LATERAL ASYMMETRY IN SHAPE OF HESCHL’S GYRUS C.M. Leonard, S.D. Towler, S. Welcome, L. Halderman, R. Otto, C. Chiarello

Department of Neuroscience, McKnight Brain Institute, University of Florida, Gainesville,Department of Psychology, University of California, Riverside & MRI Diagnostic Imaging Center, Riverside CA

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Method

ExamplesBackground

Left hemisphere: Twelve examples of Heschl’s Gyri) that were classified as “heart.” Best examples are inside red ellipses. Key features are the pinched neck joining the gyrus to the superior temporal plane and the indentation in the crown. Hearts are found more frequently in the left hemisphere. The two sides of the heart rarely become two separate HG.

200 college students were recruited for a study comparing asymmetries in linguistic processing and brain structure. They received volumetric (MPRage) brain scans in a 1.5 T GE scanner in Riverside CA. The images were sent to UF and processed with FSL software. The surface area and volume of Heschl’s gyrus were measured on consecutive 1 mm thick sagittal images between 40 and 44 mm (normalized for brain width) from the midline by raters blind to hemisphere and subject id. Each image was categorized as either round, multiple (mult), heart or snoopy (see examples below) and then a summary categorization was mad. Chi square analyses tested if the ratings differed by hemisphere, sex, consistency of hand preference or reading skill.

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References

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• This research was supported by NIDCD grant no R01 006957.

Right hemisphere: Twelve examples of Heschl’s Gyri that were classified as “heart.” The examples were randomly selected. Squares designate the best examples. Note that most right hemisphere HG that were classified as hearts lack the pronounced indentation and narrowed neck seen in the left hemisphere. Anterior is to the right.

Left hemisphere: Twelve examples of Heschl’s Gyri that were classified as “snoopy.” Key feature is the elongated Heschl’s sulcus. Snoopies are found more frequently in the left hemisphere. They are a transient feature. The examples chosen were randomly selected. The best examples are encircled.

Right hemisphere: Twelve examples of Heschl’s Gyri that were classified as “snoopy.” The examples chosen were randomly selected. Squares designate the best examples. Note that few right hemisphere snoopies have the pronounced elevation or asymmetry that characterizes left hemisphere examples.

There is considerable evidence that the left and right auditory systems are specialized for the detection of different auditory cues. Such specialization has a long evolutionary history as rodents also show evidence of lateralized strategies for detecting specific auditory cues (Fitch et al. 1993; Wetzel et al. 2008). In humans the left hemisphere is specialized for temporal processing while the right hemisphere favors pitch or spectral processing. (Liegeois-Chauvel et al. 1999; Robin et al. 1990) More recently it has been found that in the right but not the left planum temporale, activation to different pitches is proportional to the distance between them (Hyde et al. 2008) while activation to rapid formant transitions (Belin et al. 1998; Liegeois-Chauvel et al. 1999) and auditory streaming (Deike et al. 2004) is higher in left auditory cortex.

There is a very reliable difference in the size of Heschl’s gyrus (HG) in the two hemispheres (Leonard et al. 1998; Penhune et al. 1996) and the size of the left Heschl’s gyrus predicts the ease with which English speakers can learn to use the pitch patterns of a foreign language for word identification (Wong et al. 2008). There is growing evidence for localized specializations in HG. Primary auditory cortex is restricted to the posterior medial portion (Liegeois-Chauvel et al. 1991), a correlate of pitch salience has been found in a small region of nonprimary auditory cortex on the anterolateral margin of HG (Penagos et al. 2004) and an MR correlate of koniocortex (primary cortex) is more extensive in the left HG than the right, suggesting greater myelination (Sigalovsky et al. 2006).

Here we report evidence for two anatomical specializations which are more frequent in HG in the left hemisphere. In each case a large Heschl’s gyrus is deformed slightly, either by an indentation in the superior surface (“heart”) or an elongated Heschl sulcus that causes an asymmetrical stretching in the posterior direction. We have named this second specialization “snoopy” because of its’ distinct resemblance to Schultz’ famous beagle.

Results and Discussion

A search of the literature was unsuccessful in locating earlier discussions of these asymmetries. In our previous studies we failed to distinguish between hearts and other forms of multiple HG that are more frequent on the right. These asymmetries do not appear to be limited to this particular sample as they have been found in three other samples (Leonard unpublished data).

Given the speculation that gyral morphology reflects early developmental gradients and axonal trajectories it is tempting to speculate that some functional asymmetries are associated with these morphological specializations. This idea could be tested with individualized fMRI investigations.

Graph at left shows a striking hemispheric asymmetry in Heschl’s gyrus morphology that is independent of medial –lateral position between 40 and 44 mm lateral to the midline (more laterally, HG loses definition and merges with the superior temporal gyrus). On the right, round and multiple are most frequent, while on the left, hearts and snoopies are most frequent. Chi square analysis demonstrated significant asymmetry at all lateral positions, p < .0001. There were no significant associations with sex, hand preference, reading skill or visual field asymmetry.

Heart Snoopy Round Multiple

Left Right

The examples chosen were randomly selected. In these images the cortical ribbon (gray matter) is white and white matter is black. The brightness of each pixel represents the probability that the voxel contains gray matter (partial volume estimate (PVE) image). Anterior is to the left.