A2).

The perceptual effects of interphase gap duration in cochlear implant stimulation

The most common current pulse shape used for cochlear implants is a biphasic rectangular pulse. The interphase gap (IPG) is the duration of the zero-current portion which lies between the two phases. It is known from single-nerve studies in animals that, as the IPG decreases, the biphasic pulse becomes less efficient in activating the nerve cell. Thus, it can be predicted that stimulation using smaller IPGs will necessitate the use of higher currents to maintain the loudness required by the cochlear implantee. The development of contemporary processing schemes commonly involves the maximization of the rate parameter, and to achieve this in sequential pulsatile stimulation, the IPG as well as the pulse phase duration must be minimized. This experiment investigated the effect of IPG on loudness in eight cochlear implantees who use the CI24 implant manufactured by Cochlear Ltd. An exponential increase in current level was required to maintain equal loudness when IPG is reduced from 100 to 45 and 8.4 micros. The effect of IPG was greater at lower levels, was greater for shorter pulse durations (26 micros compared to 52 micros), and was not significantly different for the rates (1 kHz or 4 kHz) tested.

Functional connectivity estimation in fMRI data: influence of preprocessing and time course selection

A number of techniques have been used to provide functional connectivity estimates for a given fMRI data set. In this study we compared two methods: a ‘rest‐like’ method where the functional connectivity was estimated for the whitened residuals after regressing out the task‐induced effects, and a within‐condition method where the functional connectivity was estimated separately for each experimental condition. In both cases four pre‐processing strategies were used: 1) time courses extracted from standard pre‐processed data (standard); 2) adjusted time courses extracted using the volume of interest routines in SPM2 from standard pre‐processed data (spm); 3) time courses extracted from ICA denoised data (standard denoised); and 4) adjusted time courses extracted from ICA denoised data (spm denoised). The temporal correlation between time series extracted from two cortical regions were statistically compared with the temporal correlation between a time series extracted from a cortical region and a time series extracted form a region placed in CSF. Since the later correlation is due to physiological noise and other artifacts, we used this comparison to investigate whether rest‐like and task modulated connectivity could be estimated from the same data set. The pre‐processing strategy had a significant effect on the connectivity estimates with the standard time courses providing larger connectivity values than the spm time courses for both estimation methods. The CSF comparison indicated that for our data set only rest‐like connectivity could be estimated. The rest‐like connectivity values were similar with connectivity estimated from resting state data. Hum Brain Mapp 2008.

The effect of rate of stimulation on perception of spectral shape by cochlear implantees

The effect of rate of stimulation on spectral shape perception was measured in six users of the Nucleus CI24 cochlear implant. Three spectral shapes were created by using three profiles of current across seven electrode positions. Each current profile was replicated in three stimuli that interleaved stimulus pulses across the seven electrodes with cycle rates (rate per electrode) of 450, 900, and 1800 Hz. The stimulus space resulting from a multidimensional scaling experiment showed a clear dimension related to the rate of stimulation that was orthogonal to the dimension related to the spectral shapes. A second experiment was performed with the same subjects to investigate whether the perceptual dimension related to rate in Experiment 1 could be attributed to different perceptual flatness of the profiles at different rates. In Experiment 2, the rate of stimulation was fixed at 900 Hz and three profiles were created for each spectral shape that differed in flatness. This experiment did not, however, result in an independent perceptual dimension related to the flatness of the profile. In conclusion, rate of stimulation provided an independent perceptual dimension in the multiple-electrode stimuli, in spite of the rates being not discriminable or barely discriminable in single-electrode stimulation.

Amplitude modulation and loudness in cochlear implantees

The effect of amplitude modulation of pulse trains on the loudness perceived by cochlear implantees was investigated for different overall levels of the signal, modulation depth and the carrier rate of the pulse train. Equally loud and threshold levels were determined for a variety of signal levels, modulation depths and carrier rates in six cochlear implantees. The pattern of results was consistent with the predictions of a previously published loudness model of McKay et al. (J Acoust Soc Am 113:2054–2063, 2003). The degree to which the loudness of modulated stimuli differed from the loudness elicited by an unmodulated pulse train with equivalent average current depended on the modulation depth and the absolute current level of the unmodulated stimulus. The effect of carrier rate on this measure was predictable solely from the effect of rate on absolute current level for equal loudness. The results have important implications for the interpretation of experiments measuring modulation detection that do not control loudness cues. We show that several previously published results regarding the effect of carrier rate and added noise on modulation detection could be reinterpreted in the light of these findings.

The use of frequency compression by cochlear implant recipients with postoperative acoustic hearing.

BACKGROUND The number of cochlear implant (CI) recipients who have usable acoustic hearing in at least one ear is continuing to grow. Many such CI users gain perceptual benefits from the simultaneous use of acoustic and electric hearing. In particular, it has been shown previously that use of an acoustic hearing aid (HA) with a CI can often improve speech understanding in noise. PURPOSE To determine whether the application of frequency compression in an HA would provide perceptual benefits to CI recipients with usable acoustic hearing, either when used in combination with the CI or when the HA was used by itself. RESEARCH DESIGN A repeated-measures experimental design was used to evaluate the effects on speech perception of using a CI either alone or simultaneously with an HA that had frequency compression either enabled or disabled. STUDY SAMPLE Eight adult CI recipients who were successful users of acoustic hearing aids in their nonimplanted ears participated as subjects. INTERVENTION The speech perception of each subject was assessed in seven conditions. These required each subject to listen with (1) their own HA alone; (2) the Phonak Naida HA with frequency compression (SoundRecover) enabled; (3) the Naida with SoundRecover disabled; (4) their CI alone; (5) their CI and their own HA; (6) their CI and the Naida with SoundRecover enabled; and (7) their CI and the Naida with SoundRecover disabled. Test sessions were scheduled over a period of about 10 wk. During part of that time, the subjects were asked to use the Phonak Naida HA with their CIs in place of their own HAs. DATA COLLECTION AND ANALYSIS The speech perception tests included measures of consonant identification from a closed set of 12 items presented in quiet, and measures of sentence understanding in babble noise. The speech materials were presented at an average level of 60 dB SPL from a loudspeaker. RESULTS Speech perception was better, on average, in all conditions that included use of the CI in comparison with any condition in which only an HA was used. For example, consonant recognition improved by approximately 50 percentage points, on average, between the HA-alone listening conditions and the CI-alone condition. There were no statistically significant score differences between conditions with SoundRecover enabled and disabled. There was a small but significant improvement in the average signal-to-noise ratio (SNR) required to understand 50% of the words in the sentences presented in noise when an HA was used simultaneously with the CI. CONCLUSIONS Although each of these CI users readily accepted the Phonak Naida HA with SoundRecover frequency compression, no benefits related specifically to the use of SoundRecover were found in the particular tests of speech understanding applied in this study. The relatively high levels of perceptual performance attained by these subjects with use of a CI by itself are consistent with the finding that the addition of an HA provided little further benefit. However, the use of an HA with the CI did provide better performance than the CI alone for understanding sentences in noise.

Interhemispheric EEG coherence is reduced in auditory cortical regions in schizophrenia patients with auditory hallucinations

Central auditory processing has been reported to be impaired in schizophrenia patients who experience auditory hallucinations, and interhemispheric transfer in auditory circuits may be compromised. In this study, we used EEG spectral coherence to examine interhemispheric connectivity between cortical areas known to be important in the processing of auditory information. Coherence was compared across three subject groups: schizophrenia patients with a recent history of auditory hallucinations (AH), schizophrenia patients who did not experience auditory hallucinations (nonAH), and healthy controls (HC). Subjects listened to pure tone and word stimuli while EEG was recorded continuously. Upper alpha and upper beta band coherence was calculated from six pairs of electrodes located over homologous auditory areas in the left and right cerebral hemispheres. Significant between-group differences were found on four electrode pairs (C3-C4, C5-C6, Ft7-Ft8 and Cp5-Cp6) in the upper alpha band. Relative to both the HC and nonAH groups, coherence was lower in the AH patients, consistent with the hypothesis that interhemispheric connectivity is reduced in these patients.

Interhemispheric transfer time in patients with auditory hallucinations: An auditory event-related potential study

Central auditory processing in schizophrenia patients with a history of auditory hallucinations has been reported to be impaired, and abnormalities of interhemispheric transfer have been implicated in these patients. This study examined interhemispheric functional connectivity between auditory cortical regions, using temporal information obtained from latency measures of the auditory N1 evoked potential. Interhemispheric Transfer Times (IHTTs) were compared across 3 subject groups: schizophrenia patients who had experienced auditory hallucinations, schizophrenia patients without a history of auditory hallucinations, and normal controls. Pure tones and single-syllable words were presented monaurally to each ear, while EEG was recorded continuously. IHTT was calculated for each stimulus type by comparing the latencies of the auditory N1 evoked potential recorded contralaterally and ipsilaterally to the ear of stimulation. The IHTTs for pure tones did not differ between groups. For word stimuli, the IHTT was significantly different across the 3 groups: the IHTT was close to zero in normal controls, was highest in the AH group, and was negative (shorter latencies ipsilaterally) in the nonAH group. Differences in IHTTs may be attributed to transcallosal dysfunction in the AH group, but altered or reversed cerebral lateralization in nonAH participants is also possible.

Central auditory processing deficits in patients with auditory hallucinations as shown by event-related potentials: preliminary results.

293 schizophrenia. Motor overfl ow, involuntary movement occurring during voluntary movement, is one such NSS found in schizophrenia. Method: Thirty-seven participants (19 with schizophrenia, 18 controls) were tested. Participants exerted 25% and 75% of their maximal force output while overfl ow was monitored in the passive hand. Three transcranial magnetic stimulation protocols were designed to investigate the cortical origin of motor overfl ow: 1) motor cortex was stimulated unilaterally at 140% RMT; MEPs were recorded bilaterally; 2) stimulation of ispilateral hemisphere at 140% RMT was performed during motor overfl ow. Resulting latencies between the cMEP onset and the iSP onset were compared; 3) facilitated MEPs produced (through stimulation of contralateral hemisphere) during voluntary contraction and facilitated MEPs produced during motor overfl ow were compared. All procedures were applied to both hemispheres. Results: Previous fi ndings of increased motor overfl ow in schizophrenia compared with controls were confi rmed (P > 0.05); neither group showed a signifi cant difference between MEPs facilitated during voluntary movement and those facilitated during motor overfl ow (P > 0.05). Conclusions: Results suggest that in both groups, motor overfl ow results from an imbalance between the transcallosal processes occurring during voluntary movement, leading to bilaterally active corticospinal tracts. Specifi c defi cits in cortical excitability are likely to be responsible for greater overfl ow seen in schizophrenia.

A dissociation of structure and function in the auditory cortex of patients with schizophrenia.

288 Methods: Cases were identifi ed based on their recorded contacts with community mental health teams between July 2003 and June 2005. The observed incidence was compared with recently published treated incidence rates through indirect standardization. The sensitivity of the EP fl ags for identifying cases was calculated. Service utilizati on, as measured by treatment days per quarter, was compared between EP clients and other groups. Results: There were 2475 cases identifi ed in 2004– 2005, which is 40% higher than expected. The EP fl ags failed to identify these cases (sensitivity 7%–39%). Psychosis clients had a signifi cantly higher level of service utilization: clients with a prior psychosis diagnosis had the highest mean treatment days (7.7), followed by fi rstepisode psychosis (6.5), bipolar/mania (5.7) and other diagnoses (3.5). These differences persisted after controlling for age and AHS (P < 0.001 for all contrasts). Conclusions: The incidence of fi rst-episode psychosis was higher than expected possibly because of inconsistencies in diagnostic recording in 2003–2004. The EP fl ags are underused and fail to identify clients with fi rst-episode psychosis, whose service utilizations differs from other clients.