Contralateral Azimuths Research Articles

Head-related transfer functions of rabbits within the front horizontal plane

The head-related transfer function (HRTF) describes the direction-dependent acoustic filtering by the head that occurs between a source signal in free-field space and the signal at the tympanic membrane. HRTFs contain information on sound source location via interaural differences of their magnitude or phase spectra and via the shapes of their magnitude spectra. The present study characterized HRTFs for source locations in the front horizontal plane for nine rabbits, which are a species commonly used in studies of the central auditory system. HRTF magnitude spectra shared several features across individuals, including a broad spectral peak at 2.6kHz that increased gain by 12 to 23dB depending on source azimuth; and a notch at 7.6kHz and peak at 9.8kHz visible for most azimuths. Overall, frequencies above 4kHz were amplified for sources ipsilateral to the ear and progressively attenuated for frontal and contralateral azimuths. The slope of the magnitude spectrum between 3 and 5kHz was found to be an unambiguous monaural cue for source azimuths ipsilateral to the ear. Average interaural level difference (ILD) between 5 and 16kHz varied monotonically with azimuth over ±31dB despite a relatively small head size. Interaural time differences (ITDs) at 0.5kHz and 1.5kHz also varied monotonically with azimuth over ±358 μs and ±260 μs, respectively. Remeasurement of HRTFs after pinna removal revealed that the large pinnae of rabbits were responsible for all spectral peaks and notches in magnitude spectra and were the main contribution to high-frequency ILDs (5–16kHz), whereas the rest of the head was the main contribution to ITDs and low-frequency ILDs (0.2–1.5kHz). Lastly, inter-individual differences in magnitude spectra were found to be small enough that deviations of individual HRTFs from an average HRTF were comparable in size to measurement error. Therefore, the average HRTF may be acceptable for use in neural or behavioral studies of rabbits implementing virtual acoustic space when measurement of individualized HRTFs is not possible.

Chronic Unilateral Hearing Loss Disrupts Neural Tuning to Sound-Source Azimuth in the Rat Primary Auditory Cortex.

Accurate sound localization requires normal binaural input and precise auditory neuronal representation of sound spatial locations. Previous studies showed that unilateral hearing loss profoundly impaired the sound localization abilities. However, the underlying neural mechanism is not fully understood. Here, we investigated how chronic unilateral conductive hearing loss (UCHL) affected the neural tuning to sound source azimuth in the primary auditory cortex (AI). The UCHL was manipulated by the removal of tympanic membrane and malleus in the right ear of young (P14) rats and adult (P57) rats. We recorded the azimuth tuning of neurons in the left AI contralateral to the operated ear in the two groups of rats that experienced 2 months of UCHL, and in the left AI of age-matched control rats. We found that AI neurons in control rats showed predominant preference to sound from contralateral azimuths. However, UCHL weakened the cortical neuronal representation of contralateral azimuths on the operated ear side and strengthened the cortical neuronal representation of ipsilateral azimuths on the intact ear side. This effect was stronger in rats with UCHL at young age than in rats with UCHL in adulthood. Moreover, UCHL degraded the azimuth selectivity and azimuth sensitivity of AI neurons, and this effect was stronger in rats with UCHL in adulthood than in rats with UCHL at young age. These findings highlight a remarkable age-related experience-dependent plasticity of neural tuning to sound source azimuth in AI, and imply a neural mechanism for the impacts of chronic UCHL on sound localization abilities.

Opponent Coding of Sound Location (Azimuth) in Planum Temporale is Robust to Sound-Level Variations.

Coding of sound location in auditory cortex (AC) is only partially understood. Recent electrophysiological research suggests that neurons in mammalian auditory cortex are characterized by broad spatial tuning and a preference for the contralateral hemifield, that is, a nonuniform sampling of sound azimuth. Additionally, spatial selectivity decreases with increasing sound intensity. To accommodate these findings, it has been proposed that sound location is encoded by the integrated activity of neuronal populations with opposite hemifield tuning (“opponent channel model”). In this study, we investigated the validity of such a model in human AC with functional magnetic resonance imaging (fMRI) and a phase-encoding paradigm employing binaural stimuli recorded individually for each participant. In all subjects, we observed preferential fMRI responses to contralateral azimuth positions. Additionally, in most AC locations, spatial tuning was broad and not level invariant. We derived an opponent channel model of the fMRI responses by subtracting the activity of contralaterally tuned regions in bilateral planum temporale. This resulted in accurate decoding of sound azimuth location, which was unaffected by changes in sound level. Our data thus support opponent channel coding as a neural mechanism for representing acoustic azimuth in human AC.

Neural population encoding and decoding of sound source location across sound level in the rabbit inferior colliculus.

At lower levels of sensory processing, the representation of a stimulus feature in the response of a neural population can vary in complex ways across different stimulus intensities, potentially changing the amount of feature-relevant information in the response. How higher-level neural circuits could implement feature decoding computations that compensate for these intensity-dependent variations remains unclear. Here we focused on neurons in the inferior colliculus (IC) of unanesthetized rabbits, whose firing rates are sensitive to both the azimuthal position of a sound source and its sound level. We found that the azimuth tuning curves of an IC neuron at different sound levels tend to be linear transformations of each other. These transformations could either increase or decrease the mutual information between source azimuth and spike count with increasing level for individual neurons, yet population azimuthal information remained constant across the absolute sound levels tested (35, 50, and 65 dB SPL), as inferred from the performance of a maximum-likelihood neural population decoder. We harnessed evidence of level-dependent linear transformations to reduce the number of free parameters in the creation of an accurate cross-level population decoder of azimuth. Interestingly, this decoder predicts monotonic azimuth tuning curves, broadly sensitive to contralateral azimuths, in neurons at higher levels in the auditory pathway.

Monaural and binaural hearing directivity in the bottlenose dolphin: Evoked-potential study

Hearing thresholds as a function of sound-source azimuth were measured in bottlenose dolphins using an auditory evoked potential (AEP) technique. AEP recording from a region next to the ear allowed recording monaural responses. Thus, a monaural directivity diagram (a threshold-vs-azimuth function) was obtained. For comparison, binaural AEP components were recorded from the vertex to get standard binaural directivity diagrams. Both monaural and binaural diagrams were obtained at frequencies ranging from 8 to 128 kHz in quarter-octave steps. At all frequencies, the monaural diagram demonstrated asymmetry manifesting itself as: (1) lower thresholds at the ipsilateral azimuth as compared to the symmetrical contralateral azimuth and (2) ipsilateral shift of the lowest-threshold point. The directivity index increased with frequency: at the ipsilateral side it rose from 4.7 to 17.8 dB from 11.2 to 128 kHz, and from 10.5 to 15.6 dB at the contralateral side. The lowest-threshold azimuth shifted from 0 degrees at 90-128 kHz to 22.5 degrees at 8-11.2 kHz. The frequency-dependent variation of the lowest-threshold azimuth indicates the presence of two sound-receiving apertures at each head side: a high-frequency aperture with the axis directed frontally, and a low-frequency aperture with the axis directed laterally.

Azimuth-dependent recovery cycle affects directional selectivity of bat inferior collicular neurons determined with sound pulses within a pulse train

In our previous study, we have shown that the recovery cycle of most neurons in the inferior colliculus (IC) of the big brown bat, Eptesicus fuscus, is typically longer at ipsilateral azimuth than at contralateral azimuth under free-field stimulation conditions. The present study is to test the hypothesis that this azimuth-dependent recovery cycle may contribute to the variation of directional selectivity of IC neurons with sequential presented sound pulses within a pulse train. A 300-ms pulse train containing nine sound pulses of 4-ms with an inter-pulse interval of 33.3 ms was delivered at several selected azimuthal angles between ±80° lateral in the frontal auditory space of a bat. A family of nine directional selectivity curves was plotted with a neuron's number of impulses in response to each individual pulse against the azimuthal angles. The type and sharpness of these directional selectivity curves were then compared in relation to pulse position within the pulse train. All 675 directional selectivity curves obtained from 75 IC neurons could be described as directionally selective (423, 63%), hemifield (220, 32%), or non-directional (32, 5%). The directional selectivity curves of 45 (60%) neurons did not vary with pulse position. However, those of the remaining neurons (30, 40%) changed from one type to another such that the number of neurons with directionally selective curves progressively increased and the number of neurons with hemifield and non-directional selectivity curves decreased with increasing pulse position within the pulse train. Among 68 IC neurons whose directional selectivity curves were compared quantitatively, directional selectivity determined with sequentially presented sound pulses significantly increased in 38 (56%) neurons; decreased in 18 (26%) neurons but did not change in 12 (18%) neurons. This change of directional selectivity was due to the variation in recovery cycle of these IC neurons with azimuthal angle as we hypothesized.

Zonal organization of the vestibulocerebellum in pigeons (Columba livia): II. Projections of the rotation zones of the flocculus.

Previous neurophysiologic research in birds and mammals has shown that there are two types of Purkinje cells in the flocculus. The first type shows maximal modulation in response to rotational optokinetic stimulation about the vertical axis (rVA neurons). The second type shows maximal modulation in response to rotational optokinetic stimulation about a horizontal axis oriented 45 degrees to contralateral azimuth (rH45c neurons). In pigeons, the rVA and rH45c are organized into four alternating parasagittal zones. In this study we investigated the projections of Purkinje cells in the rVA and rH45c zones by using the anterograde tracers biotinylated dextran amine and cholera toxin subunit B. After iontophoretic injections of tracers into the rH45c zones, heavy anterograde labeling was found in the infracerebellar nucleus and the medial margin of the superior vestibular nucleus. Some labeling was also consistently observed in the lateral cerebellar nucleus and the dorsolateral vestibular nucleus. After injections into the rVA zones, heavy anterograde labeling was found in the medial and descending vestibular nuclei, the nucleus prepositus hypoglossi, and the central region of the superior vestibular nucleus. Less labeling was seen in the tangential nucleus, the dorsolateral vestibular nucleus, and the lateral vestibular nucleus, pars ventralis. These results are compared and contrasted with findings in mammalian species.

The Coding of Spatial Location by Single Units in the Lateral Superior Olive of the Cat. I. Spatial Receptive Fields in Azimuth

The lateral superior olive (LSO) is one of the most peripheral auditory nuclei receiving inputs from both ears, and LSO neurons are sensitive to interaural level differences (ILDs), one of the primary acoustical cues for sound location. We used the virtual space (VS) technique to present over earphones broadband stimuli containing natural combinations of localization cues as a function of azimuth while recording extracellular responses from single LSO cells. The responses of LSO cells exhibited spatial receptive fields (SRFs) in azimuth consonant with their sensitivity to ILDs of stimuli presented dichotically: high discharge rates for ipsilateral azimuths where stimulus amplitude to the excitatory ear exceeded that to the inhibitory ear, rapidly declining rates near the midline, and low rates for contralateral azimuths where the amplitude to the inhibitory ear exceeded that to the excitatory ear. Relative to binaural stimulation, presentations of the VS stimuli to the ipsilateral ear alone yielded increased rates, particularly in the contralateral field, confirming that the binaural SRFs were shaped by contralateral inhibition. Our finding that LSO neurons respond to azimuth consistent with their ILD sensitivity supports the long-held hypothesis that LSO neurons compute a correlate of the ILD present in free-field stimuli. Only weak correlations between the properties of pure-tone ILD functions and the SRFs were found, indicating that ILD sensitivity measured at only one sound level is not sufficient to predict sensitivity to azimuth. Sensitivity to spatial location was also retained over a wide range of stimulus levels under binaural, but not monaural, conditions.

Responses of neurons in the medial column of the inferior olive in pigeons to translational and rotational optic flowfields.

The responses of neurons in the medial column of the inferior olive to translational and rotational optic flow were recorded from anaesthetized pigeons. Panoramic translational or rotational flowfields were produced by mechanical devices that projected optic flow patterns onto the walls, ceiling and floor of the room. The axis of rotation/translation could be positioned to any orientation in three-dimensional space such that axis tuning could be determined. Each neuron was assigned a vector representing the axis about/along which the animal would rotate/translate to produce the flowfield that elicited maximal modulation. Both translation-sensitive and rotation-sensitive neurons were found. For neurons responsive to translational optic flow, the preferred axis is described with reference to a standard right-handed coordinate system, where +x, +y and +z represent rightward, upward and forward translation of the animal, respectively (assuming that all recordings were from the right side of the brain). t(+y) neurons were maximally excited in response to a translational optic flowfield that results from self-translation upward along the vertical (y) axis. t(-y) neurons also responded best to translational optic flow along the vertical axis but showed the opposite direction preference. The two remaining groups, t(-x+z) and t(-x-z) neurons, responded best to translational optic flow along horizontal axes that were oriented 45 degrees to the midline. There were two types of neurons responsive to rotational optic flow: rVA neurons preferred rotation about the vertical axis, and rH135c neurons preferred rotation about a horizontal axis at 135 degrees contralateral azimuth. The locations of marking lesions indicated a clear topographical organization of the six response types. In summary, our results reinforce that the olivo-cerebellar system dedicated to the analysis of optic flow is organized according to a reference frame consisting of three approximately orthogonal axes: the vertical axis, and two horizontal axes oriented 45 degrees to either side the midline. Previous research has shown that the eye muscles, vestibular semicircular canals and postural control system all share a similar spatial frame of reference.

The role of GABAergic inhibition on direction-dependent sharpening of frequency tuning in bat inferior collicular neurons

This study examined the role of GABAergic inhibition on direction-dependent sharpening of frequency tuning curves (FTCs) in bat inferior collicular (IC) neurons under free field stimulation conditions. The minimum threshold (MT) at the neurons best frequency (BF) and the sharpness ( Q 10, Q 20, Q 30) of FTCs of most IC neurons increased as the sound direction changed from contralateral azimuths to ipsilateral azimuths. The application of GABA A antagonist, bicuculline, lowered all MTs but the application did not abolish direction-dependent variation in MT. MTs determined during bicuculline application at 40 ipsilateral were still significantly higher than those determined at 40° contralateral (two-tailed paired t-test, P<0.0001). In contrast, although application of bicuculline essentially had no effect on the BFs of IC neurons, it differentially broadened neurons FTCs at different azimuths abolishing the direction-dependent sharpening of frequency tuning (i.e. Q n values, two-tailed paired t-test, P<0.01). These data indicate that GABAergic inhibition makes an important contribution to the direction-dependent frequency tuning of most IC neurons.

Sensitivity of human subjects to head-related transfer-function phase spectra.

Head-related transfer functions (HRTFs) for human subjects in anechoic space were modeled with modified phase spectra, including minimum-phase-plus-delay, linear-phase, and reversed-phase-plus-delay functions. The overall (wide-band) interaural time delay (ITD) for the modeled HRTFs was made consistent with that of the empirical HRTFs by setting the position-dependent, frequency-independent delay in the HRTF for the lagging ear. Signal analysis of the minimum-phase-plus-delay reconstructions indicated that model HRTFs deviate from empirical HRTF measurements maximally for contralateral azimuths and low elevations. Subjects assessed the perceptual validity of the model HRTFs in a four-interval, two-alternative, forced-choice discrimination paradigm. Results indicate that monaural discrimination performance of subjects was at chance for all three types of HRTF models. Binaural discrimination performance was at chance for the linear-phase HRTFs, was above chance for some locations for the minimum-phase-plus-delay HRTFs, and was above chance for all tested locations for the reversed-phase-plus-delay HRTFs. An analysis of low-frequency timing information showed that all of these results are consistent with efficient use of interaural time differences in the low-frequency components of the stimulus waveforms. It is concluded that listeners are insensitive to HRTF phase spectra as long as the overall ITD of the low-frequency components does not provide a reliable cue. In particular, the minimum-phase-plus-delay approximation to the HRTF phase spectrum is an adequate approximation as long as the low-frequency ITD is appropriate. These results and conclusions are all limited to the anechoic case when the HRTFs correspond to brief impulse responses limited to a few milliseconds.

Azimuth coding in primary auditory cortex of the cat. II. Relative latency and interspike interval representation.

This study was designed to explore a potential representation of sound azimuth in the primary auditory cortex (AI) of the cat by the relative latencies of a population of neurons. An analysis of interspike intervals (ISI) was done to asses azimuth information in the firings of the neurons after the first spike. Thus latencies of simultaneously recorded single-unit (SU) spikes and local field potentials (LFP) in AI of cats were evaluated for sound presented from nine speakers arranged horizontally in the frontal half field in a semicircular array with a radius of 55 cm and the cat's head in the center. SU poststimulus time histograms (PSTH) were made for each speaker location for a 100-ms window after noise-burst onset using 1-ms bins. PSTH peak response latencies for SUs and LFPs decreased monotonically with intensity, and most of the change occurred within 15 dB of the threshold at that particular azimuth. After correction for threshold differences, all latency-intensity functions had roughly the same shape, independent of sound azimuth. Differences with the minimum spike latency observed in an animal at each intensity were calculated for all azimuth-intensity combinations. This relative latency showed a weakly sigmoidal dependence on azimuth that was independent of intensity level >40 dB SPL. SU latency differences also were measured with respect to the latencies of the LFP triggers, simultaneously recorded on the same electrode. This difference was independent of stimulus intensity and showed a nearly linear dependence on sound azimuth. The mean differences across animals for both measures, however, were only significant between contralateral azimuths on one hand and frontal and ipsilateral azimuths on the other hand. Mean unit-LFP latency differences showed a monotonic dependence on azimuth with nearly constant variance and may provide the potential for an unbiased conversion of azimuth into neural firing times. The general trend for the modal ISI was the same as for relative spike latency: the shortest ISIs were found for contralateral azimuths (ISI usually 3 ms) and the longer ones for ipsilateral azimuths (the most frequent ISI was 4 ms, occasionally 5 ms was found). This trend was also independent of intensity level. This suggests that there is little extra information in the timing of extra spikes in addition to that found in the peak PSTH latency.

Azimuth coding in primary auditory cortex of the cat. I. Spike synchrony versus spike count representations.

The neural representation of sound azimuth in auditory cortex most often is considered to be average firing rate, and azimuth tuning curves based thereupon appear to be rather broad. Coincident firings of simultaneously recorded neurons could provide an improved representation of sound azimuth compared with that contained in the firing rate in either of the units. In the present study, a comparison was made between local field potentials and several measures based on unit firing rate and coincident firing with respect to their azimuth-tuning curve bandwidth. Noise bursts, covering a 60-dB intensity range, were presented from nine speakers arranged in a semicircular array with a radius of 55 cm in the animal's frontal half field. At threshold intensities, all local field potential (LFP) recordings showed preferences for contralateral azimuths. Multiunit recordings showed in 74% a threshold for contralateral azimuths, in 16% for frontal azimuths, and in only 5% showed an ipsilateral threshold. The remaining 5% were not spatially tuned. Representations for directionally sensitive units based on coincident firings provided significantly sharper tuning (50-60 degrees bandwidth at 25 dB above the lowest threshold) than those based on firing rate (bandwidths of 80-90 degrees). The ability to predict sound azimuth from the directional information contained in the neural population activity was simulated by combining the responses of the 102 single units. Peak firing rates and coincident firings with LFPs at the preferred azimuth for each unit were used to construct a population vector. At stimulus levels of >/=40 dB SPL, the prediction function was sigmoidal with the predicted frontal azimuth coinciding with the frontal speaker position. Sound azimuths >45 degrees from the midline all resulted in predicted values of -90 or 90 degrees, respectively. No differences were observed in the performance of the prediction based on firing rate or coincident firings for these intensities. This suggests that although coincident firings produce narrower azimuth tuning curves, the information contained in the overall neural population does not increase compared with that contained in a firing rate representation. The relatively poor performance of the population vector further suggests that primary auditory cortex does not code sound azimuth by a globally distributed measure of peak firing rate or coincident firing.

Sensitivity to interaural intensity differences of neurons in primary auditory cortex of the cat. I. types of sensitivity and effects of variations in sound pressure level.

1. Interaural intensity differences (IIDs) provide the major cue to the azimuthal location of high-frequency narrowband sounds. In recent studies of the azimuthal sensitivity of high-frequency neurons in the primary auditory cortex (field AI) of the cat, a number of different types of azimuthal sensitivity have been described and the azimuthal sensitivity of many neurons was found to vary as a function of changes in stimulus intensity. The extent to which the shape and the intensity dependence of the azimuthal sensitivity of AI neurons reflects features of their IID sensitivity was investigated by obtaining data on IID sensitivity from a large sample of neurons with a characteristic frequency (CF) > 5.5 kHz in AI of anesthetized cats. IID sensitivity functions were classified in a manner that facilitated comparison with previously obtained data on azimuthal sensitivity, and the effects of changes in the base intensity at which IIDs were introduced were examined. 2. IID sensitivity functions for CF tonal stimuli were obtained at one or more intensities for a total of 294 neurons, in most cases by a method of generating IIDs that kept the average binaural intensity (ABI) of the stimuli at the two ears constant. In the standard ABI range at which a function was obtained for each unit, five types of IID sensitivity were distinguished. Contra-max neurons (50% of the sample) had maximum response (a peak or a plateau) at IIDs corresponding to contralateral azimuths, whereas ipsi-max neurons (17%) had the mirror-image form of sensitivity. Near-zero-max neurons (18%) had a clearly defined maximum response (peak) in the range of +/- 10 dB IID, whereas a small group of tough neurons (2%) had a restricted range of minimal responsiveness with near-maximal responses at IIDs on either side. A final 18% of AI neurons were classified as insensitive to IIDs. The proportions of neurons exhibiting the various types of sensitivity corresponded closely to the proportions found to exhibit corresponding types of azimuthal sensitivity in a previous study. 3. There was a strong correlation between a neuron's binaural interaction characteristics and the form of its IID sensitivity function. Thus, neurons excited by monaural stimulation of only one ear but with either inhibitory, facilitatory, or mixed facilitatory-inhibitory effects of stimulation of the other ear had predominantly contra-max IID sensitivity (if contralateral monaural stimulation was excitatory) or ipsi-max sensitivity (if ipsilateral monaural stimulation was excitatory). Neurons driven weakly or not at all by monaural stimulation but facilitated binaurally almost all exhibited near-zero-max IID sensitivity. The exception to this tight association between binaural input and IID sensitivity was provided by neurons excited by monaural stimulation of either ear (EE neurons). Although EE neurons have frequently been considered to be insensitive to IIDs, our data were in agreement with two recent reports indicating that they can exhibit various forms of IID sensitivity: only 23 of 75 EE neurons were classified as insensitive and the remainder exhibited diverse types of sensitivity. 4. IID sensitivity was examined at two or more intensities (3-5 in most cases) for 84 neurons. The form of the IID sensitivity function (defined in terms of both shape and position along the IID axis) was invariant with changes in ABI for only a small proportion of IID-sensitive neurons (approximately 15% if a strict criterion of invariance was employed), and for many of these neurons the spike counts associated with a given IID varied with ABI, particularly at near-threshold levels. When the patterns of variation in the form of IID sensitivity produced by changes in ABI were classified in a manner equivalent to that used previously to classify the effects of intensity on azimuthal sensitivity, there was a close correspondence between the effects of intensity on corresponding types of azimuthal and IID sensitivity

A comparison of the interaural time sensitivity of neurons in the inferior colliculus and thalamus of the unanesthetized rabbit

The localization of low-frequency sounds (less than 3 kHz) along the azimuth involves comparing the ongoing difference in the time of arrival of a sound at the two ears. Information about interaural time differences (ITDs) is derived from an initial comparison performed in the superior olivary complex. However, little is known about which aspects of this information are transformed as it ascends the brainstem. To address this issue, we compared the ITD sensitivity of neurons in the inferior colliculus (IC) and auditory thalamus, successive stations in the auditory pathway. We found ITD sensitivity in the IC and thalamus to be similar in several respects. At both levels, the large majority of neurons responded maximally to ITDs within the range that a rabbit would normally encounter (+/- 300 microseconds) and preferred ipsilateral delays, delays that would be created by sounds in the contralateral sound field. The range of frequencies over which ITD sensitivity was expressed was also similar in the midbrain and thalamus. Several differences were also apparent. In comparison to IC neurons, neurons in the thalamus responded over more restricted ranges of ITD, responded at lower rates, and, in response to monaural stimulation, showed an increased influence of inhibition. Finally, a greater proportion of thalamic units had characteristic delays corresponding to intermediate discharge rates. The preservation of a bias for ipsilateral delays from IC to thalamus suggests that a representation of contralateral azimuths is present at both levels. Similarities between the two levels suggest that information about ITDs is faithfully transmitted from midbrain to thalamus. Differences in ITD sensitivity, such as the sharper tuning for ITDs, suggest that the thalamus is not a simple relay. Enhanced sensitivity to ITDs should translate to better-defined azimuthal receptive fields, and therefore may be a step toward achieving an optimal representation of azimuth within the auditory pathway.

Rostrocaudal and lateromedial density distributions of superior colliculus neurons projecting in the predorsal bundle and to the spinal cord: a retrograde HRP study in the cat

Efferent neurons of the cat superior colliculus (SC) which project in the predorsal bundle (PDB) and to the spinal cord (PDB neurons) form a major pathway by which the SC controls the changes of the direction of gaze in response to stimuli of visual and other modalities. Knowledge of rostrocaudal and lateromedial density distributions of different groups of PDB neurons within the SC is necessary to analyse their relationships with the topography of sensory and motor maps. Density gradients may also bear on the efficacy of connections originating from topographically different collicular regions. In the present study, large injections of HRP/WGA-HRP were made in the C1 segment of the spinal cord and in the pontobulbar tegmentum. Judged by several morphological criteria, axons of passage, including those not subjected to a direct mechanical damage, were participating in the uptake of tracers. Therefore, labeled SC neurons corresponded to the nearly total populations of contralaterally projecting tectospinal neurons (TSNs) and neurons projecting in the PDB, respectively. Subtraction of the TSN density map from that of the whole PDB population was used to infer the distribution of tectal neurons terminating in the rhombencephalic tegmentum (TRhN). This subtotal labeling method proved useful in resolving the contradictions between the earlier HRP studies on the TSN and TRhN topography. The following density distributions were obtained for different groups of PDB neurons: 1) The mean TSN density is more than two times higher in the lateral half of the SC, representing the lower visual field. In this region the density remains constant from rostral to caudal, i.e., from the representation of vertical meridian to large contralateral azimuths. In the medial half, the average density decreases from rostral to caudal. Consequently, TSNs do not show the caudalward increment predicted by the higher efficacy of caudal stimulation points in eliciting head movements. 2) The distribution of PDB neurons is symmetrical with respect to the representation of the horizontal meridian. It is close to homogeneous at all azimuths of the retinotopic map and within the zone limited by small (10-15 degrees) upward and downward elevations.(ABSTRACT TRUNCATED AT 400 WORDS)

Spatial organization of visual messages of the rabbit's cerebellar flocculus. II. Complex and simple spike responses of Purkinje cells.

1. Complex and simple spike responses of Purkinje cells were recorded in the flocculus of anesthetized, paralyzed rabbits during rotating full-field visual stimuli produced by a three-axis planetarium projector. 2. On the basis of the spatial properties of their complex spike responses, floccular Purkinje cells could be placed into three distinct classes called Vertical Axis, Anterior (45 degrees) Axis and Posterior (135 degrees) Axis. The first two classes occurred in both monocular and binocular forms; the third class was encountered only in binocular form. For the binocular response forms, stimulation through one eye, called the dominant eye, elicited a stronger modulation of the complex spike firing rate than did stimulation of the other eye. The approximate orientation of that axis about which full-field rotation elicited the deepest modulation (the preferred axis) when presented to the dominant eye served as the class label. These classes are the same as those determined qualitatively for inferior olive neurons in the previous paper (47). The present study provides a quantitative description of their spatial tuning. 3. For Vertical Axis cells, the dominant eye was ipsilateral with respect to the flocculus recording site. The preferred axis was vertical and null (no-response) axes were in the horizontal plane. For the binocular response form of Vertical Axis cells (less than 10% of this class), the direction preferences for the two eyes were synergistic with respect to rotation about the vertical axis. 4. The dominant eye for the Anterior (45 degrees) Axis cells was contralateral, with the preferred axis oriented in the horizontal plane at approximately 45 degrees contralateral azimuth. The modulation depth showed a close to cosine relation with the angle between the preferred axis and the stimulus rotation axis. The average orientation (n = 10) for the dominant eye preferred axis, determined by the best-fit sinusoid, was 47 degrees contralateral azimuth. The preferred axis orientation for the ipsilateral (nondominant) eye in the binocular response forms was between 45 and 90 degrees azimuth in the horizontal plane. A null axis for each eye was at approximately 90 degrees to the preferred axis. 5. The Posterior (135 degrees) Axis cells were encountered only in binocular response forms. The dominant eye was ipsilateral, with the preferred axis oriented at approximately 135 degrees ipsilateral azimuth close to the horizontal plane. The modulation depth showed a close to cosine relation with the angle between the preferred axis and the stimulus rotation axis.(ABSTRACT TRUNCATED AT 400 WORDS)

The representation of stimulus azimuth by high best-frequency azimuth-selective neurons in the central nucleus of the inferior colliculus of the cat.

The responses to changes in stimulus azimuth of 220 high best-frequency (BF) (greater than 3 kHz) units in the central nucleus of the inferior colliculus of the anesthetized cat were studied with BF tones (220 units) and noise stimuli (84 units). By this means we hoped to gain some insights into the way the azimuthal locations of high BF stimuli were represented in the inferior colliculus. For each unit the discharge rate was determined for stimuli located along a plane tilted at 20 degrees above the horizontal. This plane was chosen to optimize pinna directionality. Locations in the frontal field were sampled in 10-20 degree steps around a 170 degree arc. These measurements were repeated at a number of different stimulus intensities until the directional properties of the unit became clear. Units for which the functions relating discharge rate to azimuth for a given stimulus showed a clear feature (peak or border), the azimuthal location of which varied little with intensities between 20 and 40 dB above threshold, were defined as being azimuth selective for that stimulus. Only 13% of units were azimuth selective for BF tones, whereas 44% were selective for noise. Many azimuth functions for selective units were of the plateau-shaped type for which relatively high discharge rates occurring at most contralateral azimuths declined steeply to near zero and remained low for most ipsilateral azimuths. These plateau-shaped functions were most common for tonal stimuli. Other functions showed a fixed azimuth of maximum firing (best azimuth); these were more common for noise than for tonal stimuli. Detailed azimuth functions for both tone and noise stimuli were measured for 63 units. Some exhibited the same kind of azimuth function to both stimuli. However, 18 units were azimuth selective to noise but not to tones. The borders of plateau-shaped functions obtained using both noise and tonal stimuli were concentrated within 20 degrees of the median plane. Very few units had borders that spanned peripheral ipsilateral or contralateral azimuths. Although the best azimuths of some noise azimuth functions were observed to lie at these peripheral azimuths, the majority occurred around 20 degrees contralateral to the median plane. The recording sites for units were related to a three-by-three matrix of rostrocaudal and mediolateral locations across the central nucleus. Units that were azimuth selective to noise were distributed fairly evenly throughout the central nucleus, whereas units azimuth selective to tones formed highest proportions rostrally.(ABSTRACT TRUNCATED AT 400 WORDS)

Topographic organization of interaural intensity difference sensitivity in deep layers of cat superior colliculus: implications for auditory spatial representation.

Sensitivity to interaural intensity difference (IID) was examined for 103 neurons in the deep layers of superior colliculus (SC) in ketamine barbiturate-anesthetized cats. Noise stimuli were presented dichotically, and IID sensitivity functions were generated while keeping the average binaural intensity (ABI) of stimulation constant. Neurons of three binaural classes were found to be IID sensitive. Neurons receiving excitatory contralateral input and inhibitory ipsilateral input (EO/I cells, 55% of sample) had steplike IID functions, with maximum response at IIDs corresponding to contralateral azimuths (positive IIDs), total suppression at IIDs corresponding to ipsilateral azimuths (negative IIDs), and cutoffs at different positions along the IID axis for different neurons. Neurons responsive only to binaural stimulation (OO/F cells, 15% of sample) had IID functions with a sharp peak in the range of 0 to 10 dB IID. Cells receiving excitatory input contralaterally and a facilitatory ipsilateral input (EO/F cells, 7% of sample) had IID functions of intermediate shape, with a peak in the range of 10 to 20 dB IID and a sharper cutoff near zero IID than at larger positive IIDs. The sharpness of IID cutoff for EO/I cells was quantified by measuring an 80% IID dynamic range. Neurons with 80% IID dynamic ranges of less than 26 dB were judged to have sharp cutoffs. The position along the IID axis of the IID cutoff for these cells was quantified by recording the IID at which the response was at 50% of maximum (half-maximal IID). A topographic organization of EO/I cells with sharp IID cutoffs was found along the rostrocaudal axis of SC, such that rostral EO/I cells had IID functions with half-maximal IIDs near zero, while increasingly caudal EO/I cells had progressively larger (positive) half-maximal IIDs. Although detailed maps could not be obtained in individual animals, the topography was observed in each of nine experiments in which EO/I cells were located in two or more rostrocaudal locations (P = 0.00002). The effect of stimulus level on the stability of IID cutoff was examined for 13 EO/I cells. The majority (85%) showed less than 10 dB variation in half-maximal IID across a range of suprathreshold ABIs, indicating that EO/I cells in SC generally exhibit stability in cutoff with changes in intensity of broadband stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

Topographic organization of interaural-intensity-difference sensitivity in deep superior colliculus

The sensitivity to interaural intensity differences (IIDs) of acoustically responsive neurons was examined in multiple microelectrode penetrations along the rostro-caudal axis of deep superior colliculus (SC) in anesthetized cats. The most rostrally located neurons were unresponsive to monaural stimulation but were strongly excited binaurally. IID sensitivity functions for these “predominantly binaural” (PB) neurons were sharply peaked with maximum response at or near zero IID. Caudal to the PB cell area, most neurons were excited by contralateral stimulation and inhibited by ipsilateral stimulation (EI cells). All EI cells were maximally excited over a range of IIDs favoring the contralateral ear and were totally inhibited over a range favoring the ipsilateral ear. The position of the cutoff between excitatory and inhibitory regions was located near zero IID (corresponding to the median plane) for the most rostrally located EI neurons and was shifted progressively to IIDs favoring the contralateral ear (corresponding to increasing contralateral azimuths) for more caudally located EI neurons. The consequence of this topography is that IID, and hence azimuthal location, is represented by the extent of deep SC activated.