Mapping an intrinsic MR property of gray matter in auditory cortex of living humans: a possible marker for primary cortex and hemispheric differences.

Recently, magnetic resonance properties of cerebral gray matter have been spatially mapped--in vivo--over the cortical surface. In one of the first neuroscientific applications of this approach, this study explores what can be learned about auditory cortex in living humans by mapping longitudinal relaxation rate (R1), a property related to myelin content. Gray matter R1 (and thickness) showed repeatable trends, including the following: (1) Regions of high R1 were always found overlapping posteromedial Heschl's gyrus. They also sometimes occurred in planum temporale and never in other parts of the superior temporal lobe. We hypothesize that the high R1 overlapping Heschl's gyrus (which likely indicates dense gray matter myelination) reflects auditory koniocortex (i.e., primary cortex), a heavily myelinated area that shows comparable overlap with the gyrus. High R1 overlapping Heschl's gyrus was identified in every instance suggesting that R1 may ultimately provide a marker for koniocortex in individuals. Such a marker would be significant for auditory neuroimaging, which has no standard means (anatomic or physiologic) for localizing cortical areas in individual subjects. (2) Inter-hemispheric comparisons revealed greater R1 on the left on Heschl's gyrus, planum temporale, superior temporal gyrus and superior temporal sulcus. This asymmetry suggests greater gray matter myelination in left auditory cortex, which may be a substrate for the left hemisphere's specialized processing of speech, language, and rapid acoustic changes. These results indicate that in vivo R1 mapping can provide new insights into the structure of human cortical gray matter and its relation to function.

Effects of sound level on fMRI activation in human brainstem, thalamic and cortical centers.

The dependence of fMRI activation on sound level was examined throughout the auditory pathway of normal human listeners using continuous broadband noise, a stimulus widely used in neuroscientific investigations of auditory processing, but largely neglected in neuro-imaging. Several specialized techniques were combined here for the first time to enhance detection of brainstem activation, mitigate scanner noise, and recover temporal resolution lost by the mitigation technique. The main finding was increased activation with increasing level in cochlear nucleus, superior olive, inferior colliculus, medial geniculate body and auditory cortical areas. We suggest that these increases reflect monotonically increasing activity in a preponderance of individual auditory neurons responsive to broadband noise. While the time-course of activation changed with level, the change was subtle and only significant in a part of the cortex. To our knowledge, these are the first fMRI data showing the effects of sound level in subcortical centers or for a non-tonal, non-speech stimulus at any stage of the pathway. The present results add to the body of parametric data in normal human listeners and are fundamental to the design of any fMRI experiment employing continuous noise.

Short-term sound temporal envelope characteristics determine multisecond time patterns of activity in human auditory cortex as shown by fMRI.

Functional magnetic resonance imaging (fMRI) of human auditory cortex has demonstrated a striking range of temporal waveshapes in responses to sound. Prolonged (30 s) low-rate (2/s) noise burst trains elicit "sustained" responses, whereas high-rate (35/s) trains elicit "phasic" responses with peaks just after train onset and offset. As a step toward understanding the significance of these responses for auditory processing, the present fMRI study sought to resolve exactly which features of sound determine cortical response waveshape. The results indicate that sound temporal envelope characteristics, but not sound level or bandwidth, strongly influence response waveshapes, and thus the underlying time patterns of neural activity. The results show that sensitivity to sound temporal envelope holds in both primary and nonprimary cortical areas, but nonprimary areas show more pronounced phasic responses for some types of stimuli (higher-rate trains, continuous noise), indicating more prominent neural activity at sound onset and offset. It has been hypothesized that the neural activity underlying the onset and offset peaks reflects the beginning and end of auditory perceptual events. The present data support this idea because sound temporal envelope, the sound characteristic that most strongly influences whether fMRI responses are phasic, also strongly influences whether successive stimuli (e.g., the bursts of a train) are perceptually grouped into a single auditory event. Thus fMRI waveshape may provide a window onto neural activity patterns that reflect the segmentation of our auditory environment into distinct, meaningful events.