Brain-stem auditory evoked potentials in the alligator. Effects of temperature and hypoxia

Neural Maps of Interaural Time Difference in the American Alligator: A Stable Feature in Modern Archosaurs

Detection of interaural time differences (ITDs) is crucial for sound localization in most vertebrates. The current view is that optimal computational strategies of ITD detection depend mainly on head size and available frequencies, although evolutionary history should also be taken into consideration. In archosaurs, which include birds and crocodiles, the brainstem nucleus laminaris (NL) developed into the critical structure for ITD detection. In birds, ITDs are mapped in an orderly array or place code, whereas in the mammalian medial superior olive, the analog of NL, maps are not found. As yet, in crocodilians, topographical representations have not been identified. However, nontopographic representations of ITD cannot be excluded due to different anatomical and ethological features of birds and crocodiles. Therefore, we measured ITD-dependent responses in the NL of anesthetized American alligators of either sex and identified the location of the recording sites by lesions made after recording. The measured extracellular field potentials, or neurophonics, were strongly ITD tuned, and their preferred ITDs correlated with the position in NL. As in birds, delay lines, which compensate for external time differences, formed maps of ITD. The broad distributions of best ITDs within narrow frequency bands were not consistent with an optimal coding model. We conclude that the available acoustic cues and the architecture of the acoustic system in early archosaurs led to a stable and similar organization in today's birds and crocodiles, although physical features, such as internally coupled ears, head size, or shape, and audible frequency range, vary among the two groups.SIGNIFICANCE STATEMENT Interaural time difference (ITD) is an important cue for sound localization, and the optimal strategies for encoding ITD in neuronal populations are the subject of ongoing debate. We show that alligators form maps of ITD very similar to birds, suggesting that their common archosaur ancestor reached a stable coding solution different from mammals. Mammals and diapsids evolved tympanic hearing independently, and local optima can be reached in evolution that are not considered by global optimal coding models. Thus, the presence of ITD maps in the brainstem may reflect a local optimum in evolutionary development. Our results underline the importance of comparative animal studies and show that optimal models must be viewed in the light of evolutionary processes.

Coupled ears in lizards and crocodilians

Lizard ears are coupled across the pharynx, and are very directional. In consequence all auditory responses should be directional, without a requirement for computation of sound source location. Crocodilian ears are connected through sinuses, and thus less tightly coupled. Coupling may improve the processing of low-frequency directional signals, while higher frequency signals appear to be progressively uncoupled. In both lizards and crocodilians, the increased directionality of the coupled ears leads to an effectively larger head and larger physiological range of ITDs. This increased physiological range is reviewed in the light of current theories of sound localization.

Evaluation of four methods for inducing death during slaughter of American alligators (Alligator mississippiensis)

OBJECTIVE

To evaluate physical methods for inducing death during the slaughter of American alligators (Alligator mississippiensis).

ANIMALS

24 captive hatched-and-reared American alligators.

PROCEDURES

Baseline electroencephalograms (EEGs) were obtained for awake and anesthetized alligators. Corneal reflex, spontaneous blinking, and EEGs were evaluated after severance of the spinal cord, severance of the spinal cord followed by pithing of the brain, application of a penetrating captive bolt, or application of a nonpenetrating captive bolt (6 alligators/group).

RESULTS

Overall, alligators subjected to spinal cord severance alone differed from those subjected to the other techniques. Spinal cord severance alone resulted in postprocedure EEG power values greater than those in anesthetized alligators, whereas the postprocedure EEG power values were isoelectric for the other 3 techniques. Corneal reflex and spontaneous blinking were absent in all alligators immediately after application of a penetrating or nonpenetrating captive bolt. One of 6 alligators had a corneal reflex up to 1 minute after pithing, but all others within that group had immediate cessation of reflexes after pithing. Mean time to loss of spontaneous blinking and corneal reflex for alligators subjected to spinal cord severance alone was 18 minutes (range, 2 to 37 minutes) and 54 minutes (range, 34 to 99 minutes), respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Spinal cord severance followed by pithing of the brain and application of a penetrating or nonpenetrating captive bolt appeared to be humane and effective techniques for inducing death in American alligators, whereas spinal cord severance alone was not found to be an appropriate method.

Brainstem auditory evoked potentials (BAEPs) in the cynomolgus macaque monkey. Equivalence with human BAEPs and proposal of a new nomenclature

Several groups have studied brainstem auditory evoked potentials (BAEPs) in non-human primates. However, the nomenclature of the waves elicited and their correspondence with human waves I-V differ among authors. BAEPs were recorded from six anaesthetised young cynomolgus macaques (Macaca fascicularis), using different sound stimuli parameters. A constant pattern of four main waveforms was present in all the animals with stimulus intensities over 60 dB SPL, although up to four smaller waveforms were observed in some of the individuals. Latency values increased with decreasing stimulus intensities and with increasing repetition rates. These results were similar to the BAEPs observed in other species of macaques. Although an approximate equivalence between human and monkey BAEPs is possible, some discrepancies suggest that there may be generators which contribute to different waves in both species. This is the reason for our proposal of a new nomenclature for BAEP waveforms in monkeys, following a descriptive order with Arabic numerals preceded by the letter M.

The effect of sound level, temperature and dehydration on the brainstem auditory evoked potential in anuran amphibians

Brainstem auditory evoked potentials (BAEPs) were used to examine the effects of sound level, temperature, and dehydration on the auditory pathway of three species of anuran amphibians: Rana pipiens, Bufo americanus and B. terrestris. BAEP latency, amplitude and a measure of threshold were determined for all stimulus and test conditions. Threshold values obtained with this technique were similar to other neural measures of threshold in anurans, and were stable for repeated measures within 12 h and over three days. Transient changes in temperature caused non-linear changes in BAEP threshold and latency. Above 20 degrees C small threshold shifts were elicited, while below 20 degrees C we observed rapid deterioration of threshold. Animals acclimated to a cold temperature (14 degrees C) were acoustically less sensitive than warm (21 degrees C) animals, even when both groups were tested at colder temperatures. Because peripheral components of the BAEP were most affected by both transient and acclimation (longer term) cooling and warming, the sensory epithelium appears to be the most temperature-sensitive component of the auditory pathway. Dehydrated frogs showed no auditory dysfunction until a critical level of dehydration was reached. More dehydration-resistant species (B. terrestris and B. americanus) were less susceptible to BAEP degradation near their critical dehydration level.

Thermal influences on nervous system function

The various effects of temperature change are only partially predictable. Temporal measures relevant to membrane activity, action potentials, synaptic transmission, and evoked potentials are all consistently increased with cooling and decreased by warming. However, the various measures of amplitude at different levels, and even within similar preparations, are contradictory: Some laboratories report increased amplitudes with cooling and others report decreased amplitudes under similar conditions. Emphasis is given to identifying factors that may resolve the differences. These include: (a) the rate of temperature change, (b) sites of cooling, stimulation and recording, (c) stimulus characteristics, and (d) fundamental differences in temperature sensitivities of different neural tissue. Other factors that may affect the ability to predict thermal influences on neural function from existing formulations are: relative ion permeabilities, metabolic ion pumps, the resting potential at the onset of cooling, and an animal's acclimated temperature at sacrifice.

Differential impact of hypothermia and pentobarbital on brain-stem auditory evoked responses

Two experiments were conducted to determine the effects of hypothermia and pentobarbital anesthesia, alone and in combination, on the brain-stem auditory evoked responses (BAERs) of rats. In experiment I, unanesthetized rats were cooled to colonic temperatures 0.5 and 1.0 degrees C below normal. In experiment II, 2 groups of rats were cooled and tested at 37.5, 36.0, 34.5 and 31.5 degrees C. One group was anesthetized during testing and the other group was awake. The rat BAER was sensitive to cooling of 1 degree C or less. Peak latencies were prolonged and peak-to-peak amplitudes were increased by hypothermia alone. The effect on amplitude may be related to the time course of temperature change or to stimulus level. Pentobarbital significantly affected both latencies and amplitudes over and above the effects of cooling. The specific effects of pentobarbital differed by BAER peak and by temperature. The findings point up the importance of the potential confound of anesthetic drugs in most of the evoked potential literature on hypothermia and, for the first time, quantify the complex interactions between pentobarbital and temperature which affect the BAER wave form.

Brain-stem auditory evoked potentials in squirrel monkey (Saimiri sciureus)

To more fully characterize brain-stem auditory evoked potentials (BAEPs) in non-human primates, BAEPs were recorded from chronically implanted epidural electrodes in 10 squirrel monkeys (Saimiri sciureus). The effects of stimulus intensity, repetition rate, and anesthesia (ketamine 20 mg/kg i.m.) on peak latencies and inter-peak intervals were evaluated. Monkey wave forms consisted of approximately 7 peaks (I-VII), each exhibiting similar latencies across sessions, with later peaks exhibiting greater variability. In some subjects, additional peaks (IIa, IIIa) and slow potentials were recorded. The slow potentials provided a substratum for peaks IV through VII. As with human, monkey peaks exhibited systematic changes in latency with changes in stimulus intensity or repetition rate. These shifts included significant decreases in latency with increasing intensity for peaks I-IV and increases in latency with increases in repetition rate for peaks III, V, and VI. Inter-peak intervals were similar to those observed in human. Furthermore, ketamine anesthesia significantly delayed the latencies of most peaks (except I, V, and VII). Some differences between monkey and human BAEPs were evident in the relative amplitude of specific peaks. For example, peak V is typically most prominent in human, while this was true for peak III in monkey. The similarities between unanesthetized monkey and human inter-peak intervals suggest that the times required for impulses to reach particular brain-stem areas are conserved across primate species that vary in brain size. This supports the hypothesis that comparably numbered BAEP peaks in monkey and human index homologous processes. The data also suggest that the differences between animal and human BAEPs commonly reported may result from the use of anesthetics. In summary, unanesthetized monkey BAEPs resemble human BAEPs in morphology, number of peaks, polarity, latency variability, inter-peak intervals, slow potentials superimposed on the high-frequency peaks, and variations in morphology, amplitude, and resolution of peaks as a function of recording site. Thus, unanesthetized monkey BAEPs may be an excellent model for investigating the neural substrates of human BAEP or for determining species differences in acoustic processing among primates.

Postnatal development of brainstem auditory-evoked potentials, electroretinograms, and visual-evoked potentials in the calf

Brainstem auditory-evoked potentials (BAEP), electroretinograms (ERG), and visual-evoked potentials (VEP) were recorded for eight calves from birth to 56 days and the values compared with previously determined adult responses. The BAEPs, ERGs, and VEPs recorded within the first 24 hours after birth contained all of the peaks seen in adult recordings. Varying degrees of maturation of the responses were documented as changes in latency and amplitude with age. The BAEPs were adult-like at birth, with latencies falling within the mean, plus or minus one standard deviation, for adult cows. A small but significant decrease in latency with age was seen for the first, second, and fourth peaks of the response. The ERG amplitudes were also within the adult range for the entire period of the study. Latencies to the a- and b-waves declined during the first 14 days and then stabilized at adult values. The VEP latencies decreased with age, with late peaks changing more than early peaks. Latencies of all but the first peak decreased to values less than the adult range. Two VEP amplitudes increased significantly with age. Developmental appears in the calf and other precocious species are compared to those in altricious (nonprecocious) species.

Brainstem auditory-evoked potentials in Holstein cows

Brainstem auditory-evoked potentials (BAEP) were recorded from 29 Holstein cows in a typical clinical setting. The latencies of five positive peaks in the BAEP were measured, and latency-intensity functions were determined. The BAEP was similar to that reported in humans, dogs, horses, and other species. The responses were reproducible for each cow, with low variability between cows. Four peaks (I, II, III, V) were present in all recordings, and a fifth (IV) was present infrequently. All peak latencies decreased as click-stimulus intensity increased. The threshold for detection of the BAEP was higher than expected for the cow compared with the horse.

Comparison of ongoing compound field potentials in the brains of invertebrates and vertebrates

(1) Ongoing compound field potential fluctuations of higher brain centers (the micro-EEG of some authors) are considered as a biological phenomenon, a sign of the activity in the organized assemblage of cells. Such activity has been compared in several taxa with quite different brain structure to look for possible evolution in the form of the field potentials and for possible explanations of differences and similarities. (2) Recordings were made with semimicroelectrodes in the neuropile of the cerebral ganglion of the mollusc, Aplysia, with comparative observations on Helix, and the arthropods Limulus, Melanoplus, and Cambarus, and in or on the cerebral cortex and optic tectum of rays, cats and rabbits, with comparative observations on sharks, bony fish, turtles and geckos in unstimulated resting or generalized arousal states. Manipulations of state did not alter the main findings. (3) Power spectra in the cerebral ganglia of various higher invertebrates are similar; activity is fast and spikey (with the exception of Octopus). Integrated energy above 50 Hz exceeds that from 2-50 Hz and falls slowly with frequency; in Aplysia the power spectrum falls less than 10 dB between 10 and 300 Hz. In vertebrates from fish to mammals activity is similar in being mainly slow (less than 40 Hz); it commonly falls greater than 20 dB between 10 and 50 Hz. (4) Amplitude is low in invertebrates and lower vertebrates. RMS voltage in Aplysia (3-300 Hz, reference electrode remote) is typically less than 10 microV; in the ray optic tectum less than 25 microV (2-50 Hz); in the dorsal cortex of the gecko less than 30 microV, in the cat cortex greater than 85 microV. In the vertebrates amplitude does not change greatly with small shifts in electrode position, as it does in invertebrates. (5) Coherence decline with distance, measured tangentially at different electrode separations in the millimeter range, is used as an estimator of synchrony. Averaged coherence between loci 1 mm apart is negligible in Aplysia in any band from 3 to 100 Hz; in the ray tectum it is low, 0.25-0.5 between 3 and 16 Hz. In the turtle dorsal pallium it is higher, at 2 mm, 0.6-0.75 in this band. In the rabbit cortex coherence is even higher, typically greater than 0.7 at 1 mm, and greater than 0.3 at 4 mm in this band. (6) Band-pass filtered electrograms, ca. one octave wide, in all species show constant waxing and waning in each band; amplitude is not maintained even for a second. (ABSTRACT TRUNCATED AT 400 WORDS)