![]() ![]() Along a posterior-to-anterior axis, there is a continuous mapping of preferred frequencies from high to low (A1), followed by a reversed mapping of low back to high (R), followed by a third smaller mapping of high back to low (RT). The neurons of each field respond to tones over a limited frequency range and are spatially arranged according to preferred frequencies-tonotopy ( Brugge and Merzenich, 1973 Morel et al., 1993 Kaas and Hackett, 2000). In the monkey, the primary auditory cortex is subdivided into three fields, A1, R, and RT, which together correspond to the architectonic core and each have primary-like features, including direct thalamic input (ventral medial geniculate nucleus, Rauschecker et al., 1997). However, it is commonly assumed that PAC occupies only the first (more anterior) division of HG duplications ( Rademacher et al., 1993 Penhune et al., 1996). Duplications of HG, ranging from partial to complete, are common (estimated occurrence 41%, Rademacher et al., 1993), and architectonic evidence has not been clear about whether PAC occupies one or both divisions of duplicated Heschl's gyri. Complicating the matter, HG has high morphological variability across individuals and brain hemispheres. The transverse gyrus of Heschl (HG, approximately medial two-thirds) located bilaterally on the temporal plane is an important but rough marker for PAC, not indicating exact architectonic borders ( Rademacher et al., 2001). Today PAC is still not routinely identifiable in the living human brain. Over 100 years ago human primary auditory cortex (PAC, Brodmann's Area 41) was first identified based on its dense cellular structure (koniocortex) and myelination in postmortem tissue ( Campbell, 1905 Fleschig, 1908 Brodmann, 1909 von Economo and Horn, 1930). Tonotopic mappings were based on only 16 min of fMRI data acquisition, so these methods can be used as an initial mapping step in future experiments designed to probe the function of specific auditory fields. These findings significantly revise HG as a marker for human PAC and suggest that tonotopic maps may have shaped HG during human evolution. The anatomical–functional variants of PAC appear to be part of a continuum, rather than distinct subtypes. Specifically, the central union of the two primary maps (the hA1–R border) was consistently centered on the full Heschl's structure: on the gyral crown of single HGs and within the sulcal divide of duplicated HGs. PAC spanned both divisions of HG in cases of partial and complete duplications (11/20 hemispheres), not only the anterior division as commonly assumed. In 20/20 individual hemispheres, two primary mirror-symmetric tonotopic maps were clearly observed with gradients perpendicular to HG. The data reveals a clear anatomical–functional relationship that, for the first time, indicates the location of PAC across the range of common morphological variants of HG (single gyri, partial duplications, and complete duplications). We measured the two largest tonotopic subfields of PAC (hA1 and hR) using high-resolution functional MRI at 7 T relative to the underlying anatomy of Heschl's gyrus (HG) in 10 individual human subjects. For example, a neuron did not respond to any FM sound with a constant modulation magnitude, but showed a transient response whenever the magnitude of FM varied from a continuous pure tone to a certain range of FM.These complex units often showed a response with hysteresis.The primary auditory cortex (PAC) is central to human auditory abilities, yet its location in the brain remains unclear. Single unit responses of the primary auditory cortex of unanesthetized cats were studied using three standard sets of sound stimuli, i.e., clicks, white noise bursts, and pure tone bursts.The units studied were classified into eight categories according to whether they responded to all three, two, one or none of the standard sets of stimuli there were no units responding only to clicks.This suggests that excitation of some cortical neurons requires a special sound spectrum as well as temporal factors.Frequency-modulated (FM) sounds were usually more effective eliciting responses than the standard sets of stimuli.Responses to FM in bursts seemed to be classified into three groups, i.e., sharp monopeak post-stimulus time (PST) histograms, broad monopeak, and sharp multipeaks.The existence of the last group suggests a firm convergence from the lower auditory neurons.In some units, complex stimuli were more effective in evoking responses than simple FM sounds.These units responded only to sounds with complex temporal patterns.
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