Data from: Diffusion tensor imaging of dolphin brains reveals direct auditory pathway to temporal lobe

The brains of odontocetes (toothed whales) look grossly different from
their terrestrial relatives. Because of their adaptation to the aquatic
environment and their reliance on echolocation, the odontocetes'
auditory system is both unique and crucial to their survival. Yet, scant
data exist about the functional organization of the cetacean auditory
system. A predominant hypothesis is that the primary auditory cortex lies
in the suprasylvian gyrus along the vertex of the hemispheres, with this
position induced by expansion of ‘associative′ regions in lateral and
caudal directions. However, the precise location of the auditory cortex
and its connections are still unknown. Here, we used a novel diffusion
tensor imaging (DTI) sequence in archival post-mortem brains of a common
dolphin (Delphinus delphis) and a pantropical dolphin (Stenella attenuata)
to map their sensory and motor systems. Using thalamic parcellation based
on traditionally defined regions for the primary visual (V1) and auditory
cortex (A1), we found distinct regions of the thalamus connected to V1 and
A1. But in addition to suprasylvian-A1, we report here, for the first
time, the auditory cortex also exists in the temporal lobe, in a region
near cetacean-A2 and possibly analogous to the primary auditory cortex in
related terrestrial mammals (Artiodactyla). Using probabilistic tract
tracing, we found a direct pathway from the inferior colliculus to the
medial geniculate nucleus to the temporal lobe near the sylvian fissure.
Our results demonstrate the feasibility of post-mortem DTI in archival
specimens to answer basic questions in comparative neurobiology in a way
that has not previously been possible and shows a link between the
cetacean auditory system and those of terrestrial mammals. Given that
fresh cetacean specimens are relatively rare, the ability to measure
connectivity in archival specimens opens up a plethora of possibilities
for investigating neuroanatomy in cetaceans and other species.

齿鲸类（odontocetes）的大脑与其陆生近亲外观差异悬殊。因适应水生环境且依赖回声定位（echolocation），齿鲸的听觉系统兼具独特性与生存必要性。然而，关于鲸类（cetacean）听觉系统的功能组织结构，现有研究数据仍较为匮乏。主流假说认为，初级听觉皮层（primary auditory cortex）位于沿大脑半球顶点的外侧沟上回（suprasylvian gyrus），该位置由外侧与尾侧方向的联合皮层区（associative regions）扩张所形成。但截至目前，听觉皮层的确切位置及其连接通路仍未明确。本研究采用新型弥散张量成像（diffusion tensor imaging, DTI）序列，对普通海豚（Delphinus delphis）与热带斑海豚（Stenella attenuata）的存档死后脑组织进行成像，以绘制其感觉运动系统图谱。研究基于传统定义的初级视觉皮层（V1）与初级听觉皮层（A1）的分区开展丘脑分区（thalamic parcellation），发现丘脑（thalamus）存在分别与V1和A1相连的独立区域。除外侧沟上回A1外，本研究首次报道：听觉皮层同时存在于颞叶（temporal lobe）中靠近鲸类A2区的位置，该区域或与相关陆生哺乳动物（偶蹄目Artiodactyla）的初级听觉皮层具有同源性。通过概率性纤维束追踪（probabilistic tract tracing），我们发现了一条从下丘（inferior colliculus）经内侧膝状体核（medial geniculate nucleus）通往大脑外侧裂（sylvian fissure）附近颞叶的直接通路。本研究结果证实了利用存档标本开展死后DTI成像的可行性，以此前未有的方式解答比较神经生物学（comparative neurobiology）中的基础问题，并揭示了鲸类听觉系统与陆生哺乳动物听觉系统之间的关联。鉴于新鲜鲸类标本相对稀缺，对存档标本的连接通路进行测量的能力，为研究鲸类及其他物种的神经解剖学（neuroanatomy）开辟了大量新的研究契机。