Biomedical subjects

Peter Neri

Publications and source records attributed to Peter Neri.

11 recordsLinked to original sources

PubMed2006

Temporal dynamics of figure-ground segregation in human vision.

The segregation of figure from ground is arguably one of the most fundamental operations in human vision. Neural signals reflecting this operation appear in cortex as early as 50 ms and as late as 300 ms after presentation of a visual stimulus, but it is not known when these signals are used by the brain to construct the percepts of figure and ground. We used psychophysical reverse correlation to identify the temporal window for figure-ground signals in human perception and found it to lie within the range of 100-160 ms. Figure enhancement within this narrow temporal window was transient rather than sustained as may be expected from measurements in single neurons. These psychophysical results prompt and guide further electrophysiological studies.

Action Potentials↗

PubMed2006

Receptive versus perceptive fields from the reverse-correlation viewpoint.

This brief review article brings together a series of related experiments in psychophysics and physiology that show striking similarities between measurements in human observers and in single neurons. We consider seven pairs of primary research articles, each pair consisting of one paper in physiology and one in psychophysics, and we highlight common features between receptive and perceptive fields obtained using reverse correlation. We conclude by discussing how to assess the validity of perceptive fields as predictors of human responses, and by deriving a novel expression for the maximum trial-by-trial predictability attainable by any model for any psychophysical task.

Humans↗

PubMed2006

Spatial resolution for feature binding is impaired in peripheral and amblyopic vision.

We measured spatial resolution for discriminating targets that differed from nearby distractors in either color or orientation or their conjunction. In the fovea of normal human observers, whenever both attributes are big enough to be individually visible, their conjunction is also visible. In the periphery, the two attributes may be visible, but their conjunction may be invisible. We found a similar impairment in resolving conjunctions for the fovea of deprived eyes of humans with abnormal visual development (amblyopia). These results are quantitatively explained by a model of primary visual cortex (V1) in which orientation and color maps are imperfectly co-registered topographically. Our results in persons with amblyopia indicate that the ability of the fovea to compensate for this poor co-registration is consolidated by visual experience during postnatal development.

Amblyopia↗

PubMed2006

Meaningful interactions can enhance visual discrimination of human agents.

The ability to interpret and predict other people's actions is highly evolved in humans and is believed to play a central role in their cognitive behavior. However, there is no direct evidence that this ability confers a tangible benefit to sensory processing. Our quantitative behavioral experiments show that visual discrimination of a human agent is influenced by the presence of a second agent. This effect depended on whether the two agents interacted (by fighting or dancing) in a meaningful synchronized fashion that allowed the actions of one agent to serve as predictors for the expected actions of the other agent, even though synchronization was irrelevant to the visual discrimination task. Our results demonstrate that action understanding has a pervasive impact on the human ability to extract visual information from the actions of other humans, providing quantitative evidence of its significance for sensory performance.

Cognition↗

PubMed2005

Spatial integration of optic flow signals in fly motion-sensitive neurons.

Neurons in the fly lobula plate integrate motion signals over large regions of visual space in a directionally selective manner. This study is concerned with the details of this integration process. We used a stimulus consisting of a 4 x 4 lattice of locally moving Gabor patches, in which each patch could take any direction independently. We also presented only one patch at a time or two patches at a time. Across all possible directions of motion, the firing rate response r1+2 to two simultaneously presented patches was well described by r1+2(d1, d2) = G x [r1(d1) + r2(d2)] + S, where r1 and r2 are responses to individual patches moving in directions d1 and d2, and G approximately 0.81, S approximately -23. However, this quasi-linear scaling expression failed to account for three main empirical observations: 1) the directional-tuning curve for one patch is broader in the presence of another patch moving in the neuron's preferred direction (PD); 2) the vertical compression of this curve is greater when the second patch moves in the antipreferred direction (AD) as opposed to PD; 3) the ability of the neuronal response to discriminate the direction of a patch is greater when the other patch is moving in the PD as opposed to AD, where this ability is assessed using both information theory and a standard discriminability index. To account for these departures from the simple scaling model, we used a normalization model very similar to one used for macaque area MT/V5. This model can qualitatively explain all three departures from the scaling equation described above, suggesting that a gain-control normalization network may be at work within the fly lobula plate.

Animals↗

PubMed2005

Global versus local adaptation in fly motion-sensitive neurons.

Flies, like humans, experience a well-known consequence of adaptation to visual motion, the waterfall illusion. Direction-selective neurons in the fly lobula plate permit a detailed analysis of the mechanisms responsible for motion adaptation and their function. Most of these neurons are spatially non-opponent, they sum responses to motion in the preferred direction across their entire receptive field, and adaptation depresses responses by subtraction and by reducing contrast gain. When we adapted a small area of the receptive field to motion in its anti-preferred direction, we discovered that directional gain at unadapted regions was enhanced. This novel phenomenon shows that neuronal responses to the direction of stimulation in one area of the receptive field are dynamically adjusted to the history of stimulation both within and outside that area.

Animals↗

PubMed2005

A stereoscopic look at visual cortex.

Three recent studies offer new insights into the way visual cortex handles binocular disparity signals. Two of these studies recorded from single neurons in two different visual areas of the monkey brain, one (V5/MT) in dorsal and one (V4) in ventral cortex. While V5/MT neurons respond similarly to neurons in primary visual cortex (V1), V4 neurons appear to reflect a more advanced stage in the analysis of retinal disparity, closer to the perceptual experience of stereoscopic depth. Both studies are consistent with a third study using fMRI to address similar questions in humans. Together with previous evidence, these results suggest a new framework for understanding stereoscopic processing based on the separation between ventral and dorsal streams in visual cortex.

Animals↗

PubMed2004

Stereoscopic processing of absolute and relative disparity in human visual cortex.

Stereoscopic vision relies mainly on relative depth differences between objects rather than on their absolute distance in depth from where the eyes fixate. However, relative disparities are computed from absolute disparities, and it is not known where these two stages are represented in the human brain. Using functional MRI (fMRI), we assessed absolute and relative disparity selectivity with stereoscopic stimuli consisting of pairs of transparent planes in depth in which the absolute and relative disparity signals could be independently manipulated (at a local spatial scale). In experiment 1, relative disparity was kept constant, while absolute disparity was varied in one-half the blocks of trials ("mixed" blocks) and kept constant in the remaining one-half ("same" blocks), alternating between blocks. Because neuronal responses undergo adaptation and reduce their firing rate following repeated presentation of an effective stimulus, the fMRI signal reflecting activity of units selective for absolute disparity is expected to be smaller during "same" blocks as compared with "mixed" ones. Experiment 2 similarly manipulated relative disparity rather than absolute disparity. The results from both experiments were consistent with adaptation with differential effects across visual areas such that 1) dorsal areas (V3A, MT+/V5, V7) showed more adaptation to absolute than to relative disparity; 2) ventral areas (hV4, V8/V4alpha) showed an equal adaptation to both; and 3) early visual areas (V1, V2, V3) showed a small effect in both experiments. These results indicate that processing in dorsal areas may rely mostly on information about absolute disparities, while ventral areas split neural resources between the two types of stereoscopic information so as to maintain an important representation of relative disparity.

Depth Perception↗

PubMed2004

Estimation of nonlinear psychophysical kernels.

Reverse correlation techniques have been extensively used in physiology (Marmarelis & Marmarelis 1978; Sakai, Naka, & Korenberg, 1988), allowing characterization of both linear and nonlinear aspects of neuronal processing (e.g., Emerson, Bergen, & Adelson, 1992; Emerson & Citron 1992). Over the past decades, Ahumada (1996) developed a psychophysical reverse correlation technique, termed noise image classification (NIC), for deriving the linear properties of sensory filters in the context of audition first (Ahumada, 1967; Ahumada, Marken, & Sandusky, 1975), and then vision (Ahumada, 1996, 2002; Beard & Ahumada, 1998). This work explores ways of characterizing nonlinear aspects of psychophysical filters. One approach consists of an extension of the NIC technique (ExtNIC), whereby second-order (rather than just first-order) statistics in the classified noise are used to derive sensory kernels. It is shown that under some conditions, this procedure yields a good estimate of second-order kernels. A second, different approach is also considered. This method uses functional minimization (fMin) to generate kernels that best simulate psychophysical responses for a given set of stimuli. Advantages and disadvantages of the two approaches are discussed. A mathematical appendix shows some interesting facts: (1) that nonlinearities affect the linear estimate (particularly target-present averages) obtained from the NIC method, providing a rationale for some related observations made by Ahumada (1967); (2) that for a linear filter followed by a static nonlinearity (LN system), the ExtNIC estimate of the second-order nonlinear kernel is correctly null, provided the criterion is unbiased; (3) that for a biased criterion, such an estimate may contain predictable modulations related to the linear filter; and (4) that under certain assumptions and conditions, ExtNIC does return a correct estimate for the second-order nonlinear kernel.

Humans↗

PubMed2004

Attentional effects on sensory tuning for single-feature detection and double-feature conjunction.

When humans scan their visual environment, relevant objects are selectively attended for enhanced processing. It is still unclear in what ways processing is modified by attention, and whether attentional selection operates on an individual feature (such as colour, orientation or motion) or on binding together different features. In the experiments reported in this paper, these two stages were characterized using psychophysical reverse correlation. Subjects viewed eight patches, briefly flashed and symmetrically arranged around fixation. Each patch consisted of segments that could vary in both colour and orientation. One of the patches ('target') differed from the remaining 'distractor' patches with respect to either its orientation, colour, or both (in three different experiments). Subjects were asked to detect the target patch. The stimulus was preceded by a cue. On some trials ('cued' trials), the cue informed observers that the target patch could only appear at two of the eight possible locations. On remaining ('uncued') trials, all eight positions were valid. Psychophysical reverse correlation was then applied to derive linear estimates of sensory filters for orientation only, colour only, and their conjunction. In line with the properties of single neurons in cortex, attentional cueing did not affect sensory tuning for detecting individual features. However, it affected the way in which features were subsequently (and very inefficiently) combined in a multiplicative fashion. The results are consistent with a model in which attention recalibrates internal responses to the statistics of the stimulus by having signals from different features mutually control each other through reciprocal inhibition.

Attention↗

PubMed2002

Spatiotemporal mechanisms for detecting and identifying image features in human vision.

Our visual system constantly selects salient features in the environment, so that only those features are attended and targeted by further processing efforts to identify them. Models of feature detection hypothesize that salient features are localized based on contrast energy (local variance in intensity) in the visual stimulus. This hypothesis, however, has not been tested directly. We used psychophysical reverse correlation to study how humans detect and identify basic image features (bars and short line segments). Subjects detected a briefly flashed 'target bar' that was embedded in 'noise bars' that randomly changed in intensity over space and time. By studying how the intensity of the noise bars affected performance, we were able to dissociate two processing stages: an early 'detection' stage, whereby only locations of high-contrast energy in the image are selected, followed (after approximately 100 ms) by an 'identification' stage, whereby image intensity at selected locations is used to determine the identity (whether bright or dark) of the target.

Choice Behavior↗