Latency of activation of auditory cortex

["David L. Woods" <dlwoods@xxxxxxxxxxx>]

Your 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@ucdavis.edu
Publications: http://marva4.ebire.org/hcnlab

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