The Smith Kettlewell Eye Research Institute

San Francisco, CA, United States

The Smith Kettlewell Eye Research Institute

San Francisco, CA, United States
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Tyler C.W.,The Smith Kettlewell Eye Research Institute | Likova L.T.,The Smith Kettlewell Eye Research Institute
Frontiers in Systems Neuroscience | Year: 2011

Although brain imaging methods are highly effective for localizing the effects of neural activation throughout the human brain in terms of the blood oxygenation level dependent (BOLD) response, there is currently no way to estimate the underlying neural signal dynamics in generating the BOLD response in each local activation region (except for processes slower than the BOLD time course). Knowledge of the neural signal is critical if spatial mapping is to progress to the analysis of dynamic information flow through the cortical networks as the brain performs its tasks. We introduce an analytic approach that provides a new level of conceptualization and specificity in the study of brain processing by non-invasive methods. This technique allows us to use brain imaging methods to determine the dynamics of local neural population responses to their native temporal resolution throughout the human brain, with relatively narrow confidence intervals on many response properties. The ability to characterize local neural dynamics in the human brain represents a significant enhancement of brain imaging capabilities, with potential applications ranging from general cognitive studies to assessment of neuropathologies. © 2011 Tyler and Likova.


Ales J.M.,Duke University | Cottereau B.,The Smith Kettlewell Eye Research Institute
Neuropsychologia | Year: 2010

While regions of the lateral occipital cortex (LOC) are known to be selective for objects relative to feature-matched controls, it is not known what set of cues or configurations are used to promote this selectivity. Many theories of perceptual organization have emphasized the figure-ground relationship as being especially important in object-level processing. In the present work we studied the role of perceptual organization in eliciting visual evoked potentials from the object selective LOC. To do this, we used two-region stimuli in which the regions were modulated at different temporal frequencies and were comprised of either symmetric or asymmetric arrangements. The asymmetric arrangement produced an unambiguous figure-ground relationship consistent with a smaller figure region surrounded by a larger background, while four different symmetric arrangements resulted in ambiguous figure-ground relationships but still possessed strong kinetic boundaries between the regions. The surrounded figure-ground arrangement evoked greater activity in the LOC relative to first-tier visual areas (V1-V3). Response selectivity in the LOC, however, was not present for the four different types of symmetric stimuli. These results suggest that kinetic texture boundaries alone are not sufficient to trigger selective processing in the LOC, but that the spatial configuration of a figure that is surrounded by a larger background is both necessary and sufficient to selectively activate the LOC. © 2010 Elsevier Ltd.


Cacciamani L.,The Smith Kettlewell Eye Research Institute | Likova L.T.,The Smith Kettlewell Eye Research Institute
Neurobiology of Learning and Memory | Year: 2017

The perirhinal cortex (PRC) is a medial temporal lobe structure that has been implicated in not only visual memory in the sighted, but also tactile memory in the blind (Cacciamani & Likova, 2016). It has been proposed that, in the blind, the PRC may contribute to modulation of tactile memory responses that emerge in low-level “visual” area V1 as a result of training-induced cortical reorganization (Likova, 2012, 2015). While some studies in the sighted have indicated that the PRC is indeed structurally and functionally connected to the visual cortex (Clavagnier, Falchier, & Kennedy, 2004; Peterson, Cacciamani, Barense, & Scalf, 2012), the PRC's direct modulation of V1 is unknown—particularly in those who lack the visual input that typically stimulates this region. In the present study, we tested Likova's PRC modulation hypothesis; specifically, we used fMRI to assess the PRC's Granger causal influence on V1 activation in the blind during a tactile memory task. To do so, we trained congenital and acquired blind participants on a unique memory-guided drawing technique previously shown to result in V1 reorganization towards tactile memory representations (Likova, 2012). The tasks (20 s each) included: tactile exploration of raised line drawings of faces and objects, tactile memory retrieval via drawing, and a scribble motor/memory control. FMRI before and after a week of the Cognitive-Kinesthetic training on these tasks revealed a significant increase in PRC-to-V1 Granger causality from pre- to post-training during the memory drawing task, but not during the motor/memory control. This increase in causal connectivity indicates that the training strengthened the top-down modulation of visual cortex from the PRC. This is the first study to demonstrate enhanced directed functional connectivity from the PRC to the visual cortex in the blind, implicating the PRC as a potential source of the reorganization towards tactile representations that occurs in V1 in the blind brain (Likova, 2012). © 2017 Elsevier Inc.


Cottereau B.R.,Stanford University | Mckee S.P.,The Smith Kettlewell Eye Research Institute | Ales J.M.,Stanford University | Norcia A.M.,Stanford University
Journal of Neuroscience | Year: 2012

Using cortical source estimation techniques based on high-density EEG and fMRI measurements in humans, we measured how a disparity-defined surround influenced the responses to the changing disparity of a central disk within five visual ROIs: V1, V4, lateral occipital complex (LOC), hMT+, and V3A. The responses in the V1 ROI were not consistently affected either by changes in the characteristics of the surround (correlated or uncorrelated) or by its disparity value, consistent with V1 being responsive only to absolute, not relative, disparity. Correlation in the surround increased the responses in the V4, LOC, and hMT+ROIs over those measured with the uncorrelated surround. Thus, these extrastriate areas contain neurons that are sensitive to disparity differences. However, their evoked responses did not vary systematically with the surround disparity. Responses in the V3A ROI, in contrast, were increased by correlation in the surround and varied with its disparity. We modeled these V3A responses as attributable to a gain modulation of the absolute disparity response, where the gain amplitude is proportional to the center-surround disparity difference. An additional experiment identified a nonlinear center-surround interaction in V3A that facilitates the responses when center and surround are misaligned but suppresses it when they share the same disparity plane. © 2012 the authors.


Ales J.M.,The Smith Kettlewell Eye Research Institute | Yates J.L.,The Smith Kettlewell Eye Research Institute | Norcia A.M.,The Smith Kettlewell Eye Research Institute
NeuroImage | Year: 2010

The cruciform hypothesis states that if a visual evoked potential component originates in V1, then stimuli placed in the upper versus lower visual fields will generate responses with opposite polarity at the scalp. This diagnostic has been used by many studies as a definitive marker of V1 sources. To provide an empirical test of the validity of the cruciform hypothesis, we generated forward models of cortical areas V1, V2 and V3 that were based on realistic estimates of the 3-D shape of these areas and the shape and conductivity of the brain, skull and scalp. Functional MRI was used to identify the location of early visual areas and anatomical MRI data was used to construct detailed cortical surface reconstructions and to generate boundary element method forward models of the electrical conductivity of each participant's head. These two data sets for each subject were used to generate simulated scalp activity from the dorsal and ventral subdivisions of each visual area that correspond to the lower and upper visual field representations, respectively. The predicted topographies show that sources in V1 do not fully conform to the cruciform sign-reversal. Moreover, contrary to the model, retinotopic visual areas V2 and V3 show polarity reversals for upper and lower field stimuli. The presence of a response polarity inversion for upper versus lower field stimuli is therefore an insufficient criterion for identifying responses as having originated in V1. © 2010 Elsevier Inc.


Chen C.-C.,National Taiwan University | Tyler C.W.,The Smith Kettlewell Eye Research Institute
PLoS ONE | Year: 2010

Symmetry detection is an interesting probe of pattern processing because it requires the matching of novel patterns without the benefit of prior recognition. However, there is evidence that prior knowledge of the axis location plays an important role in symmetry detection. We investigated how the prior information about the symmetry axis affects symmetry detection under noise-masking conditions. The target stimuli were random-dot displays structured to be symmetric about vertical, horizontal, or diagonal axes and viewed through eight apertures (1.2° diameter) evenly distributed around a 6° diameter circle. The information about axis orientation was manipulated by (1) cueing of axis orientation before the trial and (2) varying axis salience by including or excluding the axis region within the noise apertures. The percentage of correct detection of the symmetry was measured at for a range of both target and masking noise densities. The threshold vs. noise density function was flat at low noise density and increased with a slope of 0.75-0.8 beyond a critical density. Axis cueing reduced the target threshold 2-4fold at all noise densities while axis salience had an effect only at high noise density. Our results are inconsistent with an ideal observer or signal-to-noise account of symmetry detection but can be explained by a multiple-channel model is which the response in each channel is the ratio between the nonlinear transform of the responses of sets of early symmetry detectors and the sum of external and intrinsic sources of noise. © 2010 Chen, Tyler.


Cottereau B.R.,French National Center for Scientific Research | McKee S.P.,The Smith Kettlewell Eye Research Institute | Norcia A.M.,Stanford University
Journal of Neurophysiology | Year: 2014

The perception of motion-in-depth is important for avoiding collisions and for the control of vergence eye-movements and other motor actions. Previous psychophysical studies have suggested that sensitivity to motion-in-depth has a lower temporal processing limit than the perception of lateral motion. The present study used functional MRI-informed EEG source-imaging to study the spatiotemporal properties of the responses to lateral motion and motion-in-depth in human visual cortex. Lateral motion and motion-in-depth displays comprised stimuli whose only difference was interocular phase: monocular oscillatory motion was either inphase in the two eyes (lateral motion) or in antiphase (motion-indepth). Spectral analysis was used to break the steady-state visually evoked potentials responses down into even and odd harmonic components within five functionally defined regions of interest: V1, V4, lateral occipital complex, V3A, and hMT+.We also characterized the responses within two anatomically defined regions: the inferior and superior parietal cortex. Even harmonic components dominated the evoked responses and were a factor of approximately two larger for lateral motion than motion-in-depth. These responses were slower for motion-in-depth and were largely independent of absolute disparity. In each of our regions of interest, responses at odd-harmonics were relatively small, but were larger for motion-in-depth than lateral motion, especially in parietal cortex, and depended on absolute disparity. Taken together, our results suggest a plausible neural basis for reduced psychophysical sensitivity to rapid motion-in-depth. © 2014 the American Physiological Society.


Lee B.-T.,The Smith Kettlewell Eye Research Institute | McPeek R.M.,New York University
Vision Research | Year: 2013

Covert visual search has been studied extensively in humans, and has been used as a tool for understanding visual attention and cueing effects. In contrast, much less is known about covert search performance in monkeys, despite the fact that much of our understanding of the neural mechanisms of attention is based on these animals. In this study, we characterize the covert visual search performance of monkeys by training them to discriminate the orientation of a briefly-presented, peripheral Landolt-C target embedded within an array of distractor stimuli while maintaining fixation. We found that target discrimination performance declined steeply as the number of distractors increased when the target and distractors were of the same color, but not when the target was an odd color (color pop-out). Performance was also strongly affected by peripheral spatial precues presented before target onset, with better performance seen when the precue coincided with the target location (valid precue) than when it did not (invalid precue). Moreover, the effectiveness of valid precues was greatest when the delay between precue and target was short (∼80-100. ms), and gradually declined with longer delays, consistent with a transient component to the cueing effect. Discrimination performance was also significantly affected by prior knowledge of the target location in the absence of explicit visual precues. These results demonstrate that covert visual search performance in macaques is very similar to that of humans, indicating that the macaque provides an appropriate model for understanding the neural mechanisms of covert search. © 2012 Elsevier Ltd.


Heinen S.J.,The Smith Kettlewell Eye Research Institute
Journal of vision | Year: 2011

Smooth pursuit of natural objects requires flexible allocation of attention to inspect features. However, it has been reported that attention is focused at the fovea during pursuit. We ask here if foveal attention is obligatory during pursuit, or if it can be disengaged. Observers tracked a stimulus composed of a central dot surrounded by four others and identified one of the dots when it dimmed. Extinguishing the center dot before the dimming improved task performance, suggesting that attention was released from it. To determine if the center dot automatically usurped attention, we provided the pursuit system with an alternative sensory signal by adding peripheral motion that moved with the stimulus. This also improved identification performance, evidence that a central target does not necessarily require attention during pursuit. Identification performance at the central dot also improved, suggesting that the spatial extent of the background did not attract attention to the periphery; instead, peripheral motion freed pursuit attention from the central dot, affording better identification performance. The results show that attention can be flexibly allocated during pursuit and imply that attention resources for pursuit of small and large objects come from different sources.


Likova L.T.,The Smith Kettlewell Eye Research Institute
Frontiers in Human Neuroscience | Year: 2012

In a memory-guided drawing task under blindfolded conditions, we have recently used functional Magnetic Resonance Imaging (fMRI) to demonstrate that the primary visual cortex (V1) may operate as the visuo-spatial buffer, or "sketchpad," for working memory. The results implied, however, a modality-independent or a modal form of its operation. In the present study, to validate the role of V1 in non-visual memory, we eliminated not only the visual input but all levels of visual processing by replicating the paradigm in a congenitally blind individual. Our novel Cognitive-Kinesthetic method was used to train this totally blind subject to draw complex images guided solely by tactile memory. Control tasks of tactile exploration and memorization of the image to be drawn, and memory-free scribbling were also included. FMRI was run before training and after training. Remarkably, V1 of this congenitally blind individual, which before training exhibited noisy, immature, and non-specific responses, after training produced full-fledged response time-courses specific to the tactile-memory drawing task. The results reveal the operation of a rapid training-based plasticity mechanism that recruits the resources of V1 in the process of learning to draw. The learning paradigm allowed us to investigate for the first time the evolution of plastic re-assignment in V1 in a congenitally blind subject. These findings are consistent with a non-visual memory involvement of V1, and specifically imply that the observed cortical reorganization can be empowered by the process of learning to draw. © 2012 Likova.

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