Sensitivity of single neurons in auditory cortex of cat to binaural tonal stimulation; effects of varying interaural time and intensity

Corticocortical connections of cat primary auditory cortex (AI): laminar organization and identification of supragranular neurons projecting to area AII

The laminar distribution and structure of the supragranular cells projecting from primary auditory cortex (AI) to the second auditory cortex (AII) in the cat were studied with horseradish peroxidase. Injections in AII retrogradely labeled somata in ipsilateral cortical layers I-VI of AI. A bimodal laminar disposition was found, with approximately 40% of the labeled cells in layer III, 25% in layer V, and 10-15% each in layers II, IV, and VI; only a few cells were found in layer I. The labeled cells were scattered in small aggregates between which unlabeled neurons were interspersed. There was some, though not a strict, topographical distribution of the labeled cells according to the locus of the injection in AII. Injections in the caudal part of AII labeled cells in more rostral AI, while rostral AII injections labeled cells in more caudal AI. Ventral AII injections labeled more ventrally located AI cells, while more dorsal AII injections labeled more dorsally situated AI cells. AII injections also labeled cells in other auditory cortex subdivisions, including the posterior ectosylvian, ventroposterior, temporal, and dorsal auditory zone/suprasylvian fringe cortical areas, and in some non-auditory cortical areas. In layers II and III, both pyramidal and non-pyramidal cells were labeled. More pyramidal cells were labeled in layer III than layer II (80% vs. 62%), and the proportion of non-pyramidal cells in layer II was more than twice that in layer IV (27% vs. 12%). The types of labeled cells were distinguished from one another on the basis of size, somatic and dendritic shape, and laminar distribution. The profiles of labeled cells in these experiments were compared to, and correlated with, those in Golgi-impregnated material. In layer II, the classes of corticocortical projecting cells consisted of small and medium-sized pyramidal, bipolar, and multipolar cells. Those in layer III included small, medium-sized, and large pyramidal neurons, and bipolar and multipolar cells. The average somatic area of the labeled cells did not differ significantly from that of the unlabeled cells, and both were about equal in somatic size to neurons accumulating tritiated gamma-aminobutyric acid in layers II and III. These findings suggest that there is convergent, ipsilateral input onto AII from every layer in AI, and from other cortical auditory and non-auditory areas. A morphologically heterogeneous population of cells in AI contributes to these projections. Diversity in the cytological origins of corticocortical projections implies functional differences between layers II and III since the latter also projects commissural, while layer II in the cat, does not.

Scalp potentials of normal and hearing-impaired subjects in response to sinusoidally amplitude-modulated tones

None of the current electrical audiometric procedures, alone or in combination, has yet achieved the precision of conventional audiometric testing that is used to assess hearing in verbally capable children and adults. The reason for this, in part, lies in the use of stimuli which have a wide frequency content. We have measured scalp potentials which follow the envelopes of sinusoidally amplitude-modulated tones: a frequency-specific stimulus. In normal subjects such amplitude-modulation following responses (AMFRs) appear to be generated by two sources. One source has a latency of about 30 ms, generates large responses and is only observed at modulations below 55 Hz, while the other source has a latency of 7-9 ms, generates smaller responses, and is only observed at modulations from 100-350 Hz. The latencies of these two sources are consistent with origins in the cortex and midbrain, respectively. We examined AMFRs to low frequency (50 Hz) modulations as a possible audiometric tool. In normal subjects, the amplitude of the AMFR increased as a function of intensity, decreased as a function of carrier frequency, and could be evoked across the whole audiometric range (250-8000 Hz). In hearing-impaired subjects, the AMFR amplitudes as a function of carrier frequency accurately reflected the pattern of hearing loss on a frequency-by-frequency basis. In most subjects, the threshold for evoking the AMFR was within 0-25 dB of hearing threshold. It therefore appears that the AMFR may be a potentially useful tool to assess hearing in those unable to undergo conventional audiometric testing.

Commissural neurons in layer III of cat primary auditory cortex (AI): pyramidal and non-pyramidal cell input

The types of layer III neurons in cat primary auditory cortex (AI) projecting to the contralateral AI were studied with horseradish peroxidase or horseradish peroxidase conjugated to wheat germ agglutinin. Injections between the anterior and posterior ectosylvian sulci retrogradely labeled both pyramidal and non-pyramidal somata in contralateral cortical layers III, V, and VI in AI, and in the ventral nucleus of the ipsilateral medial geniculate body. Three-quarters (72%) of the retrogradely labeled cells were found in layer III and one-quarter (28%) lay in layers V and VI. Every part of AI was innervated by commissural neurons. The topographical distribution of the labeled cells varied systematically. Injections in the caudal part of AI labeled cells in the caudal part of the opposite AI, while more rostral injections labeled cells in the contralateral, rostral AI. Injections covering the rostro-caudal extent of AI labeled cells throughout the opposite AI. Each part of AI thus projects most strongly to a contralateral, homotypic area, and less strongly to other, adjacent sectors of AI. The types of labeled cells were distinguished from one another on the basis of size, somatic and dendritic morphology, laminar distribution, and nuclear membrane morphology. Their somatodendritic profiles were compared to, and correlated with, those in Golgi-impregnated material from adult animals. Among the pyramidal cells of origin were small, medium-sized, and large neurons, and star pyramidal cells. The non-pyramidal cells of origin included bipolar and multipolar cells. Thus, at least six of the 12 kinds of neurons, as defined by morphological methods, participate in the interhemispheric pathway. Pyramidal cells comprised 65% of the cells of origin, 14% of the labeled cells in layer III were non-pyramidal, and 21% of the neurons could not be classified. It is unknown if these different types of commissural neurons have the same laminar or cytological targets in AI, or if they represent more than one functional or parallel pathway within AI. In any case, cytologically diverse layer III neurons contribute to the commissural system.

Some neural mechanisms in the cat's auditory cortex underlying sensitivity to combined tone and wide-spectrum noise stimuli

In the auditory cortex of nitrous oxide-anesthetized, muscle-relaxed cats, single neurons were studied for their responsiveness to pure tones that were mixed acoustically with simultaneously gated wide-spectrum noise bursts presented using a calibrated sealed stimulating system. The intensities of both the tone and the noise were systematically varied, with a view to ascertaining the sensitivity of cortical cells to a characteristic frequency tone delivered in the presence of a noise mask. Neurons for which wide-spectrum noise provided a net excitatory influence typically displayed a 'strong-signal capture' effect; that is, the cell's responses were dominated by whichever of the two elements of the combined stimulus was the more effective when tested separately. These cells generally had monotonic tone rate intensity functions. Most of the cells that were suppressed by the noise displayed nonmonotonic pure tone rate intensity functions. When nonmonotonic cells were studied with the combined stimulus, the noise was found to produce an intensity-dependent suppression of their tone-evoked responses that could not be overcome by elevating the tone intensity. In contrast, for the minority of monotonic neurons whose tone-evoked responses were suppressed by noise, that suppression could be overcome by raising the tone intensity. None of the cells in the sample responded in a sustained fashion to continuous noise. In each of 11 cases examined, the effect of a continuous noise mask was to elevate tone thresholds and to prolong latent periods for tones; the magnitude of both of these effects depended on the intensity of the continuous noise mask.

Neurons in the cat's primary auditory cortex distinguished by their responses to tones and wide-spectrum noise

In the cortex of barbiturate-anesthetized cats, area AI was identified by its tonotopic organization, and single neurons in that field were examined with regard to the shapes of their spike count-versus-intensity functions, the organization of their frequency-intensity response areas, and their responses to wide-spectrum noise, using calibrated sealed stimulating systems. Neurons whose pure tone rate intensity functions were monotonic in shape displayed V-shaped response areas that were open-ended at high tone intensities. In contrast, cells displaying nonmonotonic tone intensity functions tended to have circumscribed response areas; these cells were responsive to tones over limited ranges of both frequency and intensity. Monotonic neurons almost always responded to wide-spectrum noise stimuli, while nonmonotonic neurons often did not. The mean minimum latent period of monotonic cells (14.0 ms) was significantly shorter than that for nonmonotonic neurons (19.1 ms). For those cells that responded to both tones and noise, minimum latent periods for the two stimuli were similar or identical. Monotonic neurons tended to be horizontally segregated from nonmonotonic neurons across AI's middle cortical layers. The implications of these data for the nature of some neural mechanisms underlying the stimulus selectivity of cortical cells are discussed.

Binaural interactions in cortical area AI of cats reared with unilateral atresia of the external ear canal

Binaural interactions were recorded in auditory cortical (AI) neurons of anesthetized adult cats that had unilateral atresias created shortly after birth by surgically ligating and cutting one external ear canal. At the time of the recording experiment, the atresia and associated debris were removed and tones were delivered to both tympanic membranes via a sealed and calibrated acoustic system. The majority of neurons recorded were in the cortex ipsilateral to the previously occluded ear. Thresholds for monaural stimulation of either the operated or unoperated ear were within normal range although thresholds to stimulation of the previously operated ear tended to occupy the upper end of the normal distribution. Monotonic and nonmonotonic spike count-vs-intensity functions derived from responses to monaural stimulation of the atretic ear were indistinguishable in their shape from those recorded in normal cats. All binaural classes were represented in our sample in proportions similar to those reported in cats with two normal ears. The forms of the functions relating spike count to interaural intensity differences and interaural phase differences were essentially the same as those seen in normal animals. The main binaural deficit observed under these conditions was a shift in the interaural intensity difference to which an AI neuron was most sensitive. The intensity needed for a stimulus to the atretic ear to participate in the binaural response was as much as 50 dB higher than that at the opposite normal ear in contrast to the nearly equal SPLs required for binaural interactions in cats with two ears intact. It was suggested that elevated thresholds at the previously operated ear could account for much of the shift observed.

Temporal response features of cat auditory cortex neurons contributing to sensitivity to tones delivered in the presence of continuous noise

Single cat auditory cortex neurons have limited intensity dynamic ranges for characteristic frequency (CF) tones. In the presence of continuous wide-spectrum noise, these cells' tone responses undergo a dynamic range shift towards higher SPLs. In the present study, the mechanisms underlying this dynamic range shift were examined by probing the sensitivity of the cells to CF tones delivered at various delays after the onset and/or offset of a long duration noise mask. Fifty cells were studied in the cortex of 7 anesthetized cats using acoustically mixed tonal and noise stimuli presented monaurally to the contralateral ear through a calibrated, sealed stimulating system. For most neurons, the dynamic range shift induced by continuous noise was fully developed in the responses to CF tones delivered 100-250 ms after the onset of a noise mask. For nonmonotonic cells, shorter delays between noise and tone onsets resulted in a profound suppression of tone responses that was consistent with the view that noise stimuli evoke a short latency, but transient, inhibitory response in these neurons. Studies of monotonic cells with short tone delays revealed that the usual excitatory response to noise onset was sometimes followed by a period of inhibition. In most cells, as soon after mask onset that CF tones were able to evoke spike discharges, those responses had latent periods comparable to those of responses to tones of the same SPL delivered in continuous noise. After the offset of an 800 ms noise mask effecting a 15-25 dB dynamic range shift for CF tones, recovery of tone sensitivity to within 5 dB of control levels typically took 50-200 ms. On the basis of these observations, it is argued that in order for a CF tone to excite a cortical neuron after the onset of a noise mask, the tone amplitude must be sufficient to overcome both the transient central neural consequences of noise onset, and a short-term adaptation that is probably peripheral in origin. The implications of these data for the sensitivity of cortical cells to temporally varying stimuli are discussed.

Patterns of inhibition in auditory cortical cells in awake squirrel monkeys

Two-tone interactions are recorded in the responses of single units in the superior temporal gyrus to contralateral acoustic stimulation of the awake squirrel monkey. Four response types are distinguished based primarily on the nature of the inhibitory responses elicited by two-tone stimuli, and secondarily on such criteria as the patterns of response to single tones and noise stimuli, thresholds, and spontaneous activity levels. Type A units display strong lateral inhibitory influences which may extend up to 2 octaves on either side, or both sides, of the BF. They are sharply tuned at all intensities and exhibit sustained response to single tone stimuli at the BF. The units have nonmonotonic rate-level functions, and show little or no response to broad band noise. Type A units have low spontaneous rates (less than 3 spikes/s) and relatively high thresholds (greater than or equal to 30 dB SPL). Type B units are characterized by relatively high spontaneous rates of activity (greater than 20 spikes/s) and inhibitory responses to single tone stimuli. Broad band noise may evoke strong excitatory response. Type C units summate the responses to the two-tone stimulus, and show little or no inhibitory influences. They have V-shaped tuning curves, monotonic rate-level functions, low thresholds (greater than or equal to 30 dB SPL), moderate spontaneous rates (ca. 10 spikes/s), and a strong and sustained response to noise and single tone stimuli. Type D units show 'temporal inhibition' to two-tone stimuli, in that an excitatory response to the first tone suppresses (adapts or inhibits) the response to the second tone. These units generally have moderate to broad frequency tuning and phasic responses to single tone stimuli. Histological examination of electrode tracks suggests that Type A units are restricted to A1 (and possibly the rostral field) while other types are distributed over all auditory fields.

Anatomy of layer IV in cat primary auditory cortex (AI)

The structure of neurons and axons in layer IV was studied as part of a larger inquiry into the organization of primary auditory cortex (AI) in the cat. Tissue from the convexity of the middle ectosylvian gyrus between the anterior and posterior ectosylvian sulci was studied in Golgi, Nissl, Bodian, plastic-embedded, and other preparations from adult animals. Layer IV is defined as a strip about 200-250 micron thick consisting predominantly of small non-pyramidal neurons intercalated between the pyramidal somata of layers III and V, and in which few commissurally projecting cells occur. Lying some 800-900 micron beneath the pia, layer IV has six types of neurons, as seen in Golgi-stained material from anatomically and physiologically defined AI. These include three varieties (small, medium-sized, and large) of tufted neurons with intracortically branching axons and vertically polarized, cylindrical dendritic fields. Besides the tufted cells, which are the most numerous neurons in layer IV, large multipolar, double bouquet, and spiny stellate cells are scattered through layer IV. Each has a characteristic neuronal architecture and intracortical axonal branches. Smaller tufted cell somata dominate the upper half of layer IV (IVa), larger tufted cells are more common deep in layer IV (IVb). The average somatic area in midnuclear , plastic-embedded sections is 158 micron2. Layer IV (and layer IIIb) receive thick, probably ascending fibers, forming narrow, vertical terminal fields. These axons may be of thalamic origin and overlap with alternating, 50-75-micron-wide columns of somata and neuropil in layer IV. Layer III pyramidal cell axons often project to layer IV and ramify vertically and horizontally. The average height-width ratio of the dendritic domains of layer IV cells is about 3.6:1. The vertically disposed dendrites of layer IV cells, the columnar arrangement of their local axonal branches, and the polarized form of intrinsic and extrinsic axons collectively reinforces the columnar pattern in layer IV. Many layer IV cells structurally resemble neurons in layer IV in the primary visual and somatic sensory cortex. However, most AI cells have a pronounced columnar arrangement of their somata and an elongated, narrow form. The axons of many layer IV cells preserve this vertical arrangement and often branch in layer III and, to a lesser degree, in layer V.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of spatial receptive fields in the central nucleus of the cat inferior colliculus. II. Stimulus intensity effects

Single units in the central nucleus of the inferior colliculus (ICC) of barbiturate-anaesthetized cats were studied using pure-tone, best-frequency stimulation presented in the free field. At low stimulus intensities almost all neurones responded most strongly to stimuli positioned along the acoustical axis of the pinna contralateral to the recording electrode and there was little or no response to stimuli positioned in the ipsilateral hemifield. Four major classes of spatial response were distinguished when tones of moderate to high intensity were used. The simplest response (24% of the sample) to increasing intensity consisted of a monotonic increase in discharge level at all effective speaker positions and an expansion of the area of space from which a stimulus influenced the response (receptive field). A second class (21%) of units had a nonmonotonic increase in discharge level and an expanding receptive field with increasing intensity. Neither of these classes showed evidence of influence from the ipsilateral ear. The third class (26%) developed, at higher intensities, a second excitatory response region in the ipsilateral hemifield. The fourth class (20%) had receptive fields with fixed medial borders, irrespective of intensity. The third and fourth classes of units were thought to be binaurally influenced and to be sensitive to interaural phase and intensity differences, respectively.

Spatial receptive fields in the cat inferior colliculus

Auditory spatial receptive fields of 122 single units in the inferior colliculus of 8 anesthetised cats were studied with free-field acoustic stimuli presented in the frontal hemisphere. The best frequency and best frequency threshold were determined for each unit with the speaker located in a position where the unit responded strongly. The intensity was then raised to 10 dB above threshold at the best frequency and the boundaries of the spatial receptive field were determined. For sounds of low intensity, receptive field size appeared to be a continuum with respect to best frequency. Units of high best frequency had small circumscribed fields located in the contralateral frontal hemifield. The boundaries of receptive fields for units of progressively lower best frequency expanded in all directions. Thus for intermediate frequencies, fields typically filled the contralateral hemifield whereas for low frequencies, units could be activated by stimulation from any position tested. At higher intensities, the boundaries of the receptive fields of units expanded. Circumscribed receptive field centres lay on a line corresponding to the acoustical axis of the contralateral pinna. For these units with small receptive fields, the free-field response to low intensity sounds appeared to be attributable more to the directional properties of the contralateral pinna than to significant binaural interaction.

Directionality of sound pressure transformation at the cat's pinna

The directionality of the cat's pinna was studied by using the amplitude of the cochlear microphonic (CM) as a quantitative indicator of tympanic sound pressure level (SPL). It was found that tympanic SPL varied with the location of a free field stimulator in anechoic space. For high (tonal) frequencies, there was a circumscribed optimal area for tympanic SPL in the frontal ipsilateral sound field, in confirmation of previous findings with other techniques that the pinna has an acoustical axis. The directionality of the pinna, determined from the solid angle enclosed by the 5 dB isointensity-decrement line with respect to the optimal position, increased with frequency. For low tonal frequencies, no circumscribed optimal area in the frontal sound field could be distinguished, and tympanic SPL fell by only 10-12 dB for displacements of 90 degrees into the contralateral sound field. Excision of the pinna abolished the circumscribed optimal areas for tympanic SPL and revealed the pinna produces up to 28 dB amplification of acoustic signals delivered 'on-axis'.

Representation of the two ears in the auditory cortex: a re-examination

The current theory of the representation of the two ears in the auditory cortex, namely that both ears are represented in each cerebral cortex and that the contralateral ear receives a stronger representation, is critically reviewed in the light of more recent evidence. On the basis of neurophysiological and behavioral studies in animals and a variety of clinical studies, it is concluded that the notion of contralateral ear dominance in the cortex requires revision, and it is proposed that the contralateral sound field, rather than contralateral ear as such, is represented in each cortex. It is suggested that the contralateral and ipsilateral inputs to the auditory cortex do not so much differ in their relative strengths, but in the qualitative nature of the information they transmit.

Single-unit activity in the auditory cortex of monkeys actively localizing sound sources: spatial tuning and behavioral dependency

Single-unit recordings were made from the auditory cortex of unanesthetized rhesus monkeys which were trained to perform a second localization task. Recordings were made from 196 units in 4 animals. Five sound sources were located at 0 degrees (midline) and on either side at azimuths of 37.5 degrees and 75 degrees. Almost most units responded to each of the 5 sources, 49% of the units had peak firing rates for the source on the contralateral side at 75% azimuth. Another 26% of the units had peak firing rates at the 37.5 degrees contralateral location, while only 11% had peak rates for the ipsilateral locations. In order to determine whether the behavior of actively locating a sound source affected units in auditory cortex, response rates compared under two behavioral conditions: one which required the detection of a sound regardless of location and another condition which required identification of a sound's location. of the 196 units, 16 had different response rates for the two conditions, with 15 of the 16 units having higher evoked rates in the localization task. For the 16 units, the difference in firing rate was typically observed for a single speaker location. No correlation could be determined between a unit's location within the subdivisions of auditory cortex and its spatial tuning response properties or its dependency upon behavioral condition.

Responses of single neurons in physiologically defined area AI of cat cerebral cortex: sensitivity to interaural intensity differences

In 15 cats, cortical area AI was defined by its frequency organization, and cells within that field were tested for sensitivity to interaural intensity differences (IIDs) using sealed stimulus delivery systems. Of 39 cells tested quantitatively, 26 were sensitive to IIDs. In 70% of cases, sensitivity to IIDs reflected suppressive binaural interactions, and was manifested as a sigmoidal relation of spike count to IID. For 8 other cells, facilitative binaural interactions generated unit sensitivity to IIDS; three of these neurons demonstrated nonmonotonic dependency of spike count on IID, with peak firing rates at or near 0 dB IID. Analysis of spike count versus IID functions in terms of the auditory azimuths known to generate the IIDs used revealed that the majority of cells were most sensitive to IIDs associated with azimuths in the contralateral sound field. These data are compatible with other evidence on the sensitivity of cortical and brainstem cells to binaural sound localization cues, and suggest that each side of the auditory brain is independently capable of localizing sound sources in the contralateral field.

Location of rabbit auditory cortex and description of single unit activity

The following data were obtained from microelectrode exploration of rabbit auditory cortex (AC). (1) Location of AC--Major portions of the rabbit's right hemisphere were stereotaxically mapped for auditory responsive and non-responsive regions. Auditory responsive regions were found to lie behind the rhinal sulcus, extending from its midpoint to its inferior border. (2) Frequency selectivity--Characteristic frequencies (CFs) ranged from 0.4 to 33 kHz. Sharp, multipeaked and broad tuning curves were described. (3) Response patterns--A variety of post-stimulus-time (PST) histogram shapes were obtained in response to tonal stimuli. The response of a given neuron often varied with changes in stimulus frequency or intensity. (4) Latency--Neural response latencies ranged from 10 to 100 msec following stimulus onset. (5) Spontaneous rate--In the absence of acoustic stimulation, spike discharges ranged from less than 1 spike/sec to 48 spikes/sec. Spontaneous rates of less than 5 spikes/sec were measured in 42% of the neurons. Rates in excess of 15 spikes/sec occurred in fewer than 18% of the cells examined. (6) Response variability--Stimulus-evoked activity was seen to vary over time. Changes consisted of an overall increase or attenuation of discharge rate, as well as modifications of selected portions of the response pattern (PST histogram shape). The characteristics of single unit activity in rabbit auditory cortex are consistent with findings reported in other species.

Patterns of responses of cortical cells to binaural stimulation

The relationship between stimulus intensity and response latency implies that the relative time of arrival of input to binaurally sensitive loci can vary over a range significantly longer than the interaural delays resulting from the travel time of sound between the ears. To investigate the effect of such potentially long inter-arrival time intervals, responses of binaurally sensitive neurons in auditory cortex of cats were examined over relatively long interaural delays. Two kinds of interaural delay functions were observed. One kind involved a sharp transition to a reduced response rate over a narrow range of interaural delays, while the other involved a gradual reduction of response rate over a relatively prolonged range of interaural delays. The amount of suppression was found to be a sensitive function of both interaural delay and the relative intensity of the stimuli delivered to the two ears. The pattern of suppression across a binaural response was stationary with respect to the excitatory contralateral stimulus over wide ranges of interaural delay. In addition, certain characteristics of a group of cells generally responsive only to binaural stimulation are described. A model is proposed which accounts for the observed stationarity of the response suppression.

Binaural response-specific bands in primary auditory cortex (AI) of the cat: topographical organization orthogonal to isofrequency contours

The spatial distribution of neurons with different binaural response properties has been studied within the three dimensions of the primary auditory cortex (AI) in the cat. Using dichotic stimulation, 92% of neurons encountered could be classified into either the excitatory/excitatory (EE) or excitatory/inhibitory (EI) interaction class. In nearly all of almost 800 penetrations introduced along radial axes, all neurons encountered along a given penetration were of the same binaural response class. Neurons of different binaural interaction classes were spatially segregated within the plane of the cortex. Electrode penetrations made parallel to isofrequency contours traversed the mediolateral extent of AI through the middle layers of the cortex. A sharp segregation of units by binaural response class was observed in these penetrations, i.e. sequences of neurons that were all of the EE class alternated with sequences of EI neurons. The regions of uniform response to binaural stimulation formed radially organized topographical subunits that were elongated along the rostrocaudal dimension of AI. These binaural interaction bands intersect the lines of re-representation of the cochlear sensory epithelium ('isofrequency contours') and, thus, create subdivisions of AI that each contain a representation of the entire audible frequency domain. The implications of these results for the concept of AI as a unitary element in auditory processing are discussed.

Response of cat inferior colliculus neurons to binaural beat stimuli: possible mechanisms for sound localization

The interaural phase sensitivity of neurons was studied through the use of binaural beat stimuli. The response of most cells was phase-locked to the beat frequency, which provides a possible neural correlate to the human sensation of binaural beats. In addition, this stimulus allowed the direction and rate of interaural phase change to be varied. Some neurons in our sample responded selectively to manipulations of these two variables, which suggests a sensitivity to direction or speed of movement.

Binaural columns in the primary field (A1) of cat auditory cortex

Responses of single neurons and neuron clusters were studied at short intervals along penetrations into the high-frequency (4-25 kHz) representation of A1. The monaural and binaural sensitivities of neurons at best frequency were studied. With the exception of some neurons responsive only to binaural stimulation, the monaural responses of most were classified as contralateral dominant, ipsilateral dominant or equidominant. Binaural interactions of most neurons were classified as summation (binaural response size greater than the monaural response size) or suppression (dominant ear response size greater than the binaural response size). Most neurons arrayed in a column perpendicular to the cortical surface display the same aural dominance and binaural interaction. Summation columns occupy about two-thirds of the area sampled; suppression columns, about one-third. Within most suppression columns, the contralateral ear was dominant. Within summation columns, aural dominance varied. Summation columns appear to be composed of smaller columns differing in aural dominance. The sizes of binaural interaction columns vary considerably; some occupy several square millimeters of cortical surface. At least some binaural interaction columns occupy strips of cortex oriented orthogonal to isofrequency contours.

Evoked unit activity in auditory cortex of monkeys performing a selective attention task

Single-unit responses were recorded from the auditory cortex of rhesus monkeys performing a selective atteintion task which used combined light and sound stimuli. The animals were first trained to push a lever to the left for a noise burst and to the right for a tone burst, and then trained to push left for a left light and right for a right light. Subsequently, one of the four possible light and sound stimulus combinations (Noise + Left Light, Noise + Right Light, Tone + Left Light, Tone + Right Light) was randomly presented on each trial. In blocks of 100 trials only one part of the combined stimulus (either the light or the sound) determined the direction of lever push that would be reinforced. Responses of single units to identical sound stimuli were compared for blocks in which sound was the relevant cue and blocks in which light was the relevant cue. Typically, differences were in response strength without alteration of response pattern. Even the earliest response components (15-20 msec latency) could show changes. Two-thirds of the response comparisons showed differences in strength depending upon whether sound or light was the relevant cue, with about as many responses stronger for light relevant as for sound relevant. Independent of which modality was relevant, unit responses for trials in which both parts of the combined light and sound stimulus indicated the same direction of lever push were generally stronger than unit responses for trials in which the two parts of the combined stimulus signalled opposite directions of lever pushes.

Single unit study of binaural interaction in the auditory cortex of the chinchilla

The primary aim of this investigation was to systematically compare for various stimulus conditions the relative influences of contralateral and ipsilateral acoustic stimulation on cortical single units in an unanesthetized preparation and to study the effects upon single unit responses of the dominant stimulus cues for sound localization--interaural intensity difference (deltaI) and interaural time difference (deltat). Recordings were obtained from 133 units in chinchillas immobilized by gallamine triethiodide. All units were found to be influenced by input from both ears. Unit thresholds for contralateral stimulation were lower and more discharges were elicited than for ipsilateral stimulation over a range of intensities from unit threshold intensity to 80 dB sound-pressure level. A predominance of contralateral influence was also observed when the number of stimulus-evoked discharges was plotted as a function of the deltaI or deltat. For 62% of the deltaI functions maximal responsiveness occurred for binaural stimuli that were more intense at the contralateral ear. Similarly, of the 36 units that showed sensitivity to deltat parameters for tone stimuli, 22 (61%) were maximally responsive at the contralateral-leading deltat intervals. For click stimuli, maximal responsiveness for all 21 deltat-sensitive units also occured for contralateral-leading stimuli. Certain observations in the study question the generality of the hypothesis that a particular cell invariantly encodes a specific deltat, i.e., that cells have 'characteristic delays'. First, most units tested at more than two frequencies showed maximal responsiveness at different deltat intervals depending upon stimulus frequency. Second, the deltat intervals for maximal responsiveness for half of the units tested were greater than the maximal interaural delays the animal could encounter naturally. Third, deltat functions from the same unit for click and tone stimuli showed poor correspondence. These findings suggest that the encoding of interaural time and intensity might depend on an inter-hemispheric comparison of the activity of neural populations as originally proposed by von Bekesy.

Organization of the cat trapezoid body and the discharge characteristics of its fibers

The laminae of the midline cat trapezoid body (TB) are delimited and named on the basis of their characteristic fiber diameters. Proceeding ventrally from the dorsal boundary of the TB are found the medium, mixed, large, and small diameter populations (the MDP, XDP, LDP, and SDP respectively). Specific cell types of the cochlear nuclear complex give rise to the fibers of the TB. Medium diameter fibers originate from spherical cells while globular cells are the source of the large diameter fibers. Bilateral cochlear nuclei ablations demonstrate that, withe the possible exception of a few thin fibers, all midline TB fibers are second order. The discharge characteristics of single units recorded near the midline in the MDP, XDP, and LDP are presented. The MDP displays a dorso-ventral progression of best frequencies, from low to high. Very high best frequencies (greater than 12 kHz) do not seem to be represented in the MDP but are found in the XDP and LDP. Except for latent periods, the properties of units in the MDP are the same as those of spherical cells and eight nerve fibers for all parameters where comparisons are available. The excitatory response characteristics of units in the XDP and LDP are similar to those of the MDP. They differ from MDP fibers in having tuning curves with inhibitory sidebands as well as a lower mean spontaneous rate of discharge. The results confirm previous observations on the spherical cells and define the discharge characteristics of the globular cells.

Synaptic relationships of Golgi type II cells in the medial geniculate body of the cat

A combined analysis with the Golgi and silver-degeneration methods and electron microscopy in the ventral nucleus of the medial geniculate body has confirmed that the Golgi type II neuron forms dendro-dendritic synapses with the principal neuron in terminal aggregates called synaptic nests. Both types of neurons receive synaptic contacts from the afferent axons that ascend from the posterior colliculus and from those that descend from the auditory cortex. Only the principal neuron projects to the auditory cortex. The Golgi type II cells that receive endings from afferent axons send presynaptic processes to principal cells that are also contacted by the very same afferent axons. The axons of Golgi type II cells project to synaptic nests other than those supplied by the dendrites of the parent cell and link the Golgi type II cells with each other. On the surface of the Golgi type II cell there is a segregation of the different types of synaptic endings and a consistent sequence in their synaptic relationships. The endings of colliculogeniculate and Golgi type II axons predominate on the distal dendrites in the synaptic nests. Corticogeniculate endings congregate more on the soma and proximal dendrites. In the synaptic nests the Golgi type II dendrites are presynaptic to the principal cell dendrites, whereas both kinds of dendrites are postsynaptic to the very same axons, which project either from the posterior colliculus or from Golgi II cells...