Biomedical subjects

Michael H Herzog

Publications and source records attributed to Michael H Herzog.

At least 19 recordsLinked to original sources

PubMed2006

The flight path of the phoenix--the visible trace of invisible elements in human vision.

How features are attributed to objects is one of the most puzzling issues in the neurosciences. A deeply entrenched view is that features are perceived at the locations where they are presented. Here, we show that features in motion displays can be systematically attributed from one location to another although the elements possessing the features are invisible. Furthermore, features can be integrated across locations. Feature mislocalizations are usually treated as errors and limits of the visual system. On the contrary, we show that the nonretinotopic feature attributions, reported herein, follow rules of grouping precisely suggesting that they reflect a fundamental computational strategy and not errors of visual processing.

Attention↗

PubMed2006

Reverse feedback induces position and orientation specific changes.

To investigate the mechanisms of perceptual learning, we recently introduced a paradigm in which incorrect, reverse feedback followed after some but not all vernier presentations. This feedback paradigm exerted a strong effect on performance that seemed to bias decisions rather than to yield perceptual learning. Here, we show that observers can develop independent decision biases for different stimulus orientations as well as for different visual field positions. Our results demonstrate that the effects of incorrect, reverse feedback are surprisingly specific.

Cognition↗

PubMed2006

Perceptual grouping induces non-retinotopic feature attribution in human vision.

The human visual system computes features of moving objects with high precision despite the fact that these features can change or blend into each other in the retinotopic image. Very little is known about how the human brain accomplishes this complex feat. Using a Ternus-Pikler display, introduced by Gestalt psychologists about a century ago, we show that human observers can perceive features of moving objects at locations these features are not present. More importantly, our results indicate that these non-retinotopic feature attributions are not errors caused by the limitations of the perceptual system but follow rules of perceptual grouping. From a computational perspective, our data imply sophisticated real-time transformations of retinotopic relations in the visual cortex. Our results suggest that the human motion and form systems interact with each other to remap the retinotopic projection of the physical space in order to maintain the identity of moving objects in the perceptual space.

Eye Movements↗

PubMed2006

Perceptual learning with spatial uncertainties.

In perceptual learning, stimuli are usually assumed to be presented to a constant retinal location during training. However, due to tremor, drift, and microsaccades of the eyes, the same stimulus covers different retinal positions on sequential trials. Because of these variations the mathematical decision problem changes from linear to non-linear (). This non-linearity implies three predictions. First, varying the spatial position of a stimulus within a moderate range does not deteriorate perceptual learning. Second, improvement for one stimulus variant can yield negative transfer to other variants. Third, interleaved training with two stimulus variants yields no or strongly diminished learning. Using a bisection task, we found psychophysical evidence for the first and last prediction. However, no negative transfer was found as opposed to the second prediction.

Attention↗

PubMed2006

Long lasting effects of unmasking in a feature fusion paradigm.

In spite of more than 100 years of research, the mechanisms underlying visual masking are still unknown. In recent publications, we introduced an unmasking paradigm involving the fusion of features that revealed interesting spatial characteristics. Here, we investigate the temporal aspects of this paradigm showing very long lasting effects that impose serious restrictions on models of masking. We used a simple feed-forward neural network model to explain these results.

Humans↗

PubMed2005

Intact and deficient feature fusion in schizophrenia.

In patients with schizophrenia, early as well as late stages of information processing can be deficient. Therefore, it is important to determine the earliest occurrences of aberrant processing since deficits on these stages may cause abnormal processing on later, e. g. cognitive, levels. In order to investigate this issue in the visual domain, we studied one of the most basic feature integration mechanisms, namely feature fusion. Our results indicate that in schizophrenic patients this integration mechanism is qualitatively intact but reveals quantitative impairments that may influence later processing stages.

Adult↗

PubMed2005

Attention and feature integration in the feature inheritance effect.

Features of neighboring elements are not processed independently. Often, it is assumed that nearby features are integrated by a (pre-attentive) pooling mechanism. Here, we show that in the feature inheritance effect some features are integrated across space whereas others are not. This result may be partly explained by a very focused spatial attention. Our findings challenge models based on a simple pooling mechanism.

Attention↗

PubMed2004

Intact figure-ground segmentation in schizophrenia.

As revealed by backward masking studies, schizophrenic patients show strong impairments of early visual processing. However, the underlying temporal mechanisms are not yet well understood. To shed light on the exact timing of these deficits, we employed a paradigm in which two masks follow each other. We investigated 16 medicated schizophrenic patients and a matched group of 14 controls with a new backward masking technique, shine-through. In accordance with other masking studies, schizophrenic patients require a dramatically longer processing time to reach a predefined performance level compared with healthy subjects. However, patients are surprisingly sensitive to subtle differences in the timing of the two masks, revealing good temporal resolution. This good temporal resolution indicates intact and fast perceptual grouping and figure-ground segmentation in spite of high susceptibility to masking procedures in schizophrenia.

Adult↗

PubMed2004

Valences in contextual vision.

Elements in the neighborhood of a stimulus can modulate both the subjective perception of and the neural responses to this stimulus. Investigations of this contextual modulation usually focus on low level features such as the orientation difference between the target and its context. Recently, we introduced a paradigm in which contextual modulation cannot be explained by orientation differences between target and context per se. Instead, the overall structure of the context seemed to determine contextual modulation. Here, we show that edges of contextual gratings as well as isolated contextual lines are the main source of contextual suppression in this effect. Such suppressive contextual elements can be blocked by non-suppressive ones. We suggest that contextual elements reveal valences in loose analogy to chemical valences.

Adult↗

PubMed2004

Lorazepam strongly prolongs visual information processing.

Lorazepam is a drug that has been widely used for over 30 years. Whereas its therapeutic and amnestic effects are fairly well known, the visuo-perceptual impairments induced by this drug have been studied to a much lesser degree and only little is known about the influence of lorazepam on the time course of visual information processing. To gain a better insight specifically on these temporal characteristics, we used the recently discovered backward masking technique, 'shine-through', in which a vernier target precedes a grating. We tested subjects, treated with lorazepam, diazepam, or a placebo, with masking gratings of various spatial layouts. Our experiments reveal surprising results. First, for the unmasked vernier target, lorazepam induced a strong deterioration of performance compared to both diazepam and placebo. Performance deteriorated even more significantly if a masking grating was presented following the vernier. We observed that vernier discrimination could be completely abolished even if the grating appeared 400 ms after the vernier presentation. Such long time intervals are beyond usual visual masking effects. When performing the task under placebo, the participants perceived the vernier target and the masking grating as two independent time events rather than as a single event. It appears that lorazepam prolongs dramatically the processing of visual targets. The masking effects revealed here are specific to the type of grating and are much stronger under lorazepam than under diazepam.

Adult↗

PubMed2004

Testing quantitative models of backward masking.

We analyzed the relationship between U-shaped and monotonic-shaped masking functions, using both computer simulations of quantitative models and experimental data Our analysis revealed that quantitative models of backward masking predict that U-shaped masking functions should appear for weak masks and monotonic masking functions should appear for strong masks. The models predict, moreover, that for a fixed target and experimental task, as the mask changes it is possible to go from U-shaped to monotonic-shaped masking functions. Significantly, the models predict that at each stimulus onset asynchrony between the target and the mask, the U-shaped function must have weaker masking than the monotonic-shaped function. Contrary to the predictions of the models, we show an experimental situation that generates masking functions that violate this prediction.

Form Perception↗

PubMed2003

Combining backward masking and transcranial magnetic stimulation in human observers.

Both backward masking and transcranial magnetic stimulation (TMS) are capable of hindering perception of a visual target. To study the relationship between these two methods we applied TMS over the occipital pole in combination with the visual backward masking technique shine-through. The recently discovered weak shine-through mask consists of a horizontal grating with 25 vernier elements. In three subjects we determined discrimination thresholds for vernier acuity without and with the shine-through mask. Modulation of the vernier discrimination threshold was determined in both conditions with TMS at various stimulus onset asynchronies (SOAs). In the unmasked condition TMS deteriorates discrimination moderately around 120 ms TMS SOA from 25" (arc seconds) to about 130". If in addition to TMS the vernier is backward-masked, discrimination of the vernier offset is completely abolished (>300"). Therefore, TMS and backward masking can interact in a non-linear manner, strongly interfering with early visual processing.

Adult↗

PubMed2003

Collinear contextual suppression.

The context of a target can modulate behavioral as well as neural responses to that target. For example, target processing can be suppressed by iso-oriented surrounds whereas it can be facilitated by collinear contextual elements. Here, we present experiments in which collinear elements exert strong suppression whereas iso-oriented contextual surrounds yield no contextual modulation--contrary to most studies in this field. We suggest that contextual suppression depends strongly on the spatial arrangement of the context pointing to the influence of Gestalt factors in contextual modulation.

Afterimage↗

PubMed2003

Fusion of competing features is not serial.

How features of an object are bound into a unique percept is one of the puzzling problems in the cognitive and neuro-sciences. In order to investigate the spatio-temporal mechanisms of feature binding, we serially present two verniers with opposite offset directions for very short durations. Only one vernier is perceived with its offset dominated by the vernier presented second. This dominance reverses if the two verniers are followed by masking gratings, i.e. the first presented vernier dominates performance. Therefore, feature fusion can neither be explained completely by spatially local mechanisms nor by the temporal order of appearance of elements.

Adult↗

PubMed2003

Timing of contextual modulation in the shine-through effect.

Contextual elements can fundamentally change the perception of an embedded target. A recently discovered masking effect, shine-through, allows one to investigate the precise dynamics of contextual modulation of the human visual system. In this shine-through effect, a vernier precedes a grating comprising more than seven elements for display times as short as 10 ms. The vernier appears as a "shine-through" element superimposed on the grating. However, if additional single lines are presented above and below the grating, visibility of the shine-through element dramatically diminishes. Recent publications focused mainly on the spatial aspects of this contextual modulation. Here, we investigate its temporal characteristics. We show that contextual suppression can occur for context durations of 5-10 ms, even if contextual elements appear 100 ms before target onset. This contextual suppression is not due to the presentation of the contextual elements themselves since without the grating contextual elements exert only weak masking power. Only the combination of contextual elements and grating causes the contextual suppression.

Afterimage↗

PubMed2003

Extending the shine-through effect to classical masking paradigms.

A vernier, presented for a short time, shines through a following grating if the grating contains nine and more elements but remains largely invisible for smaller gratings. Therefore, extended grating masks yield, surprisingly, less masking than smaller ones. Here, we show that this mask size effect is not unique to grating masks. Masking diminishes if the size of classical pattern-, noise-, light-, and metacontrast masks increases and if these masks are regular, i.e. highly ordered.

Afterimage↗

PubMed2003

Local interactions in neural networks explain global effects in Gestalt processing and masking.

One of the fundamental and puzzling questions in vision research is how objects are segmented from their backgrounds and how object formation evolves in time. The recently discovered shine-through effect allows one to study object segmentation and object formation of a masked target depending on the spatiotemporal Gestalt of the masking stimulus (Herzog & Koch, 2001). In the shine-through effect, a vernier (two abutting lines) precedes a grating for a very short time. For small gratings, the vernier remains invisible while it regains visibility as a shine-through element for extended and homogeneous gratings. However, even subtle deviations from the homogeneity of the grating diminish or even abolish shine-through. At first glance, these results suggest that explanations of these effects have to rely on high-level Gestalt terminology such as homogeneity rather than on low-level properties such as luminance (Herzog, Fahle, & Koch, 2001). Here, we show that a simple neural network model of the Wilson-Cowan type qualitatively and quantitatively explains the basic effects in the shine-through paradigm, although the model does not contain any explicit, global Gestalt processing. Visibility of the target vernier corresponds to transient activation of neural populations resulting from the dynamics of local lateral interactions of excitatory and inhibitory layers of neural populations.

Calibration↗

PubMed2003

Nonlinear ideal observation and recurrent preprocessing in perceptual learning.

Residual micro-saccades, tremor and fixation errors imply that, on different trials in visual tasks, stimulus arrays are inevitably presented at different positions on the retina. Positional variation is likely to be specially important for tasks involving visual hyperacuity, because of the severe demands that these tasks impose on spatial resolution. In this paper, we show that small positional variations lead to a structural change in the nature of the ideal observer's solution to a hyperacuity-like visual discrimination task such that the optimal discriminator depends quadratically rather than linearly on noisy neural activities. Motivated by recurrent models of early visual processing, we show how a recurrent preprocessor of the noisy activities can produce outputs which, when passed through a linear discriminator, lead to better discrimination even when the positional variations are much larger than the threshold acuity of the task. Since, psychophysically, hyperacuity typically improves greatly over the course of perceptual learning, we discuss our model in the light of results on the speed and nature of learning.

Animals↗