Direction-dependent visual cortex activation during horizontal optokinetic stimulation (fMRI study)

Abstract

Looking at a moving pattern induces optokinetic nystagmus (OKN) and activates an assembly of cortical areas in the visual cortex, including lateral occipitotemporal (motion-sensitive area MT/V5) and adjacent occipitoparietal areas as well as ocular motor areas such as the prefrontal cortex, frontal, supplementary, and parietal eye fields. The aim of this functional MRI (fMRI) study was to investigate (1) whether stimulus direction-dependent effects can be found, especially in the cortical eye fields, and (2) whether there is a hemispheric dominance of ocular motor areas. In a group of 15 healthy subjects, OKN in rightward and leftward directions was visually elicited and statistically compared with the control condition (stationary target) and with each other. Direction-dependent differences were not found in the cortical eye fields, but an asymmetry of activation occurred in paramedian visual cortex areas, and there were stronger activations in the hemisphere contralateral to the slow OKN phase (pursuit). This can be explained by a shift of the mean eye position of gaze (beating field) in the direction of the fast nystagmus phases of approximately 2.6 degrees, causing asymmetrical visual cortex stimulation. The absence of a significant difference in the activation pattern of the cortical eye fields supports the view that the processing of eye movements in both horizontal directions is mediated in the same cortical ocular motor areas. Furthermore, no hemispheric dominance for OKN processing was found in right-handed volunteers.

Figure 1

Original recording of horizontal left eye movement for single subject during fMRI scanning. The arrow indicates the start of visual optokinetic stimulation which induces an OKN with its fast phase toward the left. With OKN there is a detectable gaze shift to the left in this single subject. The mean gaze shift for the group is 2.6°.

Figure 2

OKN during rightward (upper row) and leftward (lower row) small‐field visual stimulation vs. rest condition (stationary screen) in a group of 15 healthy right‐handed volunteers elicited very similar bilateral activations of the visual cortex, which merged into the adjacent occipitotemporal (motion‐sensitive area MT/V5) and parietooccipital areas including the parietal eye field (PEF) along the intraparietal sulcus. Additional activations were located nearly symmetrically in the anterior insular region and adjacent parts of the inferior frontal gyri (GFi) as well as in different ocular motor structures such as the prefrontal cortex (PFC, GFm = middle frontal gyrus), frontal (FEF), and supplementary eye fields (SEF). For illustrative purposes, voxels above a threshold of P ≤ 0.005 uncorrected are shown.

Figure 3

OKN during rightward (upper row) and leftward (lower row) small‐field visual stimulation in a group of 15 healthy right‐handed volunteers caused deactivations in the posterior corpus callosum which partly merged into adjacent parts of the posterior cingulate gyrus and optic radiation. Additional bilateral deactivations were found in the parieto‐insular vestibular cortex (PIVC) in the posterior insula, in the central sulcus region (best attributed to the somatosensory cortex), and in the frontal‐most and medial part of the right middle frontal gyrus (BA 8, GFm). For illustrative purposes, voxels above a threshold of P ≤ 0.005 uncorrected are shown.

Figure 4

For the contrast of leftward vs. rightward OKN (upper row) or vice versa (lower row), the group analysis of healthy volunteers showed significant differences only in parts of the paramedian visual cortex in the hemisphere contralateral to the slow phase of OKN (pursuit) (P ≤ 0.001 uncorrected).

Figure 5

Results of the evaluation of the maximum t‐values of the right‐ and left‐hemispheric activation of the frontal eye field (FEF) in 15 healthy volunteers. The activation of the right FEF tends to be stronger than of the left FEF, but in the one‐factor ANOVA no significant difference was found.