Cortical fMRI activation produced by attentive tracking of moving targets

All numbers are not equal: an electrophysiological investigation of small and large number representations

Behavioral and brain imaging research indicates that human infants, humans adults, and many nonhuman animals represent large nonsymbolic numbers approximately, discriminating between sets with a ratio limit on accuracy. Some behavioral evidence, especially with human infants, suggests that these representations differ from representations of small numbers of objects. To investigate neural signatures of this distinction, event-related potentials were recorded as adult humans passively viewed the sequential presentation of dot arrays in an adaptation paradigm. In two studies, subjects viewed successive arrays of a single number of dots interspersed with test arrays presenting the same or a different number; numerical range (small numerical quantities 1-3 vs. large numerical quantities 8-24) and ratio difference varied across blocks as continuous variables were controlled. An early-evoked component (N1), observed over widespread posterior scalp locations, was modulated by absolute number with small, but not large, number arrays. In contrast, a later component (P2p), observed over the same scalp locations, was modulated by the ratio difference between arrays for large, but not small, numbers. Despite many years of experience with symbolic systems that apply equally to all numbers, adults spontaneously process small and large numbers differently. They appear to treat small-number arrays as individual objects to be tracked through space and time, and large-number arrays as cardinal values to be compared and manipulated.

Low- and high-level motion perception deficits in anisometropic and strabismic amblyopia: evidence from fMRI

Maximum motion displacement (Dmax) is the largest dot displacement in a random-dot kinematogram (RDK) at which direction of motion can be correctly discriminated [Braddick, O. (1974). A short-range process in apparent motion. Vision Research, 14, 519-527]. For first-order RDKs, Dmax gets larger as dot size increases and/or dot density decreases. It has been suggested that this increase in Dmax reflects greater involvement of high-level feature-matching motion mechanisms and less dependence on low-level motion detectors [Sato, T. (1998). Dmax: Relations to low- and high-level motion processes. In T. Watanabe (Ed.), High-level motion processing, computational, neurobiological, and psychophysical perspectives (pp. 115-151). Boston: MIT Press]. Recent psychophysical findings [Ho, C. S., & Giaschi, D. E. (2006). Deficient maximum motion displacement in amblyopia. Vision Research, 46, 4595-4603; Ho, C. S., & Giaschi, D. E. (2007). Stereopsis-dependent deficits in maximum motion displacement. Vision Research, 47, 2778-2785] suggest that this "switch" from low-level to high-level motion processing is also observed in children with anisometropic and strabismic amblyopia as RDK dot size is increased and/or dot density is decreased. However, both high- and low-level Dmax were reduced relative to controls. In this study, we used functional MRI to determine the motion-sensitive areas that may account for the reduced Dmax in amblyopia In the control group, low-level RDKs elicited stronger responses in low-level (posterior occipital) areas and high-level RDKs elicited a greater response in high-level (extra-striate occipital-parietal) areas when activation for high-level RDKs was compared to that for low-level RDKs. Participants with anisometropic amblyopia showed the same pattern of cortical activation although extent of activation differences was less than in controls. For those with strabismic amblyopia, there was almost no difference in the cortical activity for low-level and high-level RDKs, and activation was reduced relative to the other groups. Differences in the extent of cortical activation may be related to amblyogenic subtype.

Dopamine transporters in striatum correlate with deactivation in the default mode network during visuospatial attention

Background

Dopamine and dopamine transporters (DAT, which regulate extracellular dopamine in the brain) are implicated in the modulation of attention but their specific roles are not well understood. Here we hypothesized that dopamine modulates attention by facilitation of brain deactivation in the default mode network (DMN). Thus, higher striatal DAT levels, which would result in an enhanced clearance of dopamine and hence weaker dopamine signals, would be associated to lower deactivation in the DMN during an attention task.

Methodology/Principal Findings

For this purpose we assessed the relationship between DAT in striatum (measured with positron emission tomography and [¹¹C]cocaine used as DAT radiotracer) and brain activation and deactivation during a parametric visual attention task (measured with blood oxygenation level dependent functional magnetic resonance imaging) in healthy controls. We show that DAT availability in caudate and putamen had a negative correlation with deactivation in ventral parietal regions of the DMN (precuneus, BA 7) and a positive correlation with deactivation in a small region in the ventral anterior cingulate gyrus (BA 24/32). With increasing attentional load, DAT in caudate showed a negative correlation with load-related deactivation increases in precuneus.

Conclusions/Significance

These findings provide evidence that dopamine transporters modulate neural activity in the DMN and anterior cingulate gyrus during visuospatial attention. Our findings suggest that dopamine modulates attention in part by regulating neuronal activity in posterior parietal cortex including precuneus (region involved in alertness) and cingulate gyrus (region deactivated in proportion to emotional interference). These findings suggest that the beneficial effects of stimulant medications (increase dopamine by blocking DAT) in inattention reflect in part their ability to facilitate the deactivation of the DMN.

Posterior parietal cortex and episodic retrieval: convergent and divergent effects of attention and memory

Functional neuroimaging studies of humans engaged in retrieval from episodic memory have revealed a surprisingly consistent pattern of retrieval-related activity in lateral posterior parietal cortex (PPC). Given the well-established role of lateral PPC in subserving goal-directed and reflexive attention, it has been hypothesized that PPC activation during retrieval reflects the recruitment of parietal attention mechanisms during remembering. Here, we evaluate this hypothesis by considering the anatomical overlap of retrieval and attention effects in lateral PPC. We begin by briefly reviewing the literature implicating dorsal PPC in goal-directed attention and ventral PPC in reflexive attention. We then discuss the pattern of dorsal and ventral PPC activation during episodic retrieval, and conclude with consideration of the degree of anatomical convergence across the two domains. This assessment revealed that predominantly divergent subregions of lateral PPC are engaged during acts of episodic retrieval and during goal-directed and reflexive attention, suggesting that PPC retrieval effects reflect functionally distinct mechanisms from these forms of attention. Although attention must play a role in aspects of retrieval, the data reviewed here suggest that further investigation into the relationship between processes of attention and memory, as well as alternative accounts of PPC contributions to retrieval, is warranted.

Low- and high-level first-order random-dot kinematograms: evidence from fMRI

Maximum motion displacement (Dmax) represents the largest dot displacement in a random-dot kinematogram (RDK) at which direction of motion can be discriminated. Direction discrimination thresholds for maximum motion displacement (Dmax) are not fixed but are stimulus dependent. For first-order RDKs, Dmax is larger as dot size increases and/or dot density decreases. Dmax may be limited by the receptive field size of low-level motion detectors when the dots comprising the RDK are small and densely spaced. With RDKs of increased dot size/decreased dot density, however, Dmax exceeds the spatial limits of these detectors and is likely determined by high-level feature-matching mechanisms. Using functional MRI, we obtained greater activation in posterior occipital areas for low-level RDKs and greater activation in extra-striate occipital and parietal areas for high-level RDKs. This is the first reported neuroimaging evidence supporting proposed low-level and high-level models of motion processing for first-order random-dot stimuli.

Using fMRI to distinguish components of the multiple object tracking task

Multiple object tracking (MOT) has proven to be a powerful technique for studying sustained selective attention. However, surprisingly little is known about its underlying neural mechanisms. Previous fMRI investigations have identified several brain areas thought to be involved in MOT, but there were disagreements between the studies, none distinguished between the act of tracking targets and the act of attending targets, and none attempted to determine which of these brain areas interact with each other. Here we address these three issues. First, using more observers and a random effects analysis, we show that some of the previously identified areas may not play a specific role in MOT. Second, we show that the frontal eye fields (FEF), the anterior intraparietal sulcus (AIPS), the superior parietal lobule (SPL), the posterior intraparietal sulcus (PIPS) and the human motion area (MT+) are differentially activated by the act of tracking, as distinguished from the act of attention. Finally, by using an algorithm modified from the computer science literature, we were able to map the interactions between these brain areas.

Multiple parietal reach regions in humans: cortical representations for visual and proprioceptive feedback during on-line reaching

Reaching toward a visual target involves at least two sources of information. One is the visual feedback from the hand as it approaches the target. Another is proprioception from the moving limb, which informs the brain of the location of the hand relative to the target even when the hand is not visible. Where these two sources of information are represented in the human brain is unknown. In the present study, we investigated the cortical representations for reaching with or without visual feedback from the moving hand, using functional magnetic resonance imaging. To identify reach-dominant areas, we compared reaching with saccades. Our results show that a reach-dominant region in the anterior precuneus (aPCu), extending into medial intraparietal sulcus, is equally active in visual and nonvisual reaching. A second region, at the superior end of the parieto-occipital sulcus (sPOS), is more active for visual than for nonvisual reaching. These results suggest that aPCu is a sensorimotor area whose sensory input is primarily proprioceptive, while sPOS is a visuomotor area that receives visual feedback during reaching. In addition to the precuneus, medial, anterior intraparietal, and superior parietal cortex were also activated during both visual and nonvisual reaching, with more anterior areas responding to hand movements only and more posterior areas responding to both hand and eye movements. Our results suggest that cortical networks for reaching are differentially activated depending on the sensory conditions during reaching. This indicates the involvement of multiple parietal reach regions in humans, rather than a single homogenous parietal reach region.

Is that within reach? fMRI reveals that the human superior parieto-occipital cortex encodes objects reachable by the hand

Macaque neurophysiology and human neuropsychology results suggest that parietal cortex encodes a unique representation of space within reach of the arm. Here, we used slow event-related functional magnetic resonance imaging (fMRI) to investigate whether human brain areas involved in reaching are more activated by objects within reach versus beyond reach. In experiment 1, graspable objects were placed at three possible locations on a platform: two reachable locations and one beyond reach. On some trials, participants reached to touch or grasp objects at the reachable location; on other trials participants passively viewed objects at one of the three locations. A reach-related area in the superior parieto-occipital cortex (SPOC) was more activated for targets within reach than beyond. In experiment 2, we investigated whether this SPOC response occurred when visual and motor confounds were controlled and whether it was modulated when a tool extended the effective range of the arm. On some trials, participants performed grasping and reaching actions to a reachable object location using either the hand alone or a tool; on other trials, participants passively viewed reachable and unreachable object locations. SPOC was significantly more active for passively viewed objects within reach of the hand versus beyond reach, regardless of whether or not a tool was available. Interestingly, these findings suggest that neural responses within brain areas coding actions (such as SPOC for reaching) may reflect automatic processing of motor affordances (such as reachability with the hand).

Selecting and perceiving multiple visual objects

To explain how multiple visual objects are attended and perceived, we propose that our visual system first selects a fixed number of about four objects from a crowded scene based on their spatial information (object individuation) and then encode their details (object identification). We describe the involvement of the inferior intra-parietal sulcus (IPS) in object individuation and the superior IPS and higher visual areas in object identification. Our neural object-file theory synthesizes and extends existing ideas in visual cognition and is supported by behavioral and neuroimaging results. It provides a better understanding of the role of the different parietal areas in encoding visual objects and can explain various forms of capacity-limited processing in visual cognition such as working memory.

Impairment of attentional networks after 1 night of sleep deprivation

Here, we assessed the effects of sleep deprivation (SD) on brain activation and performance to a parametric visual attention task. Fourteen healthy subjects underwent functional magnetic resonance imaging of ball-tracking tasks with graded levels of difficulty during rested wakefulness (RW) and after 1 night of SD. Self-reports of sleepiness were significantly higher and cognitive performance significantly lower for all levels of difficulty for SD than for RW. For both the RW and the SD sessions, task difficulty was associated with activation in parietal cortex and with deactivation in visual and insular cortices and cingulate gyrus but this pattern of activation/deactivation was significantly lower for SD than for RW. In addition, thalamic activation was higher for SD than for RW, and task difficulty was associated with increases in thalamic activation for the RW but not the SD condition. This suggests that thalamic resources, which under RW conditions are used to process increasingly complex tasks, are being used to maintain alertness with increasing levels of fatigue during SD. Thalamic activation was also inversely correlated with parietal and prefrontal activation. Thus, the thalamic hyperactivation during SD could underlie the reduced activation in parietal and blunted deactivation in cingulate cortices, impairing the attentional networks that are essential for accurate visuospatial attention performance.

Connectivity and signal intensity in the parieto-occipital cortex predicts top-down attentional effect in visual masking: an fMRI study based on individual differences

Top-down attention affects even the early stages of visual processing. For example, several studies have reported that instructions prior to the presentation of visual stimuli can both enhance and reduce visual masking. The finding that top-down processing influences perceptual processing is called the attentional effect. However, the magnitude of the attentional effect differs between individuals, and how these differences relate to brain activation remains to be explained. One possibility would be that activation intensity predicts the magnitude of the attentional effect. Another possible explanation would be that effective connectivity among activated areas determines the attentional effect. In the present study, we used structural equation modeling to analyze individual differences in the attentional effect on visual masking, in relation to the signal and connectivity strength of activated brain regions prior to presentation of the visual stimuli. The results showed that signal intensity was positively correlated with attentional effect in the occipital areas, but not in fronto-parietal areas, and the effect was also positively correlated with connective efficiency from the right intraparietal sulcus (IPS) to the bilateral fusiform gyrus (GF). Furthermore, a higher degree of effective connections from the right IPS to the GF led to greater neural activity in the GF. We therefore propose that the effective modulator in the parietal areas and strong activation in the visual areas together and in cooperation predict higher attentional effects in visual processing.

The motion aftereffect reloaded

The motion aftereffect is a robust illusion of visual motion resulting from exposure to a moving pattern. There is a widely accepted explanation of it in terms of changes in the response of cortical direction-selective neurons. Research has distinguished several variants of the effect. Converging recent evidence from different experimental techniques (psychophysics, single-unit recording, brain imaging, transcranial magnetic stimulation, visual evoked potentials and magnetoencephalography) reveals that adaptation is not confined to one or even two cortical areas, but occurs at multiple levels of processing involved in visual motion analysis. A tentative motion-processing framework is described, based on motion aftereffect research. Recent ideas on the function of adaptation see it as a form of gain control that maximises the efficiency of information transmission at multiple levels of the visual pathway.

Age-related changes in attentional tracking of multiple moving objects

In a multiple-object tracking (MOT) task, young and older adults attentively tracked a subset of 10 identical, randomly moving disks for several seconds, and then tried to identify those disks that had comprised the subset. Young adults who habitually played video games performed significantly better than those who did not. Compared to young subjects (mean age = 20.6 years) with whom they were matched for video game experience, older subjects (mean age = 75.3 years) showed much reduced ability to track multiple moving objects, particularly with faster movement or longer tracking times. Control measurements with stationary disks show that the age-related decline in MOT was not caused by a general change in memory per se. To generate an item-wise performance measure, we examined older subjects' proportion correct according to the serial order in which individual disks were identified. Correct identification of target disks declined with the order in which targets were reported, suggesting that attentional tracking produced graded, rather than all-or-none, outcomes.

Privileged coding of convex shapes in human object-selective cortex

What is the neural code for object shape? Despite intensive research, the precise nature of object representations in high-level visual cortex remains elusive. Here we use functional magnetic resonance imaging (fMRI) to show that convex shapes are encoded in a privileged fashion by human lateral occipital complex (LOC), a region that has been implicated in object recognition. On each trial, two convex or two concave shapes that were either identical or different were presented sequentially. Critically, the convex and concave stimuli were the same except for a binocular disparity change that reversed the figure-ground assignment. The fMRI response in LOC for convex stimuli was higher for different than that for identical shape pairs, indicating sensitivity to differences in convex shape. However, when the same stimuli were seen as concave, the response for different and identical pairs was the same, indicating lower sensitivity to changes in concave shape than convex shape. This pattern was more pronounced in the anterior than that in the posterior portion of LOC. These results suggest that convex contours could be important elements in cortical object representations.

State anxiety and visual attention: the role of the quiet eye period in aiming to a far target

In this study, we examined how individuals controlled their gaze behaviour during execution of a far aiming task and whether the functional relationship between perception and action was disrupted by increased anxiety. Twenty participants were trained on a simulated archery task, using a joystick to aim and shoot arrows at the target, and then competed in two counterbalanced experimental conditions designed to manipulate the anxiety they experienced. The specific gaze behaviour measured was the duration of the quiet eye period. As predicted, accuracy was affected by the duration of the quiet eye period, with longer quiet eye periods being associated with better performance. The manipulation of anxiety resulted in reductions in the duration of quiet eye. Our results show that the quiet eye period is sensitive to increases in anxiety and may be a useful index of the efficiency of visual orientation in aiming tasks.

Contrast detection in infants with fragile X syndrome

Studies have reported that a selective deficit in visual motion processing is present in certain developmental disorders, including Williams syndrome and autism. More recent evidence suggests a visual motion impairment is also present in adults with fragile X syndrome (FXS), the most common form of inherited mental retardation. The goal of the current study was to examine low-level cortical visual processing in infants diagnosed with FXS in order to explore the developmental origin of this putative deficit. We measured contrast detection of first-order (luminance-defined) and second-order (contrast-defined) gratings at two levels of temporal frequency, 0 Hz (static) and 4 Hz (moving). Results indicate that infants with FXS display significantly higher detection thresholds only for the second-order, moving stimuli compared to mental age-matched typically developing controls.

Neural measures of individual differences in selecting and tracking multiple moving objects

Attention can be divided so that multiple objects can be tracked simultaneously as they move among distractors. Although attentional tracking is known to be highly limited, such that most individuals can track only approximately four objects simultaneously, the neurophysiological mechanisms that underlie this capacity limitation have not been established. Here, we provide electrophysiological measures in humans of the initial selection and sustained attention processes that facilitate attentional tracking. Each measure was modulated by the number of objects the subject was tracking and was highly sensitive to each individual's specific tracking capacity. Consequently, these measures provide strong neurophysiological predictors of an individual's attentional tracking capacity. Moreover, by manipulating the difficulty of these two phases of the task, we observe that the limiting factor underlying tracking capacity can flexibly shift between these two attentional mechanisms depending on the requirements of the task.

Tracking the changing features of multiple objects: progressively poorer perceptual precision and progressively greater perceptual lag

To measure the limits on attentive tracking of continuously changing features, in our task objects constantly changed smoothly and unpredictably in orientation, spatial period or position. Observers reported the last state of one of the objects. We observed a gradual decline in performance as the number of tracked objects increased, implicating a graded processing resource. Additionally, responses were more similar to previous states of the tracked object than its final state, especially in the case of spatial frequency. Indeed for spatial frequency, this perceptual lag reached 250ms when tracking four objects. The pattern of the perceptual lags, the graded effect of set size, and the double-report performance suggest the presence of both serial and parallel processing elements.

A spatio-temporal interaction on the apparent motion trace

During the perception of apparent motion, activity along the apparent motion trace has been found in the primary visual cortex. It has been hypothesized that this activity interferes with stimuli presented on the apparent motion trace ("motion masking"). We investigated whether this perceptual interference varies with regard to the trajectory of a moving object token in a detection task. We found a general decrease of detectability of targets presented on the trace. Surprisingly, targets presented in time with the trajectory were detected significantly more often than targets which appeared out of time. We relate this finding to a spatio-temporally specific prediction of visual events along the apparent motion trace.

Sex differences in sensory gating of the thalamus during auditory interference of visual attention tasks

Men and women have different cognitive abilities that might reflect sex-specific neural organization. Here we studied sex effects on brain function using functional magnetic resonance imaging (fMRI) with variable acoustic noise (AN) to modulate the cognitive challenge and enhance the sensitivity for the detection of sex differences in brain activation. During the performance of a visual attention (VA) task that requires the tracking of multiple moving objects and has graded levels of difficulty, women (n=15) but not men (n=13) had shorter reaction times for "Loud" than for "Quiet" scans. Men activated more than women in the superior prefrontal and occipital cortices and the anterior thalamus. The latent connectivity of the prefrontal cortex was higher with the anterior thalamus but lower with the auditory cortex for men than for women. Increases in activation with visual attention load were larger for men than for women in the superior parietal and auditory cortices. Increased AN reduced brain activation in the parietal cortex and the anterior thalamus for men but not for women. Together, these sex-specific differences in brain activation during the VA task, at different cognitive and acoustic levels suggest differences in auditory gating of the thalamus for men and women.

Lateralization of the Posterior Parietal Cortex for Internal Monitoring of Self- versus Externally Generated Movements

Internal monitoring or state estimation of movements is essential for human motor control to compensate for inherent delays and noise in sensorimotor loops. Two types of internal estimation of movements exist: self-generated movements, and externally generated movements. We used functional magnetic resonance imaging to investigate differences in brain activity for internal monitoring of self- versus externally generated movements during visual occlusion. Participants tracked a sinusoidally moving target with a mouse cursor. On some trials, vision of either target (externally generated) or cursor (self-generated) movement was transiently occluded, during which subjects continued tracking by estimating current position of either the invisible target or cursor on screen. Analysis revealed that both occlusion conditions were associated with increased activity in the presupplementary motor area and decreased activity in the right lateral occipital cortex compared to a control condition with no occlusion. Moreover, the right and left posterior parietal cortex (PPC) showed greater activation during occlusion of target and cursor movements, respectively. This study suggests lateralization of the PPC for internal monitoring of internally versus externally generated movements, fully consistent with previously reported clinical findings.

Antiretroviral treatment is associated with increased attentional load-dependent brain activation in HIV patients

OBJECTIVE

The purpose of this paper was to determine whether antiretroviral medications, especially the nucleoside analogue reverse transcriptase inhibitors, lead to altered brain activation due to their potential neurotoxic effects in patients with human immunodeficiency virus (HIV) infection.

METHODS

Forty-two right-handed men were enrolled in three groups: seronegative controls (SN, n = 18), HIV subjects treated with antiretroviral medications (HIV+ARV, n = 12), or not treated with antiretroviral medications (HIV+NARV, n = 12). Each subject performed a set of visual attention tasks with increasing difficulty or load (tracking two, three or four balls) during functional magnetic resonance imaging.

RESULTS

HIV subjects, both groups combined, showed greater load-dependent increases in brain activation in the right frontal regions compared to SN (p-corrected = 0.006). HIV+ARV additionally showed greater load-dependent increases in activation compared to SN in bilateral superior frontal regions (p-corrected = 0.032) and a lower percent accuracy on the performance of the most difficult task (tracking four balls). Region of interest analyses further demonstrated that SN showed load-dependent decreases (with repeated trials despite increasing difficulty), while HIV subjects showed load-dependent increases in activation with the more difficult tasks, especially those on ARVs.

INTERPRETATION

These findings suggest that chronic ARV treatments may lead to greater requirement of the attentional network reserve and hence less efficient usage of the network and less practice effects in these HIV patients. As the brain has a limited reserve capacity, exhausting the reserve capacity in HIV+ARV would lead to declined performance with more difficult tasks that require more attention.

Neural substrates of dynamic object occlusion

In everyday environments, objects frequently go out of sight as they move and our view of them becomes obstructed by nearer objects, yet we perceive these objects as continuous and enduring entities. Here, we used functional magnetic resonance imaging with an attentive tracking paradigm to clarify the nature of perceptual and cognitive mechanisms subserving this ability to fill in the gaps in perception of dynamic object occlusion. Imaging data revealed distinct regions of cortex showing increased activity during periods of occlusion relative to full visibility. These regions may support active maintenance of a representation of the target's spatiotemporal properties ensuring that the object is perceived as a persisting entity when occluded. Our findings may shed light on the neural substrates involved in object tracking that give rise to the phenomenon of object permanence.

Human cortical representations for reaching: mirror neurons for execution, observation, and imagery

We used functional magnetic resonance imaging (fMRI) to map the cortical representations of executed reaching, observed reaching, and imagined reaching in humans. Whereas previous studies have mostly examined hand actions related to grasping, hand-object interactions, or local finger movements, here we were interested in reaching only (i.e. the transport phase of the hand to a particular location in space), without grasping. We hypothesized that mirror neuron areas specific to reaching-related representations would be active in all three conditions. An overlap between executed, observed, and imagined reaching activations was found in dorsal premotor cortex as well as in the superior parietal lobe and the intraparietal sulcus, in accord with our hypothesis. Activations for observed reaching were more dorsal than activations typically reported in the literature for observation of hand-object interactions (grasping). Our results suggest that the mirror neuron system is specific to the type of hand action performed, and that these fronto-parietal activations are a putative human homologue of the neural circuits underlying reaching in macaques. The parietal activations reported here for executed, imagined, and observed reaching are also consistent with previous functional imaging studies on planned reaching and delayed pointing movements, and extend the proposed localization of human reach-related brain areas to observation as well as imagery of reaching.

Gaze pursuit, ‘attention pursuit’ and their effects on cortical activations

A moving object draws our attention to it and we can track the object with smooth pursuit eye movements (SPEM). Gaze and attention are usually directed to the same object during SPEM. In this study we investigated whether gaze and attention can be divided during pursuit. We explored the cortical control of ocular tracking and attentive tracking and the role of focused and divided attention. We presented a sinusoidally moving target for pursuit and simultaneously a stationary target for fixation. Gaze could be directed to the pursuit target and attention to the fixation target or vice versa, or gaze and attention were directed to the same (moving or stationary) target. We found that gaze (overt) and attentive (covert) pursuit similarly activated the cortical oculomotor network. Gaze pursuit showed higher activations than attentive pursuit. Activations, specific to the dissociation of attention from gaze and independent of eye movements, were found solely in the posterior parietal cortex. A cue indicating a forthcoming attention task activated large parts of the cortical SPEM network, as a kind of preparatory mechanism. We did not find any attention-related regions outside the well-known visuo-oculomotor network. We conclude that attention control during gaze pursuit and gaze fixation occur within the cortical SPEM network, supporting the premotor theory of attention [Rizzolatti, G., Riggio, L., Dascola, I. & Umilta, C. (1987) Neuropsychologia, 25, 31-40].

Low frequency rTMS over posterior parietal cortex impairs smooth pursuit eye tracking

The role of the posterior parietal cortex in smooth pursuit eye movements remains unclear. We used low frequency repetitive transcranial magnetic stimulation (rTMS) to study the cognitive and neural systems involved in the control of smooth pursuit eye movements. Eighteen participants were tested on two separate occasions. On each occasion we measured smooth pursuit eye tracking before and after 6 min of 1 Hz rTMS delivered at 90% of motor threshold. Low frequency rTMS over the posterior parietal cortex led to a significant reduction in smooth pursuit velocity gain, whereas rTMS over the motor cortex had no effect on gain. We conclude that low frequency offline rTMS is a potentially useful tool with which to explore the cortical systems involved in oculomotor control.

Stereopsis-dependent deficits in maximum motion displacement in strabismic and anisometropic amblyopia

Direction discrimination thresholds for maximum motion displacement (D(max)) have been previously reported to be abnormal in amblyopic children [Ho, C. S., Giaschi, D. E., Boden, C., Dougherty, R., Cline, R., & Lyons, C. (2005). Deficient motion perception in the fellow eye of amblyopic children. Vision Research, 45, 1615-1627; Ho, C. S., & Giaschi, D. E. (2006). Deficient maximum motion displacement in amblyopia. Vision Research, 46, 4595-4603]. We looked at D(max) thresholds for random dot kinematograms (RDKs) biased toward low- or high-level motion mechanisms. D(max) is thought to be limited, for high-level motion mechanisms, by the efficiency of object feature tracking and probability of false matches. To reduce the influence of low-level mechanisms, we determined thresholds also for a high-pass filtered version of the RDKs. Performance did not significantly differ between strabismic and anisometropic groups with amblyopia, although both groups performed significantly worse than the age-matched control group. D(max) thresholds were higher for children with poor stereoacuity. This was significant in both anisometropic and strabismic groups, and more robust for high-pass filtered RDKs than for unfiltered RDKs. The results imply that impairment of the extra-striate dorsal stream is a likely part of the neural deficit underlying both strabismic and anisometropic amblyopia. This deficit appears to be more dependent on extent of binocularity than etiology. Our findings suggest a possible relationship between fine stereopsis, coarse stereopsis, and motion correspondence mechanisms.

Thalamo-cortical dysfunction in cocaine abusers: implications in attention and perception

Cocaine affects sensory perception and attention, but little is known about the neural substrates underlying these effects in the human brain. We used functional magnetic resonance imaging (fMRI) and a sustained visuospatial attention task to assess if the visual attention network is dysfunctional in cocaine abusers (n=14) compared to age-, gender-, and education-matched controls (n=14). Compared with controls, cocaine abusers showed (1) hypo-activation of the thalamus, which may reflect noradrenergic and/or dopaminergic deficits; (2) hyper-activation in occipital and prefrontal cortices, which may reflect increased visual cortical processing to compensate for inefficient visual thalamic processing; and (3) larger deactivation of parietal and frontal regions possibly to support the larger hemodynamic supply to the hyper-activated brain regions. These findings provide evidence of abnormalities in thalamo-cortical responses in cocaine abusers that are likely to contribute to the impairments in sensory processing and in attention. The development of therapies that diminish these thalamo-cortical deficits could improve the treatment of cocaine addiction.

Quadrantic deficit reveals anatomical constraints on selection

Our conscious experience is of a seamless visual world, but many of the cortical areas that underlie our capacity for vision have a fragmented or asymmetrical representation of visual space. In fact, the representation of the visual field is fragmented into quadrants at the level of V2, V3, and possibly V4. In theory, this division could have no functional consequences and therefore no impact on behavior. Contrary to this expectation, we find robust quadrant-level interference effects when attentively tracking two moving targets. Performance improves when target objects appear in separate quadrants (straddling either the horizontal or vertical meridian) compared with when they appear the same distance apart but within a single quadrant. These quadrant-level interference effects would not be predicted by cognitive theories of attention and tracking that do not take anatomical constraints into account. Quadrant-level interference strongly suggests that cortical areas containing a noncontiguous representation of the four quadrants of the visual field (i.e., V2, V3, and V4) impose an important constraint on attentional selection and attentive tracking.

Natural vision reveals regional specialization to local motion and to contrast-invariant, global flow in the human brain

Visual changes in feature movies, like in real-live, can be partitioned into global flow due to self/camera motion, local/differential flow due to object motion, and residuals, for example, due to illumination changes. We correlated these measures with brain responses of human volunteers viewing movies in an fMRI scanner. Early visual areas responded only to residual changes, thus lacking responses to equally large motion-induced changes, consistent with predictive coding. Motion activated V5+ (MT+), V3A, medial posterior parietal cortex (mPPC) and, weakly, lateral occipital cortex (LOC). V5+ responded to local/differential motion and depended on visual contrast, whereas mPPC responded to global flow spanning the whole visual field and was contrast independent. mPPC thus codes for flow compatible with unbiased heading estimation in natural scenes and for the comparison of visual flow with nonretinal, multimodal motion cues in it or downstream. mPPC was functionally connected to anterior portions of V5+, whereas laterally neighboring putative homologue of lateral intraparietal area (LIP) connected with frontal eye fields. Our results demonstrate a progression of selectivity from local and contrast-dependent motion processing in V5+ toward global and contrast-independent motion processing in mPPC. The function, connectivity, and anatomical neighborhood of mPPC imply several parallels to monkey ventral intraparietal area (VIP).

Temporal limits of long-range phase discrimination across the visual field

When two flickering sources are far enough apart to avoid low-level motion signals, phase judgment relies on the temporal individuation of the light and dark phases of each source. The highest rate at which the individuation can be maintained has been referred to as Gestalt flicker fusion [Van de Grind, W. A., Grüsser, O. -J., & Lunkenheimer, H. U. (1973). Temporal transfer properties of the afferent visual system. Psychophysical, neurophysiological and theoretical investigations. In R. Jung (Ed.), Handbook of sensory physiology (Vol. VII/3, pp. 431-573). Berlin: Springer, Chapter 7] and this has been taken as a measure of the temporal resolution of attention [Verstraten, F. A., Cavanagh, P., & Labianca, A. T. (2000). Limits of attentive tracking reveal temporal properties of attention. Vision Research, 40, 3651-3664; Battelli, L., Cavanagh, P., Intriligator, J., Tramo, M. J., Henaff, M. A., Michel, F., et al. (2001). Unilateral right parietal damage leads to bilateral deficit for high-level motion. Neuron, 32, 985-995]. Here we examine the variation of the temporal resolution of attention across the visual field using phase judgments of widely spaced pairs of flickering dots presented either in the upper or lower visual field and at either 4 degrees or 14 degrees eccentricity. We varied inter-dot separation to determine the spacing at which phase discriminations are no longer facilitated by low-level motion signals. Our data for these long-range phase judgments showed that temporal resolution decreases only slightly with increased distance from center of gaze (decrease from 11.4 to 8.9 Hz between 4 degrees to 14 degrees ), and does not differ between upper and lower visual fields. We conclude that the variation of the temporal limits of visual attention across the visual field differs markedly from that of the spatial resolution of attention.

Visual topography of human intraparietal sulcus

Human parietal cortex is implicated in a wide variety of sensory and cognitive functions, yet its precise organization remains unclear. Visual field maps provide a potential structural basis for descriptions of functional organization. Here, we detail the topography of a series of five maps of the contralateral visual hemifield within human posterior parietal cortex. These maps are located along the medial bank of the intraparietal sulcus (IPS) and are revealed by direct visual stimulation during functional magnetic resonance imaging, allowing these parietal regions to be routinely and reliably identified simultaneously with occipital visual areas. Two of these maps (IPS3 and IPS4) are novel, whereas two others (IPS1 and IPS2) have previously been revealed only by higher-order cognitive tasks. Area V7, a previously identified visual map, is observed to lie within posterior IPS and to share a foveal representation with IPS1. These parietal maps are reliably observed across scan sessions; however, their precise topography varies between individuals. The multimodal organization of posterior IPS mirrors this variability in visual topography, with complementary tactile activations found immediately adjacent to the visual maps both medially and laterally. These visual maps may provide a practical framework in which to characterize the functional organization of human IPS.

Perceptual completion in a dynamic scene: an investigation with an ambiguous motion paradigm

In this study we employed the streaming-bouncing stimulus to investigate aspects of dynamic occlusion, e.g., of objects that temporarily move under occlusion while covertly being tracked. Two occluders, both either luminance-defined or invisible (virtual), were placed on the trajectories of the moving objects in the streaming-bouncing stimulus. We found that the bouncing percept was dominant when the objects moved under luminance-defined occluders but not when they moved under virtual occluders. Perceived motion direction thus varied with occluder visibility. The results seem to suggest that perceptual completion of a moving object interferes with constant motion processing of the same object.

Deafness and visual enumeration: not all aspects of attention are modified by deafness

Previous studies have demonstrated that early deafness causes enhancements in peripheral visual attention. Here, we ask if this cross-modal plasticity of visual attention is accompanied by an increase in the number of objects that can be grasped at once. In a first experiment using an enumeration task, Deaf adult native signers and hearing non-signers performed comparably, suggesting that deafness does not enhance the number of objects one can attend to simultaneously. In a second experiment using the Multiple Object Tracking task, Deaf adult native signers and hearing non-signers also performed comparably when required to monitor several, distinct, moving targets among moving distractors. The results of these experiments suggest that deafness does not significantly alter the ability to allocate attention to several objects at once. Thus, early deafness does not enhance all facets of visual attention, but rather its effects are quite specific.

Differentiation between external and internal cuing: an fMRI study comparing tracing with drawing

Externally cued movement is thought to preferentially involve cerebellar and premotor circuits whereas internally generated movement recruits basal ganglia, pre-supplementary motor cortex (pre-SMA) and dorsolateral prefrontal cortex (DLPFC). Tracing and drawing are exemplar externally and internally guided actions and Parkinson's patients and cerebellar patients show deficits in tracking and drawing, respectively. In this study we aimed to examine this external/internal distinction in healthy subjects using functional imaging. Ten healthy subjects performed tracing and drawing of simple geometric shapes using pencil and paper while in a 3-T fMRI scanner. Results indicated that compared to tracing, drawing generated greater activation in the right cerebellar crus I, bilateral pre-SMA, right dorsal premotor cortex and right frontal eye field. Tracing did not recruit any additional activation compared to drawing except in striate and extrastriate visual areas. Therefore, drawing recruited areas more frequently associated with cognitively challenging tasks, attention and memory, but basal ganglia and cerebellar activity did not differentiate tracing from drawing in the hypothesised manner. As our paradigm was of a simple, repetitive and static design, these results suggest that the task familiarity and the temporal nature of visual feedback in tracking tasks, compared to tracing, may be important contributing factors towards the degree of cerebellar involvement. Future studies comparing dynamic with static external cues and visual feedback may clarify the role of the cerebellum and basal ganglia in the visual guidance of drawing actions.

Neural substrates of driving behaviour

Driving a vehicle is an indispensable daily behaviour for many people, yet we know little about how it is supported by the brain. Given that driving in the real world involves the engagement of many cognitive systems that rapidly change to meet varying environmental demands, identifying its neural basis presents substantial problems. By employing a unique combination of functional magnetic resonance imaging (fMRI), an accurate interactive virtual simulation of a bustling central London (UK) and a retrospective verbal report protocol, we surmounted these difficulties. We identified different events that characterise the driving process on a second by second basis and the brain regions that underlie them. Prepared actions such as starting, turning, reversing and stopping were associated with a common network comprised of premotor, parietal and cerebellar regions. Each prepared action also recruited additional brain areas. We also observed unexpected hazardous events such as swerving and avoiding collisions that were associated with activation of lateral occipital and parietal regions, insula, as well as a more posterior region in the medial premotor cortex than prepared actions. By contrast, planning future actions and monitoring fellow road users were associated with activity in superior parietal, lateral occipital cortices and the cerebellum. The anterior pre-SMA was also recruited during action planning. The right lateral prefrontal cortex was specifically engaged during the processing of road traffic rules. By systematically characterising the brain dynamics underlying naturalistic driving behaviour in a real city, our findings may have implications for how driving competence is considered in the context of neurological damage.

Selective Visual Dimension Weighting Deficit after Left Lateral Frontopolar Lesions

The left lateral frontopolar (LFP) cortex showed dimension change-related activation in previous event-related functional magnetic resonance imaging studies of visual singleton feature search with non-brain-lesioned participants. Here, we tested the hypothesis that LFP actively supports changes of attention from the old to the new target-defining dimension in singleton feature search. Singleton detection was selectively slowed in this task when the target-defining dimension changed in patients with left LFP lesions, compared with patients with frontomedian lesions as well as with matched controls without brain lesions. We discuss a potential role of LFP in change detection when the optimal allocation of dimension-based attention is not clearly defined by the task.

An oculomotor decision process revealed by functional magnetic resonance imaging

It is not known how the brain decides to act on moving objects. We demonstrated previously that neurons in the macaque supplementary eye field (SEF) reflect the rule of ocular baseball, a go/nogo task in which eye movements signal the rule-guided interpretation of the trajectory of a target. In ocular baseball, subjects must decide whether to pursue a moving spot target with an eye movement after discriminating whether the target will cross a distal, visible line segment. Here we identify cortical regions active during the ocular baseball task using event-related human functional magnetic resonance imaging (fMRI) and concurrent eye-movement monitoring. Task-related activity was observed in the SEF, the frontal eye field (FEF), the superior parietal lobule (SPL), and the right ventrolateral prefrontal cortex (VLPFC). The SPL and right VLPFC showed heightened activity only during ocular baseball, despite identical stimuli and oculomotor demands in the control task, implicating these areas in the decision process. Furthermore, the right VLPFC but not the SPL showed the greatest activation during the nogo decision trials. This suggests both a functional dissociation between these areas and a role for the right VLPFC in rule-guided inhibition of behavior. In the SEF and FEF, activity was similar for ocular baseball and a control eye-movement task. We propose that, although the SEF reflects the ocular baseball rule, both areas in humans are functionally closer to motor processing than the SPL and the right VLPFC. By recording population activity with fMRI during the ocular baseball task, we have revealed the cortical substrate of an oculomotor decision process.

Endoscopic eye tracking system for fMRI

Here we introduce a new video-based real-time eye tracking system suitable for functional magnetic resonance imaging (fMRI) application. The described system monitors the subject's eye, which is illuminated with infrared light, directly at the headcoil using an endoscopic fibre optical system. This endoscopic technique assures reliable, easy-to-use and fast adjustment. It requires only a minimal amount of equipment at the headcoil and inside the examination room. Moreover, the short distance between the image acquisition optics and the eye provides high spatial tracking resolution. Interference from physiological head movement is effectively reduced by simultaneous tracking of both eye and head movements.

Tracking unique objects

Is content addressable in the representation that subserves performance in multiple-object-tracking (MOT) experiments? We devised an MOT variant that featured unique, nameable objects (cartoon animals) as stimuli. There were two possible response modes: standard, in which observers were asked to report the locations of all target items, and specific, in which observers had to report the location of a particular object (e.g., "Where is the zebra?"). A measure of capacity derived from accuracy allowed for comparisons of the results between conditions. We found that capacity in the specific condition (1.4 to 2.6 items across several experiments) was always reliably lower than capacity in the standard condition (2.3 to 3.4 items). Observers could locate specific objects, indicating a content-addressable representation. However, capacity differences between conditions, as well as differing responses to the experimental manipulations, suggest that there may be two separate systems involved in tracking, one carrying only positional information, and one carrying identity information as well.