Reciprocal connectivity in visual cortex: evidence from fMRI

Two experiments are summarized from the viewpoint that visual cortex activity is partly the result of interactions between multiple cortical areas rather than of feedforward processing only. Such interactions can occur through the multitude of reciprocal connections, propagating in both directions, which are known to exist between many visual brain areas (Weller and Kaas, 1981). We used a modified Kanizsa-square illusion as the basis for a novel task with a difficulty that we could manipulate, and which required spatial attention to be focused near an illusory contour. Functional magnetic resonance imaging (fMRI) of the occipital lobe was acquired while subjects performed this task at two levels of difficulty. An area in the dorsal occipital cortex bordering the parietal lobe was found to be more active in the more difficult condition. Given that the stimulus was identical in both conditions, except for the position of a dot which changed by only 0.2 deg, an increase in fMRI signals in the occipital cortex cannot be explained by hierarchical, bottom-up, processing only. Thus, we hypothesize that the activated area is part of a feedback pathway from the superior parietal lobe with the purpose of directing spatial attention or enhancing illusory-contour coding in lower visual areas. Using a very similar control task without illusory contours, the dorsal occipital area was not sensitive to task difficulty. In another experiment, fMRI was performed before, during, and after the wearing of left-right reversing goggles by human subjects, as in the electrophysiological work of Sugita (1996). Before wearing, hemifield stimulation resulted in only contralateral V1 activity. After only 3 days of wearing reversing goggles, ipsilateral V1 activity appeared and increased throughout 5 weeks of wearing. Since V1 receives thalamic input only from the contralateral visual field, the ipsilateral activity must be the result of interhemispheric connections between left and right V1, which are found only along the vertical meridian representation, or of reciprocal connections from higher areas which have large receptive fields spanning both visual hemifields or which may be more widely connected through the corpus callosum.