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“I was up in my bedroom, getting off a spot of correspondence which I had neglected of late, and from where I sat looked down on the shady lawn with its gay flower beds. There was a bird or two hopping about, a butterfly or so fluttering to and fro and an assortment of bees buzzing hither and thither. In a garden chair sat old Mr Anstruther, getting his eight hours.”
PG Wodehouse, Very Good, Jeeves!
We seamlessly process objects, words, shapes, colours, movement and faces simultaneously within milliseconds with the mechanism rarely, if at all, flickering into our consciousness. Perhaps because of the apparent ease with which we are able to see, we live with the illusion that seeing is easy.
This review aims to consider the complex network comprising ‘higher visual function’ or ‘extrastriate vision’. It seeks to explore facial and object recognition, as well as reading and colour perception; problems traditionally regarded as disorders of the ‘ventral stream’ (see below).
Retina to the primary visual area…
Although the pathway taken by visual information between the retina and the primary visual area of the cortex is not the focus of this article, a brief summary of this path helps to place higher visual function in its context.
Visual information leaving the eyes in the optic nerve converges at the chiasm (Greek for ‘crossing’). Here, the fibres partially decussate (with information from the temporal field (nasal retina) in each eye crossing from left to right and vice versa). The optic tract sends most of its fibres to the lateral geniculate nucleus (LGN) of the thalamus. The remaining fibres pass to the superior colliculus (involved in directing eye movements) and onwards to the pulvinar nucleus of the thalamus (important in regulating visual attention). There are many theories about the exact function of the LGN, other than …
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