Ayumu Furuta

Task-Dependent V1 Responses in Human Retinitis Pigmentosa

PURPOSE During measurement with functional MRI (fMRI) during passive viewing, subjects with macular degeneration (MD) have a large unresponsive lesion projection zone (LPZ) in V1. fMRI responses can be evoked from the LPZ when subjects engage in a stimulus-related task. The authors report fMRI measurements on a different class of subjects, those with retinitis pigmentosa (RP), who have intact foveal vision but peripheral visual field loss. METHODS The authors measured three RP subjects and two control subjects. fMRI was performed while the subjects viewed drifting contrast pattern stimuli. The subjects passively viewed the stimuli or performed a stimulus-related task. RESULTS During passive viewing, the BOLD response in the posterior calcarine cortex of all RP subjects was in phase with the stimulus. A bordering, anterior LPZ could be identified by responses that were in opposite phase to the stimulus. When the RP subjects made stimulus-related judgments, however, the LPZ responses changed: the responses modulated in phase with the stimulus and task. In control subjects, the responses in a simulated V1 LPZ were unchanged between the passive and the stimulus-related judgment conditions. CONCLUSIONS Task-dependent LPZ responses are present in RP subjects, similar to responses measured in MD subjects. The results are consistent with the hypothesis that deleting the retinal input to the LPZ unmasks preexisting extrastriate feedback signals that are present across V1. The authors discuss the implications of this hypothesis for visual therapy designed to replace the missing V1 LPZ inputs and to restore vision.

Objective perimetry using functional magnetic resonance imaging in patients with visual field loss

In ophthalmic clinics, subjective perimetry is a standard examination method. However, for certain patients, objective perimetry is useful since it avoids the need for subjective judgments. The purpose of the present study is to demonstrate the feasibility of objective perimetry using functional magnetic resonance imaging (fMRI). fMRI was performed in 8 patients with visual field defects caused by cerebral lesions. The composite stimulus was either the combination of an expanding ring and a clockwise rotating wedge, or a contracting ring and a counter-clockwise rotating wedge. The largest radius was a 10 degrees visual angle with magnifying glasses. The cycle period for the ring and wedge components differed, enabling us to distinguish the two targets within a single time series. Data were analyzed using custom software that interprets the two stimuli and estimates visual field maps. Regions of interest (ROIs) were set covering the entirety of the occipital lobes, and the most effective visual field location for each voxel was calculated from these two response components. The visual field maps obtained with fMRI were compared with the 10-2 Humphrey visual field (HVF) program. While some divergences were observed, in most subjects the visual field defects on fMRI agreed with those on HVF. Cross-correlation coefficients between grayscale values of visual field maps obtained with fMRI and decibel values obtained with HVF were significant (P<0.05) in all subjects. fMRI in conjunction with our method is feasible for objectively and efficiently measuring the visual field of patients with visual field loss.

Activation in left primary visual cortex representing parafoveal visual field during reading Japanese texts.

Activation in the left primary visual cortex (V1) representing the parafoveal field during text reading has been interpreted as attentional modulation in the process of deciding saccadic target for reading ahead. Kanji words serve the main cue to decide the goal of saccades in Japanese. We aimed to determine the exact location of this modulation in the V1 and to determine whether the area of the modulation changes according to the location where the next Kanji word appears or it is fixed on a certain region in V1. Using functional magnetic resonance imaging, we determined the area in V1 representing each eccentricity on the horizontal meridian of the visual field for each participant. Then we investigated brain activation while they were reading two sets of Japanese texts that scrolled leftward as the participants. In set 1, the distance between the heads of adjacent Kanji words was about 3°. In set 2, the distance was about 5°. From the results of these experiments, we obtained activation amplitude of the area corresponding to each eccentricity. We recorded eye movements simultaneously with the acquisition of fMRI data. The maximum peak of the activation was found in the region representing about 4.5° of eccentricity on the horizontal meridian in the left V1 for each participant. The activation pattern did not essentially differ between the two text conditions, although the location of the saccades made for reading next section of the text corresponds to the head of the next Kanji word. The activation modulation during reading Japanese texts occurs in the parafoveal V1 of the left hemisphere. The attentional modulation did not change with the distance to the next goal of saccade but was fixed on the area representing about 4.5° of eccentricity.