Gardner is a left-handed adult. it is most likely that he has a thicker-than-normal:

The corpus callosum of the left handed people tends to be thicker as compared to those of the right handed people. The corpus callosum is the band of the nerves connecting the left and right hemispheres of the brain. The communication between the two hemispheres of the brain is aided by this structure. Since, the gardner is left-handed, he will have a thicker-than-normal corpus callosum.

Hence, the answer is 'corpus callosum'.