Tuning Curve Shift by Attention Modulation in Cortical Neurons: a Computational Study of its Mechanisms

Spatial Tuning in Nociceptive Processing Is Driven by Attention

When the source of nociception expands across a body area, the experience of pain increases due to the spatial integration of nociceptive information. This well-established effect is called spatial summation of pain (SSp) and has been the subject of multiple investigations. Here, we used cold-induced SSp to investigate the effect of attention on the spatial tuning of nociceptive processing. Forty pain-free volunteers (N = 40, 20 females) participated in this experiment. They took part in an SSp paradigm based on three hand immersions into cold water (5°C): Participants either immersed the radial segment ("a"), ulnar segment ("b") or both hand segments ("a+b") and provided overall pain ratings. In some trials based on "a+b" immersions, they were also asked to provide divided (ie, first pain in "a" then in "b"; or reversed) and directed attention ratings (ie, pain only in "a" or "b"). Results confirmed a clear SSp effect in which reported pain during immersions of "a" or "b" was less intense than pain during immersions of "a+b" (P < .001). Data also confirmed that spatial tuning was altered. SSp was abolished when participants provided two ratings in a divided fashion (P < .001). Furthermore, pain was significantly lower when attention was directed only to one segment ("a" OR "b") during "a+b" immersion (P < .001). We conclude that spatial tuning is dynamically driven by attention as reflected in abolished SSp. Directed attention was sufficient to focus spatial tuning and abolish SSp. Results support the role of cognitive processes such as attention in spatial tuning. PERSPECTIVE: This article presents experimental investigation of spatial tuning in pain and offers mechanistic insights of contiguous spatial summation of pain in healthy volunteers. Depending on how pain is evaluated in terms of attentional derivative (overall pain, directed, divided attention) the pain is reduced and spatial summation abolished.

Feature-based attention warps the perception of visual features

Selective attention improves sensory processing of relevant information but can also impact the quality of perception. For example, attention increases visual discrimination performance and at the same time boosts apparent stimulus contrast of attended relative to unattended stimuli. Can attention also lead to perceptual distortions of visual representations? Optimal tuning accounts of attention suggest that processing is biased towards "off-tuned" features to maximize the signal-to-noise ratio in favor of the target, especially when targets and distractors are confusable. Here, we tested whether such tuning gives rise to phenomenological changes of visual features. We instructed participants to select a color among other colors in a visual search display and subsequently asked them to judge the appearance of the target color in a 2-alternative forced choice task. Participants consistently judged the target color to appear more dissimilar from the distractor color in feature space. Critically, the magnitude of these perceptual biases varied systematically with the similarity between target and distractor colors during search, indicating that attentional tuning quickly adapts to current task demands. In control experiments we rule out possible non-attentional explanations such as color contrast or memory effects. Overall, our results demonstrate that selective attention warps the representational geometry of color space, resulting in profound perceptual changes across large swaths of feature space. Broadly, these results indicate that efficient attentional selection can come at a perceptual cost by distorting our sensory experience.

Feature-based attention processes in primate prefrontal cortex do not rely on feature similarity

Feature-based attention enables privileged processing of specific visual properties. During feature-based attention, neurons in visual cortices show "gain modulation" by enhancing neuronal responses to the features of attended stimuli due to top-down signals originating from prefrontal cortex (PFC). Attentional modulation in visual cortices requires "feature similarity:" neurons only increase their responses when the attended feature variable and the neurons' preferred feature coincide. However, whether gain modulation based on feature similarity is a general attentional mechanism is currently unknown. To address this issue, we record single-unit activity from PFC of macaques trained to switch attention between two conjunctive feature parameters. We find that PFC neurons experience gain modulation in response to attentional demands. However, this attentional gain modulation in PFC is independent of the feature-tuning preferences of neurons. These findings suggest that feature similarity is not a general mechanism in feature-based attention throughout the cortical processing hierarchy.

The Distributed Nociceptive System: A Framework for Understanding Pain

Chronic pain remains challenging to both diagnose and treat. These challenges, in part, arise from limited systems-level understanding of the basic mechanisms that process nociceptive information and ultimately instantiate a subjectively available experience of pain. Here, I provide a framework, the distributed nociceptive system, for understanding nociceptive mechanisms at a systems level by integrating the concepts of neural population coding with distributed processing. Within this framework, wide-spread engagement of populations of neurons produces representations of nociceptive information that are highly resilient to disruption. The distributed nociceptive system provides a foundation for understanding complex spatial aspects of chronic pain and provides an impetus for nonpharmacological cognitive and physical therapies that can effectively target the highly distributed system that gives rise to an experience of pain.

Strategic deployment of feature-based attentional gain in primate visual cortex

Attending to visual stimuli enhances the gain of those neurons in primate visual cortex that preferentially respond to the matching locations and features (on-target gain). Although this is well suited to enhance the neuronal representation of attended stimuli, it is nonoptimal under difficult discrimination conditions, as in the presence of similar distractors. In such cases, directing attention to neighboring neuronal populations (off-target gain) has been shown to be the most efficient strategy, but although such a strategic deployment of attention has been shown behaviorally, its underlying neural mechanisms are unknown. Here, we investigated how attention affects the population responses of neurons in the middle temporal (MT) visual area of rhesus monkeys to bidirectional movement inside the neurons' receptive field (RF). The monkeys were trained to focus their attention onto the fixation spot or to detect a direction or speed change in one of the motion directions (the "target"), ignoring the distractor motion. Population activity profiles were determined by systematically varying the patterns' directions while maintaining a constant angle between them. As expected, the response profiles show a peak for each of the 2 motion directions. Switching spatial attention from the fixation spot into the RF enhanced the peak representing the attended stimulus and suppressed the distractor representation. Importantly, the population data show a direction-dependent attentional modulation that does not peak at the target feature but rather along the slopes of the activity profile representing the target direction. Our results show that attentional gains are strategically deployed to optimize the discriminability of target stimuli, in line with an optimal gain mechanism proposed by Navalpakkam and Itti.

Transforming the Choice Outcome to an Action Plan in Monkey Lateral Prefrontal Cortex: A Neural Circuit Model

In economic decisions, we make a good-based choice first, then we transform the outcome into an action to obtain the good. To elucidate the network mechanisms for such transformation, we constructed a neural circuit model consisting of modules representing choice, integration of choice with target locations, and the final action plan. We examined three scenarios regarding how the final action plan could emerge in the neural circuit and compared their implications with experimental data. Our model with heterogeneous connectivity predicts the coexistence of three types of neurons with distinct functions, confirmed by analyzing the neural activity in the lateral prefrontal cortex (LPFC) of behaving monkeys. We obtained a much more distinct classification of functional neuron types in the ventral than the dorsal region of LPFC, suggesting that the action plan is initially generated in ventral LPFC. Our model offers a biologically plausible neural circuit architecture that implements good-to-action transformation during economic choice.

Spatial receptive field shift by preceding cross-modal stimulation in the cat superior colliculus

KEY POINTS

It has been known for some time that sensory information of one type can bias the spatial perception of another modality. However, there is a lack of evidence of this occurring in individual neurons. In the present study, we found that the spatial receptive field of superior colliculus multisensory neurons could be dynamically shifted by a preceding stimulus in a different modality. The extent to which the receptive field shifted was dependent on both temporal and spatial gaps between the preceding and following stimuli, as well as the salience of the preceding stimulus. This result provides a neural mechanism that could underlie the process of cross-modal spatial calibration.

ABSTRACT

Psychophysical studies have shown that the different senses can be spatially entrained by each other. This can be observed in certain phenomena, such as ventriloquism, in which a visual stimulus can attract the perceived location of a spatially discordant sound. However, the neural mechanism underlying this cross-modal spatial recalibration has remained unclear, as has whether it takes place dynamically. We explored these issues in multisensory neurons of the cat superior colliculus (SC), a midbrain structure that involves both cross-modal and sensorimotor integration. Sequential cross-modal stimulation showed that the preceding stimulus can shift the receptive field (RF) of the lagging response. This cross-modal spatial calibration took place in both auditory and visual RFs, although auditory RFs shifted slightly more. By contrast, if a preceding stimulus was from the same modality, it failed to induce a similarly substantial RF shift. The extent of the RF shift was dependent on both temporal and spatial gaps between the preceding and following stimuli, as well as the salience of the preceding stimulus. A narrow time gap and high stimulus salience were able to induce larger RF shifts. In addition, when both visual and auditory stimuli were presented simultaneously, a substantial RF shift toward the location-fixed stimulus was also induced. These results, taken together, reveal an online cross-modal process and reflect the details of the organization of SC inter-sensory spatial calibration.

Deactivation of Association Cortices Disrupted the Congruence of Visual and Auditory Receptive Fields in Superior Colliculus Neurons

Physiological and behavioral studies in cats show that corticotectal inputs play a critical role in the information-processing capabilities of neurons in the deeper layers of the superior colliculus (SC). Among them, the sensory inputs from functionally related associational cortices are especially critical for SC multisensory integration. However, the underlying mechanism supporting this influence is still unclear. Here, results demonstrate that deactivation of relevant cortices can both dislocate SC visual and auditory spatial receptive fields (RFs) and decrease their overall size, resulting in reduced alignment. Further analysis demonstrated that this RF separation is significantly correlated with the decrement of neurons' multisensory enhancement and is most pronounced in low stimulus intensity conditions. In addition, cortical deactivation could influence the degree of stimulus effectiveness, thereby illustrating the means by which higher order cortices may modify the multisensory activity of SC.

Spatial working memory alters the efficacy of input to visual cortex

Prefrontal cortex modulates sensory signals in extrastriate visual cortex, in part via its direct projections from the frontal eye field (FEF), an area involved in selective attention. We find that working memory-related activity is a dominant signal within FEF input to visual cortex. Although this signal alone does not evoke spiking responses in areas V4 and MT during memory, the gain of visual responses in these areas increases, and neuronal receptive fields expand and shift towards the remembered location, improving the stimulus representation by neuronal populations. These results provide a basis for enhancing the representation of working memory targets and implicate persistent FEF activity as a basis for the interdependence of working memory and selective attention.

Frontal eye field (FEF) is a visual prefrontal area involved in top-down attention. Here the authors report that FEF neurons projecting to V4/MT are persistently active during spatial working memory, and V4/MT neurons show changes in receptive field and gain at the location held in working memory.

Interaction between Spatial and Feature Attention in Posterior Parietal Cortex

Lateral intraparietal (LIP) neurons encode a vast array of sensory and cognitive variables. Recently, we proposed that the flexibility of feature representations in LIP reflect the bottom-up integration of sensory signals, modulated by feature-based attention (FBA), from upstream feature-selective cortical neurons. Moreover, LIP activity is also strongly modulated by the position of space-based attention (SBA). However, the mechanisms by which SBA and FBA interact to facilitate the representation of task-relevant spatial and non-spatial features in LIP remain unclear. We recorded from LIP neurons during performance of a task that required monkeys to detect specific conjunctions of color, motion direction, and stimulus position. Here we show that FBA and SBA potentiate each other's effect in a manner consistent with attention gating the flow of visual information along the cortical visual pathway. Our results suggest that linear bottom-up integrative mechanisms allow LIP neurons to emphasize task-relevant spatial and non-spatial features.

Feature-Based Attention by Lateral Spike Synchronization

We introduce a neural model capable of feature selectiveness by spike-mediated synchronization through lateral synaptic couplings. For a stimulus containing two features, the attended one elicits a higher response. In the case of sequential single-feature stimuli, repetition of the attended feature also results in an enhanced response, exhibited by greater synchrony and higher spiking rates.

A Feedback Model of Attention Explains the Diverse Effects of Attention on Neural Firing Rates and Receptive Field Structure

Visual attention has many effects on neural responses, producing complex changes in firing rates, as well as modifying the structure and size of receptive fields, both in topological and feature space. Several existing models of attention suggest that these effects arise from selective modulation of neural inputs. However, anatomical and physiological observations suggest that attentional modulation targets higher levels of the visual system (such as V4 or MT) rather than input areas (such as V1). Here we propose a simple mechanism that explains how a top-down attentional modulation, falling on higher visual areas, can produce the observed effects of attention on neural responses. Our model requires only the existence of modulatory feedback connections between areas, and short-range lateral inhibition within each area. Feedback connections redistribute the top-down modulation to lower areas, which in turn alters the inputs of other higher-area cells, including those that did not receive the initial modulation. This produces firing rate modulations and receptive field shifts. Simultaneously, short-range lateral inhibition between neighboring cells produce competitive effects that are automatically scaled to receptive field size in any given area. Our model reproduces the observed attentional effects on response rates (response gain, input gain, biased competition automatically scaled to receptive field size) and receptive field structure (shifts and resizing of receptive fields both spatially and in complex feature space), without modifying model parameters. Our model also makes the novel prediction that attentional effects on response curves should shift from response gain to contrast gain as the spatial focus of attention drifts away from the studied cell.

Exerting visual attention results in profound changes in the activity of neurons in visual areas of the brain. Attention increases the firing of some neurons, decreases that of others, moves and resizes the receptive fields of individual neurons, and changes their preferred features according to what is being attended. How are these complex, subtle effects generated? While several models explain various subsets of these effects, a consistent explanation compatible with anatomical and physiological observations remains elusive. Here we show that the apparently complex and multifaceted effects of attention on neural responses can be explained as the automatic consequence of a top-down modulation, falling on higher visual areas (as suggested by anatomical observations), and interacting with short-range inhibition and feedback connections between areas. Our model only assumes the existence of well-known features of brain organization (reciprocal inter-area connections, mutual inhibition between neighboring neurons) to explain a wide range of attentional effects, including apparently finely-tuned effects (complex shifts in feature preferences, automatic scaling of competitive effects to receptive field size, resizing or shifting of receptive fields, etc). Our model also makes novel, testable predictions about the effect of certain attentional manipulations on neural responses.

Modulation of pain via expectation of its location

BACKGROUND

The spatial precision of expectancy effects on pain is unclear. We hypothesized that expecting nociceptive stimuli at particular skin sites would have an analgesic effect on nociceptive stimuli presented between them (middle zone).

METHODS

Laser stimuli (evoking pin-prick pain) were delivered to three discrete skin zones on the forearm, under two conditions. During 'Localization', participants' expectation of stimuli was spatially divided between two locations (expected stimuli in only the outer two skin zones): pain intensity and stimulus location were judged. During 'No-localization' (control condition), participants had no expectation concerning stimulus location; only pain intensity was rated. Additional experiments assessed the importance of the actual location on the forearm by: shifting all skin zones proximally towards the elbow (control for joint proximity, Experiment 2); adding a fourth zone distally (control for interaction between joint proximity and enhanced distal inhibition, Experiment 3).

RESULTS

All experiments demonstrated spatially specific pain modulation, but only Experiment 2 (near elbow) supported our hypothesis: middle zone pain intensity was significantly lower (p = 0.02) during Localization than No-localization. Experiment 1 (near wrist) found reduced pain intensity during Localization only for the distal zone (p = 0.04). Experiment 3 confirmed this effect: reduced pain during Localization occurred only for the most distal zone (p = 0.046).

CONCLUSION

Expecting a painful stimulus in non-adjacent skin sites has spatially specific effects on pain modulation, but this reflects an interaction between the expected location of stimulation and the actual location. This suggests a more complex connection between somatotopic maps and nociceptive modulation than previously thought; several distinct mechanisms likely contribute.

How Attention Affects Spatial Resolution

We summarize and discuss a series of psychophysical studies on the effects of spatial covert attention on spatial resolution, our ability to discriminate fine patterns. Heightened resolution is beneficial in most, but not all, visual tasks. We show how endogenous attention (voluntary, goal driven) and exogenous attention (involuntary, stimulus driven) affect performance on a variety of tasks mediated by spatial resolution, such as visual search, crowding, acuity, and texture segmentation. Exogenous attention is an automatic mechanism that increases resolution regardless of whether it helps or hinders performance. In contrast, endogenous attention flexibly adjusts resolution to optimize performance according to task demands. We illustrate how psychophysical studies can reveal the underlying mechanisms of these effects and allow us to draw linking hypotheses with known neurophysiological effects of attention.

A mechanistic cortical microcircuit of attention for amplification, normalization and suppression

Computational models of visual attention have replicated a large number of data from visual attention experiments. However, typically each computational model has been shown to account for only a few data sets. We developed a novel model of attention, particularly focused on explaining single cell recordings in multiple brain areas, to better understand the underlying computational circuits of attention involved in spatial- and feature-based biased competition, modulation of the contrast response function, modulation of the neuronal tuning curve, and modulation of surround suppression. In contrast to previous models, we use a two layer structure inspired by the layered cortical architecture which implements amplification, divisive normalization and suppression as well as spatial pooling.

Effects of surround suppression on response adaptation of V1 neurons to visual stimuli

The influence of intracortical inhibition on the response adaptation of visual cortical neurons remains in debate. To clarify this issue, in the present study the influence of surround suppression evoked through the local inhibitory interneurons on the adaptation effects of neurons in the primary visual cortex (V1) were observed. Moreover, the adaptations of V1 neurons to both the high-contrast visual stimuli presented in the classical receptive field (CRF) and to the costimulation presented in the CRF and the surrounding nonclassical receptive field (nCRF) were compared. The intensities of surround suppression were modulated with different sized grating stimuli. The results showed that the response adaptation of V1 neurons decreased significantly with the increase of surround suppression and this adaptation decrease was due to the reduction of the initial response of V1 neurons to visual stimuli. However, the plateau response during adaptation showed no significant changes. These findings indicate that the adaptation effects of V1 neurons may not be directly affected by surround suppression, but may be dynamically regulated by a negative feedback network and be finely adjusted by its initial spiking response to stimulus. This adaptive regulation is not only energy efficient for the central nervous system, but also beneficially acts to maintain the homeostasis of neuronal response to long-presenting visual signals.

Moving sensory adaptation beyond suppressive effects in single neurons

How an object is perceived depends on the temporal context in which it is encountered. Sensory signals in the brain also depend on temporal context, a phenomenon often referred to as adaptation. Traditional descriptions of adaptation effects emphasize various forms of response fatigue in single neurons, which grow in strength with exposure to a stimulus. Recent work on vision, and other sensory modalities, has shown that this description has substantial shortcomings. Here we review our emerging understanding of how adaptation alters the balance between excitatory and suppressive signals, how effects depend on adaptation duration, and how adaptation influences representations that are distributed within and across multiple brain structures. This work points to a sophisticated set of mechanisms for adjusting to recent sensory experience, and suggests new avenues for understanding their function.

Dynamic integration of task-relevant visual features in posterior parietal cortex

The primate visual system consists of multiple hierarchically organized cortical areas, each specialized for processing distinct aspects of the visual scene. For example, color and form are encoded in ventral pathway areas such as V4 and inferior temporal cortex, while motion is preferentially processed in dorsal pathway areas such as the middle temporal area. Such representations often need to be integrated perceptually to solve tasks that depend on multiple features. We tested the hypothesis that the lateral intraparietal area (LIP) integrates disparate task-relevant visual features by recording from LIP neurons in monkeys trained to identify target stimuli composed of conjunctions of color and motion features. We show that LIP neurons exhibit integrative representations of both color and motion features when they are task relevant and task-dependent shifts of both direction and color tuning. This suggests that LIP plays a role in flexibly integrating task-relevant sensory signals.

Alterations to global but not local motion processing in long-term ecstasy (MDMA) users

RATIONALE

Growing evidence indicates that the main psychoactive ingredient in the illegal drug "ecstasy" (methylendioxymethamphetamine) causes reduced activity in the serotonin and gamma-aminobutyric acid (GABA) systems in humans. On the basis of substantial serotonin input to the occipital lobe, recent research investigated visual processing in long-term users and found a larger magnitude of the tilt aftereffect, interpreted to reflect broadened orientation tuning bandwidths. Further research found higher orientation discrimination thresholds and reduced long-range interactions in the primary visual area of ecstasy users.

OBJECTIVES

The aim of the present research was to investigate whether serotonin-mediated V1 visual processing deficits in ecstasy users extend to motion processing mechanisms.

METHOD

Forty-five participants (21 controls, 24 drug users) completed two psychophysical studies: A direction discrimination study directly measured local motion processing in V1, while a motion coherence task tested global motion processing in area V5/MT.

RESULTS

"Primary" ecstasy users (n = 18), those without substantial polydrug use, had significantly lower global motion thresholds than controls [p = 0.027, Cohen's d = 0.78 (large)], indicating increased sensitivity to global motion stimuli, but no difference in local motion processing (p = 0.365).

CONCLUSION

These results extend on previous research investigating the long-term effects of illicit drugs on visual processing. Two possible explanations are explored: defuse attentional processes may be facilitating spatial pooling of motion signals in users. Alternatively, it may be that a GABA-mediated disruption to V5/MT processing is reducing spatial suppression and therefore improving global motion perception in ecstasy users.

A unifying mechanistic model of selective attention in spiking neurons

Visuospatial attention produces myriad effects on the activity and selectivity of cortical neurons. Spiking neuron models capable of reproducing a wide variety of these effects remain elusive. We present a model called the Attentional Routing Circuit (ARC) that provides a mechanistic description of selective attentional processing in cortex. The model is described mathematically and implemented at the level of individual spiking neurons, with the computations for performing selective attentional processing being mapped to specific neuron types and laminar circuitry. The model is used to simulate three studies of attention in macaque, and is shown to quantitatively match several observed forms of attentional modulation. Specifically, ARC demonstrates that with shifts of spatial attention, neurons may exhibit shifting and shrinking of receptive fields; increases in responses without changes in selectivity for non-spatial features (i.e. response gain), and; that the effect on contrast-response functions is better explained as a response-gain effect than as contrast-gain. Unlike past models, ARC embodies a single mechanism that unifies the above forms of attentional modulation, is consistent with a wide array of available data, and makes several specific and quantifiable predictions.

At a given moment, a tremendous amount of visual information falls on the retinae, far more than the brain is capable of processing. By directing attention to a spatial location, stimuli at that position can be selectively processed, while irrelevant information from non-attended locations can be largely ignored. We present a detailed model that describes the mechanisms by which visual spatial attention may be implemented in the brain. Using this model, we simulated three previous studies of spatial attention in primates, and analysed the simulation data using the same methods as in the original experiments. Across these simulations, and without altering model parameters, our model produces results that are statistically indistinguishable from those recorded in primates. Unlike previous work, our model provides greater biological detail of how the brain performs selective visual processing, while also accurately demonstrating numerous forms of selective attention. Our results suggest that these seemingly different forms of attentional effects may result from a single mechanism for selectively processing attended stimuli.

Bump attractor dynamics in prefrontal cortex explains behavioral precision in spatial working memory

Prefrontal persistent activity during the delay of spatial working memory tasks is thought to maintain spatial location in memory. A 'bump attractor' computational model can account for this physiology and its relationship to behavior. However, direct experimental evidence linking parameters of prefrontal firing to the memory report in individual trials is lacking, and, to date, no demonstration exists that bump attractor dynamics underlies spatial working memory. We analyzed monkey data and found model-derived predictive relationships between the variability of prefrontal activity in the delay and the fine details of recalled spatial location, as evident in trial-to-trial imprecise oculomotor responses. Our results support a diffusing bump representation for spatial working memory instantiated in persistent prefrontal activity. These findings reinforce persistent activity as a basis for spatial working memory, provide evidence for a continuous prefrontal representation of memorized space and offer experimental support for bump attractor dynamics mediating cognitive tasks in the cortex.

Similar adaptation effects in primary visual cortex and area MT of the macaque monkey under matched stimulus conditions

Recent stimulus history, or adaptation, can alter neuronal response properties. Adaptation effects have been characterized in a number of visually responsive structures, from the retina to higher visual cortex. However, it remains unclear whether adaptation effects across stages of the visual system take a similar form in response to a particular sensory event. This is because studies typically probe a single structure or cortical area, using a stimulus ensemble chosen to provide potent drive to the cells of interest. Here we adopt an alternative approach and compare adaptation effects in primary visual cortex (V1) and area MT using identical stimulus ensembles. Previous work has suggested these areas adjust to recent stimulus drive in distinct ways. We show that this is not the case: adaptation effects in V1 and MT can involve weak or strong loss of responsivity and shifts in neuronal preference toward or away from the adapter, depending on stimulus size and adaptation duration. For a particular stimulus size and adaptation duration, however, effects are similar in nature and magnitude in V1 and MT. We also show that adaptation effects in MT of awake animals depend strongly on stimulus size. Our results suggest that the strategies for adjusting to recent stimulus history depend more strongly on adaptation duration and stimulus size than on the cortical area. Moreover, they indicate that different levels of the visual system adapt similarly to recent sensory experience.

Spatial and feature-based attention in a layered cortical microcircuit model

Directing attention to the spatial location or the distinguishing feature of a visual object modulates neuronal responses in the visual cortex and the stimulus discriminability of subjects. However, the spatial and feature-based modes of attention differently influence visual processing by changing the tuning properties of neurons. Intriguingly, neurons' tuning curves are modulated similarly across different visual areas under both these modes of attention. Here, we explored the mechanism underlying the effects of these two modes of visual attention on the orientation selectivity of visual cortical neurons. To do this, we developed a layered microcircuit model. This model describes multiple orientation-specific microcircuits sharing their receptive fields and consisting of layers 2/3, 4, 5, and 6. These microcircuits represent a functional grouping of cortical neurons and mutually interact via lateral inhibition and excitatory connections between groups with similar selectivity. The individual microcircuits receive bottom-up visual stimuli and top-down attention in different layers. A crucial assumption of the model is that feature-based attention activates orientation-specific microcircuits for the relevant feature selectively, whereas spatial attention activates all microcircuits homogeneously, irrespective of their orientation selectivity. Consequently, our model simultaneously accounts for the multiplicative scaling of neuronal responses in spatial attention and the additive modulations of orientation tuning curves in feature-based attention, which have been observed widely in various visual cortical areas. Simulations of the model predict contrasting differences between excitatory and inhibitory neurons in the two modes of attentional modulations. Furthermore, the model replicates the modulation of the psychophysical discriminability of visual stimuli in the presence of external noise. Our layered model with a biologically suggested laminar structure describes the basic circuit mechanism underlying the attention-mode specific modulations of neuronal responses and visual perception.

Cortical control of VTA function and influence on nicotine reward

Tobacco use is a major public health problem. Nicotine acts on widely distributed nicotinic acetylcholine receptors (nAChRs) in the brain and excites dopamine (DA) neurons in the ventral tegmental area (VTA). The elicited increase of DA neuronal activity is thought to be an important mechanism for nicotine reward and subsequently the transition to addiction. However, the current understanding of nicotine reward is based predominantly on the data accumulated from in vitro studies, often from VTA slices. Isolated VTA slices artificially terminate communications between neurons in the VTA and other brain regions that may significantly alter nicotinic effects. Consequently, the mechanisms of nicotinic excitation of VTA DA neurons under in vivo conditions have received only limited attention. Building upon the existing knowledge acquired in vitro, it is now time to elucidate the integrated mechanisms of nicotinic reward on intact systems that are more relevant to understanding the action of nicotine or other addictive drugs. In this review, we summarize recent studies that demonstrate the impact of prefrontal cortex (PFC) on the modulation of VTA DA neuronal function and nicotine reward. Based on existing evidence, we propose a new hypothesis that PFC-VTA functional coupling serves as an integration mechanism for nicotine reward. Moreover, addiction may develop due to nicotine perturbing the PFC-VTA coupling and thereby eliminating the PFC-dependent cognitive control over behavior.

Altered visual perception in long-term ecstasy (MDMA) users

RATIONALE

The present study investigated the long-term consequences of ecstasy use on visual processes thought to reflect serotonergic functions in the occipital lobe. Evidence indicates that the main psychoactive ingredient in ecstasy (methylendioxymethamphetamine) causes long-term changes to the serotonin system in human users. Previous research has found that amphetamine-abstinent ecstasy users have disrupted visual processing in the occipital lobe which relies on serotonin, with researchers concluding that ecstasy broadens orientation tuning bandwidths. However, other processes may have accounted for these results.

OBJECTIVES

The aim of the present research was to determine if amphetamine-abstinent ecstasy users have changes in occipital lobe functioning, as revealed by two studies: a masking study that directly measured the width of orientation tuning bandwidths and a contour integration task that measured the strength of long-range connections in the visual cortex of drug users compared to controls.

METHOD

Participants were compared on the width of orientation tuning bandwidths (26 controls, 12 ecstasy users, 10 ecstasy + amphetamine users) and the strength of long-range connections (38 controls, 15 ecstasy user, 12 ecstasy + amphetamine users) in the occipital lobe.

RESULTS

Amphetamine-abstinent ecstasy users had significantly broader orientation tuning bandwidths than controls and significantly lower contour detection thresholds (CDTs), indicating worse performance on the task, than both controls and ecstasy + amphetamine users.

CONCLUSION

These results extend on previous research, which is consistent with the proposal that ecstasy may damage the serotonin system, resulting in behavioral changes on tests of visual perception processes which are thought to reflect serotonergic functions in the occipital lobe.

Dynamic excitatory and inhibitory gain modulation can produce flexible, robust and optimal decision-making

Behavioural and neurophysiological studies in primates have increasingly shown the involvement of urgency signals during the temporal integration of sensory evidence in perceptual decision-making. Neuronal correlates of such signals have been found in the parietal cortex, and in separate studies, demonstrated attention-induced gain modulation of both excitatory and inhibitory neurons. Although previous computational models of decision-making have incorporated gain modulation, their abstract forms do not permit an understanding of the contribution of inhibitory gain modulation. Thus, the effects of co-modulating both excitatory and inhibitory neuronal gains on decision-making dynamics and behavioural performance remain unclear. In this work, we incorporate time-dependent co-modulation of the gains of both excitatory and inhibitory neurons into our previous biologically based decision circuit model. We base our computational study in the context of two classic motion-discrimination tasks performed in animals. Our model shows that by simultaneously increasing the gains of both excitatory and inhibitory neurons, a variety of the observed dynamic neuronal firing activities can be replicated. In particular, the model can exhibit winner-take-all decision-making behaviour with higher firing rates and within a significantly more robust model parameter range. It also exhibits short-tailed reaction time distributions even when operating near a dynamical bifurcation point. The model further shows that neuronal gain modulation can compensate for weaker recurrent excitation in a decision neural circuit, and support decision formation and storage. Higher neuronal gain is also suggested in the more cognitively demanding reaction time than in the fixed delay version of the task. Using the exact temporal delays from the animal experiments, fast recruitment of gain co-modulation is shown to maximize reward rate, with a timescale that is surprisingly near the experimentally fitted value. Our work provides insights into the simultaneous and rapid modulation of excitatory and inhibitory neuronal gains, which enables flexible, robust, and optimal decision-making.

Perceptual decision-making involves not only simple transformation of sensory information to a motor decision, but can also be modulated by high-level cognition. For example, the latter may include strategic allocation of limited attentional resources over time in a decision task to improve performance. At the neurophysiological level, there is evidence supporting attention-induced neuronal gain modulation of both excitatory and inhibitory cortical neurons. In the context of perceptual discrimination tasks performed by animals, we make use of a biologically inspired computational model of decision-making to understand the computational capabilities of such co-modulation of neuronal gains. We find that dynamic co-modulation of both excitatory and inhibitory neurons is important for flexible, and cognitively demanding decision-making while also enhancing robustness in the decision circuit's functions. Our model captures the neuronal activity and behavioural data in the animal experiments remarkably well. Decision performance in a reaction time task can be optimized, maximizing the rate of receiving reward by using fast gain recruitment. Our experimentally fitted timescale is near the optimal one, suggesting that the animals performed almost optimally. By providing both computational simulations and theoretical analyses, our computational model sheds light into the multiple functions of rapid co-modulation of neuronal gains during decision-making.

Attentional enhancement of spatial resolution: linking behavioural and neurophysiological evidence

Attention allows us to select relevant sensory information for preferential processing. Behaviourally, it improves performance in various visual tasks. One prominent effect of attention is the modulation of performance in tasks that involve the visual system's spatial resolution. Physiologically, attention modulates neuronal responses and alters the profile and position of receptive fields near the attended location. Here, we develop a hypothesis linking the behavioural and electrophysiological evidence. The proposed framework seeks to explain how these receptive field changes enhance the visual system's effective spatial resolution and how the same mechanisms may also underlie attentional effects on the representation of spatial information.

Working memory and decision processes in visual area v4

Recognizing and responding to a remembered stimulus requires the coordination of perception, working memory, and decision-making. To investigate the role of visual cortex in these processes, we recorded responses of single V4 neurons during performance of a delayed match-to-sample task that incorporates rapid serial visual presentation of natural images. We found that neuronal activity during the delay period after the cue but before the images depends on the identity of the remembered image and that this change persists while distractors appear. This persistent response modulation has been identified as a diagnostic criterion for putative working memory signals; our data thus suggest that working memory may involve reactivation of sensory neurons. When the remembered image reappears in the neuron’s receptive field, visually evoked responses are enhanced; this match enhancement is a diagnostic criterion for decision. One model that predicts these data is the matched filter hypothesis, which holds that during search V4 neurons change their tuning so as to match the remembered cue, and thus become detectors for that image. More generally, these results suggest that V4 neurons participate in the perceptual, working memory, and decision processes that are needed to perform memory-guided decision-making.

Separable mechanisms underlying global feature-based attention

Feature-based attention is known to operate in a spatially global manner, in that the selection of attended features is not bound to the spatial focus of attention. Here we used electromagnetic recordings in human observers to characterize the spatiotemporal signature of such global selection of an orientation feature. Observers performed a simple orientation-discrimination task while ignoring task-irrelevant orientation probes outside the focus of attention. We observed that global feature-based selection, indexed by the brain response to unattended orientation probes, is composed of separable functional components. One such component reflects global selection based on the similarity of the probe with task-relevant orientation values ("template matching"), which is followed by a component reflecting selection based on the similarity of the probe with the orientation value under discrimination in the focus of attention ("discrimination matching"). Importantly, template matching occurs at ∼150 ms after stimulus onset, ∼80 ms before the onset of discrimination matching. Moreover, source activity underlying template matching and discrimination matching was found to originate from ventral extrastriate cortex, with the former being generated in more anterolateral and the latter in more posteromedial parts, suggesting template matching to occur in visual cortex higher up in the visual processing hierarchy than discrimination matching. We take these observations to indicate that the population-level signature of global feature-based selection reflects a sequence of hierarchically ordered operations in extrastriate visual cortex, in which the selection based on task relevance has temporal priority over the selection based on the sensory similarity between input representations.

Traveling waves in visual cortex

Electrode recordings and imaging studies have revealed that localized visual stimuli elicit waves of activity that travel across primary visual cortex. Traveling waves are present also during spontaneous activity, but they can be greatly reduced by widespread and intensive visual stimulation. In this Review, we summarize the evidence in favor of these traveling waves. We suggest that their substrate may lie in long-range horizontal connections and that their functional role may involve the integration of information over large regions of space.

The influence of surround suppression on adaptation effects in primary visual cortex

Adaptation, the prolonged presentation of stimuli, has been used to probe mechanisms of visual processing in physiological, imaging, and perceptual studies. Previous neurophysiological studies have measured adaptation effects by using stimuli tailored to evoke robust responses in individual neurons. This approach provides an incomplete view of how an adapter alters the representation of sensory stimuli by a population of neurons with diverse functional properties. We implanted microelectrode arrays in primary visual cortex (V1) of macaque monkeys and measured orientation tuning and contrast sensitivity in populations of neurons before and after prolonged adaptation. Whereas previous studies in V1 have reported that adaptation causes stimulus-specific suppression of responsivity and repulsive shifts in tuning preference, we have found that adaptation can also lead to response facilitation and shifts in tuning toward the adapter. To explain this range of effects, we have proposed and tested a simple model that employs stimulus-specific suppression in both the receptive field and the spatial surround. The predicted effects on tuning depend on the relative drive provided by the adapter to these two receptive field components. Our data reveal that adaptation can have a much richer repertoire of effects on neuronal responsivity and tuning than previously considered and suggest an intimate mechanistic relationship between spatial and temporal contextual effects.

Expansion of MT neurons excitatory receptive fields during covert attentive tracking

Primates can attentively track moving objects while keeping gaze stationary. The neural mechanisms underlying this ability are poorly understood. We investigated this issue by recording responses of neurons in area MT of two rhesus monkeys while they performed two different tasks. During the Attend-Fixation task, two moving random dot patterns (RDPs) translated across a screen at the same speed and in the same direction while the animals directed gaze to a fixation spot and detected a change in its luminance. During the Tracking task, the animals kept gaze on the fixation spot and attentively tracked the two RDPs to report a change in the local speed of one of the patterns' dots. In both conditions, neuronal responses progressively increased as the RDPs entered the neurons' receptive field (RF), peaked when they reached its center, and decreased as they translated away. This response profile was well described by a Gaussian function with its center of gravity indicating the RF center and its flanks the RF excitatory borders. During Tracking, responses were increased relative to Attend-Fixation, causing the Gaussian profiles to expand. Such increases were proportionally larger in the RF periphery than at its center, and were accompanied by a decrease in the trial-to-trial response variability (Fano factor) relative to Attend-Fixation. These changes resulted in an increase in the neurons' performance at detecting targets at longer distances from the RF center. Our results show that attentive tracking dynamically changes MT neurons' RF profiles, ultimately improving the neurons' ability to encode the tracked stimulus features.

Spatial attention, feature-based attention, and saccades: three sides of one coin?

The last three decades has seen a steady growth of neuroscience research aimed at understanding the functions and sources of top-down attentional modulation in the brain. This correlates with recognition that attention may be a necessary component of sensory systems to support natural behaviors in natural environments. Complexity and clutter are two of the most recognizable hallmarks of natural environments, which can simultaneously contain vitally important and completely irrelevant stimuli. Attention serves as an adaptive filter providing each sensory modality preferential processing routes for important stimuli while suppressing responses to distracters, thus optimizing use of limited neural resources. In other words, attention is the family of mechanisms by which organisms are able to effectively and selectively allocate limited neural resources to achieve specific behavioral goals. This review provides some historical context for considering attentional frameworks and modern neurophysiological attention research, focusing on visual attention. A taxonomy of common attentional effects and neural mechanisms is provided, along with consideration of the specific relationship between attention and saccade planning. We examine the validity of premotor theories of attention, which posit that attention and saccade planning are one and the same. While there is strong evidence that attention and oculomotor planning are similar, with shared neural substrates, there is also evidence that these two functions are not synonymous. Finally, we examine neurophysiological explanations for dysfunction in attention-deficit/hyperactivity disorder and the hypothesis that social impairment in autism spectrum disorders is partially attributable to perturbations of attentional control circuitry.

The spatial distribution of receptive field changes in a model of peri-saccadic perception: predictive remapping and shifts towards the saccade target

At the time of an impending saccade receptive fields (RFs) undergo dynamic changes, that is, their spatial profile is altered. This phenomenon has been observed in several monkey visual areas. Although their link to eye movements is obvious, neither the exact pattern nor their function is fully clear. Several RF shifts have been interpreted in terms of predictive remapping mediating visual stability. In particular, even prior to saccade onset some cells become responsive to stimuli presented in their future, post-saccadic RF. In visual area V4, however, the overall effect of RF dynamics consists of a shrinkage and shift of RFs towards the saccade target. These observations have been linked to a pre-saccadically enhanced processing of the future fixation. In order to better understand these seemingly different outcomes, we analyzed the RF shifts predicted by a recently proposed computational model of peri-saccadic perception (Hamker, Zirnsak, Calow, & Lappe, 2008). This model unifies peri-saccadic compression, pre-saccadic attention shifts, and peri-saccadic receptive field dynamics in a common framework of oculomotor reentry signals in extrastriate visual cortical maps. According to the simulations that we present in the current paper, a spatially selective oculomotor feedback signal leads to RF dynamics which are both consistent with the observations made in studies aiming to investigate predictive remapping and saccade target shifts. Thus, the seemingly distinct experimental observations could be grounded in the same neural mechanism leading to different RF dynamics dependent on the location of the RF in visual space.

Modeling nicotinic neuromodulation from global functional and network levels to nAChR based mechanisms

Neuromodulator action has received increasing attention in theoretical neuroscience. Yet models involving both neuronal populations dynamics at the circuit level and detailed receptor properties are only now being developed. Here we review recent computational approaches to neuromodulation, focusing specifically on acetylcholine (ACh) and nicotine. We discuss illustrative examples of models ranging from functional top-down to neurodynamical bottom-up. In the top-down approach, a computational theory views ACh as encoding the uncertainty expected in an environment. A different line of models accounts for neural population dynamics treating ACh as toggling neuronal networks between read-in of information and recall of memory. Building on the neurodynamics idea we discuss two models of nicotine's action with increasing degree of biological realism. Both consider explicitly receptor-level mechanisms but with different scales of detail. The first is a large-scale model of nicotine-dependent modulation of dopaminergic signaling that is capable of simulating nicotine self-administration. The second is a novel approach where circuit-level neurodynamics of the ventral tegmental area (VTA) are combined with explicit models of the dynamics of specific nicotinic ACh receptor subtypes. We show how the model is constructed based on local anatomy, electrophysiology and receptor properties and provide an illustration of its potential. In particular, we show how the model can shed light on the specific mechanisms by which nicotine controls dopaminergic neurotransmission in the VTA. This model serves us to conclude that detailed accounts for neuromodulator action at the basis of behavioral and cognitive models are crucial to understand how neuromodulators mediate their functional properties.

Task difficulty and performance induce diverse adaptive patterns in gain and shape of primary auditory cortical receptive fields

Attention is essential for navigating complex acoustic scenes, when the listener seeks to extract a foreground source while suppressing background acoustic clutter. This study explored the neural correlates of this perceptual ability by measuring rapid changes of spectrotemporal receptive fields (STRFs) in primary auditory cortex during detection of a target tone embedded in noise. Compared with responses in the passive state, STRF gain decreased during task performance in most cells. By contrast, STRF shape changes were excitatory and specific, and were strongest in cells with best frequencies near the target tone. The net effect of these adaptations was to accentuate the representation of the target tone relative to the noise by enhancing responses of near-target cells to the tone during high-signal-to-noise ratio (SNR) tasks while suppressing responses of far-from-target cells to the masking noise in low-SNR tasks. These adaptive STRF changes were largest in high-performance sessions, confirming a close correlation with behavior.

Attention reshapes center-surround receptive field structure in macaque cortical area MT

Directing spatial attention to a location inside the classical receptive field (cRF) of a neuron in macaque medial temporal area (MT) shifts the center of the cRF toward the attended location. Here we investigate the influence of spatial attention on the profile of the inhibitory surround present in many MT neurons. Two monkeys attended to the fixation point or to 1 of 2 random dot patterns (RDPs) placed inside or next to the cRF, whereas a third RDP (the probe) was briefly presented in quick succession across the cRF and surround. The probe presentation responses were used to compute a map of the excitatory receptive field and its inhibitory surround. Attention systematically reshapes the receptive field profile, independently shifting both center and surround toward the attended location. Furthermore, cRF size is changed as a function of relative distance to the attentional focus: attention inside the cRF shrinks it, whereas directing attention next to the cRF expands it. In addition, we find systematic changes in surround inhibition and cRF amplitude. This nonmultiplicative push-pull modulation of the receptive field's center-surround structure optimizes processing at and near the attentional focus to strengthen the representation of the attended stimulus while reducing influences from distractors.

Receptive field shift and shrinkage in macaque middle temporal area through attentional gain modulation

Selective attention is the top-down mechanism to allocate neuronal processing resources to the most relevant subset of the information provided by an organism's sensors. Attentional selection of a spatial location modulates the spatial-tuning characteristics (i.e., the receptive fields of neurons in macaque visual cortex). These tuning changes include a shift of receptive field centers toward the focus of attention and a narrowing of the receptive field when the attentional focus is directed into the receptive field. Here, we report that when attention is directed into versus of receptive fields of neurons in the middle temporal visual area (area MT), the magnitude of the shift of the spatial-tuning functions is positively correlated with a narrowing of spatial tuning around the attentional focus. By developing and applying a general attentional gain model, we show that these nonmultiplicative attentional modulations of basic neuronal-tuning characteristics could be a direct consequence of a spatially distributed multiplicative interaction of a bell-shaped attentional spotlight with the spatially fined-grained sensory inputs of MT neurons. Additionally, the model lets us estimate the spatial spread of the attentional top-down signal impinging on visual cortex. Consistent with psychophysical reports, the estimated size of the "spotlight of attention" indicates a coarse spatial resolution of attention. These results illustrate how spatially specific nonmultiplicative attentional changes of neuronal-tuning functions can be the result of multiplicative gain modulation affecting sensory neurons in a widely distributed region in cortical space.

Attention to stimulus features shifts spectral tuning of V4 neurons during natural vision

Previous neurophysiological studies suggest that attention can alter the baseline or gain of neurons in extrastriate visual areas but that it cannot change tuning. This suggests that neurons in visual cortex function as labeled lines whose meaning does not depend on task demands. To test this common assumption, we used a system identification approach to measure spatial frequency and orientation tuning in area V4 during two attentionally demanding visual search tasks, one that required fixation and one that allowed free viewing during search. We found that spatial attention modulates response baseline and gain but does not alter tuning, consistent with previous reports. In contrast, feature-based attention often shifts neuronal tuning. These tuning shifts are inconsistent with the labeled-line model and tend to enhance responses to stimulus features that distinguish the search target. Our data suggest that V4 neurons behave as matched filters that are dynamically tuned to optimize visual search.

Attentional modulation of spatial integration of pain: evidence for dynamic spatial tuning

In many sensory modalities, afferent processing is dynamically modulated by attention and this modulation produces altered sensory experiences. Attention is able to alter perceived pain, but the mechanisms involved in this modulation have not been elucidated. To determine whether attention alters spatial integration of nociceptive information, subjects were recruited to evaluate pain from pairs of noxious/innocuous thermal stimuli during different spatial attentional tasks. Divided attention was able to abolish spatial summation and produce inhibition of pain. In contrast, directed attention enhanced pain intensity by partially integrating both stimuli. This dynamic modulation of spatial integration indicates that attention alters spatial dimensions of afferent nociceptive processing to optimize the perceptual response to input from a particular body region or stimulus feature. This dynamic spatial tuning of nociceptive processing provides a new conceptual insight into the functional significance of endogenous pain inhibitory and facilitatory mechanisms.

Asymmetric synaptic depression in cortical networks

Synaptic depression is essential for controlling the balance between excitation and inhibition in cortical networks. Several studies have shown that the depression of intracortical synapses is asymmetric, that is, inhibitory synapses depress less than excitatory ones. Whether this asymmetry has any impact on cortical function is unknown. Here we show that the differential depression of intracortical synapses provides a mechanism through which the gain and sensitivity of cortical circuits shifts over time to improve stimulus coding. We examined the functional consequences of asymmetric synaptic depression by modeling recurrent interactions between orientation-selective neurons in primary visual cortex (V1) that adapt to feedforward inputs. We demonstrate analytically that despite the fact that excitatory synapses depress more than inhibitory synapses, excitatory responses are reduced less than inhibitory ones to increase the overall response gain. These changes play an active role in generating selective gain control in visual cortical circuits. Specifically, asymmetric synaptic depression regulates network selectivity by amplifying responses and sensitivity of V1 neurons to infrequent stimuli and attenuating responses and sensitivity to frequent stimuli, as is indeed observed experimentally.

Visual adaptation: physiology, mechanisms, and functional benefits

Recent sensory experience affects both perception and the response properties of visual neurons. Here I review a rapid form of experience-dependent plasticity that follows adaptation, the presentation of a particular stimulus or ensemble of stimuli for periods ranging from tens of milliseconds to minutes. Adaptation has a rich history in psychophysics, where it is often used as a tool for dissecting the perceptual mechanisms of vision. Although we know comparatively little about the neurophysiological effects of adaptation, work in the last decade has revealed a rich repertoire of effects. This review focuses on this recent physiological work, the cellular and biophysical mechanisms that may underlie the observed effects, and the functional benefit that they may afford. I conclude with a brief discussion of some important open questions in the field.

V4 receptive field dynamics as predicted by a systems-level model of visual attention using feedback from the frontal eye field

Visual attention is generally considered to facilitate the processing of the attended stimulus. Its mechanisms, however, are still under debate. We have developed a systems-level model of visual attention which predicts that attentive effects emerge by the interactions between different brain areas. Recent physiological studies have provided evidence that attention also alters the receptive field structure. For example, V4 receptive fields typically shrink and shift towards the saccade target around saccade onset. We show that receptive field dynamics are inherently predicted by the mechanism of feedback in our model. According to the model an oculomotor feedback signal from an area involved in the competition for the saccade target location, e.g. the frontal eye field, enhances the gain of V4 cells. V4 receptive field dynamics can be observed after pooling the gain modulated responses to obtain a certain degree of spatial invariance. The time course of the receptive field dynamics in the model resemble those obtained from macaque V4.