Temporal dynamics of binocular disparity processing in the central visual pathway

To solve the stereo correspondence problem (i.e., find the matching features of a visual scene in both eyes), it is advantageous to combine information across spatial scales. The details of how this is accomplished are not clear. Psychophysical studies and mathematical models have suggested various types of interactions across spatial scale, including coarse to fine, fine to coarse, averaging, and population coding. In this study, we investigate dynamic changes in disparity tuning of simple and complex cells in the cat's striate cortex over a short time span. We find that disparity frequency increases and disparity ranges decrease while optimal disparity remains constant, and this conforms to a coarse-to-fine mechanism. We explore the origin of this mechanism by examining the frequency and size dynamics exhibited by binocular simple cells and neurons in the lateral geniculate nucleus (LGN). The results suggest a strong role for a feed-forward mechanism, which could originate in the retina. However, we find that the dynamic changes seen in the disparity range of simple cells cannot be predicted from their left and right eye monocular receptive field (RF) size changes. This discrepancy suggests the possibility of a dynamic nonlinearity or disparity specific feedback that alters tuning or a combination of both mechanisms.     

Visuo-haptic object-related activation in the ventral visual pathway

The ventral pathway is involved in primate visual object recognition. In humans, a central stage in this pathway is an occipito-temporal region termed the lateral occipital complex (LOC), which is preferentially activated by visual objects compared to scrambled images or textures. However, objects have characteristic attributes (such as three-dimensional shape) that can be perceived both visually and haptically. Therefore, object-related brain areas may hold a representation of objects in both modalities. Using fMRI to map object-related brain regions, we found robust and consistent somatosensory activation in the occipito-temporal cortex. This region showed clear preference for objects compared to textures in both modalities. Most somatosensory object-selective voxels overlapped a part of the visual object-related region LOC. Thus, we suggest that neuronal populations in the occipito-temporal cortex may constitute a multimodal object-related network.     

Temporal processing across multiple topographic maps in the electrosensory system

Multiple topographic representations of sensory space are common in the nervous system and presumably allow organisms to separately process particular features of incoming sensory stimuli that vary widely in their attributes. We compared the response properties of sensory neurons within three maps of the body surface that are arranged strictly in parallel to two classes of stimuli that mimic prey and conspecifics, respectively. We used information-theoretic approaches and measures of phase locking to quantify neuronal responses. Our results show that frequency tuning in one of the three maps does not depend on stimulus class. This map acts as a low-pass filter under both conditions. A previously described stimulus-class-dependent switch in frequency tuning is shown to occur in the other two maps. Only a fraction of the information encoded by all neurons could be recovered through a linear decoder. Particularly striking were low-pass neurons the information of which in the high-frequency range could not be decoded linearly. We then explored whether intrinsic cellular mechanisms could partially account for the differences in frequency tuning across maps. Injection of a Ca2+ chelator had no effect in the map with low-pass characteristics. However, injection of the same Ca2+ chelator in the other two maps switched the tuning of neurons from band-pass/high-pass to low-pass. These results show that Ca2+-dependent processes play an important part in determining the functional roles of different sensory maps and thus shed light on the evolution of this important feature of the vertebrate brain.     

Temporal dynamics of object location processing in allocentric reference frame

The spatial location of objects is processed in egocentric and allocentric reference frames, the early temporal dynamics of which have remained relatively unexplored. Previous experiments focused on ERP components related only to egocentric navigation. Thus, we designed a virtual reality experiment to see whether allocentric reference frame‐related ERP modulations can also be registered. Participants collected reward objects at the end of the west and east alleys of a cross maze, and their ERPs to the feedback objects were measured. Participants made turn choices from either the south or the north alley randomly in each trial. In this way, we were able to discern place and response coding of object location. Behavioral results indicated a strong preference for using the allocentric reference frame and a preference for choosing the rewarded place in the next trial, suggesting that participants developed probabilistic expectations between places and rewards. We also found that the amplitude of the P1 was sensitive to the allocentric place of the reward object, independent of its value. We did not find evidence for egocentric response learning. These results show that early ERPs are sensitive to the location of objects during navigation in an allocentric reference frame.     

Information processing limitations with aging in the visual scaling of isometric force

The experiment examined if age-related increases in force variability were due to decreases in visual acuity and/or visual-motor information processing deficits. Visual information scale was manipulated over a 250-fold range as young (20–29 years old) and old (60–79 years old) participants produced isometric force output to a visually presented target. Older adults were found to have a very small decrement in visual acuity, but there was no relation between visual acuity and force variability. Force variability exhibited a U-shaped trend as a function of visual information scale. Young adults had less relative variability and higher visual information transfer than the oldest old and these age differences increased with visual information scale. It is concluded that the age-related declines in visual-motor information processing influence changes in neuromuscular function and the emergent differences in force variability at the behavioral level.     

Perceptual continuity and the emergence of perceptual persistence in the ventral visual pathway

Perceptual continuity is an important aspect of our experience of the visual world. In this study, we focus on an example of perceptual continuity involving the maintenance of figure-ground segregation despite the removal of binding cues that initiated the segregation. Fragmented line drawings of objects were superimposed on a background of randomly oriented lines. Global forms could be discriminated from the background based on differences in motion or differences in color/brightness. Furthermore, perception of a global form persisted after the binding cue had been removed. A comparison between the persistence of forms constructed from motion or color demonstrated that both forms produced persistence after the object defining cues were removed. Functional imaging showed a gradual increase in the persistence of brain activity in the lower visual areas (V1, V2, VP), which reached significance in V4v and peaked in the lateral occipital area. There was no difference in the location of persistence for color- or motion-defined forms. These results suggest that the retention of a global percept is an emerging property of the ventral visual processing stream and the maintenance of grouped visual elements is independent of cue type. We postulated that perceptual persistence depends on a system of perceptual memory reflecting the state of perceptual organization.     

The relationship between visual object exploration and action processing in schizophrenia

Introduction. Abnormalities in visual exploration and action processing are widely reported in schizophrenia. The aim of this study was to investigate whether object exploration (in order to recognise an action or the object) modulates visuomotor behaviour differently in schizophrenic patients and controls.

Methods. Visual scan paths were monitored in 36 patients and 36 controls. Participants performed three tasks, in which they were asked to either (1) name the object (the object-naming task), (2) picture themselves interacting with the object and then name the action (the action-naming task), or (3) explore the object (the free-viewing task).

Results. Patients explored objects less than controls did. Controls explored the part needed to identify an object in the object-naming task and the whole object in the action-naming and free-viewing tasks. In contrast, the patients maintained their gaze on the “identity” part of the object in all three tasks.

Conclusion. Our results were consistent with the literature findings on impaired action processing in schizophrenia but also extend the known impairment to implicit action processing when the subject is visually exploring an object. We discuss our results in terms of motivation, the effect of dopamine on eye movement, attentional capture, and frontal lobe dysfunction.     

TMS can reveal contrasting functions of the dorsal and ventral visual processing streams

In order to investigate the functional specificity of the dorsal and ventral visual processing steams we used transcranial magnetic stimulation (TMS) to briefly disrupt one or the other while subjects performed three tasks, involving discrimination of colour or shape or relative position. TMS was delivered over right posterior parietal cortex (PPC) or right lateral occipital (LO) cortex, regions known to have visuo-spatial and object processing properties respectively. LO but not PPC stimulation had a significant effect on reaction time when subjects were asked to make a discrimination of relative shape. PPC stimulation had a significant effect when subjects were asked to discriminate relative position of the same shapes. Stimulation of LO also lengthened reaction times on the position task. There were no effects of stimulation at either site on colour discrimination. Results are discussed within the framework of how the dorsal stream and ventral stream are dissociated following their damage in neurological patients and possible ways in which they may interact in the normal brain.     

Temporal properties of binocular mechanisms in the human visual system

Summary A dichoptic stimulation paradigm was used to determine the degree to which the two monocular images must match in terms of the temporal properties to yield facilitation in binocular grating detection. Several converging lines of evidence point to the existence of two separate neural mechanisms in binocular detection. One of these mechanisms is selective for temporal frequency and limited in its capacity to integrate information from the two eyes over time. The other mechanism is much less selective for temporal frequency and integrates over a longer period of time. At threshold these two separate mechanisms behave independently and exhibit similar degrees of binocular summation.     

Asymmetry and ventral course of the human geniculostriate pathway as determined by hippocampal visual evoked potentials and subsequent visual field defects after temporal lobectomy

Summary Twenty-two patients with psychomotor epilepsy were implanted with depth electrodes along the axis of the mesial temporal lobe to identify an operable unilateral epileptic focus. Neuronal and field potentials were recorded in response to diffuse retinal illumination and clear short-latency responses were found in parahippocampal gyrus. These visual afferents in the mesial temporal lobe are assumed to be both from subcortical and cortical visual areas. There was a clear asymmetry in the ventral trajectory of the geniculostriate pathway as evidenced by asymmetric neuronal and field potential responses to brief flashes in right vs. left hippocampal gyrus and confirmed by a corresponding partial visual field deficit following therapeutic anterior temporal lobectomy. These results demonstrate that there is a retinotopic organization of fibers in the human geniculostriate pathway and that this pathway may have considerable variability in the anterior and ventral course these fibers take through the temporal lobe. These findings adequately account for the presence of direct projections from geniculate to hippocampal cortex and for unexpected hemianopsias with standard resections of the temporal lobe when there is a deviant detour of the geniculostriate pathway.Supported by grant no. NS 02808 and the Ralph Smith Foundation     

Temporal neocortical injuries in rats impair attending but not complex visual processing

The effects of variations of the distance between the relevant stimuli and the animals' response sites were observed upon the performances of a black-white discrimination habit for normal rats or subjects prepared with either bilateral injuries to the visual or temporal neocortex. In addition, the animals were given a strict test of visual form perception. Subjects with injuries to the visual cortex failed the test of visual form perception but performed like normals in discriminating a spatially discontiguous problem. In contrast, subjects with temporal injuries exhibited enormous performance deficits when trained on a spatially discontiguous problem but performed like normals on the test of form perception. The findings parallel the results of studies using primates and suggest that bitemporal injuries result in impairments of attending and not of complex visual processing.     

Single-neuron responses to emotional visual stimuli recorded in human ventral prefrontal cortex

Both lesion and functional imaging studies in humans, as well as neurophysiological studies in nonhuman primates, demonstrate the importance of the prefrontal cortex in representing the emotional value of sensory stimuli. Here we investigated single-neuron responses to emotional stimuli in an awake person with normal intellect. Recording from neurons within healthy tissue in ventral sites of the right prefrontal cortex, we found short-latency (120-160 ms) responses selective for aversive visual stimuli.     

Temporal processing of saccade targets in parietal cortex area LIP during visual search

We studied whether the lateral intraparietal (LIP) area-a subdivision of parietal cortex anatomically interposed between visual cortical areas and saccade executive centers-contains neurons with activity patterns sufficient to contribute to the active process of selecting saccade targets in visual search. Visually responsive neurons were recorded while monkeys searched for a color-different target presented concurrently with seven distractors evenly distributed in a circular search array. We found that LIP neurons initially responded indiscriminately to the presentation of a visual stimulus in their response fields, regardless of its feature and identity. Their activation nevertheless evolved to signal the search target before saccade initiation: an ideal observer could reliably discriminate the target from the individual activation of 60% of neurons, on average, 138 ms after stimulus presentation and 26 ms before saccade initiation. Importantly, the timing of LIP neuronal discrimination varied proportionally with reaction times. These findings suggest that LIP activity reflects the selection of both the search target and the targeting saccade during active visual search.     

Enhanced and bilateralized visual sensory processing in the ventral stream may be a feature of normal aging

Evidence has emerged for age-related amplification of basic sensory processing indexed by early components of the visual evoked potential (VEP). However, since these age-related effects have been incidental to the main focus of these studies, it is unclear whether they are performance dependent or alternately, represent intrinsic sensory processing changes. High-density VEPs were acquired from 19 healthy elderly and 15 young control participants who viewed alphanumeric stimuli in the absence of any active task. The data show both enhanced and delayed neural responses within structures of the ventral visual stream, with reduced hemispheric asymmetry in the elderly that may be indicative of a decline in hemispheric specialization. Additionally, considerably enhanced early frontal cortical activation was observed in the elderly, suggesting frontal hyper-activation. These age-related differences in early sensory processing are discussed in terms of recent proposals that normal aging involves large-scale compensatory reorganization. Our results suggest that such compensatory mechanisms are not restricted to later higher-order cognitive processes but may also be a feature of early sensory-perceptual processes.     

Neural correlates of gesture processing across human development

Co-speech gesture facilitates learning to a greater degree in children than in adults, suggesting that the mechanisms underlying the processing of co-speech gesture differ as a function of development. We suggest that this may be partially due to children's lack of experience producing gesture, leading to differences in the recruitment of sensorimotor networks when comparing adults to children. Here, we investigated the neural substrates of gesture processing in a cross-sectional sample of 5-, 7.5-, and 10-year-old children and adults and focused on relative recruitment of a sensorimotor system that included the precentral gyrus (PCG) and the posterior middle temporal gyrus (pMTG). Children and adults were presented with videos in which communication occurred through different combinations of speech and gesture during a functional magnetic resonance imaging (fMRI) session. Results demonstrated that the PCG and pMTG were recruited to different extents in the two populations. We interpret these novel findings as supporting the idea that gesture perception (pMTG) is affected by a history of gesture production (PCG), revealing the importance of considering gesture processing as a sensorimotor process.     

Contrast sensitivity in human visual areas and its relationship to object recognition

An important characteristic of visual perception is the fact that object recognition is largely immune to changes in viewing conditions. This invariance is obtained within a sequence of ventral stream visual areas beginning in area V1 and ending in high order occipito-temporal object areas (the lateral occipital complex, LOC). Here we studied whether this transformation could be observed in the contrast response of these areas. Subjects were presented with line drawings of common objects and faces in five different contrast levels (0, 4, 6, 10, and 100%). Our results show that indeed there was a gradual trend of increasing contrast invariance moving from area V1, which manifested high sensitivity to contrast changes, to the LOC, which showed a significantly higher degree of invariance at suprathreshold contrasts (from 10 to 100%). The trend toward increased invariance could be observed for both face and object images; however, it was more complete for the face images, while object images still manifested substantial sensitivity to contrast changes. Control experiments ruled out the involvement of attention effects or hemodynamic "ceiling" in producing the contrast invariance. The transition from V1 to LOC was gradual with areas along the ventral stream becoming increasingly contrast-invariant. These results further stress the hierarchical and gradual nature of the transition from early retinotopic areas to high order ones, in the build-up of abstract object representations.