Samson Annie-Hélène, Pollack Gerald S
Department of Biology, McGill University, Montreal, Quebec H3A 1B1, Canada.
J Neurophysiol. 2002 Nov;88(5):2322-8. doi: 10.1152/jn.00119.2002.
An important cue for sound localization is binaural comparison of stimulus intensity. Two features of neuronal responses, response strength, i.e., spike count and/or rate, and response latency, vary with stimulus intensity, and binaural comparison of either or both might underlie localization. Previous studies at the receptor-neuron level showed that these response features are affected by the stimulus temporal pattern. When sounds are repeated rapidly, as occurs in many natural sounds, response strength decreases and latency increases, resulting in altered coding of localization cues. In this study we analyze binaural cues for sound localization at the level of an identified pair of interneurons (the left and right AN2) in the cricket auditory system, with emphasis on the effects of stimulus temporal pattern on binaural response differences. AN2 spike count decreases with rapidly repeated stimulation and latency increases. Both effects depend on stimulus intensity. Because of the difference in intensity at the two ears, binaural differences in spike count and latency change as stimulation continues. The binaural difference in spike count decreases, whereas the difference in latency increases. The proportional changes in response strength and in latency are greater at the interneuron level than at the receptor level, suggesting that factors in addition to decrement of receptor responses are involved. Intracellular recordings reveal that a slowly building, long-lasting hyperpolarization is established in AN2. At the same time, the level of depolarization reached during the excitatory postsynaptic potential (EPSP) resulting from each sound stimulus decreases. Neither these effects on membrane potential nor the changes in spiking response are accounted for by contralateral inhibition. Based on comparison of our results with earlier behavioral experiments, it is unlikely that crickets use the binaural difference in latency of AN2 responses as the main cue for determining sound direction, leaving the difference in response strength, i.e., spike count and/or rate, as the most likely candidate.
声音定位的一个重要线索是刺激强度的双耳比较。神经元反应的两个特征，即反应强度（即放电计数和/或频率）和反应潜伏期，会随刺激强度而变化，对其中一个或两者进行双耳比较可能是定位的基础。之前在感受器 - 神经元水平的研究表明，这些反应特征会受到刺激时间模式的影响。当声音快速重复时，就像在许多自然声音中那样，反应强度会降低，潜伏期会增加，从而导致定位线索的编码改变。在本研究中，我们在蟋蟀听觉系统中一对已确定的中间神经元（左右AN2）水平上分析声音定位的双耳线索，重点关注刺激时间模式对双耳反应差异的影响。AN2的放电计数会随着快速重复刺激而减少，潜伏期会增加。这两种效应都取决于刺激强度。由于双耳的强度差异，随着刺激持续，放电计数和潜伏期的双耳差异会发生变化。放电计数的双耳差异减小，而潜伏期的差异增加。中间神经元水平上反应强度和潜伏期的比例变化比感受器水平上更大，这表明除了感受器反应的衰减之外，还有其他因素参与其中。细胞内记录显示，AN2中会建立起一个缓慢增强、持续时间长的超极化。同时，每次声音刺激引起的兴奋性突触后电位（EPSP）期间达到的去极化水平会降低。对膜电位的这些影响以及放电反应的变化都不能用对侧抑制来解释。基于将我们的结果与早期行为实验进行比较，蟋蟀不太可能将AN2反应潜伏期的双耳差异用作确定声音方向的主要线索，而反应强度的差异，即放电计数和/或频率，是最有可能的候选线索。
