Smith Kettlewell Eye Research Institute

Valley Center, CA, United States

Smith Kettlewell Eye Research Institute

Valley Center, CA, United States
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Missal M.,Catholic University of Leuven | Heinen S.J.,Smith Kettlewell Eye Research Institute
Philosophical Transactions of the Royal Society B: Biological Sciences | Year: 2017

If a visual object of interest suddenly starts tomove,we will try to followit with a smooth movement of the eyes. This smooth pursuit response aims to reduce image motion on the retina that could blur visual perception. In recent years, our knowledge of the neural control of smooth pursuit initiation has sharply increased. However, stopping smooth pursuit eye movements is less well understood and will be discussed in this paper. The most straightforward way to study smooth pursuit stopping is by interrupting image motion on the retina. This causes eye velocity to decay exponentially towards zero. However, smooth pursuit stopping is not a passive response, as shown by behavioural and electrophysiological evidence. Moreover, smooth pursuit stopping is particularly influenced by active prediction of the upcoming end of the target. Here, we suggest that a particular class of inhibitory neurons of the brainstem, the omnipause neurons, could play a central role in pursuit stopping. Furthermore, the role of supplementary eye fields of the frontal cortex in smooth pursuit stopping is also discussed. © 2017 The Author(s) Published by the Royal Society. All rights reserved.

Samonds J.M.,Carnegie Mellon University | Potetz B.R.,University of Kansas | Tyler C.W.,Smith Kettlewell Eye Research Institute | Lee T.S.,Carnegie Mellon University
Journal of Neuroscience | Year: 2013

Disparity tuning measured in the primary visual cortex (V1) is described well by the disparity energy model, but not all aspects of disparity tuning are fully explained by the model. Such deviations from the disparity energy model provide us with insight into how network interactions may play a role in disparity processing and help to solve the stereo correspondence problem. Here, we propose a neuronal circuit model with recurrent connections that provides a simple account of the observed deviations. The model is based on recurrent connections inferred from neurophysiological observations on spike timing correlations, and is in good accord with existing data on disparity tuning dynamics. We further performed two additional experiments to test predictions of the model. First, we increased the size of stimuli to drive more neurons and provide a stronger recurrent input. Our model predicted sharper disparity tuning for larger stimuli. Second, we displayed anticorrelated stereograms, where dots of opposite luminance polarity are matched between the left-and right-eye images and result in inverted disparity tuning in the disparity energy model. In this case, our model predicted reduced sharpening and strength of inverted disparity tuning. For both experiments, the dynamics of disparity tuning observed from the neurophysiological recordings in macaque V1 matched model simulation predictions. Overall, the results of this study support the notion that, while the disparity energy model provides a primary account of disparity tuning in V1 neurons, neural disparity processing in V1 neurons is refined by recurrent interactions among elements in the neural circuit. © 2013 the authors.

Cottereau B.R.,Smith Kettlewell Eye Research Institute | Cottereau B.R.,Stanford University | McKee S.P.,Smith Kettlewell Eye Research Institute | Ales J.M.,Stanford University | Norcia A.M.,Stanford University
Journal of Neuroscience | Year: 2011

We used source imaging of visual evoked potentials to measure neural population responses over a wide range of horizontal disparities (0.5-64 arcmin). The stimulus was a central disk that moved back and forth across the fixation plane at 2 Hz, surrounded either by binocularly uncorrelated dots (disparity noise) or by correlated dots presented in the fixation plane. Both disk and surround were composed of dynamic random dots to remove coherent monocular information. Disparity tuning was measured in five visual regions of interest (ROIs) [V1, human middle temporal area (hMT+), V4, lateral occipital complex (LOC), and V3A], defined in separate functional magnetic resonance imaging scans. The disparity tuning functions peaked between 2 and 16 arcmin for both types of surround in each ROI. Disparity tuning in the V1 ROI was unaffected by the type of surround, but surround correlation altered both the amplitude and phase of the disparity responses in the other ROIs. Response amplitude increased when the disk was in front of the surround in the V3A and LOC ROIs, indicating that these areas encode figure-ground relationships and object convexity. The correlated surround produced a consistent phase lag at the second harmonic in the hMT+ and V4 ROIs without a change in amplitude, while in the V3A ROI, both phase and amplitude effects were observed. Sensitivity to disparity context is thus widespread in visual cortex, but the dynamics of these contextual interactions differ across regions. Copyright © 2011 the authors.

Verghese P.,Smith Kettlewell Eye Research Institute | Kim Y.-J.,Smith Kettlewell Eye Research Institute | Wade A.R.,Smith Kettlewell Eye Research Institute | Wade A.R.,University of York
Journal of Neuroscience | Year: 2012

In a neural population driven by a simple grating stimulus, different subpopulations are maximally informative about changes to the grating's orientation and contrast. In theory, observers should attend to the optimal subpopulation when switching between orientation and contrast discrimination tasks. Here we used source-imaged, steady-state visual evoked potentials and visual psychophysics to determine whether this is the case. Observers fixated centrally while static targets were presented bilaterally along with a cue indicating task type (contrast or orientation modulation detection) and task location (left or right). Changes in neuronal activity were measured by quantifying frequency-tagged responses from flickering "reporter" gratings surrounding the targets. To determine the orientation tuning of attentionally modulated neurons, we measured responses for three different probe-reporter angles: 0, 20, and 45°.Weestimated frequency-tagged cortical activity using a minimum norm inverse procedure combined with realistic MR-derived head models and retinotopically mapped visual areas. Estimates of neural activity from regions of interest centered on V1 showed that attention to a spatial location clearly increased the amplitude of the neural response in that location. More importantly, the pattern of modulation depended on the task. For orientation discrimination, attentional modulation showed a sharp peak in the population tuned 20° from the target orientation, whereas for contrast discrimination the enhancement was more broadly tuned. Similar tuning functions for orientation and contrast discrimination were obtained from psychophysical adaptation studies. These findings indicate that humans attend selectively to the most informative neural population and that these populations change depending on the nature of the task. © 2012 the authors.

Wade A.R.,Smith Kettlewell Eye Research Institute | Wade A.R.,University of California at San Francisco | Rowland J.,Smith Kettlewell Eye Research Institute
Journal of Neuroscience | Year: 2010

Functional magnetic resonance imaging (fMRI) studies of early sensory cortex often measure stimulus-driven increases in the blood oxygenation level-dependent (BOLD) signal. However, these positive responses are frequently accompanied by reductions in the BOLD signal in adjacent regions of cortex. Although this negative BOLD response (NBR) is thought to result from neuronal suppression, the precise relationship between local activity, suppression, and perception remains unknown. By measuring BOLD signals in human primary visual cortex while varying the baseline contrast levelsinthe region affected bythe NBR, we tested three physiologically plausible computational models of neuronal modulation that could explain this phenomenon: a subtractive model, a response gain model, and a contrast gain model. We also measured the ability of isoluminant contrast togeneratean NBR. Weshow that the NBR can bemodeled as a pathway-specific contrast gain modulation thatis strongest outside the fovea.We foundasimilar spatial biasina psychophysical study usingidentical stimuli, although these dataindicatedaresponse gainrather thanacontrast gain mechanism.Wereconcile these findings by proposing (1) that the NBR is associated with a long-range suppressive mechanism that hyperpolarizes a subset of magnocellularly driven neurons atthe input to V1, (2) that this suppressionis broadly tuned tomatch the spatial features of the mask region, and (3) that increasing the baseline contrast in the suppressed region drives all neurons in the input layer, reducing the relative contribution of the suppressing subpopulation in the fMRI signal. Copyright © 2010 the authors.

Kim Y.-J.,Smith Kettlewell Eye Research Institute | Verghese P.,Smith Kettlewell Eye Research Institute
Journal of Neuroscience | Year: 2012

Attention is thought to operate by enhancing the target of interest and suppressing the surroundings. We hypothesized that the spatial profile of attention depends on the surround's relationship to the target. Using high-density electroencephalographic measurements, we examined the spatial profile of attention to a grating target surrounded by an annular grating that was either coextensive with the target (un segmented) or appeared segmented from it due to a gap or phase offset. We directly probed the spread of attention from the central target into the surround by flickering the surround and monitoring frequency-tagged steady-state visual-evoked potentials. Observers were required to detect a contrast increment that occurred only on the target. Successful detection of the increment required selecting the target and suppressing the surround, particularly when the target did not readily segment from the surround. The profile of attention was investigated in five visual regions of interest (ROIs) (V1, V4, V3A, lateral occipital complex, and human middle temporal area), mapped in a separate anatomical magnetic resonance imaging scan. We found that in most ROIs, attention to the target generated smaller responses from the surrounding annulus when it was contiguous compared with when it was clearly segmented. This result shows that the profile of attention depends on task demands and on surrounding context; attention is tightly focused when the target region needs to be isolated but loosely focused when the target region is clearly segmented. © 2012 the authors.

Morash V.S.,Smith Kettlewell Eye Research Institute
IEEE World Haptics Conference, WHC 2015 | Year: 2015

Movement strategies were investigated in a one-handed haptic search task where blindfolded sighted participants used either one or five fingers to find a landmark on an unstructured tactile map. Search theory predicts that systematic strategies, such as parallel sweeps and spirals, should be more prevalent when the searcher's detection radius is small (one finger) than when the detection radius is large (five fingers). Movement patterns were classified as either non-systematic or systematic. As predicted by search theory, systematic strategies were more common in one-finger than five-finger searches. Overall, these results indicate that systematic haptic search strategies are used and modulated by detection radius for untrained sighted participants. © 2015 IEEE.

Song J.-H.,Smith Kettlewell Eye Research Institute | McPeek R.M.,New York University
Journal of Neurophysiology | Year: 2010

Most visual scenes are complex and crowded, with several different objects competing for attention and action. Thus a complete understanding of the production of goal-directed actions must incorporate the higher-level process of target selection. To examine the neural substrates of target selection for visually guided reaching, we recorded the activity of isolated neurons in the dorsal premotor area (PMd) of monkeys performing a reaction-time visual search task. In this task, monkeys reached to an odd-colored target presented with three distractors. We found that PMd neurons typically discriminate the target before movement onset, ∼150-200 ms after the appearance of the search array. In one subset of neurons, discrimination occurred at a consistent time after search array onset regardless of when the reaching movement occurred, suggesting that these neurons are involved in target selection. In a second group of neurons, discrimination time depended on reach reaction time, consistent with involvement in movement production but not in target selection. To look for physiological corroboration of these two functionally defined groups, we analyzed the extracellular spike waveforms of recorded neurons. This analysis showed a population of neurons with narrow action potentials that carried signals related to target selection. A second population with broader action potentials was more heterogeneous, with some neurons showing activity related to target selection and others showing only movement production activity. These results suggest that PMd contains signals related to target selection and movement execution and that different signals are carried by distinct neural subpopulations. Copyright © 2010 The American Physiological Society.

Colenbrander A.,Smith Kettlewell Eye Research Institute
Acta Ophthalmologica | Year: 2010

This article, based on a report prepared for the International Council of Ophthalmology (ICO) and the International Society for Low Vision Research and Rehabilitation (ISLRR), explores the assessment of various aspects of visual functioning as needed to document the outcomes of vision rehabilitation. Documenting patient abilities and functional vision (how the person functions) is distinct from the measurement of visual functions (how the eye functions) and also from the assessment of quality of life. All three areas are important, but their assessment should not be mixed. Observation of task performance offers the most objective measure of functional vision, but it is time-consuming and not feasible for many tasks. Where possible, timing and error rates provide an easy score. Patient response questionnaires provide an alternative. They may save time and can cover a wider area, but the responses are subjective and proper scoring presents problems. Simple Likert scoring still predominates but Rasch analysis, needed to provide better result scales, is gaining ground. Selection of questions is another problem. If the range of difficulties does not match the range of patient abilities, and if the difficulties are not distributed evenly, the results are not optimal. This may be an argument to use different outcome questions for different conditions. Generic questionnaires are appropriate for the assessment of generic quality of life, but not for specific rehabilitation outcomes. Different questionnaires are also needed for screening, intake and outcomes. Intake questions must be relevant to actual needs to allow prioritization of rehabilitation goals; the activity inventory presents a prototype. Outcome questions should be targeted at predefined rehabilitation goals. The Appendix cites some promising examples. The Low Vision Intervention Trial (LOVIT) is an example of a properly designed randomized control study, and has demonstrated the remarkable effectiveness of vision rehabilitation. It is hoped that further similar studies will follow. © 2009 Acta Ophthalmol.

Verghese P.,Smith Kettlewell Eye Research Institute
Vision Research | Year: 2012

This study examines saccade strategy in a novel task where observers actively search a display to find multiple targets in a limited time. Theory predicts that the relative merit of different saccade strategies depends on the prior probability of the target at a location: when the target prior is low and multiple-target trials are rare, making a saccade to the most likely target location is close to the optimal strategy, but when the target prior is high and multiple-target trials are frequent, selecting uncertain locations is more informative. The prior probability of the target was varied from 0.17 to 0.67 to determine whether observers adjusted their saccades strategies to maximize information. Observers actively searched a noisy display with six potential target locations. Each location had an independent probability of a target, so the number of targets in a trial ranged from 0 to 6. For all target priors ranging from low to high, a trial-by-trial analysis of saccade strategy indicated that observers made saccades to the most likely target location more often than the most uncertain location. Fixating likely locations is efficient only when multiple targets are rare, as in the case of a low target prior, or in the case of the more standard single-target search task. Yet it is the preferred saccade strategy in all our conditions, even when multiple targets are frequent. These findings indicate that humans are far from ideal searchers in multiple-target search. © 2012 Elsevier Ltd.

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