Shufflebrain

Optic tracts end where they make connections with a highly organized collection of cells known as the LGB (lateral geniculate body). The LGB has the job of communicating visual signals to the visual cortex of the occipital lobe. Now there is every anatomical reason to predict that destruction of one occipital lobe will split a visual field map into seen and blank halves, as sometimes occurs.

Usually, though, a person with a lesion beyond the LGB will lose the peripheral parts of the opposite field but retain a whole, un-split view of the central field. The macula, a yellowish spot on the center of the retina, receives the projection from the central field. Thus the term macular sparing means that an otherwise split visual field remains un-split on both sides of the central zone, which is precisely as it should

not

be.

If the visual pathways were haphazardly arranged, with fibers coursing everywhere, macular sparing would be understandable. But clinical records, autopsy reports, the results of direct stimulation of conscious human brains during surgery, and probings into ape and monkey brains with minute electrodes--all means of gathering evidence--consistently show that the visual system is minutely precise in organization. For a while, some authors explained away macular sparing by assuming that central retinal fibers violate the crossing rule. But in 1934, a famous ophthalmologist, Stephen Polyak, studied the chimpanzee's visual pathways and found that central fibers

do

obey crossing rules, just like fibers of the rest of the retina: nasals cross, temporals don't! And repeated searches of human pathways has led to an identical conclusion-- namely that crossing doesn't explain macular sparing.

[2 ]

Until 1940, one could assume either or both of two additional hypotheses to explain macular sparing: that of partial survival of the visual pathways, and/or that of sloppy examination of the visual fields. But in that year, Ward Halstead and his colleagues published data in the Archives of Ophthalmology that eliminated these simple hypotheses as well.

Halstead's group reported the case history of a twenty-five year old filing clerk who arrived at a clinic in Chicago in the autumn of 1937 with a massive tumor in her left occipital lobe. Summarizing what the surgeons had to cut out to save the woman's life, Halstead et al. wrote, "The ablation had removed completely the left striate [visual] cortex and areas 18 and 19 of the occipital lobe posterior to the parieto-occipital fissure." Translated, this means that the young woman lost her entire left optic lobe--the entire half of her brain onto which the right visual field projects and in which information is processed into higher-order percepts. Visual field maps showed that the operation caused blindness in the young woman's right visual field (homonymous hemianopsia, as it is called), but with macular sparing.

***

If macular sparing always occurred after occipital-lobe damage, one might explain the phenomenon by assuming that the macular-projection area of one LGB somehow sends fibers to both occipital lobes. But the Halstead article nullified this explanation, too, with an almost identical case history of a twenty-two year old stenographer. A patient in the same hospital, she also had a massive tumor, but in her right occipital lobe. After surgery, visual field mapping showed that she was totally blind to the left field of view--without macular sparing!

In other words, not only did Halstead's group document macular sparing as a genuine anatomical paradox; they even showed that one cannot apply simple, linear cause-and-effect reasoning to it: in the case of the two young women, the same antecedents had produced decidedly different consequences.

In no way does macular sparing detract from the orderliness of the visual system. Indeed, this was part of the mystery. Specific places on the retina excite particular cells in both the LGB and the visual cortex. When stimulated, the macular zone on the retina does excite specific cells of the occipital cortex--in the rear tip of the lobe, to be exact--on the side opposite the half visual field. But the phenomenon of macular sparing (and thousands of people have exhibited the sign) shows that there is not an exclusive center in the brain for seeing the central field of view. If the message can make it into the LGB, it may make it to the mind.

***