Responses of cochlear nucleus units in the chinchilla to iterated rippled noises: analysis of neural autocorrelograms.

Temporal encoding of stimulus features related to the pitch of iterated rippled noises was studied for single units in the chinchilla cochlear nucleus. Unlike other periodic complex sounds that produce pitch, iterated rippled noises have neither periodic waveforms nor highly modulated envelopes. Infinitely iterated rippled noise (IIRN) is generated when wideband noise (WBN) is delayed (tau), attenuated, and then added to (+) or subtracted from (-) the undelayed WBN through positive feedback. The pitch of IIRN[+, tau, -1 dB] is at 1/tau, whereas the pitch of IIRN[-, tau, -1 dB] is at 1/2tau. Temporal responses of cochlear nucleus units were measured using neural autocorrelograms. Synchronous responses as shown by peaks in neural autocorrelograms that occur at time lags corresponding to the IIRN tau can be observed for both primarylike and chopper unit types. Comparison of the neural autocorrelograms in response to IIRN[+, tau, -1 dB] and IIRN[-, tau, -1 dB] indicates that the temporal discharge of primarylike units reflects the stimulus waveform fine structure, whereas the temporal discharge patterns of chopper units reflect the stimulus envelope. The pitch of IIRN[+/-, tau, -1 dB] can be accounted for by the temporal discharge patterns of primarylike units but not by the temporal discharge of chopper units. To quantify the temporal responses, the height of the peak in the neural autocorrelogram at a given time lag was measured as normalized rate. Although it is well documented that chopper units give larger synchronous responses than primarylike units to the fundamental frequency of periodic complex stimuli, the largest normalized rates in response to IIRN[+, tau, -1 dB] were obtained for primarylike units, not chopper units. The results suggest that if temporal encoding is important in pitch processing, then primarylike units are likely to be an important cochlear nucleus subsystem that carries the pitch-related information to higher auditory centers.     

Properties of correlated neural activity clusters in cat auditory cortex resemble those of neural assemblies

Spiking activity was recorded from cat auditory cortex using multi-electrode arrays. Cross-correlograms were calculated for spikes recorded on separate microelectrodes. The pair-wise cross-correlation matrix was constructed for the peak values of the correlograms. Hierarchical clustering was performed on the cross-correlation matrix for six stimulus conditions. These were silence, three multi-tone stimulus ensembles with different spectral densities, low-pass amplitude-modulated noise, and Poisson-distributed click trains that each lasted 15 min. The resulting neuron clusters reflect patches in cortex of up to several mm(2) in size that expand and contract in response to different stimuli. Cluster positions and size were very similar for spontaneous activity and multi-tone stimulus-evoked activity but differed between those conditions and the noise and click stimuli. Cluster size was significantly larger in posterior auditory field (PAF) compared with primary auditory cortex (AI), whereas the fraction of common spikes (within a 10-ms window) across all electrode activity participating in a cluster was significantly higher in AI compared with PAF. Clusters crossed area boundaries in <5% of the cases were simultaneous recording were made in AI and PAF. Clusters are therefore similar to but not synonymous with the traditional view of neural assemblies. Common-spike spectrotemporal receptive fields (STRFs) were obtained for common-spike activity and all-spike activity within a cluster. Common-spike STRFs had higher signal-to-noise ratio than all-spike STRFs and showed generally spectral and temporal sharpening. The coincident and noncoincident output of the clusters could potentially act in parallel and may serve different modes of stimulus coding.     

Responses of auditory cortex to complex stimuli: functional organization revealed using intrinsic optical signals

We used optical imaging of intrinsic signals to study the large-scale organization of ferret auditory cortex in response to complex sounds. Cortical responses were collected during continuous stimulation by sequences of sounds with varying frequency, period, or interaural level differences. We used a set of stimuli that differ in spectral structure, but have the same periodicity and therefore evoke the same pitch percept (click trains, sinusoidally amplitude modulated tones, and iterated ripple noise). These stimuli failed to reveal a consistent periodotopic map across the auditory fields imaged. Rather, gradients of period sensitivity differed for the different types of periodic stimuli. Binaural interactions were studied both with single contralateral, ipsilateral, and diotic broadband noise bursts and with sequences of broadband noise bursts with varying level presented contralaterally, ipsilaterally, or in opposite phase to both ears. Contralateral responses were generally largest and ipsilateral responses were smallest when using single noise bursts, but the extent of the activated area was large and comparable in all three aural configurations. Modulating the amplitude in counter phase to the two ears generally produced weaker modulation of the optical signals than the modulation produced by the monaural stimuli. These results suggest that binaural interactions seen in cortex are most likely predominantly due to subcortical processing. Thus our optical imaging data do not support the theory that the primary or nonprimary cortical fields imaged are topographically organized to form consistent maps of systematically varying sensitivity either to stimulus pitch or to simple binaural properties of the acoustic stimuli.     

Depth electrode recordings show double dissociation between pitch processing in lateral Heschl’s gyrus and sound onset processing in medial Heschl’s gyrus

Pitch is a fundamental perceptual attribute of sounds. Our ability to discriminate, separate, and identify sounds relies heavily on pitch. Recent neuroimaging studies in humans have provided converging evidence for the existence of a “pitch center”—a region on the superior temporal plane (STP) lateral to Heschl’s gyrus specialized in pitch extraction—but a direct confirmation is still missing. Intracerebral recordings in humans are ideally suited for such a confirmation. Here we report results from depth electrode recordings in a patient undergoing investigation for epilepsy. We demonstrate a double dissociation between responses from the medial and lateral STP around Heschl’s gyrus to the onset of sound energy and the onset of pitch. Three pieces of evidence support this finding: (1) the response to sounds that do not contain pitch is small in the lateral STP compared to the medial STP; (2) sounds that contain pitch evoke a strong response in the lateral STP; (3) at the transition from noise to a specialised noise-like, but tonal, sound referred to as iterated ripple noise, where the onset of pitch is the sole acoustic event, only the lateral contact showed a response. Our results provide direct evidence for a pitch-specific area on lateral STP with intracranial recordings from the human auditory cortex.     

Click train encoding in primary and non-primary auditory cortex of anesthetized macaque monkeys

We studied encoding of temporally modulated sounds in 28 multiunits in the primary auditory cortical field (AI) and in 35 multiunits in the secondary auditory cortical field (caudomedial auditory cortical field, CM) by presenting periodic click trains with click rates between 1 and 300 Hz lasting for 2-4 s. We found that all multiunits increased or decreased their firing rate during the steady state portion of the click train and that all except two multiunits synchronized their firing to individual clicks in the train. Rate increases and synchronized responses were most prevalent and strongest at low click rates, as expressed by best modulation frequency, limiting frequency, percentage of responsive multiunits, and average rate response and vector strength. Synchronized responses occurred up to 100 Hz; rate response occurred up to 300 Hz. Both auditory fields responded similarly to low click rates but differed at click rates above approximately 12 Hz at which more multiunits in AI than in CM exhibited synchronized responses and increased rate responses and more multiunits in CM exhibited decreased rate responses. These findings suggest that the auditory cortex of macaque monkeys encodes temporally modulated sounds similar to the auditory cortex of other mammals. Together with other observations presented in this and other reports, our findings also suggest that AI and CM have largely overlapping sensitivities for acoustic stimulus features but encode these features differently.     

Anesthesia suppresses nonsynchronous responses to repetitive broadband stimuli

Although many aspects of sensory processing are qualitatively similar in awake and anesthetized subjects, important state-dependent differences are known to exist. To investigate the effects of anesthesia on temporal processing in rat auditory cortex, multi-unit neural responses to trains of broadband clicks were recorded prior to, 15 min following, and 5 h following the administration of a ketamine-based anesthetic. While responses to clicks in isolation were relatively stable between states, responses to subsequent clicks exhibited increases in latency, peak latency, response duration, and post-onset suppression under anesthesia. Ketamine anesthetic reduced the maximum rate at which multi-unit clusters entrained to repeated clicks. No multi-unit clusters entrained to stimulus presentation rates greater than 33 Hz under anesthesia, compared with 85% and 81% in the pre- and post-anesthetic condition, respectively. Anesthesia also induced oscillatory activity that was not present in awake subjects. Finally, ketamine anesthesia abolished all tonic excitatory and suppressive nonsynchronous responses to click trains. The results of this study suggest that ketamine-based anesthesia significantly alters neural coding of broadband click trains in auditory cortex.     

Temporal modulation transfer functions in cat primary auditory cortex: separating stimulus effects from neural mechanisms.

We present here a comparison between the local field potentials (LFP) and multiunit (MU) responses, comprising 401 single units, in primary auditory cortex (AI) of 31 cats to periodic click trains, gamma-tone and time-reversed gamma-tone trains, AM noise, AM tones, and frequency-modulated (FM) tones. In a large number of cases, the response to all six stimuli was obtained for the same neurons. We investigate whether cortical neurons are likely to respond to all types of repetitive transients and modulated stimuli and whether a dependence on modulating waveform, or tone or noise carrier, exists. In 97% of the recordings, a temporal modulation transfer function (tMTF) for MU activity was obtained for gamma-tone trains, in 92% for periodic click trains, in 83% for time-reversed gamma-tone trains, in 82% for AM noise, in 71% for FM tones, and only in 53% for AM tones. In 31% of the cases, the units responded to all six stimuli in an envelope-following way. These particular units had significantly larger onset responses to each stimulus compared with all other units. The overall response distribution shows the preference of AI units for stimuli with short rise times such as clicks and gamma tones. It also shows a clear asymmetry in the ability to respond to AM noise and AM tones and points to a strong effect of the frequency content of the carrier on the subcortical processing of AM stimuli. Yet all temporal response properties were independent of characteristic frequency and frequency-tuning curve bandwidth. We show that the observed differences in the tMTFs for different stimuli are to a large extent produced by the different degree of phase locking of the neuronal firings to the envelope of the first stimulus in the train or first modulation period. A normalization procedure, based on these synchronization differences, unified the tMTFs for all stimuli except clicks and allowed the identification of a largely stimulus-invariant, low-pass temporal filter function that most likely reflects the properties of synaptic depression and facilitation. For nonclick stimuli, the low-pass filter has a cutoff frequency of approximately 10 Hz and a slope of approximately 6 dB/octave. For nonclick stimuli, there was a systematic difference between the vector strength for LFPs and MU activity that can likely be attributed to postactivation suppression mechanisms.     

Short-term sound temporal envelope characteristics determine multisecond time patterns of activity in human auditory cortex as shown by fMRI

Functional magnetic resonance imaging (fMRI) of human auditory cortex has demonstrated a striking range of temporal waveshapes in responses to sound. Prolonged (30 s) low-rate (2/s) noise burst trains elicit "sustained" responses, whereas high-rate (35/s) trains elicit "phasic" responses with peaks just after train onset and offset. As a step toward understanding the significance of these responses for auditory processing, the present fMRI study sought to resolve exactly which features of sound determine cortical response waveshape. The results indicate that sound temporal envelope characteristics, but not sound level or bandwidth, strongly influence response waveshapes, and thus the underlying time patterns of neural activity. The results show that sensitivity to sound temporal envelope holds in both primary and nonprimary cortical areas, but nonprimary areas show more pronounced phasic responses for some types of stimuli (higher-rate trains, continuous noise), indicating more prominent neural activity at sound onset and offset. It has been hypothesized that the neural activity underlying the onset and offset peaks reflects the beginning and end of auditory perceptual events. The present data support this idea because sound temporal envelope, the sound characteristic that most strongly influences whether fMRI responses are phasic, also strongly influences whether successive stimuli (e.g., the bursts of a train) are perceptually grouped into a single auditory event. Thus fMRI waveshape may provide a window onto neural activity patterns that reflect the segmentation of our auditory environment into distinct, meaningful events.     

Cortical oscillations related to processing congruent and incongruent grapheme-phoneme pairs

In this study, we investigated changes in cortical oscillations following congruent and incongruent grapheme-phoneme stimuli. Hiragana graphemes and phonemes were simultaneously presented as congruent or incongruent audiovisual stimuli to native Japanese-speaking participants. The discriminative reaction time was 57 ms shorter for congruent than incongruent stimuli. Analysis of MEG responses using synthetic aperture magnetometry (SAM) revealed that congruent stimuli evoked larger 2-10 Hz activity in the left auditory cortex within the first 250 ms after stimulus onset, and smaller 2-16 Hz activity in bilateral visual cortices between 250 and 500 ms. These results indicate that congruent visual input can modify cortical activity in the left auditory cortex.     

Neural connectivity only accounts for a small part of neural correlation in auditory cortex

In order to allow the relation of functional connectivity patterns (inferred from cross-correlograms) to structural connectivity (the anatomical substrate), we analyzed cross-correlogram peaks for spontaneous and stimulated activity in the auditory cortex. It was assumed that the broad correlograms, usually encountered, represent neural connectivity as well as secondary effects such as intrinsic firing patterns, global synchrony related to the ongoing electroencephalographic activity, and stimulus-related effects. Data were collected from 604 neuron pairs recorded under spontaneous conditions in primary auditory cortex of seven juvenile (30–70 days) and nine adult cats. Three hundred and six pairs (51%) had a peak cross-correlation coefficient significantly different from zero. For 113 neuron pairs out of this subgroup, correlations were calculated also for spike trains recorded during click stimulation. After a combined burst-correction and deconvolution procedure was carried out, the correlation peak strengths were not significantly changed for spontaneous activity, but peak width was narrower for single-electrode pairs than for dual-electrode pairs, suggesting a better synchronization for neighboring neurons. Under click stimulation conditions, overall peak synchronization strength was independent of interelectrode distance, whereas, after correction for secondary and stimulus effects, peak synchronization was significantly lower for dual-electrode pairs. However, the primary peak width for single-electrode pairs under stimulus conditions was no longer different from that of dual-electrode pairs. This implies that both under spontaneous and stimulus conditions secondary effects largely obscure any underlying correlation produced by anatomical connectivity. The secondary effects may be the result of intrinsic as well as network properties in auditory cortex and may functionally be more important than the weak primary effects resulting from anatomical connections. Cross-interval analysis suggests that the correlations in auditory cortex are dynamic and may show random switching between states of stronger and weaker synchronization.     

Cortical representations of temporal structure in sound

Pitch and spatial width are two sound attributes that can be coded by temporal acoustic structure. In this study, periodicity pitch was created by temporal iteration in a regular-interval noise, whereas spatial width was determined by the degree of interaural correlation. Previous results suggest that nonprimary auditory cortex, particularly lateral Heschl's gyrus (HG), plays an important role in the analysis of both acoustic properties. It has been argued that this role might reflect a common computational process. One proposed candidate is that of integrating the temporal pattern information across frequency channels. This paper reports the results of a systematic test for whether different classes of temporal structure do indeed engage a common neural architecture in the human auditory cortex by presenting both classes of sound stimuli to a single group of listeners. Activations related to the pitch and spatial width of the sound were partly co-localized in two distinct cortical regions: close to lateral HG and in planum temporale (PT). Lateral HG was more responsive to temporal pitch than to spatial width. This difference plus the variability across listeners for spatial width dispute the claim that the activity in lateral HG reflects a common neural computational step that encodes the temporal patterns associated with pitch and spatial width. Rather, the activity patterns are consistent with a role for lateral HG in perceptual analysis as opposed to temporal acoustic structure. In PT, the superadditive relationship between pitch and spatial width is also consistent with the concept that the auditory cortex plays an important role in integrating different classes of sound information to form auditory objects.     

Information content of auditory cortical responses to time-varying acoustic stimuli

The present study explores the issue of cortical coding by spike count and timing using statistical and information theoretic methods. We have shown in previous studies that neurons in the auditory cortex of awake primates have an abundance of sustained discharges that could represent time-varying signals by temporal discharge patterns or mean firing rates. In particular, we found that a subpopulation of neurons can encode rapidly occurring sounds, such as a click train, with discharges that are not synchronized to individual stimulus events, suggesting a temporal-to-rate transformation. We investigated whether there were stimulus-specific temporal patterns embedded in these seemingly random spike times. Furthermore, we quantitatively analyzed the precision of spike timing at stimulus onset and during ongoing acoustic stimulation. The main findings are the following. 1) Temporal and rate codes may operate at separate stimulus domains or encode the same stimulus domain in parallel via different neuronal populations. 2) Spike timing was crucial to encode stimulus periodicity in "synchronized" neurons. 3) "Nonsynchronized" neurons showed little stimulus-specific spike timing information in their responses to time-varying signals. Such responses therefore represent processed (instead of preserved) information in the auditory cortex. And 4) spike timing on the occurrence of acoustic events was more precise at the first event than at successive ones and more precise with sparsely distributed events (longer time intervals between events) than with densely packed events. These results indicate that auditory cortical neurons mark sparse acoustic events (or onsets) with precise spike timing and transform rapidly occurring acoustic events into firing rate-based representations.     

Neuromagnetic responses to binaural beat in human cerebral cortex

The dichotic presentation of two sinusoids with a slight difference in frequency elicits subjective fluctuations called binaural beat (BB). BBs provide a classic example of binaural interaction considered to result from neural interaction in the central auditory pathway that receives input from both ears. To explore the cortical representation of the fluctuation of BB, we recorded magnetic fields evoked by slow BB of 4.00 or 6.66 Hz in nine normal subjects. The fields showed small amplitudes; however, they were strong enough to be distinguished from the noise accompanying the recordings. Spectral analyses of the magnetic fields recorded on single channels revealed that the responses evoked by BBs contained a specific spectral component of BB frequency, and the magnetic fields were confirmed to represent an auditory steady-state response (ASSR) to BB. The analyses of spatial distribution of BB-synchronized responses and minimum-norm current estimates revealed multiple BB ASSR sources in the parietal and frontal cortices in addition to the temporal areas, including auditory cortices. The phase of synchronized waveforms showed great variability, suggesting that BB ASSR does not represent changing interaural phase differences (IPD) per se, but instead it reflects a higher-order cognitive process corresponding to subjective fluctuations of BB. Our findings confirm that the activity of the human cerebral cortex can be synchronized with slow BB by using information on the IPD.     

Analysis of temporal structure in sound by the human brain

For over a century, models of pitch perception have been based on the frequency composition of the sound. Pitch phenomena can also be explained, however, in terms of the time structure, or temporal regularity, of sounds. To locate the mechanism for the detection of temporal regularity in humans, we used functional imaging and a 'delay-and-add' noise, which activates all frequency regions uniformly, like noise, but which nevertheless produces strong pitch perceptions and tuneful melodies. This stimulus has temporal regularity that can be systematically altered. We found that the activity of primary auditory cortex increased with the regularity of the sound. Moreover, a melody composed of delay-and-add 'notes' produced a distinct pattern of activation in two areas of the temporal lobe distinct from primary auditory cortex. These results suggest a hierarchical analysis of time structure in the human brain.     

Auditory processing indexed by stimulus-induced alpha desynchronization in children.

By means of magnetoencephalography (MEG), we investigated event-related synchronization and desynchronization (ERS/ERD) in auditory cortex activity, recorded from twelve children aged four to six years, while they passively listened to a violin tone and a noise-burst stimulus. Time-frequency analysis using Wavelet Transform was applied to single-trials of source waveforms observed from left and right auditory cortices. Stimulus-induced changes in non-phase-locked activities were evident. ERS in the beta range (13-30 Hz) lasted only for 100 ms after stimulus onset. This was followed by prominent alpha ERD, which showed a clear dissociation between the upper (12 Hz) and lower (8 Hz) alpha range in both left and right auditory cortices for both stimuli. The time courses of the alpha ERD (onset around 300 ms, peak at 500 ms, offset after 1500 ms) were similar to those previously found for older children and adults with auditory memory related tasks. For the violin tone only, the ERD lasted longer in the upper than the lower alpha band. The findings suggest that induced alpha ERD indexes auditory stimulus processing in children without specific cognitive task requirement. The left auditory cortex showed a larger and longer-lasting upper alpha ERD than did the right auditory cortex, likely reflecting hemispheric differences in maturational stages of neural oscillatory mechanisms.     

Optical imaging of intrinsic signals in ferret auditory cortex: responses to narrowband sound stimuli

This paper describes optical imaging of the auditory cortex in the anesthetized ferret, particularly addressing optimization of narrowband stimuli. The types of sound stimuli used were tone-pip trains and sinusoidal frequency and amplitude modulated (SFM and SAM) tones. By employing short illumination wavelengths (546 nm), we have successfully characterized the tonotopic arrangement, in agreement with the well-established electrophysiological tonotopic maps of the ferret auditory primary field (AI). The magnitude of the optical signal increased with sound level, was maximal for a modulation frequency (MF) of 2-4 Hz, and was larger for tone-pip trains and SFM sounds than for SAM sounds. Accordingly, an optimal narrowband stimulus was defined. Thus optical imaging can be used successfully to obtain frequency maps in auditory cortex by an appropriate choice of stimulus parameters. In addition, background noise consisting of 0.1-Hz oscillations could be reduced by introduction of blood pressure enhancing drugs. The optical maps were largely independent of 1) the type of narrowband stimulus, 2) the sound level, and 3) the MF. This stability of the optical maps was not predicted from the electrophysiological literature.     

Sound-induced synchronization of neural activity between and within three auditory cortical areas

Neural synchrony within and between auditory cortical fields is evaluated with respect to its potential role in feature binding and in the coding of tone and noise sound pressure level. Simultaneous recordings were made in 24 cats with either two electrodes in primary auditory cortex (AI) and one in anterior auditory field (AAF) or one electrode each in AI, AAF, and secondary auditory cortex. Cross-correlograms (CCHs) for 1-ms binwidth were calculated for tone pips, noise bursts, and silence (i.e., poststimulus) as a function of intensity level. Across stimuli and intensity levels the total percentage of significant stimulus onset CCHs was 62% and that of significant poststimulus CCHs was 58% of 1,868 pairs calculated for each condition. The cross-correlation coefficient to stimulus onsets was higher for single-electrode pairs than for dual-electrode pairs and higher for noise bursts compared with tone pips. The onset correlation for single-electrode pairs was only marginally larger than the poststimulus correlation. For pairs from electrodes across area boundaries, the onset correlations were a factor 3-4 higher than the poststimulus correlations. The within-AI dual-electrode peak correlation was higher than that across areas, especially for spontaneous conditions. Correlation strengths for between area pairs were independent of the difference in characteristic frequency (CF), thereby providing a mechanism of feature binding for broadband sounds. For noise-burst stimulation, the onset correlation for between area pairs was independent of stimulus intensity regardless the difference in CF. In contrast, for tone-pip stimulation a significant dependence on intensity level of the peak correlation strength was found for pairs involving AI and/or AAF with CF difference less than one octave. Across all areas, driven rate, between-area peak correlation strength, or a combination of the two did not predict stimulus intensity. However, between-area peak correlation strength performs better than firing rate to decide if a stimulus is present or absent.     

P300-response: possible psychophysiological correlates in delta and theta frequency channels. A review.

The present paper combines a review of event-related potentials (ERPs) with empirical data concerning the question: what are the differences between auditory evoked potentials (EPs) and two types of ERPs with respect to their frequency components? In this study auditory EPs were elicited by 1500 Hz tones. The first type of ERPs was responses to 3rd attended tones in an omitted stimulus paradigm where every 4th stimulus was omitted. The second type of ERPs was responses to rare 1600 Hz tones in an oddball paradigm. The amplitudes of delta and theta components of EPs and ERPs showed significant differences: in responses to 3rd attended tones there was a significant increase in the theta frequency band (frontal and parietal locations; 0-250 ms). In the delta frequency band there was no significant change. In contrast a diffuse delta increase occurred in oddball responses and an additional prolongation of theta oscillations was observed (late theta response: 250-500 ms). These results are discussed in the context of ERPs as induced rhythmicities. The intracranial sources of ERPs, their psychological correlates and the role of theta rhythms in the cortico-hippocampal interaction are reviewed. From these results and from the literature a working hypothesis is derived assuming that delta responses are mainly involved in signal matching, decision making and surprise, whereas theta responses are more related to focused attention and signal detection.     

Reduced event-related low frequency EEG activity in schizophrenia during an auditory oddball task

This study examines EEG low frequency characteristics which have been linked to specific cognitive functions such as stimulus encoding and attention during an auditory oddball task in schizophrenia patients and healthy controls. EEG data was recorded from 17 young schizophrenia patients in a stable phase of their illness and 17 healthy controls performing an auditory oddball task. Evoked and induced delta and theta activity, N100, P300 amplitude were computed. Between 200–500 ms after a stimulus was presented, patients displayed significantly reduced P300, less evoked and induced delta and theta activity than controls. We conclude that the well known finding of P300 reduction in schizophrenia can be linked to reductions in delta and theta activity, which are a manifestation of impaired stimulus evaluation, memory retrieval, and a lack of sustained attention.     

Brain wave synchronization and entrainment to periodic acoustic stimuli

As known, different brainwave frequencies show synchronies related to different perceptual, motor or cognitive states. Brainwaves have also been shown to synchronize with external stimuli with repetition rates of ca. 10–40 Hz. However, not much is known about responses to periodic auditory stimuli with periodicities found in human rhythmic behavior (i.e. 0.5–5 Hz). In an EEG study we compared responses to periodic stimulations (drum sounds and clicks with repetition rates of 1–8 Hz), silence, and random noise. Here we report inter-trial coherence measures taken at the Cz-electrode that show a significant increase in brainwave synchronization following periodic stimulation. Specifically, we found (1) a tonic synchronization response in the delta range with a maximum response at 2 Hz, (2) a phasic response covering the theta range, and (3) an augmented phase synchronization throughout the beta/gamma range (13–44 Hz) produced through increased activity in the lower gamma range and modulated by the stimulus periodicity. Periodic auditory stimulation produces a mixture of evoked and induced, rate-specific and rate-independent increases in stimulus related brainwave synchronization that are likely to affect various cognitive functions. The synchronization responses in the delta range may form part of the neurophysiological processes underlying time coupling between rhythmic sensory input and motor output; the tonic 2 Hz maximum corresponds to the optimal tempo identified in listening, tapping synchronization, and event-interval discrimination experiments. In addition, synchronization effects in the beta and gamma range may contribute to the reported influences of rhythmic entrainment on cognitive functions involved in learning and memory tasks.