Neural correlates of correct and errant attentional selection revealed through N2pc and frontal eye field activity

Effects of different speed-accuracy instructions on perception in psychology experiments: evidence from event-related potential and oscillation

Introduction

In cognitive behavioral experiments, we often asked participants to make judgments within a deadline. However, the most common instruction of "do the task quickly and accurately" does not highlight the importance of the balance between being fast and accurate.

Methods

Our research aimed to explore how instructions about speed or accuracy affect perceptual process, focus on event-related potentials (ERPs) and event-related oscillations (EROs) of two brain responses for visual stimuli, known as P1 and N1. Additionally, we compared the conventional analysis approach with principal component analysis (PCA) based methods to analyze P1 and N1 ERP amplitude and ERO power.

Results

The results showed that individuals instructed to respond quickly had lower P1 amplitude and alpha ERO than those who prioritized accuracy, using the PCA-based approach. However, these two groups had no differences between groups in the N1 theta band using both methods. The traditional time-frequency analysis method could not detect any ERP or ERO distinctions between groups due to limitations in detecting specific components in time or frequency domains. That means PCA is effective in separating these components.

Discussion

Our findings indicate that the instructions given regarding speed and accuracy impact perceptual process of subjects during cognitive behavioral experiments. We suggest that future researchers should choose their instructions carefully, considering the purpose of study.

Auditory change detection and visual selective attention: association between MMN and N2pc

While the auditory and visual systems each provide distinct information to our brain, they also work together to process and prioritize input to address ever-changing conditions. Previous studies highlighted the trade-off between auditory change detection and visual selective attention; however, the relationship between them is still unclear. Here, we recorded electroencephalography signals from 106 healthy adults in three experiments. Our findings revealed a positive correlation at the population level between the amplitudes of event-related potential indices associated with auditory change detection (mismatch negativity) and visual selective attention (posterior contralateral N2) when elicited in separate tasks. This correlation persisted even when participants performed a visual task while disregarding simultaneous auditory stimuli. Interestingly, as visual attention demand increased, participants whose posterior contralateral N2 amplitude increased the most exhibited the largest reduction in mismatch negativity, suggesting a within-subject trade-off between the two processes. Taken together, our results suggest an intimate relationship and potential shared mechanism between auditory change detection and visual selective attention. We liken this to a total capacity limit that varies between individuals, which could drive correlated individual differences in auditory change detection and visual selective attention, and also within-subject competition between the two, with task-based modulation of visual attention causing within-participant decrease in auditory change detection sensitivity.

Neural mechanisms for executive control of speed-accuracy trade-off

The medial frontal cortex (MFC) plays an important but disputed role in speed-accuracy trade-off (SAT). In samples of neural spiking in the supplementary eye field (SEF) in the MFC simultaneous with the visuomotor frontal eye field and superior colliculus in macaques performing a visual search with instructed SAT, during accuracy emphasis, most SEF neurons discharge less from before stimulus presentation until response generation. Discharge rates adjust immediately and simultaneously across structures upon SAT cue changes. SEF neurons signal choice errors with stronger and earlier activity during accuracy emphasis. Other neurons signal timing errors, covarying with adjusting response time. Spike correlations between neurons in the SEF and visuomotor areas did not appear, disappear, or change sign across SAT conditions or trial outcomes. These results clarify findings with noninvasive measures, complement previous neurophysiological findings, and endorse the role of the MFC as a critic for the actor instantiated in visuomotor structures.

Feedforward attentional selection in sensory cortex

Salient objects grab attention because they stand out from their surroundings. Whether this phenomenon is accomplished by bottom-up sensory processing or requires top-down guidance is debated. We tested these alternative hypotheses by measuring how early and in which cortical layer(s) neural spiking distinguished a target from a distractor. We measured synaptic and spiking activity across cortical columns in mid-level area V4 of male macaque monkeys performing visual search for a color singleton. A neural signature of attentional capture was observed in the earliest response in the input layer 4. The magnitude of this response predicted response time and accuracy. Errant behavior followed errant selection. Because this response preceded top-down influences and arose in the cortical layer not targeted by top-down connections, these findings demonstrate that feedforward activation of sensory cortex can underlie attentional priority.

Rapid Input-Output Transformation between Local Field Potential and Spiking Activity during Sensation but not Action in the Superior Colliculus

Sensorimotor transformation is the sequential process of registering a sensory signal in the environment and then responding with the relevant movement at an appropriate time. For visually guided eye movements, neural signatures in the form of spiking activity of neurons have been extensively studied along the dorsoventral axis of the superior colliculus (SC). In contrast, the local field potential (LFP), which represents the putative input to a region, remains largely unexplored in the SC. We therefore compared amplitude levels and onset times of both spike bursts and LFP modulations recorded simultaneously with a laminar probe along the dorsoventral axis of SC in 3 male monkeys performing the visually guided delayed saccade task. Both signals displayed a gradual transition from sensory activity in the superficial layers to a predominantly motor response in the deeper layers, although the transition from principally sensory to mostly motor response occurred ∼500 μm deeper for the LFP. For the sensory response, LFP modulation preceded spike burst onset by <5 ms in the superficial and intermediate layers and only when data were analyzed on a trial-by-trial basis. The motor burst in the spiking activity led LFP modulation by >25 ms in the deeper layers. The results reveal a fast and efficient input-output transformation between LFP modulation and spike burst in the visually responsive layers activity during sensation but not during action. The spiking pattern observed during the movement phase is likely dominated by intracollicular processing that is not captured in the LFP.SIGNIFICANCE STATEMENT What is the sequence of events between local field potential (LFP) modulation and spiking activity during sensorimotor transformation? A trial-by-trial analysis reveals that the LFP activity leads the spike burst in the superficial and intermediate layers of the superior colliculus during visual processing, while both trial-by-trial and the average analyses show that the spike burst leads the LFP modulation during movement generation. These results suggest an almost instantaneous LFP input, spike burst output transformation in the visually responsive layers of the superior colliculus when registering the stimulus. In contrast, substantial intracollicular processing likely results in a saccade-related spike burst that leads LFP modulation.

Resolving the mesoscopic missing link: Biophysical modeling of EEG from cortical columns in primates

Event-related potentials (ERP) are among the most widely measured indices for studying human cognition. While their timing and magnitude provide valuable insights, their usefulness is limited by our understanding of their neural generators at the circuit level. Inverse source localization offers insights into such generators, but their solutions are not unique. To address this problem, scientists have assumed the source space generating such signals comprises a set of discrete equivalent current dipoles, representing the activity of small cortical regions. Based on this notion, theoretical studies have employed forward modeling of scalp potentials to understand how changes in circuit-level dynamics translate into macroscopic ERPs. However, experimental validation is lacking because it requires in vivo measurements of intracranial brain sources. Laminar local field potentials (LFP) offer a mechanism for estimating intracranial current sources. Yet, a theoretical link between LFPs and intracranial brain sources is missing. Here, we present a forward modeling approach for estimating mesoscopic intracranial brain sources from LFPs and predict their contribution to macroscopic ERPs. We evaluate the accuracy of this LFP-based representation of brain sources utilizing synthetic laminar neurophysiological measurements and then demonstrate the power of the approach in vivo to clarify the source of a representative cognitive ERP component. To that end, LFP was measured across the cortical layers of visual area V4 in macaque monkeys performing an attention demanding task. We show that area V4 generates dipoles through layer-specific transsynaptic currents that biophysically recapitulate the ERP component through the detailed forward modeling. The constraints imposed on EEG production by this method also revealed an important dissociation between computational and biophysical contributors. As such, this approach represents an important bridge between laminar microcircuitry, through the mesoscopic activity of cortical columns to the patterns of EEG we measure at the scalp.

Neural mechanism of priming in visual search

Selective attention affords scrutinizing items in our environment. However, attentional selection changes over time and across space. Empirically, repetition of visual search conditions changes attentional processing. Priming of pop-out is a vivid example. Repeatedly searching for the same pop-out search feature is accomplished with faster response times and fewer errors. We review the psychophysical background of priming of pop-out, focusing on the hypothesis that it arises through changes in visual selective attention. We also describe research done with macaque monkeys to understand the neural mechanisms supporting visual selective attention and priming of pop-out, and survey research on priming of pop-out using noninvasive brain measures with humans. We conclude by hypothesizing three alternative neural mechanisms and highlighting open questions.

Parallel fast and slow recurrent cortical processing mediates target and distractor selection in visual search

Visual search has been commonly used to study the neural correlates of attentional allocation in space. Recent electrophysiological research has disentangled distractor processing from target processing, showing that these mechanisms appear to operate in parallel and show electric fields of opposite polarity. Nevertheless, the localization and exact nature of this activity is unknown. Here, using MEG in humans, we provide a spatiotemporal characterization of target and distractor processing in visual cortex. We demonstrate that source activity underlying target- and distractor-processing propagates in parallel as fast and slow sweep from higher to lower hierarchical levels in visual cortex. Importantly, the fast propagating target-related source activity bypasses intermediate levels to go directly to V1, and this V1 activity correlates with behavioral performance. These findings suggest that reentrant processing is important for both selection and attenuation of stimuli, and such processing operates in parallel feedback loops.

Sarah E. Donohue et al. characterize the spatiotemporal propagation of target and distractor processing in the human visual cortex. They show that these signals propagate in parallel as fast and slow sweeps from higher to lower hierarchical levels, and that the fast target processing signal can bypass intermediate levels correlating with behavioral performance.

TRIM32 Deficiency Impairs Synaptic Plasticity by Excitatory-Inhibitory Imbalance via Notch Pathway

Synaptic plasticity is the neural basis of physiological processes involved in learning and memory. Tripartite motif-containing 32 (TRIM32) has been found to play many important roles in the brain such as neural stem cell proliferation, neurogenesis, inhibition of nerve proliferation, and apoptosis. TRIM32 has been linked to several nervous system diseases including autism spectrum disorder, depression, anxiety, and Alzheimer's disease. However, the role of TRIM32 in regulating the mechanism of synaptic plasticity is still unknown. Our electrophysiological studies using hippocampal slices revealed that long-term potentiation of CA1 synapses was impaired in TRIM32 deficient (KO) mice. Further research found that dendritic spines density, AMPA receptors, and synaptic plasticity-related proteins were also reduced. NMDA receptors were upregulated whereas GABA receptors were downregulated in TRIM32 deficient mice, explaining the imbalance in excitatory and inhibitory neurotransmission. This caused overexcitation leading to decreased neuronal numbers in the hippocampus and cortex. In summary, this study provides this maiden evidence on the synaptic plasticity changes of TRIM32 deficiency in the brain and proposes that TRIM32 relates the notch signaling pathway and its related mechanisms contribute to this deficit.

Priming of Attentional Selection in Macaque Visual Cortex: Feature-Based Facilitation and Location-Based Inhibition of Return

Visual search performance varies with stimulus and response history. Priming of pop-out refers to increased accuracy and reduced response time with repeated presentation of particular singleton and distractor features (e.g., a red target among green distractor stimuli), which are abruptly impaired when singleton and distractor features swap (e.g., green target among red distractors). Meanwhile, inhibition of return refers to the slowing of response time when target location repeats. Neurophysiological correlates of both these phenomena have been reported in the frontal eye field (FEF), an area in the frontal lobe contributing to attentional selection and eye movement planning. To understand the mechanistic origin of these adaptive behaviors, we investigated visual cortical area V4, an area providing input to and receiving feedback from FEF, during feature-based priming of pop-out and location-based inhibition of return. Performing a color pop-out task, monkeys exhibited pronounced priming of pop-out and inhibition of return. Neural spiking from V4 revealed earlier target selection associated with priming of pop-out and delayed selection associated with inhibition of return. These results demonstrate substantial involvement of extrastriate visual cortex in behavioral priming and inhibition of return.

Performance Monitoring during Visual Priming

Repetitive performance of single-feature (efficient or pop-out) visual search improves RTs and accuracy. This phenomenon, known as priming of pop-out, has been demonstrated in both humans and macaque monkeys. We investigated the relationship between performance monitoring and priming of pop-out. Neuronal activity in the supplementary eye field (SEF) contributes to performance monitoring and to the generation of performance monitoring signals in the EEG. To determine whether priming depends on performance monitoring, we investigated spiking activity in SEF as well as the concurrent EEG of two monkeys performing a priming of pop-out task. We found that SEF spiking did not modulate with priming. Surprisingly, concurrent EEG did covary with priming. Together, these results suggest that performance monitoring contributes to priming of pop-out. However, this performance monitoring seems not mediated by SEF. This dissociation suggests that EEG indices of performance monitoring arise from multiple, functionally distinct neural generators.

Visual Search in Behavioral Variant Frontotemporal Dementia

BACKGROUND

Changes to visual search have shown specific patterns in a number of dementia subtypes. The cortical regions involved in the control of visual search overlap with the regions affected in behavioral variant frontotemporal dementia (bvFTD). Previous literature has examined visual search in bvFTD with smaller array sizes.

OBJECTIVE

To examine the pattern of behavior shown by bvFTD patients while undertaking visual search in the presence of larger numbers of distractors to model increased cognitive load.

METHODS

15 bvFTD and 17 control participants undertook three visual search tasks: color, orientation, and conjunction searches. A wide range of array sizes was used, from 16 to 100 items arranged as a square. Behavior was quantified using accuracy, response time, and eye movements.

RESULTS

BvFTD participants displayed a reduction in accuracy and an increased response time across all task types. BvFTD participants displayed an increase in number of objects examined and number of fixations made for color and conjunction tasks. Fixation duration was increased for orientation and conjunction (the more difficult tasks) but not color search. Results indicated the increase in time to response to be due to an increased intercept, with no significant difference in slope for the different tasks.

CONCLUSION

BvFTD participants display a pattern of visual search behavior consisting of a decrease in accuracy, an increase in response time, and a corresponding increase in the number and length of eye movements made during visual search. The pattern seen corresponds to studies of frontal lobe damage, while differing in pattern from that seen in a range of other cognitive conditions.

Selective Influence and Sequential Operations: A Research Strategy for Visual Search

We discuss the problem of elucidating mechanisms of visual search. We begin by considering the history, logic, and methods of relating behavioral or cognitive processes with neural processes. We then survey briefly the cognitive neurophysiology of visual search and essential aspects of the neural circuitry supporting this capacity. We introduce conceptually and empirically a powerful but underutilized experimental approach to dissect the cognitive processes supporting performance of a visual search task with factorial manipulations of singleton-distractor identifiability and stimulus-response cue discriminability. We show that systems factorial technology can distinguish processing architectures from the performance of macaque monkeys. This demonstration offers new opportunities to distinguish neural mechanisms through selective manipulation of visual encoding, search selection, rule encoding, and stimulus-response mapping.

Neurally constrained modeling of speed-accuracy tradeoff during visual search: gated accumulation of modulated evidence

Stochastic accumulator models account for response times and errors in perceptual decision making by assuming a noisy accumulation of perceptual evidence to a threshold. Previously, we explained saccade visual search decision making by macaque monkeys with a stochastic multiaccumulator model in which accumulation was driven by a gated feed-forward integration to threshold of spike trains from visually responsive neurons in frontal eye field that signal stimulus salience. This neurally constrained model quantitatively accounted for response times and errors in visual search for a target among varying numbers of distractors and replicated the dynamics of presaccadic movement neurons hypothesized to instantiate evidence accumulation. This modeling framework suggested strategic control over gate or over threshold as two potential mechanisms to accomplish speed-accuracy tradeoff (SAT). Here, we show that our gated accumulator model framework can account for visual search performance under SAT instructions observed in a milestone neurophysiological study of frontal eye field. This framework captured key elements of saccade search performance, through observed modulations of neural input, as well as flexible combinations of gate and threshold parameters necessary to explain differences in SAT strategy across monkeys. However, the trajectories of the model accumulators deviated from the dynamics of most presaccadic movement neurons. These findings demonstrate that traditional theoretical accounts of SAT are incomplete descriptions of the underlying neural adjustments that accomplish SAT, offer a novel mechanistic account of decision-making mechanisms during speed-accuracy tradeoff, and highlight questions regarding the identity of model and neural accumulators. NEW & NOTEWORTHY A gated accumulator model is used to elucidate neurocomputational mechanisms of speed-accuracy tradeoff. Whereas canonical stochastic accumulators adjust strategy only through variation of an accumulation threshold, we demonstrate that strategic adjustments are accomplished by flexible combinations of both modulation of the evidence representation and adaptation of accumulator gate and threshold. The results indicate how model-based cognitive neuroscience can translate between abstract cognitive models of performance and neural mechanisms of speed-accuracy tradeoff.

Neural oscillations in the primate caudate nucleus correlate with different preparatory states for temporal production

When measuring time, neuronal activity in the cortico-basal ganglia pathways has been shown to be temporally scaled according to the interval, suggesting that signal transmission within the pathways is flexibly controlled. Here we show that, in the caudate nuclei of monkeys performing a time production task with three different intervals, the magnitude of visually-evoked potentials at the beginning of an interval differed depending on the conditions. Prior to this response, the power of low frequency components (6–20 Hz) significantly changed, showing inverse correlation with the visual response gain. Although these components later exhibited time-dependent modification during self-timed period, the changes in spectral power for interval conditions qualitatively and quantitatively differed from those associated with the reward amount. These results suggest that alteration of network state in the cortico-basal ganglia pathways indexed by the low frequency oscillations may be crucial for the regulation of signal transmission and subsequent timing behavior.

Tomoki Suzuki and Masaki Tanaka measured local field potentials in the caudate nucleus of monkeys performing a time production task and showed that the length of the time interval modified the magnitude of visually-evoked potentials and the spectral power at low frequencies. These changes suggest that neural oscillations within the cortico-basal ganglia pathways regulate timing behavior.

Neural mechanisms of speed-accuracy tradeoff of visual search: saccade vigor, the origin of targeting errors, and comparison of the superior colliculus and frontal eye field

Balancing the speed-accuracy tradeoff (SAT) is necessary for successful behavior. Using a visual search task with interleaved cues emphasizing speed or accuracy, we recently reported diverse contributions of frontal eye field (FEF) neurons instantiating salience evidence and response preparation. Here, we report replication of visual search SAT performance in two macaque monkeys, new information about variation of saccade dynamics with SAT, extension of the neurophysiological investigation to describe processes in the superior colliculus (SC), and a description of the origin of search errors in this task. Saccade vigor varied idiosyncratically across SAT conditions and monkeys but tended to decrease with response time. As observed in the FEF, speed-accuracy tradeoff was accomplished through several distinct adjustments in the superior colliculus. In "Accurate" relative to "Fast" trials, visually responsive neurons in SC as in FEF had lower baseline firing rates and later target selection. The magnitude of these adjustments in SC was indistinguishable from that in FEF. Search errors occurred when visual salience neurons in the FEF and the SC treated distractors as targets, even in the Accurate condition. Unlike FEF, the magnitude of visual responses in the SC did not vary across SAT conditions. Also unlike FEF, the activity of SC movement neurons when saccades were initiated was equivalent in Fast and Accurate trials. Saccade-related neural activity in SC, but not FEF, varied with saccade peak velocity. These results extend our understanding of the cortical and subcortical contributions to SAT. NEW & NOTEWORTHY Neurophysiological mechanisms of speed-accuracy tradeoff (SAT) have only recently been investigated. This article reports the first replication of SAT performance in nonhuman primates, the first report of variation of saccade dynamics with SAT, the first description of superior colliculus contributions to SAT, and the first description of the origin of errors during SAT. These results inform and constrain new models of distributed decision making.

Concurrent deployment of visual attention and response selection bottleneck in a dual-task: Electrophysiological and behavioural evidence

Visual attention and response selection are limited in capacity. Here, we investigated whether visual attention requires the same bottleneck mechanism as response selection in a dual-task of the psychological refractory period (PRP) paradigm. The dual-task consisted of an auditory two-choice discrimination Task 1 and a conjunction search Task 2, which were presented at variable temporal intervals (stimulus onset asynchrony, SOA). In conjunction search, visual attention is required to select items and to bind their features resulting in a serial search process around the items in the search display (i.e., set size). We measured the reaction time of the visual search task (RT2) and the N2pc, an event-related potential (ERP), which reflects lateralized visual attention processes. If the response selection processes in Task 1 influence the visual attention processes in Task 2, N2pc latency and amplitude would be delayed and attenuated at short SOA compared to long SOA. The results, however, showed that latency and amplitude were independent of SOA, indicating that visual attention was concurrently deployed to response selection. Moreover, the RT2 analysis revealed an underadditive interaction of SOA and set size. We concluded that visual attention does not require the same bottleneck mechanism as response selection in dual-tasks.

Might cortical hyper-responsiveness in aging contribute to Alzheimer's disease?

Our goal is to understand the neural basis of functional impairment in aging and Alzheimer's disease (AD) to be able to characterize clinically significant decline and assess therapeutic efficacy. We used frequency-tagged ERPs to word and motion stimuli to study the effects of stimulus conditions and selective attention. ERPs to word or motion increase when a task-irrelevant 2nd stimulus is added, but decrease when the task is moved to that 2nd stimulus. Spectral analyses show task effects on response power without 2nd stimulus effects. However, phase coherence shows both 2nd stimulus and task effects. Thus, power and coherence are dissociably modulated by stimulus and task effects. Task-dependent phase coherence successively declines in aging and AD. In contrast, task-dependent spectral power increases in aging, only to decrease in AD. We hypothesize that age-related declines in signal coherence, associated with increased power generation, stresses neurons and contributes to the loss of response power and the development of functional impairment in AD.

Influence of monkey dorsolateral prefrontal and posterior parietal activity on behavioral choice during attention tasks

The dorsolateral prefrontal and the posterior parietal cortex have both been implicated in the guidance of visual attention. Traditionally, posterior parietal cortex has been thought to guide visual bottom-up attention and prefrontal cortex to bias attention through top-down information. More recent studies suggest a parallel time course of activation of the two areas in bottom-up attention tasks, suggesting a common involvement, though these results do not necessarily imply identical roles. To address the specific roles of the two areas, we examined the influence of neuronal activity recorded from the prefrontal and parietal cortex of monkeys as they performed attention tasks based on choice probability and on correlation between reaction time and neuronal activity. The results revealed that posterior parietal but not dorsolateral prefrontal activity correlated with behavioral choice during the fixation period, prior to the appearance of the stimulus, resembling a bias factor. This preferential influence of posterior parietal activity on behavior was transient, so that dorsolateral prefrontal activity predicted choice after the appearance of the stimulus. Additionally, reaction time was better predicted by posterior parietal activity. These findings confirm the involvement of both dorsolateral prefrontal and posterior parietal cortex in the bottom-up guidance of visual attention, but indicate different roles of the two areas in the guidance of attention and a dynamic time course of their effects, influencing behavior at different stages of the task.

The speed-accuracy tradeoff: history, physiology, methodology, and behavior

There are few behavioral effects as ubiquitous as the speed-accuracy tradeoff (SAT). From insects to rodents to primates, the tendency for decision speed to covary with decision accuracy seems an inescapable property of choice behavior. Recently, the SAT has received renewed interest, as neuroscience approaches begin to uncover its neural underpinnings and computational models are compelled to incorporate it as a necessary benchmark. The present work provides a comprehensive overview of SAT. First, I trace its history as a tractable behavioral phenomenon and the role it has played in shaping mathematical descriptions of the decision process. Second, I present a "users guide" of SAT methodology, including a critical review of common experimental manipulations and analysis techniques and a treatment of the typical behavioral patterns that emerge when SAT is manipulated directly. Finally, I review applications of this methodology in several domains.

Primary motor cortex of the parkinsonian monkey: altered neuronal responses to muscle stretch

Exaggeration of the long-latency stretch reflex (LLSR) is a characteristic neurophysiologic feature of Parkinson's disease (PD) that contributes to parkinsonian rigidity. To explore one frequently-hypothesized mechanism, we studied the effects of fast muscle stretches on neuronal activity in the macaque primary motor cortex (M1) before and after the induction of parkinsonism by unilateral administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). We compared results from the general population of M1 neurons and two antidromically-identified subpopulations: distant-projecting pyramidal-tract type neurons (PTNs) and intra-telecenphalic-type corticostriatal neurons (CSNs). Rapid rotations of elbow or wrist joints evoked short-latency responses in 62% of arm-related M1 neurons. As in PD, the late electromyographic responses that constitute the LLSR were enhanced following MPTP. This was accompanied by a shortening of M1 neuronal response latencies and a degradation of directional selectivity, but surprisingly, no increase in single unit response magnitudes. The results suggest that parkinsonism alters the timing and specificity of M1 responses to muscle stretch. Observation of an exaggerated LLSR with no change in the magnitude of proprioceptive responses in M1 is consistent with the idea that the increase in LLSR gain that contributes to parkinsonian rigidity is localized to the spinal cord.

Production, control, and visual guidance of saccadic eye movements

Primate vision is served by rapid shifts of gaze called saccades. This review will survey current knowledge and particular problems concerning the neural control and guidance of gaze shifts.

Neural chronometry and coherency across speed-accuracy demands reveal lack of homomorphism between computational and neural mechanisms of evidence accumulation

The stochastic accumulation framework provides a mechanistic, quantitative account of perceptual decision-making and how task performance changes with experimental manipulations. Importantly, it provides an elegant account of the speed-accuracy trade-off (SAT), which has long been the litmus test for decision models, and also mimics the activity of single neurons in several key respects. Recently, we developed a paradigm whereby macaque monkeys trade speed for accuracy on cue during visual search task. Single-unit activity in frontal eye field (FEF) was not homomorphic with the architecture of models, demonstrating that stochastic accumulators are an incomplete description of neural activity under SAT. This paper summarizes and extends this work, further demonstrating that the SAT leads to extensive, widespread changes in brain activity never before predicted. We will begin by reviewing our recently published work that establishes how spiking activity in FEF accomplishes SAT. Next, we provide two important extensions of this work. First, we report a new chronometric analysis suggesting that increases in perceptual gain with speed stress are evident in FEF synaptic input, implicating afferent sensory-processing sources. Second, we report a new analysis demonstrating selective influence of SAT on frequency coupling between FEF neurons and local field potentials. None of these observations correspond to the mechanics of current accumulator models.

Separate evaluation of target facilitation and distractor suppression in the activity of macaque lateral intraparietal neurons during visual search

During visual search, neurons in the lateral intraparietal area (LIP) discriminate the target from distractors by exhibiting stronger activation when the target appears within the receptive field than when it appears outside the receptive field. It is generally thought that such target-discriminative activity is produced by the combination of target-related facilitation and distractor-related suppression. However, little is known about how the target-discriminative activity is constituted by these two types of neural modulation. To address this issue, we recorded activity from LIP of monkeys performing a visual search task that consisted of target-present and target-absent trials. Monkeys had to make a saccade to a target in the target-present trials, whereas they had to maintain fixation in the target-absent trials, in which only distractors were presented. By introducing the activity from the latter trials as neutral activity, we were able to separate the target-discriminative activity into target-related elevation and distractor-related reduction components. We found that the target-discriminative activity of most LIP neurons consisted of the combination of target-related elevation and distractor-related reduction or only target-related elevation. In contrast, target-discriminative activity composed of only distractor-related reduction was observed for very few neurons. We also found that, on average, target-related elevation was stronger and occurred earlier compared with distractor-related reduction. Finally, we consider possible underlying mechanisms, including lateral inhibitory interactions, responsible for target-discriminative activity in visual search. The present findings provide insight into how neuronal modulations shape target-discriminative activity during visual search.

Macrocircuits: decision networks

Decision-making requires stimulus categorization and localization to guide accurate responses that can be produced through multiple effectors. The success of actions is monitored so that performance can be adjusted to achieve goals. This review will survey recent empirical and theoretical developments very selectively with an emphasis on neurophysiological data from nonhuman primates that provide the clearest information about neural mechanisms.

Neural mechanisms of speed-accuracy tradeoff

Intelligent agents balance speed of responding with accuracy of deciding. Stochastic accumulator models commonly explain this speed-accuracy tradeoff by strategic adjustment of response threshold. Several laboratories identify specific neurons in prefrontal and parietal cortex with this accumulation process, yet no neurophysiological correlates of speed-accuracy tradeoff have been described. We trained macaque monkeys to trade speed for accuracy on cue during visual search and recorded the activity of neurons in the frontal eye field. Unpredicted by any model, we discovered that speed-accuracy tradeoff is accomplished through several distinct adjustments. Visually responsive neurons modulated baseline firing rate, sensory gain, and the duration of perceptual processing. Movement neurons triggered responses with activity modulated in a direction opposite of model predictions. Thus, current stochastic accumulator models provide an incomplete description of the neural processes accomplishing speed-accuracy tradeoffs. The diversity of neural mechanisms was reconciled with the accumulator framework through an integrated accumulator model constrained by requirements of the motor system.

On the origin of event-related potentials indexing covert attentional selection during visual search: timing of selection by macaque frontal eye field and event-related potentials during pop-out search

Event-related potentials (ERPs) have provided crucial data concerning the time course of psychological processes, but the neural mechanisms producing ERP components remain poorly understood. This study continues a program of research in which we investigated the neural basis of attention-related ERP components by simultaneously recording intracranially and extracranially from macaque monkeys. Here, we compare the timing of attentional selection by the macaque homologue of the human N2pc component (m-N2pc) with the timing of selection in the frontal eye field (FEF), an attentional-control structure believed to influence posterior visual areas thought to generate the N2pc. We recorded FEF single-unit spiking and local field potentials (LFPs) simultaneously with the m-N2pc in monkeys performing an efficient pop-out search task. We assessed how the timing of attentional selection depends on task demands by direct comparison with a previous study of inefficient search in the same monkeys (e.g., finding a T among Ls). Target selection by FEF spikes, LFPs, and the m-N2pc was earlier during efficient pop-out search rather than during inefficient search. The timing and magnitude of selection in all three signals varied with set size during inefficient but not efficient search. During pop-out search, attentional selection was evident in FEF spiking and LFP before the m-N2pc, following the same sequence observed during inefficient search. These observations are consistent with the hypothesis that feedback from FEF modulates neural activity in posterior regions that appear to generate the m-N2pc even when competition for attention among items in a visual scene is minimal.

Neural dynamics and circuit mechanisms of decision-making

In this review, I briefly summarize current neurobiological studies of decision-making that bear on two general themes. The first focuses on the nature of neural representation and dynamics in a decision circuit. Experimental and computational results suggest that ramping-to-threshold in the temporal domain and trajectory of population activity in the state space represent a duality of perspectives on a decision process. Moreover, a decision circuit can display several different dynamical regimes, such as the ramping mode and the jumping mode with distinct defining properties. The second is concerned with the relationship between biologically-based mechanistic models and normative-type models. A fruitful interplay between experiments and these models at different levels of abstraction have enabled investigators to pose increasingly refined questions and gain new insights into the neural basis of decision-making. In particular, recent work on multi-alternative decisions suggests that deviations from rational models of choice behavior can be explained by established neural mechanisms.

From salience to saccades: multiple-alternative gated stochastic accumulator model of visual search

We describe a stochastic accumulator model demonstrating that visual search performance can be understood as a gated feedforward cascade from a salience map to multiple competing accumulators. The model quantitatively accounts for behavior and predicts neural dynamics of macaque monkeys performing visual search for a target stimulus among different numbers of distractors. The salience accumulated in the model is equated with the spike trains recorded from visually responsive neurons in the frontal eye field. Accumulated variability in the firing rates of these neurons explains choice probabilities and the distributions of correct and error response times with search arrays of different set sizes if the accumulators are mutually inhibitory. The dynamics of the stochastic accumulators quantitatively predict the activity of presaccadic movement neurons that initiate eye movements if gating inhibition prevents accumulation before the representation of stimulus salience emerges. Adjustments in the level of gating inhibition can control trade-offs in speed and accuracy that optimize visual search performance.

How local is the local field potential?

Local field potentials (LFPs) are of growing importance in neurophysiological investigations. LFPs supplement action potential recordings by indexing activity relevant to EEG, magnetoencephalographic, and hemodynamic (fMRI) signals. Recent reports suggest that LFPs reflect activity within very small domains of several hundred micrometers. We examined this conclusion by comparing LFP, current source density (CSD), and multiunit activity (MUA) signals in macaque auditory cortex. Estimated by frequency tuning bandwidths, these signals' "listening areas" differ systematically with an order of MUA < CSD < LFP. Computational analyses confirm that observed LFPs receive local contributions. Direct measurements indicate passive spread of LFPs to sites more than a centimeter from their origins. These findings appear to be independent of the frequency content of the LFP. Our results challenge the idea that LFP recordings typically integrate over extremely circumscribed local domains. Rather, LFPs appear as a mixture of local potentials with "volume conducted" potentials from distant sites.

Neural mechanisms of saccade target selection: gated accumulator model of the visual-motor cascade

We review a new computational model developed to understand how evidence about stimulus salience in visual search is translated into a saccade command. The model uses the activity of visually responsive neurons in the frontal eye field as evidence for stimulus salience that is accumulated in a network of stochastic accumulators to produce accurate and timely saccades. We discovered that only when the input to the accumulation process was gated could the model account for the variability in search performance and predict the dynamics of movement neuron discharge rates. This union of cognitive modeling and neurophysiology indicates how the visual-motor transformation can occur, and provides a concrete mapping between neuron function and specific cognitive processes.