ACOP pattern revealed that the cross-modal activations of visual cortices by sounds were critically dependent on whether the sound location was predictable or not, indicating that this particular cross-modal process is not (fully) automatic instead, it is context-contingent 26.
Recent ERP studies have shown an auditory-evoked contralateral occipital activation (ACOP) in sighted individuals 22, 23, 26, 27.
The lack of such activity was observed both in detection 19, 24 and in discrimination tasks 25. The acoustic projections in monkeys are also heterogeneously distributed across the retinotopic representations within the primary visual cortex, with more peripheral visual field representations receiving denser projections 11, 21.ĭespite the demonstration of neuronal activity and pathways in animal models, and the clear evidence from fMRI, previous studies using scalp Event Related Potentials (ERP) in sighted humans have failed to demonstrate a reliable early response specifically involving striate areas to a unimodal auditory stimulation 22, 23. Several anatomical routes that may mediate the auditory response in the occipital cortex have been identified, like a direct thalamo-occipital pathway 20 or a cortico-cortical pathway that feeds back the auditory cortical response to the visual cortex through the parietal cortex 12, 21. A neuronal response of primary the visual cortex to sound alone has been demonstrated in mice 9, in cats 10, in primates 4, 11, 12, and crucially also in humans, by fMRI 13, 14, 15, 16, TMS 17 and ECoG 18 and EEG 19 studies. The acoustic receptive fields (RFs) of the neurons of the inferior colliculus, as well as of the Superior Temporal Sulcus, are eye-centered and move with the eye 2, 4, 5, 6, 7, 8, but no data indicate that this also occurs in the primary cortex or more generally, that the occipital visual cortex’s response to sound may be important for the construction of metrics aimed at representing sounds in. Sound localization in space is bound to visual maps in many structures of the monkey brain.
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It is not clear to date whether this sophisticated retinotopic code of the early visual cortex can also be used to derive the spatial position of other sensory sources, for example for the localization of a sound. Of all the sensory systems, vision relies on the representation of space from orderly retinotopic maps. Space representation is one hard problem that the brain has to face, given the continuously changing flow of information collected by our mobile sensors, for review see refs 1, 2, 3.
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