Auditory brain stem response testing in anesthetized horses.

This study investigates how the auditory brain stem responses of horses change under different levels of sound intensity when the horses are under general anesthesia. The researchers found that the latency, or delay before response, decreases as the stimuli intensity increases. However, when the sound pressure level fell below a certain threshold, the responses were nearly unidentifiable.

Research Methodology

• The researchers conducted the auditory brain stem response testing on 10 healthy horses, all of which were under general anesthesia.
• Insert earphones were used to deliver the auditory stimuli. The procedure involved clicks of alternating polarity delivered at a rate of 25 clicks per second.
• The recorded waveforms typically comprised of five peaks. The latencies for these waves (labelled as I through V) were measured in milliseconds across all intensities.
• The responses were then analyzed between 87-dB sound pressure level (SPL) and 136-dB SPL. A least-squares regression line was determined for each horse based on these data.

Significant Findings

• The latencies of all the waves decreased as the stimulus intensity increased. This suggests that a more intense stimulus results in a quicker response time in the auditory brain stem of the horses.
• Significantly different slopes were observed for waves I through IV. The slopes were less than zero, indicating a decrease in latency with an increase in sound pressure. However, this negative slope was not seen in wave V, indicating a different response pattern compared to the other waves.
• Average latencies for each wave at 87-dB SPL for waves I through V were found to be 1.73, 2.6, 3.82, 4.80, and 5.71 ms, respectively. These decreased to 1.36, 2.2, 3.06, 3.92, and 4.71 ms, respectively at 136-dB SPL.
• The decrease in latency per decibel ranged from 0.13 to 0.004 ms/dB, indicating a varied rate of response across different wave types.
• When peak values fell below 87-dB SPL, the response waves were essentially unrecognizable, suggesting a threshold level below which the auditory brain stem response becomes ineffective.

Conclusion and Implications

• This research sheds light on the auditory response characteristics of horses, particularly the relationship between the intensity of auditory stimuli and the subsequent latency in brain stem response.
• The findings suggest that more intense stimuli result in quicker responses, to a certain level of sound pressure. Below this level, responses become almost undetectable.
• Such insights can potentially inform veterinary practice, by providing a better understanding of auditory processing in horses. This could be particularly useful when dealing with auditory impairments or understanding anesthetic effects on the auditory system in horses.