The neural response to amplitude-modulated sinus sounds (AM sound) was investigated in the auditory cortex and insula of the awake squirrel monkey. It was found that 78.1% of all acoustically driven neurons encoded the envelope of the AM sound; the remaining 21.9% displayed simple On, On/Off or Off responses at the beginning or the end of the stimulus sound. Those neurons with AM coding were able to encode the AM sound frequency in two different ways: (1) the spikes followed the amplitude modulation envelopes in a phase locked manner; (2) the spike rate changed significantly with changing modulation frequencies. As reported in other species, the modulation transfer functions for rate showed higher modulation frequencies than the phase-locked response. Both AM codings exhibited a filter characteristic for AM sound. Whereas 46.6% of all neurons had the same filter characteristic for both the spike discharge and the phase-locked response, the remaining neurons displayed combinations of different filter types. The discharge pattern of a neuron to simple tone or noise bursts suggests the behaviour of this neuron when AM sound is used as the stimulus. Neurons with strong onset responses to tone/noise bursts tended to have higher phase-locked AM responses than neurons with weak onset responses. The spike rate maxima for AM sound showed no relation to the tone/noise burst discharge patterns. Varying modulation depth was encoded by the neuron's ability to follow the envelope cycles and not by the non-phase-locked spike rate frequency. The organization of the squirrel monkey's auditory cortex has previously been established by an anatomical study. We have added two new fields using physiological parameters. All fields investigated showed a clear functional separation for time-critical information processing. The best temporal resolution was shown by the primary auditory field (AI), the first-temporal field (T1) and the parainsular auditory field (Pi). The neural data in these fields and the amplitude modulation frequency range of squirrel monkey calls suggest a similar correlation between vocalization and perception as in human psychophysical data for speech and hearing sensation. The anterior fields in particular failed to follow the AM envelopes. For the first time in a primate, the insula was tested with different sound parameters ranging from simple tone bursts to AM sound. It is suggested that this cortical region plays a role in time-critical aspects of acoustic information processing. The observed best frequencies covered the same spectrum as AI. As in the auditory fields, most neurons in the insula encoded AM sound with different filter types. The high proportion of neurons unable to encode AM sound (40.6%) and the low mean best modulation frequency (9.9 Hz) do not support a prominent role of the insula in temporal information processing.

Amplitude-modulated (AM) and frequency-modulated (FM) elements are prominent periodic sound features of squirrel monkeys' twitter calls. To investigate how the periodic FM elements are represented in the spike activity of cortical neurons, single units in the insula, primary auditory field (AI) and rostral auditory field (R) were recorded. In five monkeys, 566 units (insula, n = 181; AI, n = 221; R, n = 164) were exposed to synthesized fundamental frequencies and one natural twitter call. Neuronal encoding of periodic FM elements takes place by phase-locking to either the up- or the down-directed FM sweeps. The phase-locking was strongly influenced by the FM-period repetition rate. The ability of neurons in both auditory fields and the insula to encode all periodic FM elements showed a marked reduction at 16 Hz FM-period repetition rate. The neurons' best frequency (BF) influenced the quality of periodicity encoding, but neurons with BFs outside the frequency range of the fundamentals also responded with periodic discharge rates. Even neurons in AI (6.8%) and the insula (22.6%) that did not respond to pure tones showed clear periodic FM encoding. The percentage of neurons able to encode all periodic FM elements within the twitter fundamental was significantly higher in field R than in AI and the insula. From 58 simultaneously recorded pairs of units in AI and the insula that had positive cross-correlation coefficients of spontaneous activity, the influence of the FM-period repetition rate on neuronal correlation was investigated. Correlated firing of AI and insula neurons seems limited to low-period repetition rates. The cross-correlation coefficients obtained for spontaneous activity and six different periodic FM sounds showed a band-pass characteristic. The natural twitter call evoked stronger neuronal responses in all fields than the synthesized fundamental frequencies with corresponding bi-directional FM sweeps. The better encoding of the transient features in the natural call can be attributed to the amplitude modulation added to the FM elements in the natural call. These amplitude modulations divide the FM elements of twitter calls into syllable-like sound elements. It is probable that encoding the complex pattern in the time and frequency domains of a call must undergo some integration at a cortical level. Additionally, these data provide the first evidence that insula neurons contribute to the encoding of complex FM signals.

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SUMMARY1. A combined light and electron microscopic study was performed on neuromuscular junctions of normal adult frogs.2. In a previous investigation signs ofnew synapse formation, as well as abandoned former synaptic sites, have been observed in normal muscles (Wernig, P6cot-Dechavassine & Stover, 1980a, b). Here we performed a detailed light and electron microscopic correlation to investigate those parts of junctions which, after staining for cholinesterase (ChE) and presynaptic axon terminals, were suspected either to be newly formed or sites abandoned by the presynaptic nerve and the Schwann cell.3. Thin presynaptic nerve branches, enclosed by Schwann cell sheaths along most of their length, formed synaptic contacts with the muscle fibre only at small circumscribed areas. In these regions post-synaptic secondary folds (invariably present at mature synapses) were either missing or were less well developed. At these small contacts, binding sites for fluorescein-labelled a-bungarotoxin were usually present.4. At other sites the ChE reaction product was present but an axon could not be detected in silver-stained preparations. Electron microscopic observation revealed post-synaptic secondary folds filled with ChE reaction product while the presynaptic axon and Schwann cell were missing. The sites with ChE remnants can thus be regarded as abandoned former synaptic contacts. No binding of fluorescein-labelled a-bungarotoxin could be detected at such sites.5. These findings confirm earlier suggestions that synaptic contacts in frog muscle are normally undergoing continual remodelling.6. The lack of binding sites for fluorescein-labelled a-bungarotoxin at abandoned synaptic sites suggests that a neural or Schwann cell factor is important for the maintainance of synaptic acetylcholine receptors.

SummarySalivary cortisol is a non-invasive and easy-to-assess measure of the activity of the hypotha1amo-pituitary-adrenocortical IHPA) system. Here we report that salivary cortisol determination can be used in squirrel monkeys (Saimiri sciureus) to monitor variations in HPA system activity induced by both housing and experimental conditions. Saliva cortisol assessment has several advantages over blood cortisol analysis such as stress-free frequent sampling, laboratory independence and lower costs. Therefore, this non-invasive measure can be the method of choice in primatological research projects and routine programmes related to the well-being of these laboratory animals. KeywordsGlucocorticoids; HPA system; primate; squirrel monkey; stress Physical challenges, or psychological perturbations in situations with low predictability, low controllability, and/or novelty, can activate the hypothalamo-pituitary-adrenocortical (HPAl system resulting in elevated serum levels of the adrenocortical steroid hormones, cortisol and corticosterone which are thought to be responsible for both adaptational and maladaptational processes to perturbing situations IMunck et al. 1984, de Kloet 1991, Weiner 1992. Cortisol, the predominant corticosteroid in primates, is mostly quantified in plasma or serum. Blood sampling, however, requires the stressfulness of capture and handling of animals together with the aversiveness of venipuncture. Although analysis of urinary cortisol would allow the researcher to avoid many of these problems, this approach is unsatisfactory when information of cortisol levels within short time intervals is