David Symmes

Individuality and stability of isolation peeps in squirrel monkeys

Isolation peeps (IPs) made by captive squirrel monkeys (Saimiri sciureus) of both the Gothic and Roman type have been analysed by spectrographic and statistical methods designed to quantify the degree of individuality found in utterances from single individuals. Clustering analysis confirmed that in both groups of monkeys individually distinct combinations of acoustic features characterize the IP and that such unique combinations persist over periods of several years. Differences between the Gothic and Roman form of the IP were found in more aspects of acoustic structure than had previously been known. The evidence suggests that information about the identity of the vocalizer is contained in the the IP.

DEVELOPMENT OF THE ISOLATION PEEP IN LABORATORY-BRED SQUIRREL MONKEYS

The development of Isolation Peeps (IPs) was studied in seven laboratory-reared squirrel monkeys (Saimiri sciureus) over the age range of one day to two years. IPs were identified by specified criteria from the wide range of vocalizations recorded and analysed by descriptive and statistical methods. Despite the presence of substantial maturational changes in IPs, the data reveal individuality or vocal signature at all ages tested. Furthermore, the animals had developed by the end of the study unique and stable IPs comparable to those recorded previously from wild-caught adults.

Arousal effects on unit responsiveness to vocalizations in squirrel monkey auditory cortex

Abstract Single neurons in the auditory cortex of awake squirrel monkeys were tested with species-specific vocalizations with and without concurrent stimulation of the midbrain reticular formation (RF). Fourteen of 55 cells (25%) tested with RF stimulation alone gave time-locked responses to successive trains of RF stimulation. Fifteen of 39 neurons (36%) showed changes in response strength or pattern to vocalizations presented with concurrent RF stimulation. However, only 4 (10%) changed their selectivity to specific vocalizations. These included cells driven by trains of RF stimulation, and those which were not. Non-stationarity in spontaneous discharge rate occurred in many of the neurons studied, but was not correlated with changes in responsiveness to vocalizations. Analysis of responsiveness to vocalizations and spontaneous and RF-induced changes in cortical EEG activity likewise revealed no correlation. These results suggest that the response selectivity to specific vocalizations of auditory cortex neurons is relatively stable, and is not greatly influenced by RF-mediated changes in cortical excitability.

Neural processing of vocalizations and artificial stimulin the medial geniculate body of squirrel monkey

The responses of 256 single neurons in the medial geniculate body of awake squirrel monkeys have been examined with a variety of acoustic stimuli. Both traditional artificial sounds and tape-recorded tokens of species-specific vocalizations were employed, and one goal of the study was the comparison along a number of response parameters of these two classes. Unit responses to the first group (clicks, tone bursts and noise bursts) revealed a level of responsivity approaching 100% in the principal nucleus. Responses to tone and noise were sustained in about half the cells, and characteristic frequencies could be identified in 81%. Responses to vocal stimuli were also very prevalent, approaching 100% in this sample. A comparison of absolute thresholds, response vigor, rate-level functions, binaural interactions, and spectral composition of effective stimuli confirmed that MGB cells process vocal stimuli and artificial stimuli in similar ways. Selectivity of MGB cells to vocal stimuli was quite low, suggesting that processing of communication sounds in MGB is relatively simple and preliminary when compared to auditory cortex.

EEG, ECG, and acid-base observations during acute fetal hypoxia

Abstract The effects of hypoxia on the EEG, heart rate, and acid-base status of fetal sheep were studied in 28 experiments which permitted simultaneous blood determinations on both sides of an intact placenta. Alterations in heart rate and rhythm preceded significant elevations of lactate concentrations and occurred prior to unambiguous changes in the EEG. A bradycardia preceded the onset of the isoelectric stage of the fetal EEG. The consistency of this finding suggested an etiologic relation. The time to recovery of normal EEG rhythms was a function of the length of the isoelectric stage but not of the total period of hypoxia or time to the onset of the isoelectric stage.

Effect of eye blink on the visual evoked response in children

Abstract The frequency of reflex blinking to flash and its effect on the visual evoked response (VER) were studied in thirty-one normal children, ages 5–11 years. Recordings were made from electrodes located at the orbit, the mastoid and at Fz, Cz and Oz. The scoring of reflex blink frequency, the averaging of evoked responses and the development of triggers for averaging involuntary blinks (not flash-associated) were done off-line. The average probability of reflex blinking to moderately intense flash under alert, eyes open conditions was 0.62, with a significant trend to increased blinking during continued testing. In eight subjects who blinked to approximately half the flashes, segregation and separate averaging of blink and blink-free responses demonstrated a substantial difference in the potentials recorded at all locations. Specifically, a positive component at the occiput (scalp to mastoid) occurring approximately 200 msec after flash was enhanced in trials that included reflex blinks. Averages of EEG time-locked to the peak of involuntary blink potentials (no flash) showed penetration of artifact into posterior scalp as well. These findings suggest that many children in this age group exhibit persistent reflex blinking, and that there is a high probability of artifact in the VER therefrom.

Spectral analysis of fetal sheep EEG during hypoxia

Abstract Power spectra and coherence functions were obtained on samples of EEG recorded from fetal sheep in utero . Hypoxia was produced by respiring the ewe on gas mixtures containing reduced oxygen pressure. With both halothane and local anesthesia, the EEG changes associated with hypoxia consisted of an early power reduction in higher frequencies followed by a reduction of over-all amplitude leading to the isoelectric stage. The times at which statistically significant changes occurred were not earlier during the progressive fall of fetal arterial pO 2 than visual estiamtes of abnormality made on line from the paper record and were in all cases later than significant cardiovascular changes.

The effect of glucose loading on the fetal response to hypoxia

A glucose load was administered to the fetus in utero by maternal infusion prior to hypoxia in 12 sheep experiments. The placental transport mechanism for glucose appeared saturated at high maternal concentrations of glucose and dependent on oxygen as a source of energy. The changes observed in the fetal EEG, cardiovascular, acid-base, and metabolic parameters during and in recovery from hypoxia were similar to a group of 17 experiments in which glucose had not been given prior to hypoxia.

Two‐tone interactions in the auditory cortex of the squirrel monkey

Two‐tone interactions are recorded in the responses of single units in the superior temporal gyrus to contralateral acoustic stimulation of the awake squirrel monkey. Four response types are distinguished based primarily on the nature of the two‐tone response, and secondarily on criteria such as the patterns of response to single tones and noise stimuli, thresholds, and spontaneous activity levels. Type A units (24/80) display strong lateral inhibitory influences which may extend up to two octaves on either side, or both sides, of the BF. They are sharply tuned at all intensities, and generally exhibit sustained response to single‐tone stimuli at the BF. The units have nonmonotonic rate‐level functions, and where tested, show little or no response to broadband noise. Most type A units have low spontaneous rates ( 30 dB SPL). Type B units (22/80) are characterized by relatively high spontaneous rates of activity (>20 spikes/s) and inhibitory responses to singl...