What fMRI has taught us about human vision

Electrocortical correlates of attention differentiate individual capacity in associative learning

Associative learning abilities vary considerably among individuals, with attentional processes suggested to play a role in these variations. However, the relationship between attentional processes and individual differences in associative learning remains unclear, and whether these variations reflect in event-related potentials (ERPs) is unknown. This study aimed to investigate the relationship between attentional processes and associative learning by recording electrocortical activity of 38 young adults (18-32 years) during an associative learning task. Learning performance was assessed using the signal detection index d'. EEG topographic analyses and source localizations were applied to examine the neural correlates of attention and associative learning. Results revealed that better learning scores are associated with (1) topographic differences during early (126-148 ms) processing of the stimulus, coinciding with a P1 ERP component, which corresponded to a participation of the precuneus (BA 7), (2) topographic differences at 573-638 ms, overlapping with an increase of global field power at 530-600 ms, coinciding with a P3b ERP component and localized within the superior frontal gyrus (BA11) and (3) an increase of global field power at 322-507 ms, underlay by a stronger participation of the middle occipital gyrus (BA 19). These insights into the neural mechanisms underlying individual differences in associative learning suggest that better learners engage attentional processes more efficiently than weaker learners, making more resources available and displaying increased functional activity in areas involved in early attentional processes (BA7) and decision-making processes (BA11) during an associative learning task. This highlights the crucial role of attentional mechanisms in individual learning variability.

Three major psychiatric disorders share specific dynamic alterations of intrinsic brain activity

BACKGROUND

Increasing evidence suggests that major psychiatric disorders, including major depressive disorder (MDD), bipolar disorder (BD) and schizophrenia (SZ) share biological, neuropsychological and clinical features, despite the criteria for their respective diagnoses being different. Neuroimaging studies have shown disrupted 'static' neural connectivity in these disorders. However, the changes in brain dynamics across the three psychiatric disorders remain unknown.

METHODS

We aim to examine the connections and divergencies of the dynamic amplitude of low-frequency fluctuation (dALFF) in MDD, BD and SZ. In total, 901 participants [MDD, 229; BD, 146; SZ, 142; and healthy controls (HCs), 384] received resting-state functional magnetic resonance imaging. The dALFF was calculated using sliding-window analysis and compared across three psychiatric disorders.

RESULTS

We found significant increases of dALFF in the right fusiform, right hippocampus, right parahippocampal in participants with MDD, BD and SZ compared to HC. We also found specific increased dALFF changes in caudate and left thalamus for SZ and BD and decreased dALFF changes in calcarine and lingual for SZ and MDD.

CONCLUSION

Our study found significant intrinsic brain activity changes in the limbic system and primary visual area in MDD, BD, and SZ, which indicates these areas disruptions are core neurobiological features shared among three psychiatric disorders. Meanwhile, our findings also indicate that specific alterations in MDD, BD, and SZ provide neuroimaging evidence for the differential diagnosis of the three mental disorders.

Disrupted resting-state EEG alpha-band interactions as a novel marker for the severity of visual field deficits after brain lesion

OBJECTIVE

Homonymous visual field deficits (HFVDs) are frequent following brain lesions. Current restoration treatments aim at activating areas of residual vision through numerous stimuli, but show limited effect. Recent findings suggest that spontaneous neural α-band coupling is more efficient for enabling visual perception in healthy humans than task-induced activations. Here, we evaluated whether it is also associated with the severity of HFVD.

METHODS

Ten patients with HFVDs after brain damage in the subacute to chronic stage and ten matched healthy controls underwent visual stimulation with alternating checkerboards and electroencephalography recordings of stimulation-induced power changes and of spontaneous neural interactions during rest.

RESULTS

Visual areas of the affected hemisphere showed reduced event-related power decrease in α and β frequency bands, but also reduced spontaneous α-band interactions during rest, as compared to contralesional areas and healthy controls. A multivariate stepwise regression retained the degree of disruption of spontaneous interactions, but not the reduced task-induced power changes as predictor for the severity of the visual deficit.

CONCLUSIONS

Spontaneous α-band interactions of visual areas appear as a better marker for the severity of HFVDs than task-induced activations.

SIGNIFICANCE

Treatment attempts of HFVDs should try to enhance spontaneous α-band coupling of structurally intact ipsilesional areas.

Zooming-in on higher-level vision: High-resolution fMRI for understanding visual perception and awareness

One of the central questions in visual neuroscience is how the sparse retinal signals leaving our eyes are transformed into a rich subjective visual experience of the world. Invasive physiology studies, which offers the highest spatial resolution, have revealed many facts about the processing of simple visual features like contrast, color, and orientation, focusing on the early visual areas. At the same time, standard human fMRI studies with comparably coarser spatial resolution have revealed more complex, functionally specialized, and category-selective responses in higher visual areas. Although the visual system is the best understood among the sensory modalities, these two areas of research remain largely segregated. High-resolution fMRI opens up a possibility for linking them. On the one hand, it allows studying how the higher-level visual functions affect the fine-scale activity in early visual areas. On the other hand, it allows discovering the fine-scale functional organization of higher visual areas and exploring their functional connectivity with visual areas lower in the hierarchy. In this review, I will discuss examples of successful work undertaken in these directions using high-resolution fMRI and discuss where this method could be applied in the future to advance our understanding of the complexity of higher-level visual processing.

Spontaneous low-frequency fluctuations in the neural system for emotional perception in major psychiatric disorders: amplitude similarities and differences across frequency bands

BACKGROUND

Growing evidence indicates both shared and distinct features of emotional perception in schizophrenia, bipolar disorder and major depressive disorder. In these disorders, alterations in spontaneous low-frequency fluctuations have been reported in the neural system for emotional perception, but the similarities and differences in the amplitude of low-frequency fluctuation (ALFF) across the 3 disorders are unknown.

METHODS

We compared ALFF and its signal balance in the neural system for emotional perception at 2 frequency bands (slow-5 and slow-4) in 119 participants with schizophrenia, 100 with bipolar disorder, 123 with major depressive disorder and 183 healthy controls. We performed exploratory Pearson partial correlation analyses to determine the relationship between ALFF signal balance and clinical variables.

RESULTS

We observed commonalities in ALFF change patterns across the 3 disorders for emotional perception neural substrates, such as increased ALFF in the anterior cerebrum (including subcortical, limbic, paralimbic and heteromodal cortical regions) and decreased ALFF in the posterior visual cortices. Schizophrenia, bipolar disorder and major depressive disorder showed significantly decreased ALFF signal balance in the neural system for emotional perception at both slow-5 and slow-4 frequency bands, with the greatest alterations for schizophrenia, followed by bipolar disorder and major depressive disorder. We found a negative correlation between ALFF signal balance and negative/disorganized symptoms in slow-4 across the 3 disorders.

LIMITATIONS

The relatively broad age range in our sample and the cross-sectional study design may not account for our findings.

CONCLUSION

The extent of the commonalities we observed further support the concept of core neurobiological disruptions shared among the 3 disorders; ALFF signal balance could be an important neuroimaging marker for the diagnosis and treatment of schizophrenia, bipolar disorder and major depressive disorder.

Haptic fMRI: Reliability and performance of electromagnetic haptic interfaces for motion and force neuroimaging experiments

Haptic interfaces compatible with functional magnetic resonance imaging (Haptic fMRI) promise to enable rich motor neuroscience experiments that study how humans perform complex manipulation tasks. Here, we present a large-scale study (176 scans runs, 33 scan sessions) that characterizes the reliability and performance of one such electromagnetically actuated device, Haptic fMRI Interface 3 (HFI-3). We outline engineering advances that ensured HFI-3 did not interfere with fMRI measurements. Observed fMRI temporal noise levels with HFI-3 operating were at the fMRI baseline (0.8% noise to signal). We also present results from HFI-3 experiments demonstrating that high resolution fMRI can be used to study spatio-temporal patterns of fMRI blood oxygenation dependent (BOLD) activation. These experiments include motor planning, goal-directed reaching, and visually-guided force control. Observed fMRI responses are consistent with existing literature, which supports Haptic fMRI's effectiveness at studying the brain's motor regions.

Neurophysiological constraints on the eye-mind link

Several current computational models of eye-movement control in reading posit a tight link between the eye and mind, with lexical processing directly triggering most “decisions” about when to start programming a saccade to move the eyes from one word to the next. One potential problem with this theoretical assumption, however, is that it may violate neurophysiological constraints imposed by the time required to encode visual information, complete some amount of lexical processing, and then program a saccade. In this article, we review what has been learned about these timing constraints from studies using ERP and MEG. On the basis of this review, it would appear that the temporal constraints are too severe to permit direct lexical control of eye movements without a significant amount of parafoveal processing (i.e., pre-processing of word n+1 from word n). This conclusion underscores the degree to which the perceptual, cognitive, and motor processes involved in reading must be highly coordinated to support skilled reading, a par excellence example of a task requiring visual-cognitive expertise.

Prefrontal inhibition of threat processing reduces working memory interference

Bottom-up processes can interrupt ongoing cognitive processing in order to adaptively respond to emotional stimuli of high potential significance, such as those that threaten wellbeing. However it is vital that this interference can be modulated in certain contexts to focus on current tasks. Deficits in the ability to maintain the appropriate balance between cognitive and emotional demands can severely impact on day-to-day activities. This fMRI study examined this interaction between threat processing and cognition; 18 adult participants performed a visuospatial working memory (WM) task with two load conditions, in the presence and absence of anxiety induction by threat of electric shock. Threat of shock interfered with performance in the low cognitive load condition; however interference was eradicated under high load, consistent with engagement of emotion regulation mechanisms. Under low load the amygdala showed significant activation to threat of shock that was modulated by high cognitive load. A directed top-down control contrast identified two regions associated with top-down control; ventrolateral PFC and dorsal ACC. Dynamic causal modeling provided further evidence that under high cognitive load, top-down inhibition is exerted on the amygdala and its outputs to prefrontal regions. Additionally, we hypothesized that individual differences in a separate, non-emotional top-down control task would predict the recruitment of dorsal ACC and ventrolateral PFC during top-down control of threat. Consistent with this, performance on a separate dichotic listening task predicted dorsal ACC and ventrolateral PFC activation during high WM load under threat of shock, though activation in these regions did not directly correlate with WM performance. Together, the findings suggest that under high cognitive load and threat, top-down control is exerted by dACC and vlPFC to inhibit threat processing, thus enabling WM performance without threat-related interference.

Multi-voxel pattern analysis of selective representation of visual working memory in ventral temporal and occipital regions

While previous results from univariate analysis showed that the activity level of the parahippocampal gyrus (PHG) but not the fusiform gyrus (FG) reflects selective maintenance of the cued picture category, present results from multi-voxel pattern analysis (MVPA) showed that the spatial response patterns of both regions can be used to differentiate the selected picture category in working memory. The ventral temporal and occipital areas including the PHG and FG have been shown to be specialized in perceiving and processing different kinds of visual information, though their role in the representation of visual working memory remains unclear. To test whether the PHG and FG show spatial response patterns that reflect selective maintenance of task-relevant visual working memory in comparison with other posterior association regions, we reanalyzed data from a previous fMRI study of visual working memory with a cue inserted during the delay period of a delayed recognition task. Classification of FG and PHG activation patterns for the selected category (face or scene) during the cue phase was well above chance using classifiers trained with fMRI data from the cue or probe phase. Classification of activity in other temporal and occipital regions for the cued picture category during the cue phase was relatively less consistent even though classification of their activity during the probe recognition was comparable with the FG and PHG. In sum, these findings suggest that the FG and PHG carry information relevant to the cued visual category, and their spatial activation patterns during selective maintenance seem to match those during visual recognition.

Oscillatory modulations in human fusiform cortex during motion-induced blindness: intracranial recording

OBJECTIVE

Motion-induced blindness (MIB) is an illusory phenomenon, in which a static target surrounded by moving distracters is perceived to disappear. We determined the electrocorticographic (ECoG) correlates of MIB.

METHODS

While undergoing intracranial ECoG recording, a patient with focal epilepsy was instructed to report the transitions of a visual target, which was designed to illusorily or physically disappear and reappear. We then determined the neural modulations associated with illusory and physical transitions of the target. We also tested whether the phase of local delta activity could predict exclusively illusory transitions.

RESULTS

High-gamma activity at 80-150 Hz was attenuated in the fusiform region prior to the reports of illusory and real visual target disappearance. Conversely, such high-gamma activity was augmented prior to the report of real target reappearance. Exclusively around illusory disappearance but not around real one, the delta phases in the fusiform region showed a highly skewed distribution with preference of the negative peak.

CONCLUSIONS

Neuronal modulations in the fusiform region may be involved in visual awareness, while spontaneous fluctuations of neural states entrained on delta rhythm may be involved in generation of MIB.

SIGNIFICANCE

Our study increases our understanding of the mechanisms of visual awareness.

Deconstructing risk: separable encoding of variance and skewness in the brain

Risky choice entails a need to appraise all possible outcomes and integrate this information with individual risk preference. Risk is frequently quantified solely by statistical variance of outcomes, but here we provide evidence that individuals' choice behaviour is sensitive to both dispersion (variance) and asymmetry (skewness) of outcomes. Using a novel behavioural paradigm in humans, we independently manipulated these 'summary statistics' while scanning subjects with fMRI. We show that a behavioural sensitivity to variance and skewness is mirrored in neuroanatomically dissociable representations of these quantities, with parietal cortex showing sensitivity to the former and prefrontal cortex and ventral striatum to the latter. Furthermore, integration of these objective risk metrics with subjective risk preference is expressed in a subject-specific coupling between neural activity and choice behaviour in anterior insula. Our findings show that risk is neither monolithic from a behavioural nor neural perspective and its decomposition is evident both in distinct behavioural preferences and in segregated underlying brain representations.

Altered control of postural sway following cerebral infarction: a cross-sectional analysis

OBJECTIVE

Balance impairment is common following cerebral infarction. However, the effects of lesion hemisphere on postural control are largely unknown. We examined dependence upon vision and noninfarcted regional brain tissue volumes for postural control in individuals with right and left hemisphere middle cerebral artery (MCA) infarcts.

METHODS

Subjects with right MCA infarct (n = 17, age = 65 +/- 8 years, 7 +/- 6 years poststroke), left MCA infarct (n = 20, age = 65 +/- 8 years, 7 +/- 6 years poststroke), and controls (n = 55, age = 65 +/- 8 years) were studied. Postural control was defined by average velocity and the range and variability of mediolateral (ML) and anteroposterior (AP) sway during eyes-open and eyes-closed standing. Regional brain volumes were quantified using anatomic MRI at 3 Tesla.

RESULTS

Right and left hemisphere stroke groups had similar infarct volumes and outcomes. Subjects with right hemisphere infarcts demonstrated greater sway velocity, ML range, and ML variability with eyes closed compared to eyes open. In this group, smaller occipital lobe volumes were associated with greater eyes-open sway velocity (R = -0.64, p = 0.012) and ML range (R = -0.82, p = 0.001). Smaller cerebellar volumes were associated with greater eyes-closed sway velocity (R = -0.60, p = 0.015), ML range (R = -0.70, p = 0.007), and ML variability (R = -0.85, p < 0.001). These associations were not observed in left hemisphere infarct subjects or controls. AP sway was unaffected by infarct hemisphere or visual condition and did not correlate with regional brain volumes.

CONCLUSIONS

Right hemisphere middle cerebral artery infarcts are associated with increased dependence on vision and noninfarcted brain regions (i.e., occipital lobes, cerebellum) to control postural sway. Strategies emphasizing postural tasks under reduced visual conditions may enhance functional recovery in these individuals.

Automatic fMRI-guided MEG multidipole localization for visual responses

Previously, we introduced the use of individual cortical location and orientation constraints in the spatiotemporal Bayesian dipole analysis setting proposed by Jun et al. ([2005]; Neuroimage 28:84-98). However, the model's performance was limited by slow convergence and multimodality of the numerically estimated posterior distribution. In this paper, we present an intuitive way to exploit functional magnetic resonance imaging (fMRI) data in the Markov chain Monte Carlo sampling -based inverse estimation of magnetoencephalographic (MEG) data. We used simulated MEG and fMRI data to show that the convergence and localization accuracy of the method is significantly improved with the help of fMRI-guided proposal distributions. We further demonstrate, using an identical visual stimulation paradigm in both fMRI and MEG, the usefulness of this type of automated approach when investigating activation patterns with several spatially close and temporally overlapping sources. Theoretically, the MEG inverse estimates are not biased and should yield the same results even without fMRI information, however, in practice the multimodality of the posterior distribution causes problems due to the limited mixing properties of the sampler. On this account, the algorithm acts perhaps more as a stochastic optimizer than enables a full Bayesian posterior analysis.

Visualization of the visual cortex in minipigs using fMRI

Minipigs are widely used to examine physiologic mechanisms under experimental settings. The purpose of our study was to localize the visual cortex of minipigs using visual stimulation in functional magnetic resonance imaging (fMRI). Five male Göttinger minipigs were studied with fMRI during visual stimulation with series of light impulses conducted into the MR cabin via fibre optical wave guides (EPI sequences, block design, 1.5 T clinical routine MR-scanner). Statistics were performed using SPM'99. All minipigs showed activation of the visual cortex due to light stimulation. The activation could be superimposed to T2-weighted structural images with good spatial resolution revealing the exact location of the visual cortex. We were able to localize the visual cortex in minipigs for the first time utilizing fMRI on a routine scanner. However, this may be useful for precise positioning of electrodes or implants or be used as a further uncomplicated non-invasive method in physiologic experiments.

Functional neuroimaging findings on the human perception of illusory contours

Illusory contours (IC) have attracted a considerable interest in recent years to derive models of how sensory information is processed and integrated within the visual system. In addition to various findings from neuropsychology, neurophysiology, and psychophysics, several recent studies have used functional neuroimaging to identify the cerebral substrates underlying human perception of IC (in particular Kanizsa figures). In this paper, we review the results from more than 20 neuroimaging studies on IC perception and highlight the great diversity of findings across these studies. We then provide a detailed discussion about the localization ('where' debate) and the timing ('when' debate) of IC processing as suggested by functional neuroimaging. Cortical responses involving visual areas as early as V1/V2 and latencies as rapid as 100 ms have been reported in several studies. Particular issues concerning the role of the right hemisphere and the retinotopic encoding of IC are also discussed. These different findings are tentatively brought together to propose different hypothetical cortical mechanisms that might be responsible for the visual formation of IC. Several remaining questions on IC processing that could potentially be explored with functional neuroimaging techniques are finally emphasized.

Information theory, novelty and hippocampal responses: unpredicted or unpredictable?

Shannon's information theory provides a principled framework for the quantitative analysis of brain responses during the encoding and representation of event streams. In particular, entropy measures the expected uncertainty of events in a given context. This contextual uncertainty or unpredictability may, itself, be important for balancing [bottom-up] sensory information and [top-down] prior expectations during perceptual synthesis. Using event-related functional magnetic resonance imaging (fMRI), we found that the anterior hippocampus is sensitive to the entropy of a visual stimulus stream. In contrast, activity in an extensive bilateral cortico-thalamic network was dictated by the surprise or information associated with each particular stimulus. In short, we show that the probabilistic structure or context in which events occur is an important predictor of hippocampal activity.

The reverse hierarchy theory of visual perceptual learning

Perceptual learning can be defined as practice-induced improvement in the ability to perform specific perceptual tasks. We previously proposed the Reverse Hierarchy Theory as a unifying concept that links behavioral findings of visual learning with physiological and anatomical data. Essentially, it asserts that learning is a top-down guided process, which begins at high-level areas of the visual system, and when these do not suffice, progresses backwards to the input levels, which have a better signal-to-noise ratio. This simple concept has proved powerful in explaining a broad range of findings, including seemingly contradicting data. We now extend this concept to describe the dynamics of skill acquisition and interpret recent behavioral and electrophysiological findings.

Investigations of the human visual system using functional magnetic resonance imaging (FMRI)

The application of functional magnetic resonance imaging (fMRI) in studies of the visual system provided significant advancement in our understanding of the organization and functional properties of visual areas in the human cortex. Recent technological and methodological improvements allowed studies to correlate neuronal activity with visual perception and demonstrated the ability of fMRI to observe distributed neural systems and to explore modulation of neural activity during higher cognitive processes. Preliminary applications in patients with visual impairments suggest that this method provides a powerful tool for the assessment and management of brain pathologies. Recent research focuses on obtaining new information about the spatial localization, organization, functional specialization and participation in higher cognitive functions of visual cortical areas in the living human brain and in further establishment of the method as a useful clinical tool of diagnostic and prognostic significance for various pathologic processes affecting the integrity of the visual system. It is anticipated that the combined neuroimaging approach in patients with lesions and healthy controls will provide new insight on the topography and functional specialization of cortical visual areas and will further establish the clinical value of the method for improving diagnostic accuracy and treatment planning.

From sensory processes to conscious perception

In recent years, cognitive neuroscientists have began to explore the process of how sensory information gains access to awareness. To further probe this process, event-related functional magnetic resonance imaging (fMRI) was used while testing subjects with a paradigm known as the "attentional blink." In this paradigm, visually presented information sporadically fails to reach awareness. It was found that the magnitude and time course of activation within the anterior cingulate (BA 32), medial prefrontal cortex (BA 9), and frontopolar cortex (BA 10) predicted whether or not information was consciously perceived during the critical period for the attentional blink. These results are discussed in light of a neural framework for conscious processing.

The Human Basal Forebrain Integrates the Old and the New

Acquisition of new learning is challenged by the phenomenon of proactive interference (PI), which occurs when previous learning disrupts later learning. Whereas human neuroimaging studies have focused on the cortical contributions to interference resolution, animal studies demonstrate that efficient resolution of PI depends on cholinergic modulation from basal forebrain (BF). Whether the BF promotes PI resolution in humans is unknown. Here, we adapted a PI paradigm from animal studies for use in a functional MRI experiment. During PI resolution, neurologically intact subjects recruited a BF network that included afferent anterior and posterior cortical sites associated with efficient memory acquisition and perceptual processing. Despite normal performance, nonamnesic patients with alcoholism, which is known to disrupt BF function, did not activate a BF network but instead invoked anterior cortical sites traditionally associated with executive function. These results provide evidence for parallel neural systems, each with the potential to resolve interference in the face of competing information.

Pain imaging: future applications to integrative clinical and basic neurobiology

We have entered a new era in understanding CNS circuitry involved in acute and chronic pain. The ability to objectively measure a pain or analgesic state of the brain using non-invasive methods that define neural activation provides the possibility for top-down approaches to drug discovery. These brain maps represent the specific brain state. In the future, correlations with such states and behavioral, genetic, epigenetic or other chemical markers may help define specific diagnostic tools and novel approaches to drug discovery.

Functional magnetic resonance imaging and its clinical utility in patients with visual disturbances

Functional magnetic resonance imaging (fMRI) is a powerful, non-invasive technique for mapping human brain function. Because of the robust signal intensity changes associated with visual stimuli, fMRI is particularly useful for studying visual cortex (including both striate and extrastriate cortex). Also, activation of the lateral geniculate nuclei has been successfully demonstrated by fMRI. Therefore, fMRI may be potentially useful in patients with visual deficits by providing a non-invasive method for assessing the afferent visual pathways and higher cortical areas. Although there have been several reviews on fMRI, few have highlighted its clinical applicability in patients with visual disturbances. Our article will review fMRI principles and methodology, then focus on the possible applications and limitations of this technique in clinical ophthalmology.

Visual feature and conjunction searches of equal difficulty engage only partially overlapping frontoparietal networks

According to a classical view of visual object recognition, targets are detected "pre-attentively" if they carry unique features, whereas attention has to be deployed serially to object locations for feature binding if the targets can be distinguished from distracters only in terms of their feature conjunctions. Consistent with this view, recent reports suggest a contribution of the posterior parietal cortex (PPC; one major region controlling spatial attention) to conjunction search as opposed to feature search. However, PPC engagement in conjunction search might also reflect feature-based attention or the difficulty of target selection. The present fMRI study compared regions and amplitudes of cortical activity reflecting the attention mechanisms of a conjunction and a feature search of equal difficulty performed during maintenance of fixation. Attention-related activity was assessed by comparing each hard feature and conjunction search with an easy feature search. Hard feature and conjunction search activated overlapping regions in multiple PPC areas and in the frontal eye field (FEF). Most consistent PPC overlaps were located in the anterior and posterior intraparietal sulcus (IPS). The response amplitude of posterior IPS did not differ between both search tasks. However, the IPS junction with the transverse occipital sulcus and the FEF responded at a higher amplitude during conjunction search. Moreover, regions of the prefrontal cortex and the PPC were activated only during either hard feature or conjunction search. These findings suggest that equally difficult visual searches for features and conjunctions are controlled by overlapping frontoparietal networks, but also that both search types involve specific mechanisms.

Cortical activation during Braille reading is influenced by early visual experience in subjects with severe visual disability: a correlational fMRI study

Functional magnetic resonance imaging was performed on blind adults resting and reading Braille. The strongest activation was found in primary somatic sensory/motor cortex on both cortical hemispheres. Additional foci of activation were situated in the parietal, temporal, and occipital lobes where visual information is processed in sighted persons. The regions were differentiated most in the correlation of their time courses of activation with resting and reading. Differences in magnitude and expanse of activation were substantially less significant. Among the traditionally visual areas, the strength of correlation was greatest in posterior parietal cortex and moderate in occipitotemporal, lateral occipital, and primary visual cortex. It was low in secondary visual cortex as well as in dorsal and ventral inferior temporal cortex and posterior middle temporal cortex. Visual experience increased the strength of correlation in all regions except dorsal inferior temporal and posterior parietal cortex. The greatest statistically significant increase, i.e., approximately 30%, was in ventral inferior temporal and posterior middle temporal cortex. In these regions, words are analyzed semantically, which may be facilitated by visual experience. In contrast, visual experience resulted in a slight, insignificant diminution of the strength of correlation in dorsal inferior temporal cortex where language is analyzed phonetically. These findings affirm that posterior temporal regions are engaged in the processing of written language. Moreover, they suggest that this function is modified by early visual experience. Furthermore, visual experience significantly strengthened the correlation of activation and Braille reading in occipital regions traditionally involved in the processing of visual features and object recognition suggesting a role for visual imagery.

Modulation of sensory suppression: implications for receptive field sizes in the human visual cortex

Neurophysiological studies in monkeys show that when multiple visual stimuli appear simultaneously in the visual field, they are not processed independently, but rather interact in a mutually suppressive way. This suggests that multiple stimuli compete for neural representation. Consistent with this notion, we have previously found in humans that functional magnetic resonance imaging (fMRI) signals in V1 and ventral extrastriate areas V2, V4, and TEO are smaller for simultaneously presented (i.e., competing) stimuli than for the same stimuli presented sequentially (i.e., not competing). Here we report that suppressive interactions between stimuli are also present in dorsal extrastriate areas V3A and MT, and we compare these interactions to those in areas V1 through TEO. To exclude the possibility that the differences in responses to simultaneously and sequentially presented stimuli were due to differences in the number of transient onsets, we tested for suppressive interactions in area V4, in an experiment that held constant the number of transient onsets. We found that the fMRI response to a stimulus in the upper visual field was suppressed by the presence of nearby stimuli in the lower visual field. Further, we excluded the possibility that the greater fMRI responses to sequential compared with simultaneous presentations were due to exogeneous attentional cueing by having our subjects count T's or L's at fixation, an attentionally demanding task. Behavioral testing demonstrated that neither condition interfered with performance of the T/L task. Our previous findings suggested that suppressive interactions among nearby stimuli in areas V1 through TEO were scaled to the receptive field (RF) sizes of neurons in those areas. Here we tested this idea by parametrically varying the spatial separation among stimuli in the display. Display sizes ranged from 2 x 2 degrees to 7 x 7 degrees and were centered at 5.5 degrees eccentricity. Based on the effects of display size on the magnitude of suppressive interactions, we estimated that RF sizes at an eccentricity of 5.5 degrees were <2 degrees in V1, 2-4 degrees in V2, 4-6 degrees in V4, larger than 7 degrees (but still confined to a quadrant) in TEO, and larger than 6 degrees (confined to a quadrant) in V3A. These estimates of RF sizes in human visual cortex are strikingly similar to those measured in physiological mapping studies in the homologous visual areas in monkeys.

The effect of acute hypoglycemia on brain function and activation: a functional magnetic resonance imaging study

The authors' aim was to examine the regional anatomy of brain activation by cognitive tasks commonly used in hypoglycemia research and to assess the effect of acute hypoglycemia on these in healthy volunteers. Eight right-handed volunteers performed a set of cognitive tasks-finger tapping (FT), simple reaction time (SRT), and four-choice reaction time (4CRT)-twice during blood oxygen level-dependent (BOLD) functional magnetic resonance imaging of the brain on two occasions. In study 1 (n = 6), plasma glucose was maintained at euglycemia (5 mmol/l) throughout. In study 2 (n = 6), plasma glucose was reduced to 2.5 mmol/l for the second set. Performance of the tasks resulted in specific group brain activation maps. During hypoglycemia, FT slowed (P = 0.026), with decreased BOLD activation in right premotor cortex and supplementary motor area and left hippocampus and with increased BOLD activation in left cerebellum and right frontal pole. Although there was no significant change in SRT, BOLD activation was reduced in right cerebellum and visual cortex. The 4CRT deteriorated (P = 0.020), with reduction in BOLD activation in motor and visual systems but increased BOLD signal in a large area of the left parietal association cortex, a region involved in planning. Hypoglycemia impairs simple brain functions and is associated with task-specific localized reductions in brain activation. For a task with greater cognitive load, the increased BOLD signal in planning areas is compatible with recruitment of brain regions in an attempt to limit dysfunction. Further investigation of these mechanisms may help devise rational treatment strategies to limit cortical dysfunction during acute iatrogenic hypoglycemia.

Visual motion and the human brain: what has neuroimaging told us?

Recently, neuroimaging techniques have been applied to the study of human motion perception, complementing established techniques such as psychophysics, neurophysiology and neuropsychology. Because vision, particularly motion perception, has been studied relatively extensively, it provides an interesting case study to examine the contributions and limitations of neuroimaging to cognitive neuroscience. We suggest that in the domain of motion perception neuroimaging has: (1) revealed an extensive network of motion areas throughout the human brain, in addition to the well-studied motion complex (MT+); (2) verified and extended findings from other techniques; (3) suggested extensive top-down influences on motion perception; and (4) allowed experimenters to examine the neural correlates of awareness. We discuss these contributions, along with limitations and future directions for the neuroimaging of motion.

Central disorders of vision in humans

Over the past 20 years, researchers have discovered over 30 separate visual areas in the cortex of the macaque monkey that exhibit specific responses to visual and environmental stimuli. Many of these areas are homologous to regions of the human visual cortex, and numerous syndromes involving these areas are described in the neurologic and ophthalmic literature. The focus of this review is the anatomy and physiology of these higher cortical visual areas, with special emphasis on their relevance to syndromes in humans. The early visual system processes information primarily by way of two separate systems: parvocellular and magnocellular. Thus, even at this early stage, visual information is functionally segregated. We will trace this segregation to downstream areas involved in increasingly complex visual processing and discuss the results of lesions in these areas in humans. An understanding of these areas is important, as many of these patients will first seek the attention of the ophthalmologist, often with vague, poorly defined complaints that may be difficult to specifically define.

Sensory systems

Our understanding of sensory systems has grown impressively in recent years as a result of intense efforts to characterize the mechanisms underlying perception. A large body of evidence has accrued regarding the processes through which sensory information at the biochemical, electrophysiological, and systems levels contributes to the conscious experience of a stimulus. Our efforts to understand the function of sensory systems have been aided by the development of new techniques, including powerful methods of molecular biology, refined short- and long-term approaches to recording from single and multiple neurons, and non-invasive neuroimaging techniques that allow us to study activity within the human brain while subjects perform a variety of cognitive tasks. In future research, the last approach is likely to form a bridge between the large body of electrophysiological knowledge acquired in animal experiments and that currently being obtained in human imaging research.

Involvement of the human frontal eye field and multiple parietal areas in covert visual selection during conjunction search

Searching for a target object in a cluttered visual scene requires active visual attention if the target differs from distractors not by elementary visual features but rather by a feature conjunction. We used functional magnetic resonance imaging (fMRI) in human subjects to investigate the functional neuroanatomy of attentional mechanisms employed during conjunction search. In the experimental condition, subjects searched for a target defined by a conjunction of colour and orientation. In the baseline condition, subjects searched for a uniquely coloured target, regardless of its orientation. Eye movement recordings outside the scanner verified subjects' ability to maintain fixation during search. Reaction times indicated that the experimental condition was attentionally more demanding than the baseline condition. Differential activations between conditions were therefore ascribed to top-down modulation of neural activity. The frontal eye field, the ventral precentral sulcus and the following posterior parietal regions were consistently activated: (i) the postcentral sulcus; (ii) the posterior; and (iii) the anterior part of the intraparietal sulcus; and (iv) the junction of the intraparietal with the transverse occipital sulcus. Parietal regions were spatially distinct and displayed differential amplitudes of signal increase with a maximal amplitude in the posterior intraparietal sulcus. Less consistent activation was found in the lateral fusiform gyrus. These results suggest an involvement of the human frontal eye field in covert visual selection of potential targets during search. These results also provide evidence for a subdivision of posterior parietal cortex in multiple areas participating in covert visual selection, with a major contribution of the posterior intraparietal sulcus.

Functional imaging of the monkey brain

Functional magnetic resonance imaging (fMRI) has become an essential tool for studying human brain function. Here we describe the application of this technique to anesthetized monkeys. We present spatially resolved functional images of the monkey cortex based on blood oxygenation level dependent (BOLD) contrast. Checkerboard patterns or pictures of primates were used to study stimulus-induced activation of the visual cortex, in a 4.7-Tesla magnetic field, using optimized multi-slice, gradient-recalled, echo-planar imaging (EPI) sequences to image the entire brain. Under our anesthesia protocol, visual stimulation yielded robust, reproducible, focal activation of the lateral geniculate nucleus (LGN), the primary visual area (V1) and a number of extrastriate visual areas, including areas in the superior temporal sulcus. Similar responses were obtained in alert, behaving monkeys performing a discrimination task.

Visual activity in the human frontal eye field

Although visual information processing in the monkey frontal eye field (FEF) has been well demonstrated, the contribution of its human homologue to vision is still unknown. Here we report a study of intracranial visual evoked potentials (VEPs) recorded from the human FEF which was identified by electrical cortical stimulation. Electrical stimulations and EEG recordings were carried out via subdural grid electrodes placed over the frontal cortex in three epileptic patients. Evoked eye movements were mainly horizontal and always directed to the hemispace contralateral to the stimulation site. Intracranial VEPs showed responses predominately to stimuli in the contralateral visual field. Our findings demonstrate a close relationship between the direction of the electrically elicited eye movements and the visual stimulus location which predominantly leads to neural responses in the FEF. These findings provide evidence for the functional role of the human FEF in the analysis of visual stimuli from the contralateral visual field as well as in the generation of eye movements towards these conspicuous targets.

Functional evaluation using magnetic resonance imaging of the visual cortex in patients with retrochiasmatic lesions

OBJECT

The goal of this study was to evaluate the clinical potential of combining functional magnetic resonance (fMR) imaging with conventional morphological MR imaging and to assess its usefulness for objective evaluation of visual function as part of treatment planning in patients harboring space-occupying lesions involving the posterior afferent visual system.

METHODS

It was hypothesized that regional activation of the visual cortex during visual stimulation would show an asymmetric response consistent with the well-known retinotopical organization of the human visual cortex. To test this hypothesis, the pattern of regional cortical activity detected by fMR imaging during binocular repetitive photic stimulation (10 Hz) was compared with the findings of conventional visual field testing. Functional mapping of the visual cortex was performed using a noninvasive blood oxygen level-dependent MR technique in 10 patients with intraaxial and two with extraaxial lesions. Experiments involving two of the patients were unsuccessful because of motion artifacts. In all the remaining patients functional activity was demonstrated in the primary visual area that corresponded to the anatomical location of the calcarine cortex. In nine patients, the identified patterns of activation in the visual cortex were consistent with the visual field deficits (seven homonymous hemianopsias, one homonymous central scotoma, and one inferior quadrantanopsia) and with the traditional teaching of retinotopical representation. Discordance between fMR imaging and perimetric findings was observed in one case.

CONCLUSIONS

These results demonstrate that fMR imaging can be performed routinely and successfully in patients with visual abnormalities as part of a conventional neuroradiological evaluation. The technique provides essential information about the function-structure relationship specific to an individual patient and holds promise not only for diagnosis and therapy planning, but also for understanding the topography and functional specialization of the human visual cortex.

Object vision and visual awareness

Conscious experience involves perceiving, attending, remembering, and recognizing. Recent neuroscientific research has made significant contributions to our understanding of the mechanisms that mediate such capacities. Physiological and neuropsychological investigations have provided increasingly detailed descriptions of the location and functional properties of the brain structures involved in conscious perception, in attentive behavior and working memory, and in the recognition of objects. Such studies suggest that awareness of a visual stimulus probably reflects the interconnectivity and the type of cells involved in the representation of this stimulus, rather than the activity of specific circumscribed visual areas or processing streams.

Cue-invariant activation in object-related areas of the human occipital lobe

The extent to which primary visual cues such as motion or luminance are segregated in different cortical areas is a subject of controversy. To address this issue, we examined cortical activation in the human occipital lobe using functional magnetic resonance imaging (fMRI) while subjects performed a fixed visual task, object recognition, using three different primary visual cues: motion, texture, or luminance contrast. In the first experiment, a region located on the lateral aspect of the occipital lobe (LO complex) was preferentially activated in all 11 subjects both by luminance and motion-defined object silhouettes compared to full-field moving and stationary noise (ratios, 2.00+/-0.19 and 1.86+/-0.65, respectively). In the second experiment, all subjects showed enhanced activation in the LO complex to objects defined both by luminance and texture contrast compared to full-field texture patterns (ratios, 1.43+/-0.08 and 1.32+/-0.08, respectively). An additional smaller dorsal focus that exhibited convergence of object-related cues appeared to correspond to area V3a or a region slightly anterior to it. These results show convergence of visual cues in LO and provide strong evidence for its role in object processing.

fMRI of monkey visual cortex

While functional magnetic resonance imaging (fMRI) is now used widely for demonstrating neural activity-related signals associated with perceptual, motor, and cognitive processes in humans, to date this technique has not been developed for use with nonhuman primates. fMRI in monkeys offers a potentially valuable experimental approach for investigating brain function, which will complement and aid existing techniques such as electrophysiology and the behavioral analysis of the effects of brain lesions. There are, however, a number of significant technical challenges involved in using fMRI with monkeys. Here, we describe the procedures by which we have overcome these challenges to carry out successful fMRI experiments in an alert monkey, and we present the first evidence of activity-related fMRI signals from monkey cerebral cortex.

Brain mapping in animals and humans

Studies using functional magnetic resonance imaging (fMRI) to map cortical areas in humans have revealed many similarities with recent cortical mapping studies from nonhuman primates as well as some striking differences. Improved methods for analyzing, displaying and averaging fMRI data on an unfolded cortical surface atlas are poised to improve the integration of information across burgeoning numbers of imaging studies. By combining fMRI with electrical and passive magnetic imaging modalities, the millisecond-to-millisecond sequence of activation of different cortical regions elicited by an event can be imaged, provided the regions are sufficiently far apart.

Perceiving visually presented objects: recognition, awareness, and modularity

Object perception may involve seeing, recognition, preparation of actions, and emotional responses--functions that human brain imaging and neuropsychology suggest are localized separately. Perhaps because of this specialization, object perception is remarkably rapid and efficient. Representations of componential structure and interpolation from view-dependent images both play a part in object recognition. Unattended objects may be implicitly registered, but recent experiments suggest that attention is required to bind features, to represent three-dimensional structure, and to mediate awareness.

A neural system for human visual working memory

Working memory is the process of actively maintaining a representation of information for a brief period of time so that it is available for use. In monkeys, visual working memory involves the concerted activity of a distributed neural system, including posterior areas in visual cortex and anterior areas in prefrontal cortex. Within visual cortex, ventral stream areas are selectively involved in object vision, whereas dorsal stream areas are selectively involved in spatial vision. This domain specificity appears to extend forward into prefrontal cortex, with ventrolateral areas involved mainly in working memory for objects and dorsolateral areas involved mainly in working memory for spatial locations. The organization of this distributed neural system for working memory in monkeys appears to be conserved in humans, though some differences between the two species exist. In humans, as compared with monkeys, areas specialized for object vision in the ventral stream have a more inferior location in temporal cortex, whereas areas specialized for spatial vision in the dorsal stream have a more superior location in parietal cortex. Displacement of both sets of visual areas away from the posterior perisylvian cortex may be related to the emergence of language over the course of brain evolution. Whereas areas specialized for object working memory in humans and monkeys are similarly located in ventrolateral prefrontal cortex, those specialized for spatial working memory occupy a more superior and posterior location within dorsal prefrontal cortex in humans than in monkeys. As in posterior cortex, this displacement in frontal cortex also may be related to the emergence of new areas to serve distinctively human cognitive abilities.