Fast-track vision (original) (raw)

Neuroanatomy

Nature Reviews Neuroscience volume 4, page 779 (2003)Cite this article

The responses of dopaminergic neurons in the midbrain to visual stimuli — particularly those that predict reward — has come under scrutiny recently, but the pathway by which visual information reaches these neurons is unclear. Comoli et al. have found a direct projection from a primary visual area, the superior colliculus, to the substantia nigra that could be responsible for the very fast responses of the dopamine neurons.

Previous clues to the existence of such a pathway came from the finding that chemical stimulation of the superior colliculus could cause correlated bursting in midbrain dopaminergic nuclei. Also, cortical response latencies to visual stimuli are longer than those of dopaminergic neurons, making it unlikely that the responses of these neurons are mediated by input from the visual cortex.

Comoli and colleagues used anterograde tracing to investigate whether there was a direct link between the superior colliculus and the midbrain dopaminergic nuclei. They injected tracers into the superior colliculus and found that they were transported specifically to the dopaminergic part of the substantia nigra and to the ventral tegmental area. Electron microscopy confirmed that the labelled fibres formed synapses, apparently onto both dopaminergic and non-dopaminergic neurons in these regions. Interestingly, the synapses were both symmetric (probably excitatory) and asymmetric (probably inhibitory), and these two populations of synapses might arise from different groups of neurons in the superior colliculus.

The authors went on to investigate the electrophysiological properties of the pathway. Both the superior colliculus and the substantia nigra showed short-latency responses to visual input, with the superior colliculus responding more quickly. Pharmacological inhibition or disinhibition of the superior colliculus decreased or increased, respectively, the size of the evoked response in the substantia nigra. Finally, removal of the superficial layers of the superior colliculus abolished visual evoked responses in the substantia nigra, whereas removal of the visual cortex did not.

These findings support the idea that a direct projection from the superior colliculus carries short-latency visual input to the dopaminergic areas of the ventral midbrain. However, it remains to be seen whether this pathway is can mediate specific responses of dopaminergic neurons to reward-related stimuli. To do so, it would need to distinguish reward-predicting stimuli from other salient or unpredictable events — even before the eyes have moved to localize the event.

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  1. Rachel Jones
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Jones, R. Fast-track vision.Nat Rev Neurosci 4, 779 (2003). https://doi.org/10.1038/nrn1238

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