The end point of the ventral visual stream: face and non-face perceptual deficits following unilateral anterior temporal lobe damage

While it has been claimed that the ventral visual stream ends in the inferior aspects of the anterior temporal lobe (ATL), little is known about whether this region is important for visual perception. Here the performance of two patients with unilateral ATL damage was assessed across four visual perception tasks that parametrically varied stimulus similarity. Patients performed normally on difficult judgments of circle size or face age but were impaired on face identity and dot pattern matching tasks. Portions of the ATL, most likely the ventral surface, may have a functional role in visual perception tasks requiring detailed configural processing, most commonly used to discern facial identity.     

功能区胶质瘤的术中直接电刺激判断核心手术技术

目的分析术中直接电刺激判断大脑功能区的手术技术。方法回顾性分析25例大脑半球胶质瘤手术切除技术,及fMRI与DTI在辅助判断肿瘤与功能区的作用。通过术中直接电刺激判断大脑功能区,最大程度切除肿瘤,提高病人术后Karnofsky生活状态(KPS)评分。结果19例术前KPS 80~90分的病人(术前平均85.8分)术后恢复至平均95.3分,6例KPS 40~70分病人术后恢复至平均73.3分;MRI示肿瘤全切23例,肿瘤大部切除2例。结论术中直接电刺激有助于判断大脑功能区位置,从而以最小的损伤,最大程度切除胶质瘤。     

Task-specific recruitment of dorsal and ventral visual areas during tactile perception

Many studies have found that visual cortical areas are active during tactile perception. Here we used positron emission tomographic (PET) scanning in normally sighted humans to show that extrastriate cortical regions are recruited in a task-specific manner during perceptual processing of tactile stimuli varying in two dimensions. Mental rotation of tactile Forms activated a focus around the anterior part of the left intraparietal sulcus. Since prior studies have reported activity nearby during mental rotation of visual stimuli, this focus appears to be associated with the dorsal visual (visuospatial) pathway. Discrimination between tactile Forms activated the right lateral occipital complex, an object-selective region in the ventral visual (visual Form) pathway. Thus, tactile tasks appear to recruit cortical regions that are active during corresponding visual tasks. Activation of these areas in both visual and tactile tasks could reflect visual imagery during tactile perception, activity in multisensory representations, or both.     

The quantitative nature of a visual task differentiates between ventral and dorsal stream

The aim of the present positron emission tomography (PET) study was to investigate how visual processing in dorsal and ventral streams depends on the quantitative nature of the task. In the same-different task, participants identified the presence of an orientation difference between two gratings, presented centrally in succession. In the quantification task, participants estimated the magnitude of the difference and compared it to a fixed standard. Detection of dimming of the fixation point was used as a control task. Visual input, motor responses, and performance were equated across tasks. Subtracting same-different from quantification yielded significant activation in the left superior parietal lobule and left ventral premotor cortex, consistent with results obtained in number-processing tasks. The reverse subtraction yielded activation in the right inferior temporal gyrus, in agreement with earlier results. These results demonstrate that a single attribute can be processed either in the ventral or dorsal stream, depending on the cognitive operations required by the tasks.     

Dorsal and ventral stream mediated visual processing in genetic subtypes of Prader-Willi syndrome

Previous work has suggested that there are specific deficits in dorsal stream processing in a variety of developmental disorders. Prader–Willi syndrome (PWS) is associated with two main genetic subtypes, deletion and disomy. Relative strengths in visual processing are shown in PWS, although these strengths may be specific to the deletion subtype. We investigated visual processing in PWS using an adapted Simon task which contrasted location (dorsal stream) and shape identity (ventral stream) tasks. Compared to a group of typically developing children, children with PWS deletion showed a greater degree of impairment in the dorsal stream task than in the ventral stream task, a pattern similar to that shown in a group of boys with Fragile-X syndrome. When matched on a measure of non-verbal ability, children with PWS disomy showed the opposite pattern with better performance in the location compared to the shape task, although these task performance asymmetries may have been linked to executive control processes. It is proposed that children with PWS deletion show a relative strength in visual processing in the ventral stream along with a specific deficit in dorsal stream processing. In contrast, children with PWS disomy show neither effect.     

Ventral visual cortex in humans: cytoarchitectonic mapping of two extrastriate areas

The extrastriate visual cortex forms a complex system enabling the analysis of visually presented objects. To gain deeper insight into the anatomical basis of this system, we cytoarchitectonically mapped the ventral occipital cortex lateral to BA 18/V2 in 10 human postmortem brains. The anatomical characterization of this part of the ventral stream was performed by examination of cell-body-stained histological sections using quantitative cytoarchitectonic analysis. First, the gray level index (GLI) was measured in the ventral occipital lobe. Cytoarchitectonic borders, i.e., significant changes in the cortical lamination pattern, were then identified using an observer-independent algorithm based on multivariate analysis of GLI profiles. Two distinct cytoarchitectonic areas (hOC3v, hOC4v) were characterized in the ventral extrastriate cortex lateral to BA 18/V2. Area hOC3v was found in the collateral sulcus. hOC4v was located in this sulcus and also covered the fusiform gyrus in more occipital sections. Topographically, these areas thus seem to represent the anatomical substrates of functionally defined areas, VP/V3v and V4/V4v. Following histological analysis, the delineated cytoarchitectonic areas were transferred to 3D reconstructions of the respective postmortem brains, which in turn were spatially normalized to the Montreal Neurological Institute reference space. A probabilistic map was generated for each area which describes how many brains had a representation of this area in a particular voxel. These maps can now be used to identify the anatomical correlates of functional activations observed in neuroimaging experiments to enable a more informed investigation into the many open questions regarding the organization of the human visual cortex.     

Spatio‐temporal dynamics of adaptation in the human visual system: a high‐density electrical mapping study

When sensory inputs are presented serially, response amplitudes to stimulus repetitions generally decrease as a function of presentation rate, diminishing rapidly as inter‐stimulus intervals (ISIs) fall below 1 s. This ‘adaptation’ is believed to represent mechanisms by which sensory systems reduce responsivity to consistent environmental inputs, freeing resources to respond to potentially more relevant inputs. While auditory adaptation functions have been relatively well characterized, considerably less is known about visual adaptation in humans. Here, high‐density visual‐evoked potentials (VEPs) were recorded while two paradigms were used to interrogate visual adaptation. The first presented stimulus pairs with varying ISIs, comparing VEP amplitude to the second stimulus with that of the first (paired‐presentation). The second involved blocks of stimulation (N = 100) at various ISIs and comparison of VEP amplitude between blocks of differing ISIs (block‐presentation). Robust VEP modulations were evident as a function of presentation rate in the block‐paradigm, with strongest modulations in the 130–150 ms and 160–180 ms visual processing phases. In paired‐presentations, with ISIs of just 200–300 ms, an enhancement of VEP was evident when comparing S2 with S1, with no significant effect of presentation rate. Importantly, in block‐presentations, adaptation effects were statistically robust at the individual participant level. These data suggest that a more taxing block‐presentation paradigm is better suited to engage visual adaptation mechanisms than a paired‐presentation design. The increased sensitivity of the visual processing metric obtained in the block‐paradigm has implications for the examination of visual processing deficits in clinical populations.     

Intraoperative mapping of motor areas during brain tumor surgery: electrical stimulation patterns

Brain mapping with direct electrical stimulation is usefull when the tumor is located near or has infiltrated the central lobe. OBJECTIVE: To analyze the surgical findings with direct electrical stimulation of the cortex and white matter under general anesthesia during surgery for brain tumors related to the central lobe. METHOD: We studied 42 patients operated on from June 2000 to June 2003. We analyzed surgical findings and details of brain mapping. RESULTS: The mean value of the intensity of the stimulus was greater among those who presented motor deficit prior to surgery (p = 0.0425) and edema on MRI (p = 0.0468) or during anesthesia with continuous propofol (p = 0.001). CONCLUSION: The functional mapping of the central lobe may be influenced by severe motor deficit, edema on MRI and propofol's anesthesia.     

Effects of EEG-vigilance regulation patterns on early perceptual processes in human visual cortex

ObjectiveTo investigate influences of EEG-vigilance regulation patterns on perceptual processing during sustained visual attention in early visual areas.MethodsWe compared a subject group with stable vigilance regulation to a group with unstable EEG-vigilance regulation. A rapid serial visual presentation stream (RSVP) elicited a 7.5 Hz steady state visual evoked potential (SSVEP), a continuous sinusoidal brain response as a measure of attentional resource allocation during sustained attention in early visual cortex. Subjects performed a target discrimination task. 150 trials were divided into two parts (75 trials each, trial duration: 11 s).ResultsA significant interaction vigilance group by experimental part provided significantly greater SSVEP amplitudes for the unstable group in the second compared to the first part of the experiment. Both groups showed training effects with increased hit rates and d′-values in the second part of the experiment.ConclusionsThe unexpected finding of SSVEP amplitude increase for the unstable group might be due to competitive interactions for neural resources between the alpha response and SSVEPs.SignificanceIndividual patterns of EEG-vigilance regulation have a moderate impact on early sensory processing during sustained visual attention that is not paralleled in task performance.     

Toward a taxonomy of unconscious perceptual visual processes: from the patient to the healthy subject

Our conscious perception is not exhaustive of all the processes at work when we face a visual scene. In the light of a recent theoretical model, - the conscious global workspace model -, which states the necessary and sufficient conditions for a perceptual representation to reach conscious content, we propose here a taxonomy, which distinguishes between four types of unconscious visual processes. For each of them, we will draw close links between several neurological syndromes and experimental visual paradigms, which can be used in the laboratory with normal subjects.     

Learning-related changes of brain activation in the visual ventral stream: an fMRI study of mirror reading skill

A previous neuroimaging study has indicated that the visual dorsal stream may contribute to accurate reading of mirror-reversed words. However, the role of the visual ventral stream in the learning of mirror reading skill remains ambiguous. In the present fMRI study, we investigated learning-related changes in brain activation in the visual ventral stream in a mirror reading task. Subjects participated in three successive runs of the mirror reading task, in each of which they were asked to read mirror-reversed words and normal words as accurately and as quickly as possible. The behavioral data for the mirror reading condition showed significant improvement in reaction time but not in performance accuracy across the three runs. The activation data showed different learning-associated patterns related to the right and left visual ventral streams. On the right side, activity related to the reading of mirror stimuli was significantly greater than that related to normal stimuli in the first run only, whereas on the left side it was greater in all runs. Additional correlation analysis between response time data and percentage signal changes only in the mirror reading condition showed significant correlation on the right visual ventral stream in the first run only, whereas that on the left visual ventral stream was found only in the third run. The dissociable response between the right and left visual ventral streams may reflect learning-related changes in reading strategy and may be critical in improving the speed of reading mirror-reversed words.     

Optical mapping of electrical activity in rat somatosensory and visual cortex

We have investigated the use of optical methods for monitoring neuron activity in mammalian cortex. The cortex was stained with a voltage-sensitive dye and fluorescence was simultaneously measured from 124 areas using a photodiode array. Optical signals were detected in rat somatosensory cortex in response to small whisker movements and in visual cortex in response to light flashes to the eye. Relatively large signals were obtained during focal interictal epileptiform discharges induced by bicuculline. The measuring system had a time resolution of milliseconds and a spatial resolution of a few hundred micrometers. Simultaneous, multi-site optical recordings of activity may provide a new and potentially powerful method for studying function and dysfunction in mammalian cortex.     

Neuronal learning of invariant object representation in the ventral visual stream is not dependent on reward

Neurons at the top of primate ventral visual stream [inferior temporal cortex (IT)] have selectivity for objects that is highly tolerant to variation in the object's appearance on the retina. Previous nonhuman primate (Macaca mulatta) studies suggest that this neuronal tolerance is at least partly supported by the natural temporal contiguity of visual experience, because altering that temporal contiguity can robustly alter adult IT position and size tolerance. According to that work, it is the statistics of the subject's visual experience, not the subject's reward, that instruct the specific images that IT treats as equivalent. But is reward necessary for gating this type of learning in the ventral stream? Here we show that this is not the case--temporal tolerance learning proceeds at the same rate, regardless of reward magnitude and regardless of the temporal co-occurrence of reward, even in a behavioral task that does not require the subject to engage the object images. This suggests that the ventral visual stream uses autonomous, fully unsupervised mechanisms to constantly leverage all visual experience to help build its invariant object representation.     

Activation of color-selective areas of the visual cortex in a blind synesthete

Many areas of the visual cortex are activated when blind people are stimulated naturally through other sensory modalities (e.g., haptically; Sadato et al., 1996). While this extraneous activation of visual areas via other senses in normal blind people might have functional value (Kauffman et al., 2002; Lessard et al., 1998), it does not lead to conscious visual experiences. On the other hand, electrical stimulation of the primary visual cortex in the blind does produce illusory visual phosphenes (Brindley and Lewin, 1968). Here we provide the first evidence that high-level visual areas not only retain their specificity for particular visual characteristics in people who have been blind for long periods, but that activation of these areas can lead to visual sensations. We used fMRI to demonstrate activity in visual cortical areas specifically related to illusory colored and spatially located visual percepts in a synesthetic man who has been completely blind for 10 years. No such differential activations were seen in late-blind or sighted non-synesthetic controls; neither were these areas activated during color-imagery in the late-blind synesthete, implying that this subject's synesthesia is truly a perceptual experience.     

Genetic contributions to changes of fiber tracts of ventral visual stream in 22q11.2 deletion syndrome

Patients with 22q11.2 deletion syndrome (22q11.2DS) represent a population at high risk for developing schizophrenia, as well as learning disabilities. Deficits in visuo-spatial memory are thought to underlie some of the cognitive disabilities. Neuronal substrates of visuo-spatial memory include the inferior fronto-occipital fasciculus (IFOF) and the inferior longitudinal fasciculus (ILF), two tracts that comprise the ventral visual stream. Diffusion Tensor Magnetic Resonance Imaging (DT-MRI) is an established method to evaluate white matter (WM) connections in vivo. DT-MRI scans of nine 22q11.2DS young adults and nine matched healthy subjects were acquired. Tractography of the IFOF and the ILF was performed. DT-MRI indices, including Fractional anisotropy (FA, measure of WM changes), axial diffusivity (AD, measure of axonal changes) and radial diffusivity (RD, measure of myelin changes) of each of the tracts and each group were measured and compared. The 22q11.2DS group showed statistically significant reductions of FA in IFOF in the left hemisphere. Additionally, reductions of AD were found in the IFOF and the ILF in both hemispheres. These findings might be the consequence of axonal changes, which is possibly due to fewer, thinner, or less organized fibers. No changes in RD were detected in any of the tracts delineated, which is in contrast to findings in schizophrenia patients where increases in RD are believed to be indicative of demyelination. We conclude that reduced axonal changes may be key to understanding the underlying pathology of WM leading to the visuo-spatial phenotype in 22q11.2DS.     

Feed‐forward hierarchical model of the ventral visual stream applied to functional brain image classification

Functional brain imaging is a common tool in monitoring the progression of neurodegenerative and neurological disorders. Identifying functional brain imaging derived features that can accurately detect neurological disease is of primary importance to the medical community. Research in computer vision techniques to identify objects in photographs have reported high accuracies in that domain, but their direct applicability to identifying disease in functional imaging is still under investigation in the medical community. In particular, Serre et al. ( 2005 : In: IEEE Conference on Computer Vision and Pattern Recognition (CVPR‐05). pp 994–1000) introduced a biophysically inspired filtering method emulating visual processing in striate cortex which they applied to perform object recognition in photographs. In this work, the model described by Serre et al. [2005] is extended to three‐dimensional volumetric images to perform signal detection in functional brain imaging (PET, SPECT). The filter outputs are used to train both neural network and logistic regression classifiers and tested on two distinct datasets: ADNI Alzheimer's disease 2‐deoxy‐D ‐glucose (FDG) PET and National Football League players Tc99m HMPAO SPECT. The filtering pipeline is analyzed to identify which steps are most important for classification accuracy. Our results compare favorably with other published classification results and outperform those of a blinded expert human rater, suggesting the utility of this approach. Hum Brain Mapp 35:38–52, 2014. © 2012 Wiley Periodicals, Inc.     

The threshold of cortical electrical stimulation for mapping sensory and motor functional areas

This study aimed to investigate the threshold of cortical electrical stimulation (CES) for functional brain mapping during surgery for the treatment of rolandic epilepsy. A total of 21 patients with rolandic epilepsy who underwent surgical treatment at the Beijing Institute of Functional Neurosurgery between October 2006 and March 2008 were included in this study. Their clinical data were retrospectively collected and analyzed. The thresholds of CES for motor response, sensory response, and after discharge production along with other threshold-related factors were investigated. The thresholds (mean ± standard deviation) for motor response, sensory response, and after discharge production were 3.48 ± 0.87, 3.86 ± 1.31, and 4.84 ± 1.38 mA, respectively. The threshold for after discharge production was significantly higher than those of both the motor and sensory response (both p < 0.05). A negative linear correlation was found between the threshold of after discharge production and disease duration. Using the CES parameters at a stimulation frequency of 50 Hz and a pulse width of 0.2 ms, the threshold of sensory and motor responses were similar, and the threshold of after discharge production was higher than that of sensory and motor response.     

Activation time course of responses to illusory contours and salient region: a high-density electrical mapping comparison

Until recently, early visual areas (V1/V2) were believed to respond mainly to illusory contours (ICs). At present, however, functional neuroimaging suggests that the human lateral occipital complex (LOC), a higher tier than V2, responds strongly to ICs and that IC-related activation in V1/V2 in fact might be driven by feedback input from the LOC. When Kanizsa-type ICs are modified by rounding the corners of the inducers and misaligning them slightly, the impression of an enclosed salient region (SR) remains, although ICs no longer are perceived. Stanley and Rubin (Stanley, D.A., Rubin, N., 2003. fMRI activation in response to illusory contours and salient regions in the human lateral occipital complex. Neuron, 37, 323-331) found that the LOC responded to SR, suggesting that the LOC subserves a rapid but crude region-based segmentation process preceding boundary completion in V1/V2. The present study compared the time course of cortical responses to ICs with those to SR using high-density (74-channel) event-related potentials (ERPs). Scalp mapping and statistical analysis indicated that shared negative modulation for ICs and SR was distributed bilaterally over the lateral occipital scalp at a latency of 70 to 180 ms. Slightly later, a weak negative modulation occurred with ICs but not SR at the occipital pole scalp from 170 to 180 ms. Dipoles for early and late modulations were fitted optimally in the LOC and occipital pole, respectively. The present results suggested that IC-related cortical activation could be separated into region-based segmentation and subsequent boundary completion.     

Spatiotemporal differences between cognitive processes of spatially possible and impossible objects: a high-density electrical mapping study

Differences in cognitive processing between spatially possible and impossible figures were investigated using event-related potentials (ERPs). Two types of figures with identical luminance and equivalent spatial frequency were used as visual stimuli: possible three-dimensional figures (drawn with perspective and existing in the three-dimensional world) and impossible figures (drawn with perspective but not existing in the three-dimensional world). High-density electroencephalographic recording (72 channels) was performed for analysis of ERPs accompanying perception of each figure type; amplitude differences between the conditions were considered neurophysiologic correlates to perceptual differences between possible and impossible objects. Low-resolution brain electromagnetic tomography (LORETA) was used to identify the current source related to the differences. Compared with impossible three-dimensional figures, perception of possible figures showed a significant negative potential increase in the right inferior occipitotemporal region between 350 and 389 ms of latency. The current source was localized to the right fusiform gyrus. The results suggest that right fusiform gyrus is involved in discrimination between spatially possible and impossible objects.