Representation of amplitude modulation in the auditory cortex of the cat. I. The anterior auditory field (AAF)

The ability of cortical neurons to follow amplitude modulation (AM) of tones was examined in the anterior auditory cortical field (AAF) of anesthetized cats using multiple-unit recording techniques. Sinusoidal and rectangular modulations (100%) of a monaural carrier tone at the characteristic frequency of each location were presented to study the degree of response synchronization and changes in firing rate as a function of the modulation frequency. All investigated locations were tuned to a 'best modulation frequency' (BMF) as determined by synchronization measures. Almost all locations (94%) were tuned to a BMF as determined by spike rate. Maximal binaural-interaction strength was observed for modulation frequencies close to the BMF of neurons. For sinusoidal AM, a correlation (r = 0.63, P less than 0.01) between BMF and CF of neurons in AAF could be demonstrated for the synchronization of the response.     

Functional organization of the medial division of the medial geniculate body of the cat: tonotopic organization, spatial distribution of response properties and cortical connections

The discharge properties of 735 single units located in the pars magnocellularis (M) of the medial division of the medial geniculate body (MGB) were studied in 23 nitrous oxide anesthetized cats in response to simple acoustic stimuli (clicks, noise and tone bursts). A systematic decrease of single unit characteristic frequencies (CF) was observed along electrode track portions crossing M from dorso-medial to ventro-lateral. These data indicate that M is tonotopically organized with an arrangement of low CF units latero-ventrally and high CF units dorso-medially. This preferential arrangement of single units as a function of their CF was consistent with the location and orientation of clusters of labeled cells in M resulting from wheat-germ agglutinin labeled with horseradish peroxidase (WGA-HRP) injections in CF defined loci in the anterior (AAF) or primary (AI) auditory cortical fields. The quality of the tonotopic arrangement was low caudally and increased in the rostral direction, indicating that this tonotopicity concerns mainly the anterior half of M. Response latencies to clicks, noise and tone bursts were on average longer in the posterior part of M than in its anterior part. Time-locking of discharges in response to repetitive acoustic pulses was more frequent anteriorly than posteriorly and the upper limiting rate of locking was on average higher rostrally (up to 200-300 Hz). In contrast, other response properties such as responsiveness to the various combinations of simple acoustic stimuli, response patterns and tuning were more randomly distributed in M, showing the whole range of response properties seen in the MGB. Data derived from several injections of WGA-HRP performed in distinct auditory cortical fields in several animals indicated that M projects to the tonotopic cortical fields (AAF, AI and PAF) as well as to the non-tonotopically organized secondary auditory cortex (AII). The contribution of M to the total thalamic input reaching each field of the auditory cortex was quantitatively more important for AAF (30%) and PAF (20%) than for AI and AII (about 10% each).     

Temporal and spatial coding of periodicity information in the inferior colliculus of awake chinchilla (Chinchilla laniger)

Amplitude modulation responses and onset latencies of multi-unit recordings and evoked potentials were investigated in the central nucleus of inferior colliculus (ICC) in the awake chinchilla. Nine hundred and one recording sites with best frequencies between 60 and 30 kHz showed either phasic (18%), tonic (25%), or phasic-tonic (57%) responses. Of 554 sites tested for responses to modulation frequencies 73% were responsive and 57% showed clear preference for a narrow range of modulation frequencies. Well defined bandpass characteristics were found for 32% of rate modulation transfer functions (rate-MTFs) and 36% of synchronization MTFs (sync-MTFs). The highest best modulation frequency (BMF) of a bandpass rate-MTF was 600 Hz. Neurons with phasic responses to best-frequency tones showed strong phase coupling to modulation frequencies and were dominated by bandpass rate-MTFs and sync-MTFs. Most neurons with tonic responses showed bandpass tuning only for sync-MTFs. Both BMFs and onset latencies changed systematically across frequency-band laminae of the ICC. Low BMFs and long latencies were located medially and high BMFs and short latencies laterally. Latency distributions obtained with evoked potentials to clicks showed a similar gradient to the multi-unit data. These findings are in line with previous findings in different animals including humans and support the hypothesis that temporal processing results in a topographic arrangement orthogonal to the spectral processing axis, thus forming a second neural axis of the auditory system.     

Responses of DCN-PVCN neurons and auditory nerve fibers in unanesthetized decerebrate cats to AM and pure tones: analysis with autocorrelation/power-spectrum

We investigated amplitude-modulated (AM) tone encoding behavior of dorsal and posteroventral cochlear-nucleus (DCN and PVCN) neurons and auditory nerve (AN) fibers in decerebrate unanesthetized cats. Some of the modulation transfer functions (MTFs) were narrowly-tuned band-pass functions; these included responses at moderate and high stimulus levels of DCN pause/build-type-III neurons and the following types of DCN and PVCN chopper neurons: chop-S and/or chop-type-I/III. Other MTFs were broad low-pass or complex functions. Chop-T neurons of the DCN and PVCN tended to exhibit low-pass or flat MTFs. The band-pass MTF neurons exhibited intrinsic oscillations (IOs) in responses to AM or pure tones. The IOs, which were detected in autocorrelation functions and power spectra, were closely correlated (r = 0.863) with the best envelope frequency (BEF). All of the AN fibers showed broad low-pass MTFs with some showing a rudimentary peak in the MTF. The MTFs of DCN-PVCN neurons and AN fibers showed, respectively: (1) BEFs ranging 50-500 Hz, and 400-1300 Hz; (2) upper cut-off frequencies ranging 200-1200 Hz, and 1600-3200 Hz. At stimulus levels of 60-85 dB SPL, maximum modulation gains were as high as 12 dB for DCN-PVCN neurons but were limited to below about 0 dB for AN fibers. The median dynamic ranges of DCN and PVCN neurons (51 and 42 dB, respectively) were substantially wider than those of the low and high spontaneous rate AN fibers (30 and 31 dB, respectively). The observation of higher modulation gain, wider dynamic range, and more narrowly-tuned MTF of DCN-PVCN neurons than AN fibers supports the concept that the capabilities to encode dynamic signals are enhanced in DCN-PVCN neurons compared with AN fibers.     

Extrathalamic ascending projections to physiologically identified fields of the cat auditory cortex

The neurons of origin of ascending extrathalarnic projections to the auditory cortex were labeled retrogradely with WGA-HRP injected in physiologically identified auditory cortical fields of the cat (anterior (AAF), primary (AI), posterior (PAF) and secondary (All) fields). After injection in the tonotopically organized auditory cortical fields (AAF, AI and PAF), labeled neurons were distributed in 7 extrathalamic subcortical regions included in one or the other of 2 distinct systems of ascending projections to the neocortex. In the ‘diffuse’ system of projection, labeled neurons were observed bilaterally in the locus coeruleus, the nuclei of the raphe, the lateral hypothalamus, ipsilaterally in the ventromedial mesencephalic tegmentum and the basal forebrain; in the ‘accessory sensory’ system of projection, labeled neurons were found ipsilaterally in the nucleus of the brachium of the inferior colliculus and bilaterally in the claustrum. After injection in AH, labeled neurons were seen only in the ‘diffuse’ system of projection. For AAF and AI, the major contribution to the total extrathalamic ascending input originated from the lateral hypothalamus, whereas for All it was the locus coeruleus. In contrast, PAF received extrathalamic ascending inputs mainly from the claustrum. Anterogradely labeled corticofugal terminal fields were found only in the nucleus of the brachium of the inferior colliculus and, after injection in PAF, in the claustrum.     

Origin of afferents to physiologically defined regions of the medial geniculate body of the cat: ventral and dorsal divisions

To study the origin of afferents to the medial geniculate body (MGB), single unit recordings were first conducted to define physiologically a given region in this auditory thalamic nucleus. Horseradish peroxidase (HRP) was then injected extracellularly, in order to label retrogradely the neurons whose axon terminals end in this region. The principal inputs to the MGB are coming from various nuclei of the brain stem, the auditory cortex and the reticular complex of the thalamus. The ventral division receives its cortical inputs principally from the primary (AI) and the posterior (PAF) auditory cortical fields, with a quantitatively smaller contribution of the secondary (AII) and anterior (AAF) cortical fields. On the other hand, the dorsal division receives a majority of its cortical inputs from AII, with a less important contribution of AI and PAF. The auditory cortex sends roughly as many axons to these two divisions as does the brain stem, mainly the inferior colliculus (IC). The analysis of ascending inputs to the same regions of the MGB reveals that, on the average, 88%, 7% and 5% of them are coming from the ipsilateral IC, the contralateral IC and other nuclei of the brain stem, respectively.     

The auditory steady-state response: full-term and premature neonates

Two studies were aimed at developing the auditory steady-state response (ASSR) for universal newborn hearing screening. First, neonates who had passed auditory brainstem response, transient evoked otoacoustic emission, and distortion-product otoacoustic emission tests were also tested with ASSRs using modulated tones that varied in frequency and level. Pass rates were highest (> 90%) for amplitude-modulated tones presented at levels > or = 69 dB SPL. The effect of modulation frequency on ASSR for 500- and 2000-Hz tones was evaluated in full-term and premature infants in the second study. Full-term infants had higher pass rates for 2000-Hz tones amplitude modulated at 74 to 106 Hz compared with pass rates for a 500-Hz tone modulated at 58 to 90 Hz. Premature infants had lower pass rates than full-term infants for both carrier frequencies. Systematic investigation of ASSR threshold and the effect of modulation frequency in neonates is needed to adapt the technique for screening.     

Effects of acute pure tone induced hearing loss on response properties in three auditory cortical fields in cat.

In this study, we assessed the changes in spontaneous activity and frequency tuning by simultaneous recording of multi-units and local field potentials in primary auditory cortex (AI), anterior auditory field (AAF) and secondary auditory cortex (AII) of cats before and immediately after 30 min exposure to a loud (93 123 dB SPL) pure tone. The average difference of the pure tone and the characteristic frequency (CF) was less than one octave for 70% of the recordings. We found that the mean threshold at CF increased significantly in AI and in AAF but not in AII. The mean CF for units in AI decreased significantly, whereas no significant effect was noted in AAF and AII. The mean frequency-tuning curve bandwidth decreased significantly in AII. Spontaneous activity increased significantly in AI, did not change in AAF, and decreased significantly in AII. Inter-area neural synchrony was not affected. Multi-unit response areas were usually similarly affected as local field potentials based response areas because the 'damaged area', defined as the response surface before minus the surface after the trauma, was very similar. This suggests that the damage reflects peripheral activity changes. Enhancement of frequency response areas around CF, but at least one octave below the frequency of the traumatizing tone, was found most frequently in AAF and suggests a reduction of inhibition likely as a result of the peripheral hearing loss.     

Responses of young and aged rat inferior colliculus neurons to sinusoidally amplitude modulated stimuli

The inferior colliculus (IC) is a processing center for monaural and binaural auditory signals. Many units in the central nucleus of the inferior colliculus (CIC) respond to amplitude and frequency modulated tones, features found in communication signals. The present study examined potential effects of age on responses to sinusoidally amplitude modulated (SAM) tones in CIC and external cortex of the inferior colliculus (ECIC) units in young and aged F344 rats. Extracellular recordings from 154 localized single units of aged (24 month) rats were compared to recordings from 135 IC units from young adult (3 month) animals. SAM tones were presented at 30 dB above threshold. Comparisons were made between CIC and ECIC regarding the percentage of units responding to SAM stimuli, the relationship between SAM responsiveness and temporal response patterns, maximum discharge rates and maximum modulation gains, shapes of rate transfer functions and synchronization modulation transfer functions (MTFs) in response to SAM tones. Sixty percent of units in young and aged rat IC were selectively responsive to SAM stimuli. Eighty-one percent of units classified as onset temporal response patterns were not tonically responsive to SAM stimuli. Median maximum discharge rate in response to SAM tones was 17.6/s in young F344 rats; median maximum modulation gain was 3.85 dB. These measurements did not change significantly with age. Thirty-seven percent of young rat units displayed bandpass MTFs and 53% had lowpass MTFs. There was a significant age-related shift in the distribution of MTF shapes in both the CIC and ECIC. Aged animals showed a lower percentage of bandpass functions and a higher percentage of lowpass functions. Age-related changes observed in SAM coding may reflect an altered balance between excitatory/inhibitory neurotransmitter efficacy in the aged rat IC, and/or possibly a change in the functional dynamic range of IC neurons.     

Modeling Responses in the Superior Paraolivary Nucleus: Implications for Forward Masking in the Inferior Colliculus

A phenomenological model of the responses of neurons in the superior paraolivary nucleus (SPON) of the rodent is presented in this study. Pure tones at the characteristic frequency (CF) and broadband noise stimuli evoke offset-type responses in these neurons. SPON neurons also phase-lock to the envelope of sinusoidally amplitude-modulated (SAM) stimuli for a range of modulation frequencies. Model SPON neuron received inhibitory input that was relayed by the ipsilateral medial nucleus of the trapezoid body from the contralateral model ventral cochlear nucleus neuron. The SPON model response was simulated by detecting the slope of its inhibitory postsynaptic potential. Responses of the proposed model to pure tones at CF and broadband noise were offset-type independent of the duration of the input stimulus. SPON model responses were also synchronized to the envelope of SAM stimuli with precise timing for a range of modulation frequencies. Modulation transfer functions (MTFs) obtained from the model response to SAM stimuli resemble the physiological MTFs. The output of the proposed SPON model provides an input for models of physiological responses at higher levels of the ascending auditory pathway and can also be utilized to infer possible mechanisms underlying gap detection and duration encoding as well as forward masking at the level of the auditory midbrain.     

Comparison between local field potentials and unit cluster activity in primary auditory cortex and anterior auditory field in the cat

Multi-unit (MU) activity and local field potentials (LFP) were simultaneously recorded from 161 sites in the middle cortical layers of the primary auditory cortex (AI) and the anterior auditory field (AAF) in 51 cats. Responses were obtained for frequencies between 625 Hz and 40 kHz, at intensities from 75 dB SPL to threshold. We compared the response properties of MU activity and LFP triggers, in terms of characteristic frequency (CF), threshold at CF, minimum latency and frequency tuning-curve bandwidth 20 dB above threshold. On average, thresholds at CF were significantly lower for LFP events than those for MU spikes (4.6 dB for AI, and 3 dB for AAF). Minimum latencies were significantly shorter for LFP events than for MU spikes (1.5 ms in AI, and 1.7 ms in AAF). Frequency tuning curves were significantly broader for LFP events than those for MU spikes (1.0 octave in AI, and 1.3 octaves in AAF). In contrast, the CF was not significantly different between LFP events and MU spikes. The LFP results indicate that cortical neurons receive convergent sub-cortical inputs from a broad frequency range. The sharper tuning curves for MU activity compared to those of LFP events are likely the result of intracortical inhibitory processes.     

Spatial sensitivity of neurons in the anterior, posterior, and primary fields of cat auditory cortex

We assessed the spatial-tuning properties of units in the cat's anterior auditory field (AAF) and compared them with those observed previously in the primary (A1) and posterior auditory fields (PAF). Multi-channel, silicon-substrate probes were used to record single- and multi-unit activity from the right hemispheres of alpha-chloralose-anesthetized cats. Spatial tuning was assessed using broadband noise bursts that varied in azimuth or elevation. Response latencies were slightly, though significantly, shorter in AAF than A1, and considerably shorter in both of those fields than in PAF. Compared to PAF, spike counts and latencies were more poorly modulated by changes in stimulus location in AAF and A1, particularly at higher sound pressure levels. Moreover, units in AAF and A1 demonstrated poorer level tolerance than units in PAF with spike rates modulated as much by changes in stimulus intensity as changes in stimulus location. Finally, spike-pattern-recognition analyses indicated that units in AAF transmitted less spatial information, on average, than did units in PAF-an observation consistent with recent evidence that PAF is necessary for sound-localization behavior, whereas AAF is not.     

Temporal modulation transfer functions for single neurons in the auditory midbrain of the leopard frog. Intensity and carrier-frequency dependence

The sensitivity for amplitude modulation was investigated for 77 neurons from the auditory midbrain of the leopard frog. The results show that tuning to modulation frequencies occurs in about one-third of the units but is quite varied in its appearance. Two slightly differing characterizations for this tuning have been used; the overall response or rate-Modulation Transfer Function and the synchronized response or temporal-MTF (tMTF). The relation between the two characterizations is given by the vector-strength. Only one-third of the units showed a vector-strength that was significantly different from zero. Another synchronization measure, the synchronization factor which is based on the auto-coincidence function, was significantly different from zero in about 3/4 of the units. The Best Modulation Frequency (BMF) and tuning band-width were found to be independent of both stimulus intensity and carrier frequency, although the average BMF for band-pass units was slightly higher for the amphibian papilla range of carrier frequencies than for the basilar papilla range (66 Hz vs. 49 Hz). The most frequent BMF for band-pass units was around 55 Hz, this does not correspond with the dominant modulation frequency of the mating call which is around 20 Hz. The synchronization measures were negatively correlated with intensity and independent of carrier frequency. The phase response of the tMTF was used to calculate the group delay. In contrast to the latency of the units the group delay was independent of stimulus intensity.     

Processing of modulation frequency in the dorsal cochlear nucleus of the guinea pig: Amplitude modulated tones

The modulation frequency (Fm), particularly high Fm (> 200 Hz), in amplitude modulated (AM) tones can elicit the perception of the periodicity pitch (Langner, 1992). In this study, single unit responses to the Fms of the sinusoidal AM tones were investigated at 50 to 90 dB SPL. The recordings were made from the dorsal cochlear nucleus (DCN) of neuroleptic anesthetized guinea pigs with an intact cerebellum. The DCN units show a good capability of phase-locking to Fm at 400–1200 Hz. On-S-type II and Pauser/Buildup (P/B) units have a high modulation gain (7.2–8.3 dB). P/B units can preserve the high modulation gain (5–9 dB) up to 90 dB SPL. The modulation gain exponentially increases with decreasing modulation depth (Dm) and the phase-locking is detectable even at the Dm as low as 2–5%. The ‘central skipping’ of the phase-locking peak has been found at deep Dms in a few cases. The synchronization is independent of the discharge rate and can remain high even when the responses to AM tones are inhibited below the spontaneous activity. Such encoding behaviors over the unit's response area show that the Fm phase-locking is strong near or at its characteristic frequency (CF). The synchronization index (SI) versus carrier frequency (Fc) curve is similar to the inverse shape of tuning curve but more narrowly tuned than the iso-intensity function of pure tones at moderate to high intensity levels. The phase-locking is related to the unit's spontaneous rate (SR). The average modulation gain of the lower SR (≤ 2 spikes/s) units is 5 dB higher than that of the higher SR (> 2 spikes/s) units (8.16 and 2.92 dB, respectively) at 70 dB SPL. These results suggest that AM information is temporally encoded over broad ranges of modulation parameters in the DCN and is conveyed by the Fc channel. Such a timing mechanism can play an important role in processing of complex sounds under normal acoustic conditions.     

Stimulus properties influencing the responses of inferior colliculus neurons to amplitude-modulated sounds

The temporal pattern of the responses of neurons in the inferior colliculus of the anesthetized rat were studied using continuous tone or noise carrier signals, amplitude modulated by pseudorandom noise. Period histograms of the responses, cross-correlated with the pseudorandom noise, gave an estimate of the unit's impulse responses to modulation. The amplitude-modulation rate transfer function (MTF) was obtained by Fourier transforming the correlograms. At sound levels within approximately 15 dB of the unit threshold, the MTFs were near lowpass functions between 6 and 200 Hz but became more bandpass-like as the intensity was increased. There was a steep decline in the response to modulation at modulation frequencies above 200 Hz for all stimulus intensities. For the bandpass-type MTFs the greatest modulation of the discharge pattern occurred at modulation frequencies between 10 and 200 Hz with a maximum in the distribution of MTF peak values between 100 and 120 Hz. There was no consistent relationship with characteristic frequency of either the position of the MTF peak or the high-frequency cutoff of the MTF. The cross-correlograms obtained at high stimulus intensities (30-60 dB above threshold) often showed a negative peak, representing a decrease in the probability of firing in response to intensity increments in the stimulus, and denoting a nonmonotonic rate-intensity function. The MTFs for units responding to amplitude-modulated broadband noise were often flatter in the low frequency region than those generated with tone carriers at corresponding intensities. For some units addition of a broadband noise background to the modulated tone changed the response characteristic of the MTF from bandpass to lowpass and shifted the MTF peak to a lower modulation frequency. The results demonstrate that although neurons in the inferior colliculus are selectively sensitive to the modulation frequency of dynamic stimuli, the response characteristics are not invariant, but instead are closely dependent on the conditions under which the modulation is presented.     

Auditory neurophonic responses to amplitude-modulated tones: transfer functions and forward masking

Two auditory neurophonic responses - one recorded from the scalp (frequency following response or FFR) and one from the auditory nerve (auditory nerve neurophonic or ANN) - were obtained following stimulation of the cat cochlea with amplitude-modulated (AM) high-frequency tones. The carrier frequencies varied between 2 and 30 kHz. The modulation frequencies varied between 400 and 3000 Hz. The AM responses were compared with pure-tone neurophonic responses. The AM response waveforms were found to have a similar spectral composition, similar rates of adaptation, and similar rates of recovery from forward masking as the comparable pure-tone responses. As with the pure-tone neurophonics, an unmodulated masking stimulus can produce prolonged depression of the probe response. The amount and duration of this depression is dependent upon the level and frequency of the masker. The frequency dependence of the depression is demonstrated by forward masked tuning curves which indicate that the AM responses arise from fiber populations which have restricted characteristic frequency distributions centered on the carrier frequency. Response amplitude as a function of stimulus level (I/O) functions, response amplitude as a function of carrier frequency (carrier transfer functions or CTF) and response amplitude as a function of modulation frequency (modulation transfer functions or MTF) were also measured. It was found that the I/O functions were saturating monotonic functions of stimulus intensity, CTFs were flat for carrier frequencies from 6 to 30 kHz, and MTFs were flat for modulation frequencies from 100 to 1500 Hz. These results are compared with similar data for single units and compound action potentials.     

Modulation transfer functions: a comparison of the results of three methods

Modulation transfer functions (MTFs) were measured with three different psychoacoustical paradigms in the same normal-hearing subjects. In the temporal-probe method, the threshold of a 4-ms probe tone (frequencies of 1000 and 4000 Hz) was measured at various envelope phases within a 100% sinusoidally amplitude-modulated (SAM) noise at modulation frequencies from 2 to 256 Hz. For the derived-MTF method, the threshold of a 500-ms tone at 1000 and 4000 Hz was measured in the same noise at the same modulation frequencies. For the modulation-detection paradigm, modulation thresholds were measured as a function of modulation frequency for bandpass filtered SAM noise centered at 1000 and 4000 Hz. MTFs with lowpass shapes were observed with all three methods. Differences were observed in the cutoff frequencies and/or attenuation rates when the data were fitted with lowpass filter transfer functions. Factors influencing those differences are discussed.     

Auditory steady-state responses to exponential modulation envelopes

OBJECTIVE: This study examined the steady-state responses evoked by tones modulated with exponential envelopes. The hypothesis was that stimuli with envelopes containing more rapid changes would evoke larger responses. DESIGN: Multiple auditory steady-state responses were recorded simultaneously to eight tonal stimuli, four in each ear. The carrier frequencies of the stimuli ranged from 500 to 6000 Hz and the modulation rates were between 75 and 95 Hz. The modulation envelopes were based on functions using sin' where N was 1, 2, 3, or 4. Setting N to 1 produced the traditional sinusoidal modulation. RESULTS: Exponential envelopes with N greater than 1 produced larger steady-state responses than a sinusoidal envelope. For amplitude-modulation (AM), exponential envelopes increased response amplitudes by 21% at 55 dB pSPL, and by 29% at 35 dB pSPL. The increases were smaller for carrier frequencies of 1500 to 2000 Hz than for lower and higher carrier frequencies. Latencies calculated from phase data increased significantly with increasing N. This was likely caused by the point of maximal envelope-slope shifting later in time as N increased. For frequency modulation (FM), the steady-state responses did not significantly change with changes in the power of the exponential envelopes. CONCLUSIONS: When tones are amplitude-modulated with exponential envelopes based on sin(N), the amplitude and latency of the steady-state response increased significantly with increasing N. Using exponential envelopes with N greater than 1 should considerably shorten the time needed for responses to become significant when using steady-state responses in objective audiometry.     

Electrically evoked auditory steady-state responses in a guinea pig model: latency estimates and effects of stimulus parameters

Cochlear implant speech processors typically extract envelope information of speech signals for presentation to the auditory nerve as modulated trains of electric pulses. Recent studies showed the feasibility of recording, at the scalp, the electrically evoked auditory steady-state response using amplitude-modulated electric stimuli. Sinusoidally amplitude-modulated electric stimuli were used to elicit such responses from guinea pigs in order to characterize this response. Response latencies were derived to provide insight regarding neural generator sites. Two distinct sites, one cortical and another more peripheral, were indicated by latency estimates of 22 and 2 ms, respectively, with the former evoked by lower (13-49 Hz) and the latter by higher (55-320 Hz) modulation frequencies. Furthermore, response amplitudes declined with increasing carrier frequency, exhibited a compressive growth with increasing modulation depths, and were sensitive to modulation depths to as low as 5%.     

Detection of Modulated Tones in Modulated Noise by Non-human Primates

In natural environments, many sounds are amplitude-modulated. Amplitude modulation is thought to be a signal that aids auditory object formation. A previous study of the detection of signals in noise found that when tones or noise were amplitude-modulated, the noise was a less effective masker, and detection thresholds for tones in noise were lowered. These results suggest that the detection of modulated signals in modulated noise would be enhanced. This paper describes the results of experiments investigating how detection is modified when both signal and noise were amplitude-modulated. Two monkeys (Macaca mulatta) were trained to detect amplitude-modulated tones in continuous, amplitude-modulated broadband noise. When the phase difference of otherwise similarly amplitude-modulated tones and noise were varied, detection thresholds were highest when the modulations were in phase and lowest when the modulations were anti-phase. When the depth of the modulation of tones or noise was varied, detection thresholds decreased if the modulations were anti-phase. When the modulations were in phase, increasing the depth of tone modulation caused an increase in tone detection thresholds, but increasing depth of noise modulations did not affect tone detection thresholds. Changing the modulation frequency of tone or noise caused changes in threshold that saturated at modulation frequencies higher than 20 Hz; thresholds decreased when the tone and noise modulations were in phase and decreased when they were anti-phase. The relationship between reaction times and tone level were not modified by manipulations to the nature of temporal variations in the signal or noise. The changes in behavioral threshold were consistent with a model where the brain subtracted noise from signal. These results suggest that the parameters of the modulation of signals and maskers heavily influence detection in very predictable ways. These results are consistent with some results in humans and avians and form the baseline for neurophysiological studies of mechanisms of detection in noise.