Psychophysical Detection Thresholds Research Articles

Simultaneous measurement of intra-epidermal electric detection thresholds and evoked potentials for observation of nociceptive processing following sleep deprivation

Sleep deprivation has been shown to increase pain intensity and decrease pain thresholds in healthy subjects. In chronic pain patients, sleep impairment often worsens the perceived pain intensity. This increased pain perception is the result of altered nociceptive processing. We recently developed a method to quantify and monitor altered nociceptive processing by simultaneous tracking of psychophysical detection thresholds and recording of evoked cortical potentials during intra-epidermal electric stimulation. In this study, we assessed the sensitivity of nociceptive detection thresholds and evoked potentials to altered nociceptive processing after sleep deprivation in an exploratory study with 24 healthy male and 24 healthy female subjects. In each subject, we tracked nociceptive detection thresholds and recorded central evoked potentials in response to 180 single- and 180 double-pulse intra-epidermal electric stimuli. Results showed that the detection thresholds for single- and double-pulse stimuli and the average central evoked potential for single-pulse stimuli were significantly decreased after sleep deprivation. When analyzed separated by sex, these effects were only significant in the male population. Multivariate analysis showed that the decrease of central evoked potential was associated with a decrease of task-related evoked activity. Measurement repetition led to a decrease of the detection threshold to double-pulse stimuli in the mixed and the female population, but did not significantly affect any other outcome measures. These results suggest that simultaneous tracking of psychophysical detection thresholds and evoked potentials is a useful method to observe altered nociceptive processing after sleep deprivation, but is also sensitive to sex differences and measurement repetition.

Estimating health of the implanted cochlea using psychophysical strength-duration functions and electrode configuration

It is generally believed that the efficacy of cochlear implants is partly dependent on the condition of the stimulated neural population. Cochlear pathology is likely to affect the manner in which neurons respond to electrical stimulation, potentially resulting in differences in perception of electrical stimuli across cochlear implant recipients and across the electrode array in individual cochlear implant users. Several psychophysical and electrophysiological measures have been shown to predict cochlear health in animals and were used to assess conditions near individual stimulation sites in humans. In this study, we examined the relationship between psychophysical strength-duration functions and spiral ganglion neuron density in two groups of guinea pigs with cochlear implants who had minimally-overlapping cochlear health profiles. One group was implanted in a hearing ear (N = 10) and the other group was deafened by cochlear perfusion of neomycin, inoculated with an adeno-associated viral vector with an Ntf3-gene insert (AAV.Ntf3) and implanted (N = 14). Psychophysically measured strength-duration functions for both monopolar and tripolar electrode configurations were then compared for the two treatment groups. Results were also compared to their histological outcomes. Overall, there were considerable differences between the two treatment groups in terms of their psychophysical performance as well as the relation between their functional performance and histological data. Animals in the neomycin-deafened, neurotrophin-treated, and implanted group (NNI) exhibited steeper strength-duration function slopes; slopes were positively correlated with SGN density (steeper slopes in animals that had higher SGN densities). In comparison, the implanted hearing (IH) group had shallower slopes and there was no relation between slopes and spiral ganglion density. Across all animals, slopes were negatively correlated with ensemble spontaneous activity levels (shallower slopes with higher ensemble spontaneous activity levels). We hypothesize that differences in strength-duration function slopes between the two treatment groups were related to the condition of the inner hair cells, which generate spontaneous activity that could affect the across-fiber synchrony and/or the size of the population of neural elements responding to electrical stimulation. In addition, it is likely that spiral ganglion neuron peripheral processes were present in the IH group, which could affect membrane properties of the stimulated neurons. Results suggest that the two treatment groups exhibited distinct patterns of variation in conditions near the stimulating electrodes that altered detection thresholds. Overall, the results of this study suggest a complex relationship between psychophysical detection thresholds for cochlear implant stimulation and nerve survival in the implanted cochlea. This relationship seems to depend on the characteristics of the electrical stimulus, the electrode configuration, and other biological features of the implanted cochlea such as the condition of the inner hair cells and the peripheral processes.

Effects of electrical pulse polarity shape on intra cochlear neural responses in humans: Triphasic pulses with anodic and cathodic second phase

Modern cochlear implants employ charge-balanced biphasic and triphasic pulses. However, the effectiveness of electrical pulse shape and polarity is still a matter of debate. For this purpose, a previous study (Bahmer & Baumann, 2013) conducted electrophysiological and psychophysical measurements following triphasic pulse stimulation with constant cathodic second phase and varying anodic first and third phases. Pulse stimulation with constant anodic second phase was not investigated.Therefore, in this study, pulse stimulation with cathodic and anodic second phase was applied for the recording of electrically evoked compound action potentials (ECAPs) as well as for psychophysical thresholds in cochlear implant (CI) recipients. First it was investigated whether the temporal polarity distribution has a different effect on neuronal stimulation when the second phase is cathodic or anodic; second, whether the electrophysiological and psychophysical results show a comparable difference between triphasic stimulation with anodic and cathodic second phases.The results showed that variation of the temporal polarity distribution of the triphasic pulse had a smaller effect on the ECAP response when the second phase was anodic compared to when it was cathodic, whereas for psychophysical detection thresholds this variation had a similar effect for both polarities. While electrophysiological responses and psychophysical detection thresholds showed a high correlation for variations of the triphasic pulse with cathodic second phase, the results for variations of the triphasic pulse with anodic second phase showed only moderate correlation. Furthermore, the difference between triphasic stimulation with cathodic and anodic second phases did not correlate between the electrophysiological and psychophysical results.In summary, after stimulation with different configurations of triphasic pulses used in the present study, the polarity of the second phase has an effect on electrophysiological response at suprathreshold level but not on the psychophysical detection thresholds. Thus, at different stimulation levels a possible substitution of the psychophysical test by an electrophysiological measurement (e.g. neural health measurement of the cochlea) could not be corroborated by the present results.

A simple vector-like law for perceptual information combination is also followed by a class of cortical multisensory bimodal neurons

SummaryAn interdisciplinary approach to sensory information combination shows a correspondence between perceptual and neural measures of nonlinear multisensory integration. In psychophysics, sensory information combinations are often characterized by the Minkowski formula, but the neural substrates of many psychophysical multisensory interactions are unknown. We show that audiovisual interactions – for both psychophysical detection threshold data and cortical bimodal neurons – obey similar vector-like Minkowski models, suggesting that cortical bimodal neurons could underlie multisensory perceptual sensitivity. An alternative Bayesian model is not a good predictor of cortical bimodal response. In contrast to cortex, audiovisual data from superior colliculus resembles the ‘City-Block’ combination rule used in perceptual similarity metrics. Previous work found a simple power law amplification rule is followed for perceptual appearance measures and by cortical subthreshold multisensory neurons. The two most studied neural cell classes in cortical multisensory interactions may provide neural substrates for two important perceptual modes: appearance-based and performance-based perception.

Temporal integration of short-duration pulse trains in cochlear implant listeners: Psychophysical and electrophysiological measurements

While electrically-evoked auditory brainstem response (eABR) thresholds for low-rate pulse trains correlate well with behavioral thresholds measured at the same rate, the correlation is much weaker with behavioral thresholds measured at high rates, such as used clinically. This implies that eABRs to low-rate stimuli cannot be reliably used for objective programming of threshold levels in cochlear implant (CI) users. Here, we investigate whether the use of bunched-up pulses (BUPS), consisting of groups of closely-spaced pulses may be used as an alternative stimulus.Experiment 1 measured psychophysical detection thresholds for several stimuli having a period of 32 ms in nine CI subjects implanted with a Med-EL device. The stimuli differed in the number of pulses present in each period (from 1 to 32), the pulse rate within period (1000 pps and as high as possible for BUPS) and the electrode location (apical or basal). The correlation between psychophysical thresholds obtained for a high-rate (1000 pps) clinical stimulus and for the BUPS stimuli increased as the number of pulses per period of BUPS increased from 1 to 32. This first psychophysical experiment suggests that the temporal processes affecting the threshold of clinical stimuli are also present for BUPS.Experiment 2 measured eABRs on the apical electrode of eight CI subjects for BUPS having 1, 2, 4, 8, 16 or 32 pulses per period. For most subjects, wave V was visible for BUPS having up to 16 pulses per period. The latency of wave V at threshold increased as a function of the number of pulses per period, suggesting that the eABR reflects the integration of multiple pulses at such low levels or that the neural response to each individual pulse increases along the sequence due to facilitation processes. There was also a strong within-subject correlation between electrophysiological and behavioral thresholds for the different BUPS stimuli. This demonstrates that the drop in behavioral threshold obtained when increasing the number of pulses per period of the BUPS can be measured electrophysiologically using eABRs. In contrast, the across-subject correlation between eABR thresholds for BUPS and clinical thresholds remained relatively weak and did not increase with the number of pulses per period. Implications of the use of BUPS for objective programming of CIs are discussed.

Effects of Electrode Location on Estimates of Neural Health in Humans with Cochlear Implants.

There are a number of psychophysical and electrophysiological measures that are correlated with SGN density in animal models, and these same measures can be performed in humans with cochlear implants (CIs). Thus, these measures are potentially applicable in humans for estimating the condition of the neural population (so called "neural health" or "cochlear health") at individual sites along the electrode array and possibly adjusting the stimulation strategy in the CI sound processor accordingly. Some measures used to estimate neural health in animals have included the electrically evoked compound potential (ECAP), psychophysical detection thresholds, and multipulse integration (MPI). With regard to ECAP measures, it has been shown that the change in the ECAP response as a function of increasing the stimulus interphase gap ("IPG Effect") also reflects neural density in implanted animals. These animal studies have typically been conducted using preparations in which the electrode was in a fixed position with respect to the neural population, whereas in human cochlear implant users, the position of individual electrodes varies widely within an electrode array and also across subjects. The current study evaluated the effects of electrode location in the implanted cochlea (specifically medial-lateral location) on various electrophysiological and psychophysical measures in eleven human subjects. The results demonstrated that some measures of interest, specifically ECAP thresholds, psychophysical detection thresholds, and ECAP amplitude-growth function (AGF) linear slope, were significantly related to the distances between the electrode and mid-modiolar axis (MMA). These same measures were less strongly related or not significantly related to the electrode to medial wall (MW) distance. In contrast, neither the IPG Effect for the ECAP AGF slope or threshold, nor the MPI slopes were significantly related to MMA or MW distance from the electrodes. These results suggest that "within-channel" estimates of neural health such as the IPG Effect and MPI slope might be more suitable for estimating nerve condition in humans for clinical application since they appear to be relatively independent of electrode position.

Simultaneous tracking of psychophysical detection thresholds and evoked potentials to study nociceptive processing

Measuring altered nociceptive processing involved in chronic pain is difficult due to a lack of objective methods. Potential methods to characterize human nociceptive processing involve measuring neurophysiological activity and psychophysical responses to well-defined stimuli. To reliably measure neurophysiological activity in response to nociceptive stimulation using EEG, synchronized activation of nerve fibers and a large number of stimuli are required. On the other hand, to reliably measure psychophysical detection thresholds, selection of stimulus amplitudes around the detection threshold and many stimulus–response pairs are required. Combining the two techniques helps in quantifying the properties of nociceptive processing related to detected and non-detected stimuli around the detection threshold.The two techniques were combined in an experiment including 20 healthy participants to study the effect of intra-epidermal electrical stimulus properties (i.e. amplitude, single- or double-pulse and trial number) on the detection thresholds and vertex potentials. Generalized mixed regression and linear mixed regression were used to quantify the psychophysical detection probability and neurophysiological EEG responses, respectively.It was shown that the detection probability is significantly modulated by the stimulus amplitude, trial number, and the interaction between stimulus type and amplitude. Furthermore, EEG responses were significantly modulated by stimulus detection and trial number. Hence, we successfully demonstrated the possibility to simultaneously obtain information on psychophysical and neurophysiological properties of nociceptive processing. These results warrant further investigation of the potential of this method to observe altered nociceptive processing.

Effects of Contact Force and Vibration Frequency on Vibrotactile Sensitivity During Active Touch.

Humans require precise force control to execute fine manual tasks, which is generally facilitated to a great extent by providing adequate feedback. Currently, such dexterous manual tasks can be an input source of computing. To design appropriate vibrotactile stimuli for manual tasks, it is essential to quantify human vibrotactile sensitivity over a large range of contact forces. In this paper, we report the psychophysical detection thresholds for vibrotactile stimuli measured for five pressing forces that cover the range of forces encountered during ordinary manual tasks. The experimental results showed stark contrasts between stimulus frequencies, depending on actively exerted pressing force. The detection thresholds for 40Hz stimuli first increased and then decreased as the pressing force increased, but the detection threshold for 250Hz stimuli generally decreased as the force increased. These results have immediate consequences on the design of vibrotactile feedback for manual tasks in many applications of tangible interaction, tele-operation, and VR.

Visual Function at the Atrophic Border in Choroideremia Assessed with Adaptive Optics Microperimetry

Recent advances in retinal imaging allow visualization of structural abnormalities in retinal disease at the cellular level. This study used adaptive optics (AO) microperimetry to assess visual sensitivity with high spatial precision and to examine how function varies across 2 phenotypic features observed in choroideremia: atrophic lesion borders and outer retinal tubulations (ORTs). Cross-sectional study. Twelve choroideremia patients. A custom AO scanning light ophthalmoscope (AOSLO) equipped with both confocal and nonconfocalsplit-detection imaging methods was used to image the photoreceptor inner and outer segment mosaics. For AO microperimetry, circular 550-nm stimuli were presented through the AOSLO system; stimuli were either 9.6 or 38.3 arcmin2 (approximately 60 or 15 times smaller than a Goldman III stimulus). Test locations were identified in structural images and stimuli were targeted to these locations using real-time retinal tracking combined with measurements of transverse chromatic aberration. Psychophysical detection thresholds were measured at the atrophic border in 12 patients. Additionally, visual sensitivity was probed along ORTs in 4patients. Visual sensitivity thresholds measured with AO microperimetry at retinal locations corresponding to structural phenotypes observed on AOSLO retinal images. In choroideremia, sharp borders between intact central islands of the photoreceptor mosaic and complete atrophy of the outer retina and retinal pigment epithelium were observed in both split-detection and confocal structural images. Adaptive optics microperimetry at locations spanning these borders showed a commensurately sharp decrease in function, with readily measurable visual sensitivity on one side and dense scotoma on the other. These functional transitions often occurred over a distance smaller than the diameter of the Goldman III stimulus. Thresholds measured along ORTs showed dense scotoma over the tubule in all 4 participants, despite the visibility of remnant cone inner segments on the AO images. Choroideremia patients exhibited sharp functional transitions that collocated with structural transitions from intact to severely degenerated retina. We found no evidence of visual sensitivity over ORTs. Measuring cone function with high resolution offered insight into disease mechanisms and may enable precise assessment of whether experimental therapies, such as gene therapy, provide a functional benefit.

Sensitivity to S-Cone Stimuli and the Development of Myopia.

PurposeLongitudinal chromatic aberration (LCA) is a color signal available to the emmetropization process that causes greater myopic defocus of short wavelengths than long wavelengths. We measured individual differences in chromatic sensitivity to explore the role LCA may play in the development of refractive error.MethodsForty-four observers were tested psychophysically after passing color screening tests and a questionnaire for visual defects. Refraction was measured and only subjects with myopia or hyperopia without severe astigmatism participated. Psychophysical detection thresholds for 3 cyc/deg achromatic, L-, M-, and S-cone–isolating Gabor patches and low-frequency S-cone increment (S+) and decrement (S−) blobs were measured. Parametric Pearson correlations for refractive error versus threshold were calculated and nonparametric bootstrap 95% percentage confidence intervals (BCIs) for r were computed.ResultsS-cone Gabor detection thresholds were higher than achromatic, L-, and M-cone Gabors. S-cone Gabor thresholds were higher than either S+ or S− blobs. These results are consistent with studies using smaller samples of practiced observers. None of the thresholds for the Gabor stimuli were correlated with refractive error (RE). A negative correlation with RE was observed for both S+ (r = −0.28; P = 0.06; BCI: r = −0.5, −0.04) and S− (r = −0.23; P = 0.13; BCI = −0.46, 0.01) blobs, although this relationship did not reach conventional statistical significance.ConclusionsThresholds for S+ and S− stimuli were negatively related to RE, indicating that myopes may have reduced sensitivity to low spatial frequency S-cone stimuli. This reduced S-cone sensitivity might have played a role in their failure to emmetropize normally.

The Effect of Locomotion on Early Visual Contrast Processing in Humans.

Most of our knowledge about vision comes from experiments in which stimuli are presented to immobile human subjects or animals. In the case of human subjects, movement during psychophysical, electrophysiological, or neuroimaging experiments is considered to be a source of noise to be eliminated. Animals used in visual neuroscience experiments are typically restrained and, in many cases, anesthetized. In reality, however, vision is often used to guide the motion of awake, ambulating organisms. Recent work in mice has shown that locomotion elevates visual neuronal response amplitudes (Niell and Stryker, 2010; Erisken et al., 2014; Fu et al., 2014; Lee et al., 2014; Mineault et al., 2016) and reduces long-range gain control (Ayaz et al., 2013). Here, we used both psychophysics and steady-state electrophysiology to investigate whether similar effects of locomotion on early visual processing can be measured in humans. Our psychophysical results show that brisk walking has little effect on subjects' ability to detect briefly presented contrast changes and that co-oriented flankers are, if anything, more effective masks when subjects are walking. Our electrophysiological data were consistent with the psychophysics indicating no increase in stimulus-driven neuronal responses while walking and no reduction in surround suppression. In summary, we have found evidence that early contrast processing is altered by locomotion in humans but in a manner that differs from that reported in mice. The effects of locomotion on very low-level visual processing may differ on a species-by-species basis and may reflect important differences in the levels of arousal associated with locomotion.SIGNIFICANCE STATEMENT Mice are the current model of choice for studying low-level visual processing. Recent studies have shown that mouse visual cortex is modulated by behavioral state: primary visual cortex neurons in locomoting mice tend to be more sensitive and less influenced by long-range gain control. Here, we tested these effects in humans by measuring psychophysical detection thresholds and electroencephalography (EEG) responses while subjects walked on a treadmill. We found no evidence of increased contrast sensitivity or reduced surround suppression in walking humans. Our data show that fundamental measurements of early visual processing differ between humans and mice and this has important implications for recent work on the links among arousal, behavior, and vision in these two species.

Spatiochromatic Interactions between Individual Cone Photoreceptors in the Human Retina.

A remarkable feature of human vision is that the retina and brain have evolved circuitry to extract useful spatial and spectral information from signals originating in a photoreceptor mosaic with trichromatic constituents that vary widely in their relative numbers and local spatial configurations. A critical early transformation applied to cone signals is horizontal-cell-mediated lateral inhibition, which imparts a spatially antagonistic surround to individual cone receptive fields, a signature inherited by downstream neurons and implicated in color signaling. In the peripheral retina, the functional connectivity of cone inputs to the circuitry that mediates lateral inhibition is not cone-type specific, but whether these wiring schemes are maintained closer to the fovea remains unsettled, in part because central retinal anatomy is not easily amenable to direct physiological assessment. Here, we demonstrate how the precise topography of the long (L)-, middle (M)-, and short (S)-wavelength-sensitive cones in the human parafovea (1.5° eccentricity) shapes perceptual sensitivity. We used adaptive optics microstimulation to measure psychophysical detection thresholds from individual cones with spectral types that had been classified independently by absorptance imaging. Measured against chromatic adapting backgrounds, the sensitivities of L and M cones were, on average, receptor-type specific, but individual cone thresholds varied systematically with the number of preferentially activated cones in the immediate neighborhood. The spatial and spectral patterns of these interactions suggest that interneurons mediating lateral inhibition in the central retina, likely horizontal cells, establish functional connections with L and M cones indiscriminately, implying that the cone-selective circuitry supporting red-green color vision emerges after the first retinal synapse.SIGNIFICANCE STATEMENT We present evidence for spatially antagonistic interactions between individual, spectrally typed cones in the central retina of human observers using adaptive optics. Using chromatic adapting fields to modulate the relative steady-state activity of long (L)- and middle (M)-wavelength-sensitive cones, we found that single-cone detection thresholds varied predictably with the spectral demographics of the surrounding cones. The spatial scale and spectral pattern of these photoreceptor interactions were consistent with lateral inhibition mediated by retinal horizontal cells that receive nonselective input from L and M cones. These results demonstrate a clear link between the neural architecture of the visual system inputs-cone photoreceptors-and visual perception and have implications for the neural locus of the cone-specific circuitry supporting color vision.

Effect of Postural Control Biomechanical Gain on Psychophysical Detection Thresholds in Anterior Horizontal Translation of Standing Blindfolded Subjects

Effect of Postural Control Biomechanical Gain on Psychophysical Detection Thresholds in Anterior Horizontal Translation of Standing Blindfolded Subjects

Monopolar Detection Thresholds Predict Spatial Selectivity of Neural Excitation in Cochlear Implants: Implications for Speech Recognition.

The objectives of the study were to (1) investigate the potential of using monopolar psychophysical detection thresholds for estimating spatial selectivity of neural excitation with cochlear implants and to (2) examine the effect of site removal on speech recognition based on the threshold measure. Detection thresholds were measured in Cochlear Nucleus® device users using monopolar stimulation for pulse trains that were of (a) low rate and long duration, (b) high rate and short duration, and (c) high rate and long duration. Spatial selectivity of neural excitation was estimated by a forward-masking paradigm, where the probe threshold elevation in the presence of a forward masker was measured as a function of masker-probe separation. The strength of the correlation between the monopolar thresholds and the slopes of the masking patterns systematically reduced as neural response of the threshold stimulus involved interpulse interactions (refractoriness and sub-threshold adaptation), and spike-rate adaptation. Detection threshold for the low-rate stimulus most strongly correlated with the spread of forward masking patterns and the correlation reduced for long and high rate pulse trains. The low-rate thresholds were then measured for all electrodes across the array for each subject. Subsequently, speech recognition was tested with experimental maps that deactivated five stimulation sites with the highest thresholds and five randomly chosen ones. Performance with deactivating the high-threshold sites was better than performance with the subjects’ clinical map used every day with all electrodes active, in both quiet and background noise. Performance with random deactivation was on average poorer than that with the clinical map but the difference was not significant. These results suggested that the monopolar low-rate thresholds are related to the spatial neural excitation patterns in cochlear implant users and can be used to select sites for more optimal speech recognition performance.

Evaluating multipulse integration as a neural-health correlate in human cochlear-implant users: Relationship to spatial selectivity.

The decrease of psychophysical detection thresholds as a function of pulse rate for a fixed-duration electrical pulse train is referred to as multipulse integration (MPI). The MPI slopes correlate with anatomical and physiological indices of cochlear health in guinea pigs with cochlear implants. The aim of the current study was to assess whether the MPI slopes were related to the spatial spread of activation by electrical stimulation. The hypothesis was that MPI is dependent on the total number of excitable neurons at the stimulation site, with broader neural excitation producing a steeper threshold decrease as a function of stimulation rate. MPI functions were measured at all stimulation sites in 22-site electrode arrays in human subjects. Some sites with steep MPI functions and other sites with shallow functions were assessed for spatial spread of excitation at 900 pps using a forward-masking paradigm. The results showed a correlation between the slopes of the forward-masking functions and the steepness of MPI, with broader stimulation predicting greater integration. The results are consistent with the idea that integration of multiple pulses in a pulse train relies on the number of excitable neurons at the stimulation site.

The trade-off relationship between spatial tuning and multipulse integration in human subjects with cochlear implants

Guinea pigs studies have shown a correlation between the slope of the function relating psychophysical detection thresholds to pulse rate (known as multipulse integration) and the spiral ganglion cell density in electrical stimulation. If this relationship holds true in human subjects with cochlear implants, we hypothesize that multipulse integration should reduce (i.e., become shallow sloped) at stimulation sites with sharp tuning or when electrode configuration changes from broad (monopolar) to focused stimulation (bipolar). Multipulse integration was measured at all stimulation sites using two pulse rates with phase duration of 75 us in a monopolar stimulation mode (MP1 + 2) and bipolar stimulation mode (BP + 0). At selected stimulation sites, a psychophysical spatial tuning curve was also measured using a forward-masking paradigm. The multipulse integration slopes were steeper with MP electrode configuration than with BP. In MP mode, the multipulse integration slopes were steeper at stimulation sites ...

Normalization in human somatosensory cortex.

Functional magnetic resonance imaging (fMRI) was used to measure activity in human somatosensory cortex and to test for cross-digit suppression. Subjects received stimulation (vibration of varying amplitudes) to the right thumb (target) with or without concurrent stimulation of the right middle finger (mask). Subjects were less sensitive to target stimulation (psychophysical detection thresholds were higher) when target and mask digits were stimulated concurrently compared with when the target was stimulated in isolation. fMRI voxels in a region of the left postcentral gyrus each responded when either digit was stimulated. A regression model (called a forward model) was used to separate the fMRI measurements from these voxels into two hypothetical channels, each of which responded selectively to only one of the two digits. For the channel tuned to the target digit, responses in the left postcentral gyrus increased with target stimulus amplitude but were suppressed by concurrent stimulation to the mask digit, evident as a shift in the gain of the response functions. For the channel tuned to the mask digit, a constant baseline response was evoked for all target amplitudes when the mask was absent and responses decreased with increasing target amplitude when the mask was concurrently presented. A computational model based on divisive normalization provided a good fit to the measurements for both mask-absent and target + mask stimulation. We conclude that the normalization model can explain cross-digit suppression in human somatosensory cortex, supporting the hypothesis that normalization is a canonical neural computation.

Insertion trauma and recovery of function after cochlear implantation: Evidence from objective functional measures

Partial loss and subsequent recovery of cochlear implant function in the first few weeks following cochlear implant surgery has been observed in previous studies using psychophysical detection thresholds. In the current study, we explored this putative manifestation of insertion trauma using objective functional measures: electrically-evoked compound action potential (ECAP) amplitude-growth functions (ECAP amplitude as a function of stimulus level). In guinea pigs implanted in a hearing ear with good post-implant hearing and good spiral ganglion neuron (SGN) survival, consistent patterns of ECAP functions were observed. The slopes of ECAP growth functions were moderately steep on the day of implant insertion, decreased to low levels over the first few days after implantation and then increased slowly over several weeks to reach a relatively stable level. In parallel, ECAP thresholds increased over time after implantation and then recovered, although more quickly, to a relatively stable low level as did thresholds for eliciting a facial twitch. Similar results were obtained in animals deafened but treated with an adenovirus with a neurotrophin gene insert that resulted in good SGN preservation. In contrast, in animals implanted in deaf ears that had relatively poor SGN survival, ECAP slopes reached low levels within a few days after implantation and remained low. These results are consistent with the idea that steep ECAP growth functions require a healthy population of auditory nerve fibers and that cochlear implant insertion trauma can temporarily impair the function of a healthy SGN population.This article is part of a Special Issue entitled <IEB Kyoto>.

Central Auditory Processing of Temporal and Spectral-Variance Cues in Cochlear Implant Listeners.

Cochlear implant (CI) listeners have difficulty understanding speech in complex listening environments. This deficit is thought to be largely due to peripheral encoding problems arising from current spread, which results in wide peripheral filters. In normal hearing (NH) listeners, central processing contributes to segregation of speech from competing sounds. We tested the hypothesis that basic central processing abilities are retained in post-lingually deaf CI listeners, but processing is hampered by degraded input from the periphery. In eight CI listeners, we measured auditory nerve compound action potentials to characterize peripheral filters. Then, we measured psychophysical detection thresholds in the presence of multi-electrode maskers placed either inside (peripheral masking) or outside (central masking) the peripheral filter. This was intended to distinguish peripheral from central contributions to signal detection. Introduction of temporal asynchrony between the signal and masker improved signal detection in both peripheral and central masking conditions for all CI listeners. Randomly varying components of the masker created spectral-variance cues, which seemed to benefit only two out of eight CI listeners. Contrastingly, the spectral-variance cues improved signal detection in all five NH listeners who listened to our CI simulation. Together these results indicate that widened peripheral filters significantly hamper central processing of spectral-variance cues but not of temporal cues in post-lingually deaf CI listeners. As indicated by two CI listeners in our study, however, post-lingually deaf CI listeners may retain some central processing abilities similar to NH listeners.

The neural substrate for binaural masking level differences in the auditory cortex.

The binaural masking level difference (BMLD) is a phenomenon whereby a signal that is identical at each ear (S0), masked by a noise that is identical at each ear (N0), can be made 12-15 dB more detectable by inverting the waveform of either the tone or noise at one ear (Sπ, Nπ). Single-cell responses to BMLD stimuli were measured in the primary auditory cortex of urethane-anesthetized guinea pigs. Firing rate was measured as a function of signal level of a 500 Hz pure tone masked by low-passed white noise. Responses were similar to those reported in the inferior colliculus. At low signal levels, the response was dominated by the masker. At higher signal levels, firing rate either increased or decreased. Detection thresholds for each neuron were determined using signal detection theory. Few neurons yielded measurable detection thresholds for all stimulus conditions, with a wide range in thresholds. However, across the entire population, the lowest thresholds were consistent with human psychophysical BMLDs. As in the inferior colliculus, the shape of the firing-rate versus signal-level functions depended on the neurons' selectivity for interaural time difference. Our results suggest that, in cortex, BMLD signals are detected from increases or decreases in the firing rate, consistent with predictions of cross-correlation models of binaural processing and that the psychophysical detection threshold is based on the lowest neural thresholds across the population.