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The neural circuits associated with the human monocular near/far response (NFR) were studied in thirteen normal volunteers by changes in relative regional cerebral blood flow (rCBF) measured with H2 15O infusion and positron emission tomography (PET). Behavioral tasks were selected to differentially recruit relevant circuits in three states of activity: resting, fixating, and near/far accommodating. Neural networks selectively activated for eliciting the NFR to blur cues during constant visual fixation occupy posterior structures which include occipital visual regions, temporal cortex, cerebellar hemispheres and vermis. Of note, no significant activation occurred in prefrontal regions. The NFR recruited many regions already active during constant visual fixation, and it engaged circuits partly dissociable from those mediating other types of eye movements as well as those participating in visual fixation.

Psychophysical scales of a 3,3 c/deg monochromatic checkerboard of variable contrast were compared with steady state visually evoked potentials (VEP) recorded by gross electrodes on the scalp. These estimates of neuronal population responses grew as a power function of physical contrast having an exponent of approximately the same magnitude as the corresponding psychophysical function which gives credence to the validity of the procedures employed. The functional neuroimaging technique of positron emission tomography (PET), based on radioactive decay of a labelled tracer occurring inside the brain, was applied in normal subjects to quantitatively explore the influence of voluntary positive accommodation and also to examine the effect of reduced contrast sensitivity in human strabismic amblyopia. A great asymmetry in metabolic activity was observed in the striate cortex, that is, the Brodmann area 17 (BA 17) activation was strongest contralateral to the dominant viewing eye. The PET scans revealed, however, a high correlation between blood flow increases in the right striate cortex (BA 17) and the left extrastriate cortex (BA 18) during voluntary accommodation, possibly reflecting top-down modulation and reentrant processes. The poor contrast sensitivity in strabismic amblyopia could essentially be explained by deactivation of the ipsilateral extrastriate cortical areas BA 18 and BA 19.