hear colors, taste shapes, or experience other curious sensory modality crossings, allegedly related to abnormal functioning of the hippocampus, one of the limbic structures in the brain. It has also been suggested that synesthesia constitutes a form of "supernormal integration" involving the posterior parietal cortex. The Russian composer Alexander Scriabin and Russian-born painter Wassily Kandinsky both pioneered artistic links between sight and sound, while they may have been synesthetes themselves. Russian mnemonist Solomon Shereshevskii, studied for decades by neuropsychologist Alexander Luria, appears to have used his natural synesthesia to memorize amazing amounts of data.

in seeing with your ears when using a device that maps images into sounds, or in hearing with your eyes when using a device that maps sounds into images.

In case of "explicit" synesthesia, the sounds would induce conscious sensations (qualia) of light and visual patterns.

While neural activity of a certain kind is a necessary condition for every manifestation of consciousness, from the lightest sensation to the most exquisitely constructed sense of self, it is neither a sufficient condition of it, nor, still less, is it identical with it.

Many features of ordinary consciousness also resist neurological explanation.

There is nothing in the convergence or coherence of neural pathways that gives us this "merging without mushing", this ability to see things as both whole and separate.

This concerns the disjunction between the objects of science and the contents of consciousness. Science begins when we escape our subjective, first-person experiences into objective measurement, and reach towards a vantage point the philosopher Thomas Nagel called "the view from nowhere".

Thus measurement takes us further from experience and the phenomena of subjective consciousness to a realm where things are described in abstract but quantitative terms. To do its work, physical science has to discard "secondary qualities", such as colour, warmth or cold, taste - in short, the basic contents of consciousness. For the physicist then, light is not in itself bright or colourful, it is a mixture of vibrations in an electromagnetic field of different frequencies. The material world, far from being the noisy, colourful, smelly place we live in, is colourless, silent, full of odourless molecules, atoms, particles, whose nature and behaviour is best described mathematically. In short, physical science is about the marginalisation, or even the disappearance, of phenomenal appearance/qualia, the redness of red wine or the smell of a smelly dog.

Consciousness, on the other hand, is all about phenomenal appearances/qualia.

There is nothing in physical science that can explain why a physical object such as a brain should ascribe appearances/qualia to material objects that do not intrinsically have them.

Then their "appearings" will depend on the viewpoint of the conscious observer.

Material objects require consciousness in order to "appear".

Our failure to explain consciousness in terms of neural activity inside the brain inside the skull is not due to technical limitations which can be overcome. It is due to the self-contradictory nature of the task, of which the failure to explain "aboutness", the unity and multiplicity of our awareness, the explicit presence of the past, the initiation of actions, the construction of self are just symptoms.