A historical review of the representation of the visual field in primary visual cortex with special reference to the neural mechanisms underlying macular sparing
Introduction
Although there have been articles produced that review the literature on the representation of the visual world in primary visual cortex, this is the first to combine a historical review of this subject along with the important associated topics of both the measurement of cortical magnification factor and potential mechanisms underlying the phenomenon of macular sparing hemianopia. This latter topic is of key import to anatomically based psychological models of higher visual processing because these models need to incorporate the laterality effects observed in both language dependent and language independent tasks. If visual hemispace is truly split between each hemisphere, then when a subjects fixates a visual object, say a word, then the portion to the left of fixation is processed initially by the right hemisphere, and vice versa. Therefore, any psychological model of reading needs to include a process whereby the visual field is integrated across the vertical meridian somewhere downstream of primary visual cortex. Conversely, if central vision is bilaterally represented in the primary visual cortex of both hemispheres then, in theory, association cortex on either side can independently process visual percepts without recourse to ‘crosstalk’ between early visual processing units in both hemispheres. I leave the elucidation of these various models to others, aiming to present here an anatomically limited review of the representation of the visual world through the visual system up to and including primary visual cortex.
This article is in four sections. Section 2 deals very briefly with the components of the visual system while Section 3 comprises a historical review of the work carried out on the cortical representation of the visual hemifield within primary visual cortex. In Section 4, I review the key studies that have shed light on the disproportionate representation of central vision within primary visual cortex and in Section 5 I discuss the different models that have been proposed to explain the anatomical basis of macular sparing and macular splitting hemianopia.
Section snippets
Components of the visual system
Image formation inside the eye has been likened to the process performed by a camera, an analogy that holds truest for the refracting elements at the front of the eye. The German astronomer Johannes Kepler (1571–1630), known principally for his laws of planetary motion, maintained a keen interest in optics and was the first to describe the role of the cornea and lens in the formation of an inverted image on the retina. The similarity between the eye and the pinhole camera had previously been
Cortical representation of the visual hemifield
Francesco Gennari (1750–?) first described the striate cortex, so named because of its lamina macroscopic appearance due to the prominent band of geniculo-striate fibres. He dubbed it the “lineola albidiar” (Gennari, 1782) but it soon came to be known eponymously as the stria of Gennari. Gennari suggested that others concentrate on this area as it may have a specific function, but he had no clue as to what this was. The prevailing view at that time was that the cerebral cortex had a unitary
Disproportionate representation of central vision
The careful study of wartime victims with occipital lobe injuries filled in the cortical maps hinted at by the basic animal experiments carried out by Munk and others. However, it was a return to experiments on animals that resulted in these maps becoming further refined as experimental techniques, not least in the field of anaesthesia, allowed researchers to investigate cortical anatomy without having to destroy it first. Talbot and Marshall measured action potentials in the occipital cortex
The anatomical basis of macular sparing and macular splitting
In cases of homonymous hemianopia, the phenomenon of macular sparing (where a portion, anything between 1° and 10°, of central vision is preserved on the same side as the homonymous defect) was well recognized in the 19th Century. The majority of authors believed it to be due to the bilateral representation of central vision in each occipital lobe. Two other explanations have been proffered; firstly that sparing results from incomplete damage to the striate cortex and its connections; secondly
Conclusion
Over the last 130 years or so, key studies on animals and humans have produced clear maps of how the visual field is represented in primary visual cortex. These maps have become refined over time with the early maps consistently underrating the amount of cortex given over to central vision. This error has been due in part to differences between humans and other primates, but accurate measurements of M have been hampered by both the vagaries of pathological lesions encountered in patients, and
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