Decoding the dynamic representation of musical pitch from human brain activity.

In music, the perception of pitch is governed largely by its tonal function given the preceding harmonic structure of the music. While behavioral research has advanced our understanding of the perceptual representation of musical pitch, relatively little is known about its representational structure in the brain. Using Magnetoencephalography (MEG), we recorded evoked neural responses to different tones presented within a tonal context. Multivariate Pattern Analysis (MVPA) was applied to "decode" the stimulus that listeners heard based on the underlying neural activity. We then characterized the structure of the brain's representation using decoding accuracy as a proxy for representational distance, and compared this structure to several well established perceptual and acoustic models. The observed neural representation was best accounted for by a model based on the Standard Tonal Hierarchy, whereby differences in the neural encoding of musical pitches correspond to their differences in perceived stability. By confirming that perceptual differences honor those in the underlying neuronal population coding, our results provide a crucial link in understanding the cognitive foundations of musical pitch across psychological and neural domains.

An ideal-observer model of human sound localization.

In recent years, a great deal of research within the field of sound localization has been aimed at finding the acoustic cues that human listeners use to localize sounds and understanding the mechanisms by which they process these cues. In this paper, we propose a complementary approach by constructing an ideal-observer model, by which we mean a model that performs optimal information processing within a Bayesian context. The model considers all available spatial information contained within the acoustic signals encoded by each ear. Parameters for the optimal Bayesian model are determined based on psychoacoustic discrimination experiments on interaural time difference and sound intensity. Without regard as to how the human auditory system actually processes information, we examine the best possible localization performance that could be achieved based only on analysis of the input information, given the constraints of the normal auditory system. We show that the model performance is generally in good agreement with the actual human localization performance, as assessed in a meta-analysis of many localization experiments (Best et al. in Principles and applications of spatial hearing, pp 14-23. World Scientific Publishing, Singapore, 2011). We believe this approach can shed new light on the optimality (or otherwise) of human sound localization, especially with regard to the level of uncertainty in the input information. Moreover, the proposed model allows one to study the relative importance of various (combinations of) acoustic cues for spatial localization and enables a prediction of which cues are most informative and therefore likely to be used by humans in various circumstances.

Neck abscess and vocal cord paresis: delayed complications of a self-extruded long fishbone stuck in throat.

A 57-year-old Caucasian man, otherwise fit and well, presented with a 2-week history of dysphagia, odynophagia. Two weeks prior to the presentation, he had felt a fishbone stuck in his throat which was self-extruded after 3 days. Subsequently he developed a right anterior neck swelling and hoarseness. Transnasal endoscopic examination of larynx revealed an injected and oedematous right hemilarynx with right vocal cord paresis. An ultrasound examination of the neck confirmed a collection in the neck on the right side, and frank pus was aspirated from the neck abscess and he responded well to conservative management. Subsequent examination in follow-up had shown complete recovery of vocal cord movement. The patient did not seek medical attention immediately after getting a 5 cm fishbone extruded from the throat which resulted in significant morbidity. All patients should be alerted to the possibility of delayed complications and they should be encouraged to seek urgent medical attention.

Systematic distortions of auditory space perception following prolonged exposure to broadband noise.

Perceptual distortions referred to as aftereffects may arise following exposure to an adapting sensory stimulus. The study of aftereffects has a long and distinguished history [Kohler and Wallach, Proc. Am. Philos. Soc. 88, 269-359 (1944)] and a range of aftereffects have been well described in sensory modalities such as the visual system [Barlow, in Vision: Coding and Efficiency (Cambridge University Press, Cambridge, 1990)]. In the visual system these effects have been interpreted as evidence for a population of cells or channels specific for certain features of a stimulus. However there has been relatively little work examining auditory aftereffects, particularly in respect of spatial location. In this study we have examined the effects of a stationary adapting noise stimulus on the subsequent auditory localization in the vicinity of the adapting stimulus. All human subjects in this study were trained to localize short bursts of noise in a darkened anechoic environment. Adaptation was achieved by presenting 4 min of continuous noise at the start of each block of trials and was maintained by a further 15-s noise burst between each trial. The adapting stimulus was located either directly in front of the subject or 30 degrees to the right of the midline. Subjects were required to determine the location of noise burst stimuli (150 ms) in the proximity of the adapting stimulus following each interstimulus period of adaptation. Results demonstrated that following adaptation there was a general radial displacement of perceived sound sources away from the location of the adapting stimulus. These data are more consistent with a channel-based or place-based process of sound localization rather than a simple level-based adaptation model. A simple "distribution shift" model that assumes an array of overlapping spatial channels is advanced to explain the psychophysical data.

Neural system identification model of human sound localization.

This paper examines the role of biological constraints in the human auditory localization process. A psychophysical and neural system modeling approach was undertaken in which performance comparisons between competing models and a human subject explore the relevant biologically plausible "realism constraints." The directional acoustical cues, upon which sound localization is based, were derived from the human subject's head-related transfer functions (HRTFs). Sound stimuli were generated by convolving bandpass noise with the HRTFs and were presented to both the subject and the model. The input stimuli to the model were processed using the Auditory Image Model of cochlear processing. The cochlear data were then analyzed by a time-delay neural network which integrated temporal and spectral information to determine the spatial location of the sound source. The combined cochlear model and neural network provided a system model of the sound localization process. Aspects of humanlike localization performance were qualitatively achieved for broadband and bandpass stimuli when the model architecture incorporated frequency division (i.e., the progressive integration of information across the different frequency channels) and was trained using variable bandwidth and center-frequency sounds. Results indicate that both issues are relevant to human sound localization performance.

A performance adequate computational model for auditory localization.

A computational model of auditory localization resulting in performance similar to humans is reported. The model incorporates both the monaural and binaural cues available to a human for sound localization. Essential elements used in the simulation of the processes of auditory cue generation and encoding by the nervous system include measured head-related transfer functions (HRTFs), minimum audible field (MAF), and the Patterson-Holdsworth cochlear model. A two-layer feed-forward back-propagation artificial neural network (ANN) was trained to transform the localization cues to a two-dimensional map that gives the direction of the sound source. The model results were compared with (i) the localization performance of the human listener who provided the HRTFs for the model and (ii) the localization performance of a group of 19 other human listeners. The localization accuracy and front-back confusion error rates exhibited by the model were similar to both the single listener and the group results. This suggests that the simulation of the cue generation and extraction processes as well as the model parameters were reasonable approximations to the overall biological processes. The amplitude resolution of the monaural spectral cues was varied and the influence on the model's performance was determined. The model with 128 cochlear channels required an amplitude resolution of approximately 20 discrete levels for encoding the spectral cue to deliver similar localization performance to the group of human listeners.

The localisation of spectrally restricted sounds by human listeners.

The two principal binaural cues to sound location are interaural time differences (ITDs), which are thought to be dominant at low frequencies, and interaural level differences (ILDs), which are thought to dominate at mid to high frequencies. The outer ear also filters the sound in a location dependent manner and provides spectral cues to location. In these experiments we have examined the relative contribution of these cues to the auditory localisation performance by humans. Six subjects localised sounds by pointing their face toward the perceived location of stimuli presented in complete darkness in an anechoic chamber. Control stimuli were spectrally flat (400 Hz to 16 kHz), while the relative contribution of location cues in the low frequency channels was determined using noise high passed at 2 kHz and in the high frequency channels using stimuli low passed at 2 kHz. The removal of frequencies below 2 kHz had little effect on either the pattern of systematic errors or the distribution of localisation estimates with the exception of an increase in the size of the standard deviations associated with a few rear locations. This suggests considerable redundancy in the auditory localisation information contained within a broadband sound. In contrast, restricting the target spectrum to frequencies below 2 kHz resulted in a large increase in the cone-of-confusion errors as well as a subject dependent biasing of the front-to-back or back-to-front confusions. These biases and the reduction in localisation accuracy for high pass stimuli at some posterior locations are consistent with a contribution of spectral information at low frequencies.

Human localisation of band-pass filtered noise.

In this work we study the influence and relationship of five different acoustical cues to the human sound localisation process. These cues are: interaural time delay, interaural level difference, interaural spectrum, monaural spectrum, and band-edge spectral contrast. Of particular interest was the synthesis and integration of the different cues to produce a coherent and robust percept of spatial location. The relative weighting and role of the different cues was investigated using band-pass filtered white noise with a frequency range (in kHz) of: 0.3-5, 0.3-7, 0.3-10, 0.3-14, 3-8, 4-9, and 7-14. These stimuli provided varying amounts of spectral information and physiologically detectable temporal information, thus probing the localisation process under varying sound conditions. Three subjects with normal hearing in both ears have performed five trials of 76 test positions for each of these stimuli in an anechoic room. All subjects showed systematic mislocalisation on most of these stimuli. The location to which they are mislocalised varies among subjects but in a systematic manner related to the five different acoustical cues. These cues have been correlated with the subject's localisation responses on an individual basis with the results suggesting that the internal weighting of the spectral cues may vary with the sound condition.

Medical problem based learning supported by intranet technology: a natural student centred approach.

In response to the explosion in medical information, there have been considerable recent changes in medical curriculum development. The move to problem based learning (PBL) is, in part, a result of these changes. The Faculty of Medicine at the University of Sydney has exploited a WWW based intranet for the development, delivery, management and evaluation of it's problem based, graduate medical program (GMP). This system has been employed to develop the 72 medical problems that contribute to the first two years of the GMP. The activities of more than 400 members of the faculty have been coordinated using the intranet to develop the wide range of resources to support learning in the program. Daily management of the curriculum is also enabled using Web site posting of bulletins, e-mail and ongoing development of technology training. Coupled with the PBL problems is a formative assessment system that provides questions and feedback that cover the whole range of learning topics. Part of the student and staff evaluation is supported both informally and formally through the use of a 'Feedback' button on each web page and web delivered structured formal evaluations, respectively.

Methods for spherical data analysis and visualization.

A systematic analysis of the localization of objects in extra-personal space requires a three-dimensional method of documenting location. In auditory localization studies the location of a sound source is often reduced to a directional vector with constant magnitude with respect to the observer, data being plotted on a unit sphere with the observer at the origin. This is an attractive form of data representation as the relevant spherical statistical and graphical methods are well described. In this paper we collect together a set of spherical plotting and statistical procedures to visualize and summarize these data. We describe methods for visualizing auditory localization data without assuming that the principal components of the data are aligned with the coordinate system. As a means of comparing experimental techniques and having a common set of data for the verification of spherical statistics, the software (implemented in MATLAB) and database described in this paper have been placed in the public domain. Although originally intended for the visualization and summarization of auditory psychophysical data, these routines are sufficiently general to be applied in other situations involving spherical data.

Healthcare and the information age: implications for medical education.

The information age, combining rapidly developing information technology and massive growth in biomedical and clinical data, is placing special demands on healthcare workers. Further, radical changes in access to information in our society are affecting the doctor-patient relationship. These changes necessitate a new approach to primary and continuing medical education. A number of imperatives for medical education are identified and some practical changes to a medical curriculum are described.

Altered spectral localization cues disrupt the development of the auditory space map in the superior colliculus of the ferret.

Spectral localization cues provided by the outer ear are utilized in the construction of the auditory space map in the superior colliculus (SC). The role of the outer ear in the development of this map was examined by recording from the SC of anesthetized, adult ferrets in which the pinna and concha had been removed in infancy. The acoustical consequences of this procedure were assessed by recording outer ear impulse responses via a probe-tube microphone implanted in the wall of the ear canal. Both monaural and binaural spectral cues normally show a number of asymmetric features within the horizontal plane, which allow azimuthal locations on either side of the interaural axis to be discriminated. These features were eliminated or altered by chronic pinnectomy. The responses of auditory units in the SC to noise bursts presented in the free field were examined at sound levels of approximately 10 and 25 dB above unit threshold. After bilateral pinnectomy, the representation of auditory space was severely degraded at both sound levels. In contrast to normal ferrets, many units had bilobed azimuthal response profiles, indicating that they were unable to resolve sound locations on either side of the interaural axis. There was also much less order in the distribution of best azimuths or elevations of those units that were tuned to a single direction. Some units were tuned to locations that extended much further into the hemifield ipsilateral to the recording side than the normal range of best azimuths. Unilateral removal of the outer ear, which disrupts the monaural spectral cues for one side only, had a much smaller effect on the development of the auditory representation. At supra- and near-threshold sound levels, the representation of sound azimuth in the SC on both sides of the brain was less scattered than that found after bilateral pinna removal. Nevertheless, units with bilobed responses, broader tuning, and inappropriate best azimuths were observed in both the left and right SC of ferrets in which the left pinna and concha had been removed in infancy. These data illustrate that the localization cues provided by the outer ear play a critical role in the development of the auditory space map in the SC. In contrast to other manipulations of either auditory or visual inputs, the map does not appear to adapt to the changes in spectral cues brought about by pinna removal, suggesting that residual binaural cues are, by themselves, insufficient for its normal maturation.

The nature and distribution of errors in sound localization by human listeners.

Measurement of localization performance will reflect errors that relate to the sensory processing of the cues to sound location and the errors associated with the method by which the subject indicates the perceived location. This study has measured the ability of human subjects to localize a short noise burst presented in the free field with the subject indicating the perceived location by pointing their nose towards the source. Subjects were first trained using a closed loop training paradigm which involved instantaneous feedback as to the accuracy of head pointing which resulted in the reduction of residual localization errors and a rapid acquisition of the task by the subjects. Once trained, 19 subjects localized between 4 and 6 blocks of 76 target locations. The data were pooled and the distribution of errors associated with each target location was examined using spherical methods. Errors in the localization estimates for about one third of the locations were rotationally symmetrical about their mean but the remaining locations were best described by an elliptical distribution (Kent distributed). For about one half of the latter locations the orientations of the directions of the greatest variance of the distributions were not aligned with the azimuth and elevation coordinates used for describing the spatial location of the targets. The accuracy (systematic errors) and the distribution of the errors (variance) in localization for our population of subjects were also examined for each test location. The size of the data set and the methods of analysis provide very reliable measures of important baseline parameters of human auditory localization.

Masking produced by broadband noise presented in virtual auditory space.

An attempt has been made to relate the masking effects studied under dichotic listening conditions to masking seen in the free field. Rather than use a free-field masking paradigm combined with monaural and binaural listening conditions, broadband maskers presented in virtual auditory space (VAS) have been used. Two virtual locations were tested: One was the right interaural axis (+90 degrees from the anterior midline) and the other was 40 degrees right of the anterior midline. Narrow-band (critical bandwidth) dichotic and diotic maskers were also derived from the VAS masker by bandpass filtering around the test frequency. This procedure preserved the interaural differences within the critical band about the test frequency but removed information outside the critical band. Using a diotic target tone of 0.6 kHz with a narrow-band masker centered on 0.6 kHz there was an increase in signal detection in the dichotic conditions when compared to that attributable to either ear alone. Furthermore, there was no further advantage in signal detection at this target frequency when a broadband VAS masker was used. This suggests that for low-frequency targets, the binaural differences within the critical band about the target frequency are sufficient for effective unmasking. In contrast, for a target frequency of 4 kHz, a dichotic narrow-band masker resulted in a reduction in detection compared to that attributable to either ear. However, detection improved to the level attributable to the far ear when a broadband VAS masker was used. This suggests that information outside the critical band is involved in the unmasking of high-frequency targets.

The development of topographically-aligned maps of visual and auditory space in the superior colliculus.

The role of the superior colliculus in attending and orienting to sensory stimuli is facilitated by the presence within this midbrain nucleus of superimposed maps of different sensory modalities. We have studied the steps involved in the development of topographically-aligned maps of visual and auditory space in the ferret superior colliculus. Injections of fluorescent beads into the superficial layers showed that the projection from the contralateral retina displays topographic order on the day of birth (PO). Recordings made from these layers at the time of eye opening, approximately 1 month later, revealed the presence of an adult-like map of visual space. In contrast, the auditory space map in the deeper layers emerged gradually over a much longer period of postnatal life. In adult ferrets in which one eye had been deviated laterally just before eye opening, the auditory spatial tuning of single units recorded in the contralateral superior colliculus was shifted by a corresponding amount, so that the registration of the visual and auditory maps was maintained. Chronic application of the NMDA-receptor antagonist MK801 disrupted the normal development of the auditory space map, but had no effect on the visual map in either juvenile or adult animals, or on the auditory map once it had matured. These findings indicate that visual cues may play an instructive role, possibly via a Hebbian mechanism of synaptic plasticity, in the development of appropriately tuned auditory responses, thereby ensuring that the neural representations of both modalities share the same coordinates. Changes observed in the auditory representation following partial lesions of the superficial layers at PO suggest that these layers may provide the source of the visual signals responsible for experience-induced plasticity in auditory spatial tuning.

Responses of neurons in the ferret superior colliculus to the spatial location of tonal stimuli.

Using multi-unit recordings, we compared the azimuthal spatial selectivity of auditory neurons in the deep layers of the ferret superior colliculus (SC) to broadband and tonal stimuli. Responses to noise were tuned at different sound levels to a single location, which varied topographically along the rostrocaudal axis of the nucleus to form a map of sound azimuth. Frequency response profiles tended to be multi-peaked, so the spatial tuning was examined at two or more frequencies in each case. Some of the azimuthal response profiles obtained with tonal stimuli were bilobed, as expected from the spatially ambiguous cues available at individual frequencies, although the rest were tuned to a single region of space. The preferred sound directions usually varied with the frequency used, and the range of auditory best positions at each recording site was significantly greater with tones than with noise. Comparison with the acoustical properties of the auditory periphery suggested that the near-threshold positional selectivity of many of the tonal responses may be determined by the monaural directionality of the outer ear. When the sound level was raised by 20 dB so that both ears were stimulated at all speaker locations, the range of tonal best positions obtained at each frequency increased and some of the units responded best to pure tones located in the ipsilateral hemifield. The lack of topographic order in the distribution of tonal spatial selectivity along the rostrocaudal axis of the SC indicates the need for a broadband input, incorporating the spectral localization cues provided by the outer ear, in the construction of a neural map of auditory space.

Measuring the human head-related transfer functions: a novel method for the construction and calibration of a miniature "in-ear" recording system.

A method for the construction of a small "in-ear" system for recording the human-free field-to-eardrum and headphone-to-eardrum transfer functions is described. Customized in-ear inserts were obtained by a simple ear printing and electroplating method, resulting in a thin (< 0.25 mm) outer shell that minimized obstruction of the entrance at the ear canal. The insert can be used to position a microphone probe tube deep within the auditory canal. The effects of this recording system on the sound field in the ear canal were calibrated using a model head equipped with a second internal microphone close to the eardrum. Transfer functions were recorded for 343 different stimulus locations in free space and for a headphone sound source. For the free-field stimuli the presence of the recording system resulted in a small attenuation with maximum effects around 3.5 and 12.5 kHz (-1.5 and -2.0 dB, respectively). Passing the data through an auditory filter model reduced the averaged attenuation to less than -1.4 dB. Phase was undistorted up to 2.5 kHz. These results suggests that the perturbations produced by the insert are unlikely to be perceptually relevant.

Monaural and binaural spectrum level cues in the ferret: acoustics and the neural representation of auditory space.

1. The role of the structures of the outer ear in producing monaural and binaural spectral cues to sound location was examined acoustically in the ferret. A probe microphone was introduced across the wall of the external auditory canal and its responses to digitally constructed wideband signals were recorded for a large number of free field locations. 2. In the intact animal the patterns of both monaural and binaural cues were asymmetrical for horizontal locations about the interaural axis. For anterior sound locations the monaural transformations demonstrated relative gains at middle and high frequencies and a location-dependent frequency notch. Changing elevation resulted in variations in the corner frequencies of these spectral features. Additionally, there was greater front-back asymmetry in the binaural spectral cues for locations in lateral space when compared with locations near the midline. 3. Surgical removal of the pinna and concha (pinnectomy) eliminated all the major front-back asymmetrical features in the horizon monaural and binaural spectral transformations as well as the elevation-dependent variations in the monaural spectra. Thus the residual transformations were ambiguous for sound locations in lateral space, resulting in "cones of confusion" centered on the interaural axis. 4. These cues were reflected in the topographic representation of auditory space in the deeper layers of the superior colliculus (SC). Previous studies have shown that spatial tuning at near-threshold sound levels is based on monaural pinna cues, whereas binaural inputs are utilized at higher levels that stimulate both ears. In the intact ferret we examined statistically the topography of the representation of sound azimuth for near-threshold and suprathreshold stimuli and the alignment of the auditory and visual representations in the SC. The distributions of auditory best positions within the SC for near- and suprathreshold stimulus levels were statistically indistinguishable, suggesting that both monaural and binaural cues are integrated in this neural representation of space. 5. Pinnectomy resulted in a large increase in the number of auditory units that responded best to two distinct locations in space. One lobe of the response was tuned appropriately in terms of the position of the unit within the SC, demonstrating that the residual acoustical cues are sufficient for the construction of a topographic representation of auditory space. However, the second region of space, thereby producing an ambiguous representation.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes induced in the representation of auditory space in the superior colliculus by rearing ferrets with binocular eyelid suture.

There have been conflicting reports concerning the importance of visual experience in the development of auditory localization mechanisms. We have examined the representation of auditory space in the superior colliculus of adult ferrets that were visually deprived by binocular eyelid suture from postnatal days 25-28, prior to natural eye opening, until the time of recording. This procedure attenuated the transmission of light by a factor of a least 20-25 and blurred the image so that, as long as the eyelids were still fused, the responses of visual units in the superficial layers of the superior colliculus were labile and very poorly tuned. After the eyelids were opened, the representation of the visual field in these layers appeared to be normal. Acoustically responsive units were, as usual, almost exclusively restricted to the deeper layers of the superior colliculus. However, unlike normal animals, where responses occurring only at stimulus onset predominate, most of these units exhibited sustained or multi-peaked discharge patterns. The degree of spatial tuning of individual units recorded from the normal and deprived groups of animals was not significantly different in either azimuth or elevation. Normally orientated maps of both sound azimuth and elevation were also found in the visually deprived ferrets. However, abnormalities were present in the topography and precision of these representations and consequently in their alignment with the overlying visual map. In particular, an increase was observed in the proportion of auditory units with spatially ambiguous receptive fields, in which the maximum response occurred at two distinct locations. These results indicate that patterned visual experience is not required for establishing at least a crude map of auditory space in the superior colliculus, but suggest that it may play a role in refining this representation during development.