The ganglion cells of the retina project in an orderly fashion to the lateral geniculate nucleus (LGN) of the thalamus and from there to the primary visual cortex (V1); adjacent spots on the retina are represented by adjacent neurons in the lateral geniculate nucleus and the primary visual cortex.
The stria of Gennari – a set of heavily myelinated, horizontally projecting axons within the termination zone of lateral geniculate nucleus (LGN) input to V1 – provides an anatomical marker particular to V1.
Experiments with artificially created compound eyes in Xenopus demonstrate that not only the ganglion cells but also their axons carry these specificities.
In the cochlea, the vibrations are transduced into electrical information through the firing of hair cells in the organ of Corti.
The organ of Corti projects in an orderly fashion to structures in the brainstem (namely, the cochlear nuclei and the inferior colliculus), and from there to the medial geniculate nucleus of the thalamus and the primary auditory cortex.
In areas that are tonotopically organized, the frequency varies systematically from low to high along the surface of the cortex, but is relatively constant across cortical depth.
The general image of topographic organization in animals is multiple tonotopic maps distributed over the surface of the cortex.
[4] The somatosensory system comprises a diverse range of receptors and processing centers to produce the perception of touch, temperature, proprioception, and nociception.
Receptors are located throughout the body including the skin, epithelia, internal organs, skeletal muscles, bones, and joints.
The cutaneous receptors of the skin project in an orderly fashion to the spinal cord, and from there, via different afferent pathways (dorsal column-medial lemniscus tract and spinothalamic tract), to the ventral posterior nucleus of the thalamus and the primary somatosensory cortex.
This illustration is a fairly accurate representation of how much cortical area represents each body part or region.
[5] The higher motor centers of octopuses (large brained invertebrates) are notable for organizing (unlike vertebrates) highly skilled movements without the use of somatotopic maps of their bodies.
While refinement of the bulbar topographic code relies on activity, the development occurs partly without apparent contribution from activity-dependent processes.
Mice lacking the olfactory cyclic nucleotide-gated ion channel fail to exhibit odor-evoked electrophysiological responses in the sensory epithelium, but the pattern of convergence of like axons in the bulb is unaltered in these mutant mice, arguing strongly that olfactory experience is not necessary for the establishment or refinement of the topographic map.
These findings, however, do not exclude a role for activity-dependent processes in the maintenance or potential plasticity of the map after it is established.
Afferents from taste receptors and mechanoreceptors of the tongue access different ascending systems in the brainstem.
Details in the maps came later through microelectrode stimulation and recording techniques became commonly used in demonstrating somatotopic maps and later in the auditory and visual systems, both cortically and in subcortical structures such as the colliculi and geniculate nuclei of the thalamus.
For example, Hubel and Wiesel originally studied the retinotopic maps in the primary visual cortex using single-cell recording.
Recently, however, imaging of the retinotopic map in the cortex and in sub-cortical areas, such as the lateral geniculate nucleus, have been improved using the fMRI technique.