Question #7: Describing primary sensory pathways in humans

Visual: Path goes, photoreceptor cells in retina --> ganglion cells -- (via optic nerve) --> optic chiasm (crossing of axons) --> lateral geniculate nucleus (mostly) + superior colliculi (can mediate saccades) + pretectal area (pupillary light reflex: if light shined in one eye, both pupils constrict). Then from lateral geniculate nucleus -- (via optic radiations) --> V1 --> V2 --> parietal visual cortical areas (moving objects around in your head) + temporal visual areas (complex perception of patterns and forms). See here.

Auditory: Path starts in the hair cells of the cochlea, specifically the center axis called the spiral ganglion -- (via auditory nerve) --> cochlear nucleus --> location of sound detection: (ventral cochlear nucleus --> superior olive of medulla) + quality of sound: (dorsal cochlear nucleus: frequency differences) -- (via lateral lemniscus fiber tract) --> inferior colliculus --> auditory nucleus of the sensory thalamus (aka medial geniculate nucleus) --> primary auditory cortex of temporal lobes. See here.

Olfactory: Path is from olfactory receptors of roof of the nasal cavity -- (via axons of receptors projecting as first cranial nerve) --> olfactory bulb -- (via axons of mitral cells projecting as olfactory tract) --> olfactory cortex -- (via mediodorsal nucleus of the thalamus) --> insular cortex (taste integrates with smell to produce flavor) + orbitofrontal cortex (odor-taste association learning at single neuron level). Notice that olfaction is the one sensation that gives info directly to the cortex from receptors without first passing through the thalamus. See here.

However, there are many multisensory connections in the cerebral cortex, superior colliculus, and thalamus, such that our sensations can feedback and "correct" one another well before conscious awareness. For example, the premotor cortex achieves multisensory integration by converging visual, auditory, and somatosensory inputs, and has large amounts of overlap with axons of various sensory systems sending projections to other cortical regions. These integration mechanisms vary by behavioral task as well type of sensory input and context must be taken into account.

Inspired by CalTech’s Question #7 for cognitive scientists: “Describe the main pathways between sensory receptors and cortex (including intra-cortical circuits) for mammalian vision, hearing and olfaction.”

References

Critchley HD, Rolls ET. 1996 Olfactory neuronal responses in the primate orbitofrontal cortex: analysis in an olfactory discrimination task. Abstract.

Cappe C, et al. 2009 Multisensory anatomical pathways. doi:10.1016/j.heares.2009.04.017.

The Washington University School of Medicine Neuroscience Tutorial, here.