Subject: Latency of activation of auditory cortex From: "David L. Woods" <dlwoods(at)UCDAVIS.EDU> Date: Wed, 20 Jan 1999 06:33:47 -0800Your question on the latency of initial excitation of auditory is an interesting one from several perspectives. Direct recordings from human auditory cortex are probably the gold standard. Celesia was first to obtain these, and, if I recall correctly, found initial excitation in the 12-15 ms range (1). In later studies(2) , longer latency responses (similar to the middle latency AEPs recorded from the scalp) were obtained from lateral superior temporal plane locations (distant from primary auditory cortex). More recent intracranial studies have been performed by Liegeois-Chauvel in France. She found the initial volley arrived at about 13 ms when recordings were made in the medial portions of Heschls gyrus(3). Longer latency components with higher amplitudes are recorded also recorded at medial Heschls gyrus, and larger longer-latency AEPs can also be recorded at more lateral locations within the superior temporal plane(4). In macaques, Mitch Steinschneider at Albert Einstein has obtained similar results -- the time of arrival of the initial thalamo-cortical volley at layer 4 of auditory cortex is estimated from intracranial AEPs, current source-density analysis, and multiple unit recording is about 8 ms in the monkey(5). Larger amplitude AEPs, with longer latencies, are seen at more superficial laminae. These times of arrival are reasonably consistent with the estimated speed of conduction in auditory fibers of the brainstem estimated from brainstem auditory evoked potentials (BAEPs). Wave V of the BAEP originates in the vicinity of the inferior colliculus, while waves VI and VII (latencies about 8 and 10 ms), are often suggested to arise in the medial geniculate body and auditory cortex, respectively(6). This is consistent with estimates of the conduction velocity in the lateral lemniscus using peak latency measures of BAEP components and measures of distances between hypothetical generator regions. Incidentally, conduction velocity in humans appears to be considerably slower than in sonar-dependent dolphins, where huge auditory fibers appear specialized for particularly rapid conduction(7). With respect to intensity effects, these are already seen at wave V of the BAEP. It shortens by about 20-40 us/dB(8). Some of this shortening is due to cochlear displacement -- i.e., in the absence of masking, louder sounds excite more basilar portions of the cochlea. Wave V latencies also change, as expected, with changes in stimulus frequency (or high frequency hearing loss). This reflects in part the time for traveling waves to excite hair cells on the basilar membrane. However, there also appear to be alterations in the speed of conduction of human auditory fibers, such that a subset of high frequency fibers (possibly analogous to magnocellular projections in vision) conduct more rapidly -- as reflected in shortened interpeak latencies of AEP components(9). These effects (several ms at wave V) increase until at N1 latencies (~110 ms), 4.0 kHz tones generate N1s which are 15-20 ms shorter in latency than those elicited by loudness-matched 250 Hz tones. Finally, AEPs suggest that the bulk of auditory processing in conscious subjects occurs well after the initial cortical volley. Indeed, the initial volley is difficult to detect: its putative reflection on the scalp, wave VIII of the BAEP, is invisible in most subjects. Middle latency (10-70 ms) AEPs, while detectable, have generally small amplitudes (0.5-2.0 uV), whereas the N1 shows 4-10 uV amplitudes, and is relatively even more prominent with magnetic recording (N1m). This component occurs at about 10x the latency of initial cortical excitation, and has a complex set of subcomponents, with tonotopic and non-tonotopic generators(10). __________________________________________ 1. Celesia, G. G., Broughton, R. J., Rasmussen, T., and Branch, C. Auditory evoked responses from the exposed human cortex. Electroencephalography & Clinical Neurophysiology, 1968, 24: 458-466. 2. Celesia, G., and Puletti, F. Auditory input to the human cortex during states of drowsiness and surgical anesthesia. Electroencephalography and Clinical Neurophysiology, 1971, 31: 603-609. 3. Liegeois-Chauvel, C., Musolino, A., and Chauvel, P. Localization of the primary auditory area in man. Brain, 1991, 114: 139-51. 4. Liegeois-Chauvel, C., Musolino, A., Badier, J. M., Marquis, P., and Chauvel, P. Evoked potentials recorded from the auditory cortex in man: evaluation and topography of the middle latency components. Electroencephalography and Clinical Neurophysiology, 1994, 92: 204-14. 5. Steinschneider, M., Tenke, C. E., Schroeder, C. E., Javitt, D. C., and Vaughan, H. G. Cellular generators of the cortical auditory evoked potential initial component. Electroencephalography and Clinical Neuro- physiology, 1992, 84: 196-200. 6. Markand, O. N. Brainstem auditory evoked potentials. Journal Of Clinical Neurophysiology, 1994, 11: 319-342. 7. Ridgway, S. H., Bullock, T. H., Carder, D. A., Seeley, R. L., and Galam- bos, R. Auditory brainstem response in dolphins. Proceedings of the National Academy of Sciences, 1981, 78: 1943-47. 8. Starr, A. Auditory pathway origins of scalp-derived auditory brainstem responses. In G. Morocutti and P. A. Rizzo (Eds.), Evoked Potentials: Neurophysiological and Clinical Aspects. Amsterdam: Elsevier, 1985 : 19. 33-156. Woods, D. L., Alain, C., Covarrubias, D., and Zaidel, O. Frequency- related differences in the speed of human auditory processing. Hearing Research, 1993, 66: 46-52. 10. Woods, D. L. The component structure of the N1 wave of the human auditory evoked potential. Electroencephalography and Clinical Neurophysiology Supplement. Perspectives on Event-Related Potential Research, 1994, 44: 102-109. David L. Woods, Professor of Neurology, Dept. of Neurology,UC Davis, Neurology Service (127), NCSC, 150 Muir Rd., Martinez, CA 94553 Tel (925) 372-2571, Fax (925) 229-2315 Email:dlwoods(at)ucdavis.edu Publications: http://marva4.ebire.org/hcnlab Email to AUDITORY should now be sent to AUDITORY(at)lists.mcgill.ca LISTSERV commands should be sent to listserv(at)lists.mcgill.ca Information is available on the WEB at http://www.mcgill.ca/cc/listserv