Persistence of the dark-background-contingent gaze upshift during visual fixations of rhesus monkeys

During visual fixations, the eyes are directed so that the image of the target (object of interest) falls on the fovea. An exception to this rule was described in macaque monkeys (though not in humans): dark background induces a gaze shift upwards, sometimes large enough to shift the target's image off the fovea. In this article we address an aspect not previously rigorously studied, the time course of the upshift. The time course is critical for determining whether the upshift is indeed an attribute of visual fixation or, alternatively, of saccades that precede the fixation. These alternatives lead to contrasting predictions regarding the time course of the upshift (durable if the upshift is an attribute of fixation, transient if caused by saccades). We studied visual fixations with dark and bright background in three monkeys. We confined ourselves to a single upshift-inducing session in each monkey so as not to study changes in the upshift caused by training. Already at their first sessions, all monkeys showed clear upshift. During the first 0.5 s after the eye reached the vicinity of the target, the upshift was on average larger, but also more variable, than later in the trial; this initial high value 1) strongly depended on target location and was maximal at locations high on the screen, and 2) appears to reflect mostly the intervals between the primary and correction saccades. Subsequently, the upshift stabilized and remained constant, well above zero, throughout the 2-s fixation interval. Thus there is a persistent background-contingent upshift genuinely of visual fixation.

Priming of Head Premotor Circuits During Oculomotor Preparation

Large, rapid gaze shifts necessitate intricate coordination of the eyes and head. Brief high-frequency bursts of activity within the intermediate and deeper layers of the superior colliculus (dSC) encode desired gaze shifts regardless of component movements of the eyes and head. However, it remains unclear whether low-frequency activity emitted by oculomotor neurons within the dSC and elsewhere has any role in eye-head gaze shifts. Here we test the hypothesis that such low-frequency activity contributes to eye-head coordination by selectively priming head premotor circuits. We exploited the capacity for short-duration (10 ms, 4 pulses) dSC stimulation to evoke neck muscle responses without compromising ocular stability, stimulating at various intervals of a “gap-saccade” task. Low-frequency neural activity in many oculomotor areas (including the dSC) is known to increase during the progression of the gap-saccade task. Stimulation was passed during either a fixation-interval while a central fixation point was illuminated, a 200-ms gap-interval between fixation point offset and target onset, or a movement-interval following target onset. In the two monkeys studied, the amplitude of evoked responses on multiple neck muscles tracked the known increases in low-frequency oculomotor activity during the gap-saccade task, being greater following stimulation passed at the end of the gap- versus the fixation-interval, and greater still when the location of stimulation during the movement interval coincided with the area of the dSC generating the ensuing saccade. In one of these monkeys, we obtained a more detailed timeline of how these results co-varied with low-frequency oculomotor activity by stimulating, across multiple trials, at different times within the fixation-, gap- and movement-intervals. Importantly, in both monkeys, baseline levels of neck EMG taken immediately prior to stimulation onset did not co-vary with the known pattern of low-frequency oculomotor activity up until the arrival of a transient burst associated with visual target onset. These baseline results demonstrate that any priming of the head premotor circuits occurs without affecting the output of neck muscle motoneurons, We conclude that low-frequency oculomotor activity primes head premotor circuits well in advance of gaze shift initiation, and in a manner distinct from its effects on the eye premotor circuits. Such distinctions presumably aid the temporal coordination of the eyes and head despite fundamentally different biomechanics.

Quantitative Analysis of Substantia Nigra Pars Reticulata Activity During a Visually Guided Saccade Task

Several lines of evidence suggest that the pars reticulata subdivision of the substantia nigra (SNr) plays a role in the generation of saccadic eye movements. However, the responses of SNr neurons during saccades have not been examined with the same level of quantitative detail as the responses of neurons in other key saccadic areas. For this report, we examined the firing rates of 72 SNr neurons while awake-behaving primates correctly performed an average of 136 trials of a visually guided delayed saccade task. On each trial, the location of the visual target was chosen randomly from a grid spanning 40° of horizontal and vertical visual angle. We measured the firing rates of each neuron during five intervals on every trial: a baseline interval, a fixation interval, a visual interval, a movement interval, and a reward interval. We found four distinct classes of SNr neurons. Two classes of neurons had firing rates that decreased during delayed saccade trials. The firing rates of discrete pausers decreased after the onset of a contralateral target and/or before the onset of a saccade that would align gaze with that target. The firing rates of universal pausers decreased after fixation on all trials and remained below baseline until the delivery of reinforcement. We also found two classes of SNr neurons with firing rates that increased during delayed saccade trials. The firing rates of bursters increased after the onset of a contralateral target and/or before the onset of a saccade aligning gaze with that target. The firing rates of pause-bursters increased after the onset of a contralateral target but decreased after the illumination of an ipsilateral target. Our quantification of the response profiles of SNr neurons yielded three novel findings. First, we found that some SNr neurons generate saccade-related increases in activity. Second, we found that, for nearly all SNr neurons, the relationship between firing rate and horizontal and vertical saccade amplitude could be well described by a planar surface within the range of movements we sampled. Finally we found that for most SNr neurons, saccade-related modulations in activity were highly variable on a trial-by-trial basis.

Poor reliability of immunocytochemical localization of IgG in immersion-fixed tissue from the central nervous system.

The effect of fixation technique and post mortem-to-fixation interval in immersion-fixed tissue from the central nervous system on immunocytochemical staining for the presence of an immunoglobulin was determined in mice. Immersion-fixed tissue was found to be inferior to perfusion-fixed tissue for immunocytochemical staining of this serum protein. Unlike what has been observed for other antigens, the quality of staining for IgG in immersion-fixed tissue decreased to unacceptable levels if the post mortem-to-fixation interval was increased to more than a few hours. This effect may be secondary to the rapid post-mortem disintegration of the blood-brain barrier and a resulting diffusion of serum proteins into surrounding tissue from the vasculature.