Neuroanatomical correlates of the near response: voluntary modulation of accommodation/vergence in the human visual system

This study identifies brain regions participating in the execution of eye m ovements for voluntary positive accommodation (VPA) during open-loop vergen ce conditions. Neuronal activity was estimated by measurement of changes in regional cerebral blood flow (rCBF) with positron emission tomography and O-15-water. Thirteen naive volunteers viewed a checkerboard pattern with th eir dominant right eye, while a lens interrupted the line of gaze during al ternate 1.5s intervals. Three counterbalanced tasks required central fixati on and viewing of a stationary checkerboard pattern: (i) through a 0.0 diop ter (D) lens; (ii) through a -5.0-D lens while avoiding volitional accommod ation and permitting blur; and (iii) through a -5.0-D lens while maintainin g maximal focus. The latter required large-amplitude, high-frequency VPA. A s an additional control, seven of the subjects viewed passively a digitally blurred checkerboard through a 0.0-D lens as above. Optometric measurement s confirmed normal visual acuity and ability to perform the focusing task ( VPA). Large-amplitude saccadic eye movements, verified absent by electro-oc ulography, were inhibited by central fixation. Image averaging across subje cts demonstrated multifocal changes in rCBF during VPA: striate and extrast riate visual cortices; superior temporal cortices; and cerebellar cortex an d vermis. Decreases in rCBF occurred in the lateral intraparietal area, pre frontal and frontal and/or supplementary eye fields. Analysis of regions of interest in the visual cortex showed systematic and appropriate task depen dence of rCBF. Activations may reflect sensorimotor processing along the re flex are of the accommodation system, while deactivations may indicate inhi bition of systems participating in visual search.