The resting brain in blind subjects – an fMRI study with eyes open and closed in darkness

Abstract

Objective: It has been shown in normal subjects that opening the eyes in darkness without external visual stimulation increases activity in the ocular motor and attentional systems. By contrast, closing the eyes causes a relative signal increase in multiple sensory areas such as the visual, somatosensory, vestibular, auditory, and gustatory cortical areas. These two states of „resting“ activity of the human brain in complete darkness, which can be imaged using fMRI, were termed the „exteroceptive“ state (contrast OPEN>CLOSED) and the „interoceptive“ state (contrast CLOSED>OPEN). In the present study we investigated whether similar brain activity states can be found in blind subjects when opening and closing their eyes. Methods: Seven congenitally, totally blind individuals participated in the study. Subjects were trained outside the scanner to perform the following tasks: eyes open (OPEN) or eyes closed (CLOSED) at rest while looking straight ahead in complete darkness. Functional images were acquired on a 1.5 T standard clinical scanner using echo-planar imaging. The data were compared to data published earlier of a matched group of healthy volunteers (Marx et al. 2003). Analysis was performed using SPM2. Results: Overall, the differences between the OPEN and CLOSED conditions were less pronounced in the blind subjects than in the controls. In congenitally, totally blind subjects the contrast OPEN>CLOSED revealed a relative increase in activity in the middle occipital gyrus bilaterally as well as in the anterior cingulate cortex. The intergroup comparison (congenitally, totally blind subjects vs. controls) for this contrast showed an activity increase in regions similar to those found in the blind subjects alone. Discussion: There are also differences in brain activity patterns between eyes-open or eyes-closed at rest in congenitally, totally blind subjects, but they are not those of the above-described „intero-“/“exteroceptive“ states. They are rather diametrically opposed to those observed in sighted controls: the activity increases in the occipital cortex for the contrast OPEN>CLOSED suggest a functional reorganization of the visual cortex. While the visual cortex has been previously shown to mediate attentional tasks in congenitally blind subjects, its activation during the eyes-open condition in conjunction with the anterior cingulate might be indicative of an increased attentional load.