Consonance and dissonance of musical chords: neural correlates in auditory cortex of monkeys and humans.

Some musical chords sound pleasant, or consonant, while others sound unpleasant, or dissonant. Helmholtz's psychoacoustic theory of consonance and dissonance attributes the perception of dissonance to the sensation of "beats" and "roughness" caused by interactions in the auditory periphery between adjacent partials of complex tones comprising a musical chord. Conversely, consonance is characterized by the relative absence of beats and roughness. Physiological studies in monkeys suggest that roughness may be represented in primary auditory cortex (A1) by oscillatory neuronal ensemble responses phase-locked to the amplitude-modulated temporal envelope of complex sounds. However, it remains unknown whether phase-locked responses also underlie the representation of dissonance in auditory cortex. In the present study, responses evoked by musical chords with varying degrees of consonance and dissonance were recorded in A1 of awake macaques and evaluated using auditory-evoked potential (AEP), multiunit activity (MUA), and current-source density (CSD) techniques. In parallel studies, intracranial AEPs evoked by the same musical chords were recorded directly from the auditory cortex of two human subjects undergoing surgical evaluation for medically intractable epilepsy. Chords were composed of two simultaneous harmonic complex tones. The magnitude of oscillatory phase-locked activity in A1 of the monkey correlates with the perceived dissonance of the musical chords. Responses evoked by dissonant chords, such as minor and major seconds, display oscillations phase-locked to the predicted difference frequencies, whereas responses evoked by consonant chords, such as octaves and perfect fifths, display little or no phase-locked activity. AEPs recorded in Heschl's gyrus display strikingly similar oscillatory patterns to those observed in monkey A1, with dissonant chords eliciting greater phase-locked activity than consonant chords. In contrast to recordings in Heschl's gyrus, AEPs recorded in the planum temporale do not display significant phase-locked activity, suggesting functional differentiation of auditory cortical regions in humans. These findings support the relevance of synchronous phase-locked neural ensemble activity in A1 for the physiological representation of sensory dissonance in humans and highlight the merits of complementary monkey/human studies in the investigation of neural substrates underlying auditory perception.

Molecular epidemiology of Mycobacterium tuberculosis infection in Israel.

Restriction fragment length polymorphism analysis with IS6110 and DR-r probes was used to study 69 Mycobacterium tuberculosis isolates obtained from Israeli patients and new immigrants from the former Soviet Union and Ethiopia. DNA fingerprinting identified unique patterns for almost all isolates, indicating that most patients were infected with a unique strain imported from their country of origin and that their latent infection was reactivated in Israel.

Neural correlates of auditory stream segregation in primary auditory cortex of the awake monkey.

An important feature of auditory scene analysis is the perceptual organization of sequential sound components, or 'auditory stream segregation'. Auditory stream segregation can be demonstrated by presenting a sequence of high and low frequency tones in an alternating pattern, ABAB. When the tone presentation rate (PR) is slow or the frequency separation (DeltaF) between the tones is small (<10%), a connected alternating sequence ABAB is perceived. When the PR is fast or the DeltaF is large, however, the alternating sequence perceptually splits into two parallel auditory streams, one composed of interrupted 'A' tones, and the other of interrupted 'B' tones. The neurophysiological basis of this perceptual phenomenon is unknown. Neural correlates of auditory stream segregation were examined in A1 of the awake monkey using neuronal ensemble techniques (multiunit activity and current source density). Responses evoked by alternating frequency sequences of tones, ABAB, were studied as a function of PR (5, 10, 20 and 40 Hz). 'A' tones corresponded to the best frequency (BF) of the cortical site, while 'B' tones were situated away from the BF by an amount DeltaF. At slow PRs, 'A' and 'B' tones evoked responses that generated an overall pattern of activity at the stimulus PR. In contrast, at fast PRs, 'B' tone responses were differentially suppressed, resulting in a pattern of activity consisting predominantly of 'A' tone responses at half the PR. The magnitude of 'B' tone response suppression increased with DeltaF. Differential suppression of BF and non-BF tone responses at high PRs can be explained by physiological principles of forward masking. The effect of DeltaF is explained by the hypothesis that responses to tones distant from the BF are more susceptible to suppression by BF tones than responses to tones near the BF. These results parallel human psychoacoustics of auditory stream segregation and suggest a cortical basis for the perceptual phenomenon.

AaIT: from neurotoxin to insecticide.

AaIT is a single chain neurotoxic polypeptide derived from the venom of the Buthid scorpion Androctonus australis Hector, composed of 70 amino acids cross-linked by four disulfide bridges. Its strict selectivity for insects has been documented by toxicity, electrophysiological and ligand receptor binding assays. These last have shown that various insect neuronal membranes possess a single class of non-interacting AaIT binding sites of high affinity (K(D) = 1-3(n)M) and low capacity (0.5-2.0 pmol/mg prot.). The fast excitatory paralysis induced by AaIT is a result of a presynaptic effect, namely the induction of a repetitive firing in the terminal branches of the insect's motor nerves resulting in a massive and uncoordinated stimulation of the respective skeletal muscles. The neuronal repetitive activity is attributed to an exclusive and specific perturbation of sodium conductance as a consequence of toxin binding to external loops of the insect voltage-dependent sodium channel and modification of its gating mechanism. From a strictly agrotechnical point of view AaIT involvement in plant protection has taken the following two complementary forms: firstly, as a factor for the genetic engineering of insect infective baculoviruses resulting in potent and selective bio-insecticides. The efficacy of the AaIT-expressing, recombinant baculovirus is attributed mainly to its ability to continuously provide and translocate the gene of the expressed toxin to the insect central nervous system; secondly, based on the pharmacological flexibility of the voltage-gated sodium channel, as a device for insecticide resistance management. Channel mutations conferring resistance to a given class of insecticidal agents (such as the KDR phenomenon) may greatly increase susceptibility to the AaIT expressing bioinsecticides. Thus the AaIT is a pharmacological tool for the study of insect neuronal excitability and chemical ecology and the development of new approaches to insect control.

Complex tone processing in primary auditory cortex of the awake monkey. I. Neural ensemble correlates of roughness.

Previous physiological studies [e.g., Bieser and Muller-Preuss, Exp. Brain Res. 108, 273-284 (1996); Schulze and Langner, J. Comp. Physiol. A 181, 651-663 (1997); Steinschneider et al., J. Acoust. Soc. Am. 104, 2935-2955 (1998)] have suggested that neural activity in primary auditory cortex (A1) phase-locked to the waveform envelope of complex sounds with low (<300 Hz) periodicities may represent a neural correlate of roughness perception. However, a correspondence between these temporal response patterns and human psychophysical boundaries of roughness has not yet been demonstrated. The present study examined whether the degree of synchronized phase-locked activity of neuronal ensembles in A1 of the awake monkey evoked by complex tones parallels human psychoacoustic data defining the existence region and frequency dependence of roughness. Stimuli consisted of three consecutive harmonics of fundamental frequencies (f(0)s) ranging from 25 to 4000 Hz. The center frequency of the complex tones was fixed at the best frequency (BF) of the cortical sites, which ranged from 0.3 to 10 kHz. Neural ensemble activity in the thalamorecipient zone (lower lamina III) and supragranular cortical laminae (upper lamina III and lamina II) was measured using multiunit activity and current source density techniques and the degree of phase-locking to the f0 was quantified by spectral analysis. In the thalamorecipient zone, the stimulus f0 at which phase-locking was maximal increased with BF and reached an upper limit between 75 and 150 Hz for BFs greater than about 3 kHz. Estimates of limiting phase-locking rates also increased with BF and approximated psychoacoustic values for the disappearance of roughness. These physiological relationships parallel human perceptual data and therefore support the relevance of phase-locked activity of neuronal ensembles in A1 for the physiological representation of roughness.

Complex tone processing in primary auditory cortex of the awake monkey. II. Pitch versus critical band representation.

Noninvasive neurophysiological studies in humans support the existence of an orthogonal spatial representation of pure tone frequency and complex tone pitch in auditory cortex [Langner et al., J. Comp. Physiol. A 181, 665-676 (1997)]. However, since this topographic organization is based on neuromagnetic responses evoked by wideband harmonic complexes (HCs) of variable fundamental frequency (f0), and thus interharmonic frequency separation (deltaF), critical band filtering effects due to differential resolvability of harmonics may have contributed to shaping these responses. To test this hypothesis, the present study examined responses evoked by three-component HCs of variable f0 in primary auditory cortex (A1) of the awake monkey. The center frequency of the HCs was fixed at the best frequency (BF) of the cortical site. Auditory evoked potential (AEP), multiunit activity, and current source density techniques were used to evaluate A1 responses as a function of f0 (=deltaF). Generally, amplitudes of nearly all response components increased with f0, such that maximal responses were evoked by HCs comprised of low-order resolved harmonics. Statistically significant increases in response amplitude typically occurred at deltaFs between 10% and 20% of center frequency, suggestive of critical bandlike behavior. Complex tone response amplitudes also reflected nonlinear summation in that they could not be predicted by the pure tone frequency sensitivity curves of the cortical sites. A mechanism accounting for the observed results is proposed which involves mutual lateral inhibitory interactions between responses evoked by stimulus components lying within the same critical band. As intracortical AEP components likely to be propagated to the scalp were also strongly modulated by deltaF, these findings indicate that noninvasive recordings of responses to complex sounds may require a consideration of critical band effects in their interpretation.

Binaural interactions in primary auditory cortex of the awake macaque.

The functional organization of primary auditory cortex in non-primates is generally modeled as a tonotopic gradient with an orthogonal representation of independently mapped binaural interaction columns along the isofrequency contours. Little information is available regarding the validity of this model in the primate brain, despite the importance of binaural cues for sound localization and auditory scene analysis. Binaural and monaural responses of A1 to pure tone stimulation were studied using auditory evoked potentials, current source density and multiunit activity. Key findings include: (i) differential distribution of binaural responses with respect to best frequency, such that 74% of the sites exhibiting binaural summation had best frequencies below 2000 Hz; (ii) the pattern of binaural responses was variable with respect to cortical depth, with binaural summation often observed in the supragranular laminae of sites showing binaural suppression in thalamorecipient laminae; and (iii) dissociation of binaural responses between the initial and sustained action potential firing of neuronal ensembles in A1. These data support earlier findings regarding the temporal and spatial complexity of responses in A1 in the awake state, and are inconsistent with a simple orthogonal arrangement of binaural interaction columns and best frequency in A1 of the awake primate.

Fate of new neurons in adult canary high vocal center during the first 30 days after their formation.

Projection neurons are added to the high vocal center (HVC) of adult songbirds. Here we report on events associated with their initial arrival in HVC. Neurons formed in adult canaries were labeled with [(3)H]-thymidine and examined 8, 15, 22, and 31 days later. By 8 days, some [(3)H]-labeled cells with the nuclear profile of postmigratory neurons were already present in HVC but could not be retrogradely labeled by Fluoro-Gold injections in the robust nucleus of the archistriatum (RA); 7 days later, a few such cells could be backfilled from RA. Thus, new neurons may arrive in HVC as much as 1 week prior to establishing connections with RA. By 31 days, 43% of the [(3)H]-labeled neurons could be backfilled from RA. In no case were new neurons backfilled by tracer injections into Area X, suggesting that newly formed HVC cells do not establish a transient connection with this region. At all survival times, the somata of new neurons were often clustered tightly together with other HVC neurons that differed in age and projection. Between days 15 and 25 after their birth, half of the new HVC neurons disappeared. We conclude: (1) that neurons arrive in HVC earlier than previously thought, (2) that soon after their arrival they become part of cell clusters in HVC, and (3) that in addition to the previously described death of new neurons that occurs over a period of months, there is an early wave of death that occurs soon after new neurons adopt a postmigratory phenotype.

Click train encoding in primary auditory cortex of the awake monkey: evidence for two mechanisms subserving pitch perception.

Multiunit activity (MUA) and current source density (CSD) patterns evoked by click trains are examined in primary auditory cortex (A1) of three awake monkeys. Temporal and spectral features of click trains are differentially encoded in A1. Encoding of temporal features occurs at rates of 100-200 Hz through phase-locked activity in the MUA and CSD, is independent of pulse polarity pattern, and occurs in high best frequency (BF) regions of A1. The upper limit of ensemble-wide phase-locking is about 400 Hz in the input to A1, as manifested in the cortical middle laminae CSD and MUA of thalamocortical fibers. In contrast, encoding of spectral features occurs in low BF regions, and resolves both the f0 and harmonics of the stimuli through local maxima of activity determined by the tonotopic organization of the recording sites. High-pass filtered click trains decrease spectral encoding in low BF regions without modifying phase-locked responses in high BF regions. These physiological responses parallel features of human pitch perception for click trains, and support the existence of two distinct physiological mechanisms involved in pitch perception: the first using resolved harmonic components and the second utilizing unresolved harmonics that is based on encoding stimulus waveform periodicity.

Pitch vs. spectral encoding of harmonic complex tones in primary auditory cortex of the awake monkey.

Neuromagnetic studies in humans and single-unit studies in monkeys have provided conflicting views regarding the role of primary auditory cortex (A1) in pitch encoding. While the former support a topographic organization based on the pitch of complex tones, single-unit studies support the classical tonotopic organization of A1 defined by the spectral composition of the stimulus. It is unclear whether the incongruity of these findings is due to limitations of noninvasive recordings or whether the discrepancy genuinely reflects pitch representation based on population encoding. To bridge these experimental approaches, we examined neuronal ensemble responses in A1 of the awake monkey using auditory evoked potential (AEP), multiple-unit activity (MUA) and current source density (CSD) techniques. Macaque monkeys can perceive the missing fundamental of harmonic complex tones and therefore serve as suitable animal models for studying neural encoding of pitch. Pure tones and harmonic complex tones missing the fundamental frequency (f0) were presented at 60 dB SPL to the ear contralateral to the hemisphere from which recordings were obtained. Laminar response profiles in A1 reflected the spectral content rather than the pitch (missing f0) of the compound stimuli. These findings are consistent with single-unit data and indicate that the cochleotopic organization is preserved at the level of A1. Thus, it appears that pitch encoding of multi-component sounds is more complex than suggested by noninvasive studies, which are based on the assumption of a single dipole generator within the superior temporal gyrus. These results support a pattern recognition mechanism of pitch encoding based on a topographic representation of stimulus spectral composition at the level of A1.

Delayed-type hypersensitivity activity of the Brucella L7/L12 ribosomal protein depends on posttranslational modification.

The ribosomal protein L7/L12 isolated from Brucella melitensis induces a delayed-type hypersensitivity (DTH) reaction in brucella-sensitized guinea pigs. Surprisingly, the recombinant brucella L7/L12 protein expressed in Escherichia coli as a fusion protein with a six-histidine tag cannot elicit such a reaction. The six histidines tagged to the recombinant L7/L12 protein were removed enzymatically, but the resulting protein did not induce a DTH reaction in sensitized animals. Incubation of the recombinant L7/L12 fusion protein in a B. melitensis lysate endowed the recombinant protein with a DTH activity, suggesting that the recombinant protein was modified by this treatment. Glycosylation or phosphorylation of the recombinant L7/L12 protein could not be detected. On the other hand, radiolabeled palmitic acid was found to be incorporated to the recombinant protein during its incubation in the brucella lysate. This incorporation was specific for the brucella L7/L12 protein and was inhibited when the brucella lysate was frozen and thawed before the incubation. The data reported here indicate that posttranslational modification of L7/L12 protein comprising at least an acylation step is required for the brucella L7/L12 DTH activity.

Cloning of the gene encoding streptococcal immunoglobulin A protease and its expression in Escherichia coli.

We have identified and cloned a 6-kilobase-pair segment of chromosomal DNA from Streptococcus sanguis ATCC 10556 that encodes immunoglobulin A (IgA) protease activity when cloned into Escherichia coli. The enzyme specified by the iga gene in plasmid pJG1 accumulates in the periplasm of E. coli MM294 cells and has a substrate specificity for human IgA1 identical to that of native S. sanguis protease. Hybridization experiments with probes from within the encoding DNA showed no detectable homology at the nucleotide sequence level with chromosomal DNA of gram-negative bacteria that excrete IgA protease. Moreover, the S. sanguis iga gene probes showed no detectable hybridization with chromosomal DNA of S. pneumoniae, although the IgA proteases of these two streptococcal species cleaved the identical peptide bond in the human IgA1 heavy-chain hinge region.

Preferential suppression of normal exoenzyme formation by membrane-modifying agents.

Cerulenin, phenethyl alcohol, benzyl alcohol, procaine, and a series of aliphatic alcohols selectively suppressed production of active exoenzymes by various bacterial strains.

Evidence linking penicillinase formation and secretion to lipid metabolism in Bacillus licheniformis.

The formation of penicillinase by cultures of Bacillus licheniformis was preferentially suppressed by cerulenin, an antibiotic known to specifically inhibit fatty acid synthesis in microorganisms. The effect was studied at cerulenin concentrations that had almost no effect on the rate of cell growth and overall protein synthesis, but that reduced the rate of [14C]acetate incorporation (by 50 to 70%), indicating partial inhibition of lipid synthesis. The levels of both the released enzyme (exopenicillinase) and its cell-bound precursor were reduced to the same extent (70% to 80%). Enzyme formation was gradually resumed after the removal of cerulenin or the addition of a mixture of fatty acids prepared from lipids extracted from B. licheniformis. Reversal was less effective as the time interval between treatment with cerulenin and addition of fatty acids increased. We conclude that de novo synthesis of fatty acids is required for the formation of both the membrane-bound and extracellular penicillinase. Suppression of the membrane-bound enzyme is a likely consequence of the altered membrane (decreased lipid-to-lipid ratio and increased density) seen in cerulenin-treated preparations. The corresponding suppression of exopenicillinase is consistent with the view that it is derived from the membrane-bound form. A mechanism linking the general class of exportable proteins to specific aspects of lipid synthesis is discussed.