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  • 1.
    Richter, H. O
    et al.
    University of Gävle, Belastningsskadecentrum.
    Andersson, J
    Schneider, H
    Långström, B
    Neuroanatomical correlates of voluntary inhibition of accommodation/vergence under monocular open-loop viewing conditions.2005In: European Journal of Neuroscience, ISSN 0953-816X, E-ISSN 1460-9568, Vol. 21, no 11, p. 3077-3088Article in journal (Refereed)
    Abstract [en]

    The purpose of this work is to identify human neural circuits involved in inhibition of accommodation/vergence by contrasting the cortical functions subservient to negative voluntary accommodation/vergence (NVA) with those evoked by active fixation in darkness (FIX). Five subjects with normal corrected acuity were studied using positron emission tomography and the H O bolus technique. The dominant right eye viewed a laser speckle pattern (633 nm) whose direction and velocity of motion were determined by the refractive state of the eye. The speckle pattern was presented at a distance of 1.8 m (0.55 D). The non-dominant eye was patched. Subjects performed two tasks counterbalanced for order effects: (i) attempted fixation on the remembered target in darkness with the dominant eye open and ‘fixating’; and (ii) voluntary reduction of the laser speckle flow during each alternate 20-s epoch when a convex +2.0 D lens was placed in front of the right eye causing the speckle pattern to move downwards at 3 °/s. Comparison of the condition of NVA with the condition of FIX indicated widespread occipital activation. Decreases in absolute regional cerebral blood flow occurred in the superior parietal cortex (BA 5), frontal cortex (BA 8 and 10) and within the postcentral/precentral gyrus (BA 1/2/3/4) bilaterally where deactivation clusters eclipsed the presumed neck and shoulder areas. Negative accommodation/vergence appears to be driven by a reduction of parasympathetic tone, and has the effect of shutting down brain regions known to be involved in regulating visual search as well as a centrally controlled eye–head–neck–shoulder motor programme responsible for posturing gaze.

  • 2.
    Richter, Hans
    et al.
    University of Gävle, Department of Education and Psychology, Ämnesavdelningen för psykologi.
    Costello, P.
    Sponheim, S. R.
    Lee, J. T.
    Pardo, J. V.
    Functional neuroanatomy of the human near/far response to blur cues: eye-lens accommodation/vergence to point targets varying in depth2004In: European Journal of Neuroscience, ISSN 0953-816X, E-ISSN 1460-9568, Vol. 20, no 10, p. 2722-2732Article in journal (Refereed)
    Abstract [en]

    The purpose of this study was to identify the networks involved in the regulation of visual accommodation/vergence by contrasting the cortical functions subservient to eye-lens accommodation with those evoked by foveal fixation. Neural activity was assessed in normal volunteers by changes in rCBF measured with PET. Thirteen right-handed subjects participated in three monocular tasks: (i) resting with eyes closed; (ii) sustained foveal fixation upon a LED at 1.2 m (0.83 D); and (iii) accommodating alternately on a near (24 cm, 4.16 D) vs. a far (3.0 m, 0.33 D) LED alternately illuminated in sequential 2 s epochs. The contrast between the conditions of near/far accommodation and of constant foveal fixation revealed activation in cerebellar hemispheres and vermis; middle and inferior temporal cortex (BA 20, 21, 37); striate cortex and associative visual areas (BA 17/18). Comparison of the condition of constant fixation with the condition of resting with closed eyes indicated activation of cerebellar hemispheres and vermis; visual cortices (BA 17/18); a right hemisphere dominant network encompassing prefrontal (BA 6, 9, 47), superior parietal (BA 7), and superior temporal (BA 40) cortices; and bilateral thalamus. The contrast between the conditions of near/far accommodation with closed-eye rest reflected an incremental summation of the activations found in the previous comparisons (i.e. activations associated with constant fixation). Neural circuits activated selectively during the near/far response to blur cues over those during constant visual fixation, occupy posterior structures that include occipital visual regions, cerebellar hemispheres and vermis, and temporal cortex.

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