Cat Auditory Cortex Research Articles

The posterior field P of cat auditory cortex: coding of envelope transients.

The posterior field (P) of the cat auditory cortex contains a very high proportion of neurons whose responses change non-monotonically with the sound pressure level (SPL) of tonal stimuli, leading to circumscribed frequency-SPL response areas, and it has therefore been suggested that field P may be specialized for processing of sound intensity. We demonstrate here a great diversity of response areas in field P. Furthermore, by varying tone SPL and rise time, we show that, as in primary auditory cortex (AI), the onset response of a field P neuron is better described as a function of the instantaneous peak pressure (envelope) at the time of response generation than of the steady-state SPL of the stimulus. Such responses could be used to track transients or represent envelopes in more general terms, rather than to code SPL. Compared with AI, field P neurons have relatively long minimum latencies along with a large jitter in spike timing. Tracking would therefore be most effective for slowly varying envelopes, and one function of the inhibition that generates non-monotonicity in field P may be to suppress temporally sluggish responses to rapid transients, such as the onsets of high-SPL, short rise time tones. Field P may thus be specialized for coding slowly varying signals.

Real-time imaging of neural activity during binaural interaction in the guinea pig auditory cortex.

Spatio-temporal patterns of binaural interaction in the guinea pig auditory cortex (AC) were observed using optical recording with a 12 x 12 photodiode array and a voltage-sensitive dye. The amplitudes of the sound-induced light signals from the cortex were transformed into sequential two-dimensional images every 0.58 ms. Binaural sound stimuli evoked an excitatory response followed by a strong inhibition, and contralateral stimuli evoked a strong excitatory response followed by a weak inhibition. Ipsilateral sound stimuli evoked a weak response. Binaural stimulation induced two types of ipsilateral inhibition: a fast binaural inhibition which was detected only after the contralateral and ipsilateral responses were subtracted from the binaural responses, and which appeared 12-25 ms after the onset of stimulation, and a slow binaural inhibitory effect which was clearly observed in the binaural responses themselves, appearing 70-95 ms after the onset of stimulation. The fast binaural inhibition was observed in the same area as the contralateral excitatory response. The inhibited area became stronger and more widespread with increasing intensity of ipsilateral stimulation. We did not observe the specialized organization of binaural neurons as electrophysiologically found in the cat AC, in which binaural neurons of the same binaural response type are clustered together and alternate with clusters of other response types.

Temporal Integration and Duration Tuning in the Dorsal Zone of Cat Auditory Cortex

The present study examined auditory cortical neurons, the responses of which depended on the duration of noise bursts. We recorded from 150 neurons with response latencies exceeding 30 msec and from 28 neurons with OFF responses to auditory stimuli in the dorsal zone of cat auditory cortex. Of 150 long-latency neurons, 132 displayed some form of duration selectivity. Seventy-eight were classified as selective for long durations. Among the long-duration-selective neurons, 30 responded only to noise burst stimuli with durations longer than a minimal threshold and were classified further as duration threshold neurons. Of 132 duration-selective neurons, 41 responded selectively to noise bursts of short duration; 13 showed maximal responses to noise bursts of a particular duration and could be regarded as duration-tuned neurons. OFF-response neurons included ones that were long-duration-selective, duration-tuned, and nonduration-selective. Duration tuning has been described previously only at the midbrain level in amphibians and bats. The present finding of sensitivity to sound duration in at least one region of cat auditory cortex indicates that this form of neural tuning may be important for hearing in all vertebrates, and for processing of sound at multiple levels in the auditory pathway. The duration tuning in the cat auditory cortex was much broader, and the best duration was distributed over a wider range than in the bat inferior colliculus. We suggest that the duration selectivity of the long-latency neurons results from integration along the time domain of a stimulus during the latent period.

サリチル酸，キニンによるネコ聴覚皮質領神経活動の影響

サリチル酸，キニンによるネコ聴覚皮質領神経活動の影響

Optical imaging of cat auditory cortex cochleotopic selectivity evoked by acute electrical stimulation of a multi-channel cochlear implant.

We measured reflectance changes by means of optical imaging of intrinsic signals to study the effects of acute electrical cochlear stimulation on the topography of the cat auditory cortex. After single-pulse electrical stimulation at selected sites of a multichannel implant device, we found topographically restricted response areas representing mainly the high-frequency range in AI. Systematic variation of the stimulation pairs and thus of the cochlear frequency sites revealed a systematic and corresponding shift of the response areas that matched the underlying frequency organization. Intensity functions were usually very steep. Increasingly higher stimulation currents evoked increasingly larger response areas, resulting in decreasing spatial, i.e. cochleotopic, selectivity; however, we observed only slight positional shifts of the focal zones of activity. Electrophysiological recordings of local field potential maps in the same individual animals revealed close correspondence of the locations of the cortical response areas. The results suggest that the method of optical imaging can be used to map response areas evoked by electrical cochlear stimulation, thereby maintaining a profound cochleotopic selectivity. Further experiments in chronically stimulated animals will shed more light on the degree of functional and reorganizational capacities of the primary cortex and could be beneficial for our understanding of the treatment of profound deafness.

Intensity dependence of auditory evoked potentials in behaving cats

The intensity dependence of auditory evoked potentials (AEPs) recorded epidurally over the primary (AI) and secondary (AII) areas of the auditory cortex was studied in behaving cats during wakefulness, sleep and anesthesia. Four kHz tones of 50, 60, 70, and 80 dB SPL, presented in random order every 2 ± 0.2 s by a bone conductor, elicited clear changes of the AEP amplitudes with increasing stimulus intensity, but individual components displayed different responses curves. AEP components from the AI region showed saturation of their amplitude with stimulus intensity (P13, P34) or no amplitude increase (N19), while amplitude and intensity were linearly related in the AII area. The intensity dependence of the first positive component (P12/P13) was consistently stronger for the AEP recorded from the AI than from the AII area, while later components exhibited no difference between AI and AII. During slow wave sleep, the intensity dependence of this first positive component increased in the two areas, while that of later components decreased. Pentobarbital anesthesia abolished almost all later components and depressed the intensity dependence of the first positive component both in the AI and AII area. These results indicate that (1) clear intensity dependence of AEP exists in the cat auditory cortex and (2) this intensity dependence, especially that of the first positive AEP component, shares functional similarities to the human augmenting/reducing phenomenon in the auditory modality concerning regional differences and sleep-waking cycle.

Stimulus-induced spike bursts in two fields of cat auditory cortex

The sound-evoked responses of extracellularly recorded cat primary auditory cortical neurons usually consist of a single spike or a short-term burst of 2–4 spikes, irrespective of the nature of the acoustic signal. In the cat's auditory cortex, the properties of such responses have to date been described only for cells in the primary field (AI). The purpose of the present study was to describe the properties of stimulus-evoked spike-burst responses seen in neurons of the posterior auditory field (P) and to compare those properties with those of a sample of AI neurons studied under similar conditions. The data come from 80 field P and 31 AI neurons studied with tonal and noise-burst stimuli in barbiturate-anesthetized cats, using calibrated, sealed stimulus delivery systems and conventional extracellular recording techniques. The mean inter-spike intervals (ISI) seen in the transient burst responses of posterior field cells were typically short (2–5 ms) and, where it was possible to test them, independent of the rise time of tonal signals, suggesting that they were also independent of the onset spectrum of the stimulus. The mean ISIs were often independent of the stimulus amplitude, even though the signal level had profound effects on the number of spikes evoked and the latency and regularity with which the responses were initiated. Each neuron was assigned a ‘characteristic ISI’, i.e., the mean ISI seen in the most vigorous responses. The distribution of characteristic ISIs for AI and P neurons overlapped, but were significantly different, with the characteristic ISIs of field P neurons being longer. In both AI and P populations, characteristic ISI was significantly correlated with minimal first-spike latency. The slopes of the regression lines of characteristic ISI on minimal latency for AI and for P cells were not significantly different from each other. Since the minimal latencies of AI neurons were usually shorter than those of field P neurons, the shorter characteristic ISIs of AI cells may thus be interpreted as secondary to their shorter latent periods. The general properties of stimulus-evoked spike bursts seen in field P neurons were thus very similar those previously described for AI cells. These data are consistent with the view that the majority of extracellular recordings in the cat's auditory cortex come from pyramidal neurons and are appropriate as a specialization for transfer of information to nonpyramidal, inhibitory interneurons.

Stimulus-induced spike bursts in two fields of cat auditory cortex

The sound-evoked responses of extracellularly recorded cat primary auditory cortical neurons usually consist of a single spike or a short-term burst of 2–4 spikes, irrespective of the nature of the acoustic signal. In the cat's auditory cortex, the properties of such responses have to date been described only for cells in the primary field (AI). The purpose of the present study was to describe the properties of stimulus-evoked spike-burst responses seen in neurons of the posterior auditory field (P) and to compare those properties with those of a sample of AI neurons studied under similar conditions. The data come from 80 field P and 31 AI neurons studied with tonal and noise-burst stimuli in barbiturate-anesthetized cats, using calibrated, sealed stimulus delivery systems and conventional extracellular recording techniques. The mean inter-spike intervals (ISI) seen in the transient burst responses of posterior field cells were typically short (2–5 ms) and, where it was possible to test them, independent of the rise time of tonal signals, suggesting that they were also independent of the onset spectrum of the stimulus. The mean ISIs were often independent of the stimulus amplitude, even though the signal level had profound effects on the number of spikes evoked and the latency and regularity with which the responses were initiated. Each neuron was assigned a ‘characteristic ISI’, i.e., the mean ISI seen in the most vigorous responses. The distribution of characteristic ISIs for AI and P neurons overlapped, but were significantly different, with the characteristic ISIs of field P neurons being longer. In both AI and P populations, characteristic ISI was significantly correlated with minimal first-spike latency. The slopes of the regression lines of characteristic ISI on minimal latency for AI and for P cells were not significantly different from each other. Since the minimal latencies of AI neurons were usually shorter than those of field P neurons, the shorter characteristic ISIs of AI cells may thus be interpreted as secondary to their shorter latent periods. The general properties of stimulus-evoked spike bursts seen in field P neurons were thus very similar those previously described for AI cells. These data are consistent with the view that the majority of extracellular recordings in the cat's auditory cortex come from pyramidal neurons and are appropriate as a specialization for transfer of information to nonpyramidal, inhibitory interneurons.

Neural correlates of gap detection and auditory fusion in cat auditory cortex.

Responses were recorded from 130 single neurones in the primary auditory cortex of 12 ketamine-anaesthetized cats in response to double-click stimuli, to a /ba/-/pa/ phoneme continuum and to gaps inserted early (after 5 ms) and late (after 500 ms) in a 1 s duration noiseburst. Stimulus levels were between 45 and 75 dB SPL. Neural detection threshold for the 'late gap' was less than 5 ms. For the double click and 'early gap' stimuli thresholds were between 40 and 50 ms, whereas the phoneme continuum threshold for voice-onset-time (VOT) was between 10 and 25 ms. The 'late gap' and VOT thresholds are similar to psychophysical gap detection and the /ba/-/pa/ categorical perception boundary respectively.

Monaural inhibition in cat auditory cortex

1. Several studies of auditory cortex have examined the competitive inhibition that can occur when appropriate sounds are presented to each ear. However, most cortical neurons also show both excitation and inhibition in response to presentation of stimuli at one ear alone. The extent of such inhibition has not been described. Forward masking, in which a variable masking stimulus was followed by a fixed probe stimulus (within the excitatory response area), was used to examine the extent of monaural inhibition for neurons in primary auditory cortex of anesthetized cats (barbiturate or barbiturate-ketamine). Both the masking and probe stimuli were 50-ms tone pips presented to the contralateral ear. Most cortical neurons showed significant forward masking at delays beyond which masking effects in the auditory nerve are relatively small compared with those seen in cortical neurons. Analysis was primarily concerned with such components. Standard rate-level functions were also obtained and were examined for nonmonotonicity, an indication of level-dependent monaural inhibition. 2. Consistent with previous reports, a wide range of frequency tuning properties (excitatory response area shapes) was found in cortical neurons. This was matched by a wide range of forward-masking-derived inhibitory response areas. At the most basic level of analysis, these were classified according to the presence of lateral inhibition, i.e., where a probe tone at a neuron's characteristic frequency was masked by tones outside the limits of the excitatory response area. Lateral inhibition was a property of 38% of the sampled neurons. Such neurons represented 77% of those with nonmonotonic rate-level functions, indicating a strong correlation between the two indexes of monaural inhibition; however, the shapes of forward masking inhibitory response areas did not usually correspond with those required to account for the "tuning" of a neuron. In addition, it was found that level-dependent inhibition was not added to by forward masking inhibition. 3. Analysis of the discharges to individual stimulus pair presentations, under conditions of partial masking, revealed that discharges to the probe occurred independently of discharges to the preceding masker. This indicates that even when the masker is within a neuron's excitatory response area, forward masking is not a postdischarge habituation phenomenon. However, for most neurons the degree of masking summed over multiple stimulus presentations appears determined by the same stimulus parameters that determine the probability of response to the masker.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphology and spatial distribution of corticothalamic terminals originating from the cat auditory cortex

In this paper we studied the morphology and spatial distribution of corticothalamic axons and terminals originating from the auditory cortical fields of the cat. The anterograde tracer biocytin was injected at electrophysiologically characterized loci in the primary (AI) ( N = 2), anterior (AAF) ( N = 1), posterior (PAF) ( N = 1) and secondary (AII) ( N = 2) auditory fields. In all cases, two different types of labeled terminals were found in the auditory thalamus: small spherical endings (1–2 μm) and giant, finger-like endings (5–10 μm). After biocytin injections in AI and AAF, the majority of anterogradely labeled axons terminated in the rostral half of the pars lateralis (LV) of the ventral division of the medial geniculate body (vMGB). In LV, the corticothalamic axons ramified profusely, giving rise to dense terminal fields forming well delineated curved stripes, with small spherical endings. Additional terminal fields formed by small endings were observed in the medial division of the medial geniculate body (mMGB). Giant endings were observed in a small area in the dorsal nucleus (D) of the dorsal division of the medial geniculate body (dMGB), near its border with the vMGB. PAF projections were located in the caudal half of vMGB and in mMGB, where only small terminals were found. Giant endings were seen in the superficial part of dMGB emerging from labeled corticothalamic axons oriented in parallel to the dorsal surface of the MGB. Projections from AII gave rise to a main terminal field of small endings in D; a second terminal field consisting of giant endings intermingled with small endings was found in the deep dorsal nucleus (DD) of dMGB. We conclude that small terminals serve the feedback projection to the thalamic nucleus from which the injected cortical field receives its main input, whereas giant terminals cross the borders between the parallel ascending auditory pathways.

Factors shaping the tone level sensitivity of single neurons in posterior field of cat auditory cortex.

1. The posterior field (field P) of the cat's auditory cortex contains a higher proportion of neurons whose response/level functions for characteristic frequency (CF) tones are nonmonotonic than does the primary field (AI). The general purpose of the present study is to assess whether the response/level functions of field P neurons are generated by the same mechanisms as those of cells in AI. All of the data came from single neurons in the cortices of barbiturate-anesthetized cats, to which we presented tonal stimuli through sealed, calibrated stimulating systems. 2. We obtained quantitative data from 123 neurons, of which 108 were located in field P. Of the 108 field P cells, 70% had nonmonotonic response/level functions for 5-ms rise time tones of CF. For cells of any given CF, both CF thresholds and best SPLs (i.e., SPLs associated with maximal responses) varied widely. A correlation analysis revealed that a linear relation between best SPL and CF threshold accounted for 73% of the data variance in the association between those response variables. An analysis of data from 83 nonmonotonic cells in AI revealed a similar relation. 3. Field P neurons whose response/level functions were non-monotonic for 5-ms rise time CF tones became even more narrowly tuned to SPL when the rise time of the tone bursts was reduced to 1 ms. Lengthening the rise time to 20 ms reduced or eliminated the SPL tuning in almost all of these neurons. The general form of monotonic tone response/level functions was commonly unaffected by variation in signal rise time. In a few instances, cells with monotonic response/level functions for 5- and 20-ms rise time tones developed nonmonotonic functions for 1-ms rise time tones. 4. Field P neurons with nonmonotonic response/level functions for CF tones usually failed to respond to wideband noise pulses, or, less commonly, responded to noise only at low SPLs. In contrast, field P cells with a monotonic response to CF tones usually responded monotonically to noise. 5. The minimal mean first-spike latencies of field P neurons were generally longer than those of AI cells studied under similar conditions. The precision of first-spike timing, measured using the SD of the mean first-spike latency, was commonly poorer than that of AI cells. 6. The properties of field P cells followed the same rules as those seen in AI.(ABSTRACT TRUNCATED AT 400 WORDS)

誘発反応 トーンバースト刺激によるネコ聴皮質直上での誘発電位のオン反応とオフ反応 覚醒時と麻酔下での比較

誘発反応 トーンバースト刺激によるネコ聴皮質直上での誘発電位のオン反応とオフ反応 覚醒時と麻酔下での比較

Temporal modulation transfer functions for AM and FM stimuli in cat auditory cortex. Effects of carrier type, modulating waveform and intensity

For 167 single units, recorded from primary auditory cortex in 28 cats, we show that tuning to the modulation frequency (MF) of amplitude-modulated (AM) sound is strongly dependent on carrier type. In general AM noise-bursts and click-trains produce good tuning to MFs with repetition rates around 8–10 Hz. Amplitude- or frequency-modulation of tone-carriers resulted largely in low-pass temporal modulation transfer functions (tMTFs) with a best modulation frequency (BMF) around 4 Hz. Individual BMFs for noise carriers ranged from 3–26 Hz, whereas for tone carriers they were mostly below 6 Hz and rarely above 10 Hz. The sharpness of tuning for broad-band stimuli decreased with increasing duty-cycle of the modulation; it was most pronounced for clicks, next best for exponential sine-AM and broadest for sinusoidal AM. In contrast the reverse was found for tone carriers; the better modulation following was found for sinusoidal modulation and was most likely entirely due to a stronger onset response. Decreasing the modulation depth below 100% showed an increasing influence of onset transients and periodic rebounds, however, the average tMTFs for depths between 50–100% are similar. The optimal intensity level for noise carriers was usually higher than for tone carriers. Overall the modulation-sensitivity of cortical neurons regardless of carrier type and modulating waveform was in the range of modulation frequencies found in music, speech and other complex sounds.

Population responses to multifrequency sounds in the cat auditory cortex: Four-tone complexes

Population responses to two-tone and four-tone sounds were recorded in primary auditory cortex of anesthetized cats. The stimuli were delivered through a sealed, calibrated sound delivery system. The envelope of the neural signal (short time mean absolute value, MABS) was recorded extracellularly from six microelectrodes simultaneously. A new method was developed to describe the responses to the four-tone complexes. The responses were represented as sums of contributions of different orders. The first order contributions described the effect of the single frequencies appearing in the stimulus. The second order contributions described the modulatory effect of the pairs of frequencies. Higher order contributions could in principle be computed. This paper concentrates on the mean onset responses. The extent to which the first and second order contributions described the onset responses was assessed in two ways. First, the actual responses to two-tone stimuli were compared with those predicted using the contributions computed from the four-tone stimuli. Second, the residual variance in the responses, after the substraction of the first and second order contributions, was computed and compared with the variability in the responses to repetitions of the same stimulus. The first type of analysis showed good quantitative agreement between the predicted and the measured two-tone responses. The second type of analysis showed that the first and second order contributions were often sufficient to predict the responses to four-tone stimuli up to the level of the variability in the responses to repetitions of a single stimulus. In conjunction with the results of the companion paper (Nelken et al., 1994a) it is concluded that the onset responses to multifrequency sounds are shaped mainly by the single frequency content of the sound and by two-tone interactions, and that higher order interactions contribute much less to the responses. It follows that single-tone effects and two-tone interactions are necessary and sufficient to explain the mean population onset responses to the four-tone stimuli. More information can be coded in the temporal evolution of the responses.

In search of the best stimulus: An optimization procedure for finding efficient stimuli in the cat auditory cortex

Units in the auditory cortex of cats respond to a large variety of stimuli: pure tones, AM- and FM-modulated signals, clicks, wideband noise, natural sounds, and more. However, no single family of sounds was found to be optimal (in the sense that oriented lines are optimal in the visual cortex). The search for optimal complex sounds is hard because of the high dimensionality of the space of interesting sounds. In an effort to overcome this problem, an automatic search procedure for finding efficient stimuli in high-dimensional sound spaces was developed. This procedure chooses the stimuli to be presented according to the responses to past stimuli, trying to increase the strength of the response. The results of applying this method to recordings of population activity in the primary auditory cortex of cats are described. The search was applied to single tones, two-tone stimuli, four-tone stimuli and to a two-dimensional subset of nine-tone stimuli, parametrized by the center frequency and the fixed difference between adjacent frequencies. The method was able to find efficient stimuli, and its performance improved with the dimension of the sound spaces. Efficient stimuli, found in different optimization runs using population activity recorded from the same electrode, often shared similar frequencies and pairs of frequencies, and tended to evoke similar levels of activity. This result indicates that a global analysis of the location of spectral peaks is performed at the level of the auditory cortex.

Population responses to multifrequency sounds in the cat auditory cortex: One- and two-parameter families of sounds

Population responses to multi-frequency sounds were recorded in primary auditory cortex of anesthetized cats. The sounds consisted of single-tone stimuli; two-tone stimuli; and nine-tone stimuli, with the tones evenly spaced on a linear frequency scale. The stimuli were presented through a sealed, calibrated sound delivery system. Single units, cluster activity (CA) and the short-time mean absolute value of the envelope of the neural signal (MABS) were recorded extracellularly from six microelectrodes simultaneously. The CA and MABS were interpreted as measures of the activity of large populations of neurons, in contrast with the single unit activity which is presumably recorded from single neurons. The responses of the MABS signal to simple stimuli were generally similar to those of the CA, but were more stable statistically. Thus, the MABS is better suited for studying the activity of populations of neurons. The responses to tones near the best frequency were strongly influenced by a second tone, even when the second tone was outside the single-tone response area. These influences could be both facilitatory and suppressory. They could not be predicted from the responses to single tones. The responses to the nine-tone stimuli could be explained qualitatively by the responses to the two-tone stimuli. It is concluded that the population responses in primary auditory cortex are shaped by the contributions of the individual frequencies appearing in the stimulus and by the interactions between pairs of frequencies. Interactions between stimulus components are therefore a necessary component of any attempt to explain the processing of complex sounds in the auditory cortex. They may play a role in a global representation of the stimulus spectrum in the primary auditory cortex. The presence of higher-order interactions cannot be excluded by the results presented here.

Terminal morphology and distribution of corticothalamic fibers originating from layers 5 and 6 of cat primary auditory cortex.

Two types of terminations were observed on corticothalamic fibers arising from cells in different layers of cat auditory cortex. Injections of the anterograde tracers Phaseolus vulgaris leucoagglutinin (PHA-L) or biocytin were made into single cortical loci that included both superficial layer 5 (5a) and layer 6 in the primary auditory cortex (AI). These resulted in labeling of terminal fibers with small (approximately 1 micron) and large (approximately 2 microns) boutons in the medial geniculate complex (MG) and the lateral nucleus of the posterior complex. Large boutons were found in the deep and superficial dorsal nuclei, in the ventrolateral nucleus, and, less frequently, in the medial nucleus of the MG. They usually ended in grape-like clusters of boutons. By contrast, small boutons were found densely in the pars lateralis and pars ovoidea of the ventral nucleus, and to a lesser extent in the medial nucleus of MG. In the anterior third of the ventral nucleus, where the highest density of labeled fibers was observed, the small bouton terminations formed a plate-like plexus. In the inferior colliculus (IC), most terminal boutons on labeled corticotectal fibers were of large size. To reveal the cells of origin of the axons ending in the two different types of corticothalamic terminations, biocytin injections were localized in either layer 5 or layer 6 of AI or PHA-L injections were made into middle layers, including layer 5a but excluding layer 6. Virtually all labeled terminals found in the MG after layer 5 injections were of large size, while those found after layer 6 injections were of small size. The distribution of terminals of single-labeled axons was extensive and variable. For example, an axon recovered after a layer 5 injection of biocytin ended in at least seven patches of clusters of large boutons along much of the anteroposterior axis of MG. Our previous findings showed two neuronal populations situated in superficial layer 5 and in layer 6 of AI and projecting to the thalamus. The axons of these cells had different patterns of collateral distributions in the cortex. The present study shows that the extrinsically projecting axons of these two populations also have different terminal morphologies and distribution patterns in the MG. The findings suggest that the corticothalamic pathway in the cat auditory system consists of at least two feedback projections originating from different cortical layers that exert different influences on distinct thalamic neuronal populations.

Rapid changes in the frequency tuning of neurons in cat auditory cortex resulting from pure-tone-induced temporary threshold shift

The response areas (frequency by intensity) of single neurons in primary auditory cortex of anesthetized cats were studied before and after temporary threshold shifts in cochlear sensitivity induced by an intense pure tone. Cochlear temporary threshold shift was monitored through the threshold of the gross auditory nerve compound action potential and in most cases involved a notch-like loss centered at the characteristic frequency of the unit under study. Only two neurons showed changes in response area that mirrored the changes at the auditory periphery. Most neurons (14) showed more complex changes involving both expansion and contraction of response areas. Expansion of response areas was indicated by lower thresholds at some frequencies and by the emergence of sensitivity to previously ineffective frequencies. A change was classified as contraction when the response area after the intense-tone exposure was smaller than would be expected by applying the profile of the temporary threshold shift to the initial response area. Contraction of both upper (high intensity) and lower boundaries of response areas was found; in the most extreme cases, neurons were totally unresponsive after the intense-tone exposure. The complexity of effects of temporary threshold shifts on the response areas of cortical neurons is likely to be related to mechanisms that normally determine the frequency response limits of these neurons. The response areas of cortical neurons are more complex than those of auditory nerve fibers, and are thought to reflect the integration of excitatory and inhibitory inputs. The variety of effects observed in this study are consistent with the excitatory and inhibitory components of the response area of a given neuron being differentially affected by the temporary threshold shift.

Effects of bilateral and unilateral ablation of auditory cortex in cats on the unconditioned head orienting response to acoustic stimuli.

1. Reflexive head orienting responses (ORs) elicited by bursts of wide-band noise were investigated in cats after bilateral or unilateral ablation of the auditory cortex, and the cats' performance was compared with that of control cats. The OR was used as an indication of ability to orient toward the azimuthal direction of a source of sound. 2. To adequately test this ability, a unique combination of stimulus duration and position of the sound source was selected on each trial. Stimulus durations (0.1, 0.3, and 1.5 s) were selected so that the offset of a burst of noise occurred before, during, or after an OR. The stimuli were produced from speakers positioned approximately at the interaural horizontal plane within each quadrant of a cat's auditory field. The ORs were recorded on moving film and analyzed quantitatively. 3. In the control cats, the trajectory of the OR was characterized by a saccadic profile (rapid steplike movement, monophasic velocity, and biphasic acceleration). The accuracy of the OR varied directly with stimulus duration, suggesting that the response was modified by auditory feedback produced by a head movement during the stimulus. Corrective responses executed during long-duration (1.5 s) stimuli reduced residual mean error to < 7 degrees in each of the control animals. The mean error in orientation was smaller for sources located in the frontal sound field than for sources located behind the coronal plane the head (> 90 degrees). When brief (0.1 s) stimuli were presented behind the head, the cats confused back with front directions on most of the trials. 4. Compared with performance in the control cats, bilateral destruction of the auditory geniculocortical system severely impaired a cat's ability to orient consistently and accurately toward a source of sound. Latencies to the onset of ORs were increased, the magnitudes of ORs were reduced, average error in orienting to a sound source was larger under every stimulus duration-source position combination, relatively few corrective responses were executed, and residual mean error was significantly elevated (bilateral = 28.1 degrees; control = 3.1 degrees). Several animals with bilateral lesions also made vertical errors in orienting to azimuthal sources of sound. 5. However, in the bilateral lesion group, ORs were initiated in the correct right or left direction; and, rather than eliminating accurate responses altogether, the lesions reduced the probability of their occurrence. Furthermore, the saccadic profile of the response was preserved, providing evidence that the motor control system for the OR was not perturbed by the bilateral lesions.(ABSTRACT TRUNCATED AT 400 WORDS)