Neuronal correlates of multiple top-down signals during covert tracking of moving objects in macaque prefrontal cortex

Visual Attention in the Prefrontal Cortex

Voluntary attention selects behaviorally relevant signals for further processing while filtering out distracter signals. Neural correlates of voluntary visual attention have been reported across multiple areas of the primate visual processing streams, with the earliest and strongest effects isolated in the prefrontal cortex. In this article, I review evidence supporting the hypothesis that signals guiding the allocation of voluntary attention emerge in areas of the prefrontal cortex and reach upstream areas to modulate the processing of incoming visual information according to its behavioral relevance. Areas located anterior and dorsal to the arcuate sulcus and the frontal eye fields produce signals that guide the allocation of spatial attention. Areas located anterior and ventral to the arcuate sulcus produce signals for feature-based attention. Prefrontal microcircuits are particularly suited to supporting voluntary attention because of their ability to generate attentional template signals and implement signal gating and their extensive connectivity with the rest of the brain. Expected final online publication date for the Annual Review of Vision Science, Volume 8 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Common cortical areas have different neural mechanisms for covert and overt visual pursuits

Although humans can direct their attention to visual targets with or without eye movements, it remains unclear how different brain mechanisms control visual attention and eye movements together and/or separately. Here, we measured MEG and fMRI data during covert/overt visual pursuit tasks and estimated cortical currents using our previously developed extra-dipole, hierarchical Bayesian method. Then, we predicted the time series of target positions and velocities from the estimated cortical currents of each task using a sparse machine-learning algorithm. The predicted target positions/velocities had high temporal correlations with actual visual target kinetics. Additionally, we investigated the generalization ability of predictive models among three conditions: control, covert, and overt pursuit tasks. When training and testing data were the same tasks, the largest reconstructed accuracies were overt, followed by covert and control, in that order. When training and testing data were selected from different tasks, accuracies were in reverse order. These results are well explained by the assumption that predictive models consist of combinations of three computational brain functions: visual information-processing, maintenance of attention, and eye-movement control. Our results indicate that separate subsets of neurons in the same cortical regions control visual attention and eye movements differently.

Spatial and temporal adaptation of predictive saccades based on motion inference

Moving objects are often occluded behind larger, stationary objects, but we can easily predict when and where they reappear. Here, we show that the prediction of object reappearance is subject to adaptive learning. When monkeys generated predictive saccades to the location of target reappearance, systematic changes in the location or timing of target reappearance independently altered the endpoint or latency of the saccades. Furthermore, spatial adaptation of predictive saccades did not alter visually triggered reactive saccades, whereas adaptation of reactive saccades altered the metrics of predictive saccades. Our results suggest that the extrapolation of motion trajectory may be subject to spatial and temporal recalibration mechanisms located upstream from the site of reactive saccade adaptation. Repetitive exposure of visual error for saccades induces qualitatively different adaptation, which might be attributable to different regions in the cerebellum that regulate learning of trajectory prediction and saccades.

Motor facilitation during observation of implied motion: Evidence for a role of the left dorsolateral prefrontal cortex

The phenomenon of motor resonance (the increase in motor cortex excitability during observation of actions) has been previously described. Transcranial magnetic stimulation (TMS) studies have demonstrated a similar effect during perception of implied motion (IM). The left dorsolateral prefrontal cortex (DLPFC) seems to be activated during action observation. Furthermore, the role of this brain area in motor resonance to IM is yet to be investigated. Fourteen healthy volunteers were enrolled into the study. We used transcranial direct current stimulation (tDCS) to stimulate DLPFC aiming to investigate whether stimulation with different polarities would affect the amplitude of motor evoked potential collected during observation of images with and without IM. The results of our experiment indicated that Cathodal tDCS over the left DLPFC prevented motor resonance during observation of IM. On the contrary, anodal and sham tDCS did not significantly modulate motor resonance to IM. The current study expands the understanding of the neural circuits engaged during observation of IM. Our results are consistent with the hypothesis that action understanding requires the interaction of large networks and that the left DLPFC plays a crucial role in generating motor resonance to IM.

Automatic comparison of stimulus durations in the primate prefrontal cortex: the neural basis of across-task interference

Rhesus monkeys performed two tasks, both requiring a choice between a red square and a blue circle. In the duration task, the two stimuli appeared sequentially on each trial, for varying durations, and, later, during the choice phase of the task, the monkeys needed to choose the one that had lasted longer. In the matching-to-sample task, one of the two stimuli appeared twice as a sample, with durations matching those in the duration task, and the monkey needed to choose that stimulus during the choice phase. Although stimulus duration was irrelevant in the matching-to-sample task, the monkeys made twice as many errors when the second stimulus was shorter. This across-task interference supports an order-dependent model of the monkeys' choice and reveals something about their strategy in the duration task. The monkeys tended to choose the second stimulus when its duration exceeded the first and to choose the alternative stimulus otherwise. For the duration task, this strategy obviated the need to store stimulus-duration conjunctions for both stimuli, but it generated errors on the matching-to-sample task. We examined duration coding in prefrontal neurons and confirmed that a population of cells encoded relative duration during the matching-to-sample task, as expected from the order-dependent errors.

Differential neuronal representation of spatial attention dependent on relative target locations during multiple object tracking

Humans can simultaneously track multiple moving objects with attention. The number of objects that can be tracked is known to be larger when visual stimuli are presented bilaterally rather than presented unilaterally. To elucidate the underlying neuronal mechanism, we trained monkeys to covertly track a single or multiple object(s). We found that neurons in the lateral prefrontal cortex exhibited greater activity for the target passing through the receptive field (RF) than for distractors. During multiple-object tracking, response enhancement for one target presented in the RF was stronger when the other target was located in the opposite than the same visual hemifield. Because the neuronal modulation did not differ depending on relative target locations with respect to upper and lower visual hemifields, the distance between the targets does not explain the results. We propose that inherent, anatomical separation of visual processing for contralateral and ipsilateral visual fields might constrain cognitive capacity.

Different neuronal computations of spatial working memory for multiple locations within versus across visual hemifields

Spatial working memory is one of the most studied cognitive functions, serving as a model system to decipher computational principles of the brain. Although neuronal mechanisms for remembering a single location have been well elucidated, little is known about memory for multiple locations. Here, we examined the activities of prefrontal neurons during monkeys remembered positions of one or two visual cue(s). When the two cues were presented across the left and right visual fields, neurons exhibited a comparable response to the activity for the preferred cue presented alone. When the two cues were presented within the same hemifield, neurons exhibited an intermediate response between those to the individual cues. Subsequent computer simulations predicted a lower signal-to-noise ratio in the latter condition, which was further verified by behavioral experiments. Considering the separation of contralateral and ipsilateral visual processing, the lateral inhibition in local circuits might implicitly determine different neuronal computations and memory capacities for bilateral and unilateral displays.

Retrospective and prospective information coding by different neurons in the prefrontal cortex

Neurons in the lateral prefrontal cortex show sustained activity during the maintenance of visual memory. Previous studies have also indicated that prefrontal neurons show predictive activity in anticipation of upcoming visual stimuli. Because these retrospective and prospective coding of visual stimuli have been examined in separate experiments, how these processes interact in individual neurons remains unknown. To examine this, we recorded from single prefrontal neurons while monkeys performed two behavioural tasks. In one task, the animals passively viewed a moving object during fixation, whereas in the other, they remembered the location of a briefly presented visual cue for subsequent saccades. We found that many neurons were reactive and responded only after the visual stimulus appeared in their receptive field, while some neurons were predictive and increased their activity even before the moving stimulus entered the receptive field. In the memory-guided saccade trials, the reactive neurons exhibited sustained activity during the delay period, whereas the predictive neurons did not. Delays of visual response to a moving stimulus did not correlate with visual latency for a stationary stimulus. Instead, it correlated with the magnitude of sustained activity during the delay period in the memory-guided saccade task. Our data show that retrospective and prospective coding of visual information are represented by distinct neuronal populations, and that their temporal preferences are stable across different task conditions. Reactive signals may reflect the amount of temporal integration in short-term memory, whereas predictive signals may solely represent future events in isolation from the maintenance of past information.

Manipulation of object choice by electrical microstimulation in macaque frontal eye fields

For each saccade, we select an object to direct gaze and to specify the direction and amplitude of eye movement. Although these 2 processes are inevitably interdependent when visual stimuli are held stationary, several lines of evidence suggest that the neuronal signals in the frontal eye fields (FEF) that underlie the selection of visual objects are distinct from those underlying the selection of saccades. In the present study, we overtly dissociated these 2 processes spatially and temporally using the covert object-tracking paradigm, in which 4 identical objects moved randomly for 3 s before monkeys made a saccade to a previously selected target. To assess the causal role of the FEF in the 2 selection processes, we applied electrical microstimulation to the FEF at various times during the motion period. When stimulation was delivered at the motion onset, animals tended to choose an object that was initially presented at a particular location depending on the stimulation site. In contrast, the same stimulation delivered at the motion end failed to alter saccade end points. These results indicate that manipulation of FEF activity can change the selection of a visual object without affecting saccade goals, suggesting the existence of neurons solely regulating visual selection.