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Auditory / Hearing

Bizley, J.K., I. Nelken, F.R. Nodal, B. Ahmed, A.J. King, and J.W.H. Schnupp (2002). An investigation into the functional anatomy of ferret auditory cortex using optical imaging and multi - electrode recordings. In: 32nd Annual Meeting of the Society for Neuroscience, Society for Neuroscience Abstract Viewer and Itinerary Planner., November 2, 2002-November 7, 2002, Orlando, Florida, USA., Vol. 2002, p. Abstract No. 354.10.
Descriptors: auditory cortex, functional anatomy, ferret, optical imaging, area mapping, recordings, auditory stimuli, frequency tuning.

Bizley, J.K., F.R. Nodal, I. Nelken, and A.J. King (2005). Functional organization of ferret auditory cortex. Cerebral Cortex 15(10): 1637-1653. ISSN: 1047-3211.
Abstract: We characterized the functional organization of different fields within the auditory cortex of anaesthetized ferrets. As previously reported, the primary auditory cortex, A1, and the anterior auditory field, AAF, are located on the middle ectosylvian gyrus. These areas exhibited a similar tonotopic organization, with high frequencies represented at the dorsal tip of the gyrus and low frequencies more ventrally, but differed in that AAF neurons had shorter response latencies than those in A1. On the basis of differences in frequency selectivity, temporal response properties and thresholds, we identified four more, previously undescribed fields. Two of these are located on the posterior ectosylvian gyrus and were tonotopically organized. Neurons in these areas responded robustly to tones, but had longer latencies, more sustained responses and a higher incidence of non-monotonic rate-level functions than those in the primary fields. Two further auditory fields, which were not tonotopically organized, were found on the anterior ectosylvian gyrus. Neurons in the more dorsal anterior area gave short-latency, transient responses to tones and were generally broadly tuned with a preference for high (>8 kHz) frequencies. Neurons in the other anterior area were frequently unresponsive to tones, but often responded vigorously to broadband noise. The presence of both tonotopic and non-tonotopic auditory cortical fields indicates that the organization of ferret auditory cortex is comparable to that seen in other mammals.
Descriptors: ferrets, auditory cortex, acoustic stimulation, brain mapping, electrodes, implanted, electroencephalography, neurons physiology.

Bizley, J.K., F.R. Nodal, C.H. Parsons, and A.J. King (2003). Investigating the role of ferret primary auditory cortex in vertical sound localization using reversible inactivation and lesions. Society for Neuroscience Abstract Viewer and Itinerary Planner 2003: Abstract No. 488.1.
Descriptors: ferret, primary auditory cortex, lesions, sound localization, broadband noise, vertical sound localization.
Notes: 33rd Annual Meeting of the Society of Neuroscience, New Orleans, LA, USA; November 08-12, 2003.

Campbell, R.A., J.W. Schnupp, A. Shial, and A.J. King (2006). Binaural-level functions in ferret auditory cortex: Evidence for a continuous distribution of response properties. Journal of Neurophysiology 95(6): 3742-3755. ISSN: 0022-3077.
Abstract: Many previous studies have subdivided auditory neurons into a number of physiological classes according to various criteria applied to their binaural response properties. However, it is often unclear whether such classifications represent discrete classes of neurons or whether they merely reflect a potentially convenient but ultimately arbitrary partitioning of a continuous underlying distribution of response properties. In this study we recorded the binaural response properties of 310 units in the auditory cortex of anesthetized ferrets, using an extensive range of interaural level differences (ILDs) and average binaural levels (ABLs). Most recordings were from primary auditory fields on the middle ectosylvian gyrus and from neurons with characteristic frequencies >5 kHz. We used simple multivariate statistics to quantify a fundamental coding feature: the shapes of the binaural response functions. The shapes of all 310 binaural response surfaces were represented as points in a five-dimensional principal component space. This space captured the underlying shape of all the binaural response surfaces. The distribution of binaural level functions was not homogeneous because some shapes were more common than others. Despite this, clustering validation techniques revealed no evidence for the existence of discrete, or partially overlapping, clusters that could serve as a basis for an objective classification of binaural-level functions. We also examined the gradients of the response functions for the population of units; these gradients were greatest near the midline, which is consistent with free-field data showing that cortical neurons are most sensitive to changes in stimulus location in this region of space.
Descriptors: ferrets, auditory perception, physiology, auditory threshold physiology, physiology, animal models, acoustic stimulation, methods, auditory cortex.

Moore, D.R., L.R. Highton, O. Kacelnik, and a.J. King (2002). Effect of bilateral auditory cortex lesions on sound localisation by the ferret. Society for Neuroscience Abstract Viewer and Itinerary Planner 2002: Abstract No. 845.11.
Descriptors: ferret, sound localisation, cortex lesiona, surgical anblation, auditory cortex, motor problem, lesions, sensory deficit.
Notes: 32nd Annual Meeting of the Society for Neuroscience, Orlando, Florida, USA; November 02-07, 2002.

Mrsic Flogel, T.D., A.J. King, and J.W. Schnupp (2005). Encoding of virtual acoustic space stimuli by neurons in ferret primary auditory cortex. Journal of Neurophysiology 93(6): 3489-3503. ISSN: 0022-3077.
Abstract: Recent studies from our laboratory have indicated that the spatial response fields (SRFs) of neurons in the ferret primary auditory cortex (A1) with best frequencies > or =4 kHz may arise from a largely linear processing of binaural level and spectral localization cues. Here we extend this analysis to investigate how well the linear model can predict the SRFs of neurons with different binaural response properties and the manner in which SRFs change with increases in sound level. We also consider whether temporal features of the response (e.g., response latency) vary with sound direction and whether such variations can be explained by linear processing. In keeping with previous studies, we show that A1 SRFs, which we measured with individualized virtual acoustic space stimuli, expand and shift in direction with increasing sound level. We found that these changes are, in most cases, in good agreement with predictions from a linear threshold model. However, changes in spatial tuning with increasing sound level were generally less well predicted for neurons whose binaural frequency-time receptive field (FTRF) exhibited strong excitatory inputs from both ears than for those in which the binaural FTRF revealed either a predominantly inhibitory effect or no clear contribution from the ipsilateral ear. Finally, we found (in agreement with other authors) that many A1 neurons exhibit systematic response latency shifts as a function of sound-source direction, although these temporal details could usually not be predicted from the neuron's binaural FTRF.
Descriptors: ferrets, auditory cortex, sound localization, space perception, acoustic stimulation, auditory pathways, radiation, animal models, neurological, predictive value of tests.

Mrsic Flogel, T.D., H. Versnel, and A.J. King (2006). Development of contralateral and ipsilateral frequency representations in ferret primary auditory cortex. European Journal of Neuroscience 23(3): 780-792. ISSN: 0953-816X.
Abstract: Little is known about the maturation of functional maps in the primary auditory cortex (A1) after the onset of sensory experience. We used intrinsic signal imaging to examine the development of the tonotopic organization of ferret A1 with respect to contralateral and ipsilateral tone stimulation. Sound-evoked responses were recorded as early as postnatal day (P) 33, a few days after hearing onset. From P36 onwards, pure tone stimuli evoked restricted, tonotopically organized patches of activity. There was an age-dependent increase in the cortical area representing each octave, with a disproportionate expansion of cortical territory representing frequencies > 4 kHz after P60. Similar tonotopic maps were observed following stimulation of the contralateral and ipsilateral ears. During the first few weeks following hearing onset, no differences were found in the area of cortical activation or in the magnitude of the optical responses evoked by stimulation of each ear. In older animals, however, contralateral stimuli evoked stronger responses and activated a larger A1 area than ipsilateral stimuli. Our findings indicate that neither the tonotopic organization nor the representation of inputs from each ear reach maturity until approximately 1 month after hearing onset. These results have important implications for cortical signal processing in juvenile animals.
Descriptors: ferrets, auditory cortex, brain mapping, sound localization, acoustic stimulation, age factors, diagnostic imaging, radiation, imaging, three dimensional methods.

Nelken, I., J.K. Bizley, F.R. Nodal, B. Ahmed, J.W. Schnupp, and A.J. King (2004). Large-scale organization of ferret auditory cortex revealed using continuous acquisition of intrinsic optical signals. Journal of Neurophysiology 92(4): 2574-2588. ISSN: 0022-3077.
Abstract: We have adapted a new approach for intrinsic optical imaging, in which images were acquired continuously while stimuli were delivered in a series of continually repeated sequences, to provide the first demonstration of the large-scale tonotopic organization of both primary and nonprimary areas of the ferret auditory cortex. Optical responses were collected during continuous stimulation by repeated sequences of sounds with varying frequency. The optical signal was averaged as a function of time during the sequence, to produce reflectance modulation functions (RMFs). We examined the stability and properties of the RMFs and show that their zero-crossing points provide the best temporal reference points for quantifying the relationship between the stimulus parameter values and optical responses. Sequences of different duration and direction of frequency change gave rise to comparable results, although in some cases discrepancies were observed, mostly between upward- and downward-frequency sequences. We demonstrated frequency maps, consistent with previous data, in primary auditory cortex and in the anterior auditory field, which were verified with electrophysiological recordings. In addition to these tonotopic gradients, we demonstrated at least 2 new acoustically responsive areas on the anterior and posterior ectosylvian gyri, which have not previously been described. Although responsive to pure tones, these areas exhibit less tonotopic order than the primary fields.
Descriptors: ferrets, auditory cortex, acoustic stimulation, brain mapping, electrodes implanted, electrophysiology, image processing, computer assisted, neurological, nerve net, respiratory mechanics, stereotaxic techniques.

Schnupp, J.W., J. Booth, and A.J. King (2003). Modeling individual differences in ferret external ear transfer functions. Journal of the Acoustical Society of America 113(4 Pt 1): 2021-2030. ISSN: 0001-4966.
Abstract: Individual variations in head and outer ear size, as well as growth of these structures during development, can markedly alter the values of the binaural and monaural cues which form the basis for auditory localization. This study investigated individual differences in the directional component of the head-related transfer function of both adult and juvenile ferrets. In line with previous studies in humans and cats, intersubject spectral differences were found to be reduced by scaling one of the directional transfer functions on a log-frequency axis. The optimal scale factor correlated most highly with pinna cavity height. Optimal frequency scaling reduced interear spectral difference equally well for adult-juvenile comparisons as for comparisons between pairs of adult ears. This illustrates that the developmental changes in localization cue values should be at least partly predictable on the basis of the expected growth rate of the outer ear structures. Predictions of interaural time differences (ITDs) were also derived from the physical dimensions of the head. ITDs were found to be poorly fitted by the spherical head model, while much better predictions could be derived from a model based on von Mises spherical basis functions. Together, these findings show how more accurate estimates of spatial cue values can be made from knowledge of the dimensions of the head and outer ears, and may facilitate the generation of virtual acoustic space stimuli in the absence of acoustical measurements from individual subjects.
Descriptors: ferrets, ear, external physiology, ferrets, laterality, sound localization, age factors, biometry, computer simulation, sound spectrography.

Versnel, H., J.E. Mossop, T.D. Mrsic Flogel, B. Ahmed, and D.R. Moore (2002). Optical imaging of intrinsic signals in ferret auditory cortex: Responses to narrowband sound stimuli. Journal of Neurophysiology 88(3): 1545-1558. ISSN: 0022-3077.
Abstract: This paper describes optical imaging of the auditory cortex in the anesthetized ferret, particularly addressing optimization of narrowband stimuli. The types of sound stimuli used were tone-pip trains and sinusoidal frequency and amplitude modulated (SFM and SAM) tones. By employing short illumination wavelengths (546 nm), we have successfully characterized the tonotopic arrangement, in agreement with the well-established electrophysiological tonotopic maps of the ferret auditory primary field (AI). The magnitude of the optical signal increased with sound level, was maximal for a modulation frequency (MF) of 2-4 Hz, and was larger for tone-pip trains and SFM sounds than for SAM sounds. Accordingly, an optimal narrowband stimulus was defined. Thus optical imaging can be used successfully to obtain frequency maps in auditory cortex by an appropriate choice of stimulus parameters. In addition, background noise consisting of 0.1-Hz oscillations could be reduced by introduction of blood pressure enhancing drugs. The optical maps were largely independent of 1) the type of narrowband stimulus, 2) the sound level, and 3) the MF. This stability of the optical maps was not predicted from the electrophysiological literature.
Descriptors: ferrets, auditory cortex, acoustic stimulation, angiotensin II, brain mapping, optics, oscillometry, time factors, vasoconstrictor agents, vasomotor system.

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