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Manfred Fahle

Publications and source records attributed to Manfred Fahle.

17 recordsLinked to original sources
PubMed2005

Perceptual learning: specificity versus generalization.

Perceptual learning improves performance on many tasks, from orientation discrimination to the identification of faces. Although conventional wisdom considered sensory cortices as hard-wired, the specificity of improvement achieved through perceptual learning indicates an involvement of early sensory cortices. These cortices might be more plastic than previously assumed, and both sum-potential and single cell recordings indeed demonstrate plasticity of neuronal responses of these sensory cortices. However, for learning to be optimally useful, it must generalize to other tasks. Further research on perceptual learning should therefore, in my opinion, investigate first, the conditions for generalization of training-induced improvement, second, its use for teaching and rehabilitation, and third, its dependence on pharmacological agents.

Animals↗
PubMed2005

Learning to tell apples from oranges.

Recognizing images requires sorting them into the correct perceptual categories. Without categories, apples could not be discriminated from oranges. A fundamental question is whether categories are innate or learned, and if learned, how well such discriminations generalize. A recent finding by Notman et al. demonstrates that certain categorical discriminations are learned fast but vanish for slightly rotated stimuli. Rather than higher "cognitive" learning, this indicates involvement of "early" mechanisms, which cannot generalize across different stimulus orientations.

Cognition↗
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

Perceptual learning: a case for early selection.

Perceptual learning is any relatively permanent change of perception as a result of experience. Visual learning leads to sometimes dramatic and quite fast improvements of performance in perceptual tasks, such as hyperacuity discriminations. The improvement often is very specific for the exact task trained, for the precise stimulus orientation, the stimulus position in the visual field, and the eye used during training. This specificity indicates location of the underlying changes in the nervous system at least partly on the level of the primary visual cortex. The dependence of learning on error feedback and on attention, on the other hand, proves the importance of top-down influences from higher cortical centers. In summary, perceptual learning seems to rely at least partly on changes on a relatively early level of cortical information processing (early selection), such as the primary visual cortex under the influence of top-down influences (selection and shaping). An alternative explanation based on late selection is discussed.

Discrimination Learning↗
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

Figure-ground segregation can rely on differences in motion direction.

If the elements within a figure move synchronously while those in the surround move at a different time, the figure is easily segregated from the surround and thus perceived. Lee and Blake (1999) [Visual form created solely from temporal structure. Science, 284, 1165-1168] demonstrated that this figure-ground separation may be based not only on time differences between motion onsets, but also on the differences between reversals of motion direction. However, Farid and Adelson (2001) [Synchrony does not promote grouping in temporally structured displays. Nature Neuroscience, 4, 875-876] argued that figure-ground segregation in the motion-reversal experiment might have been based on a contrast artefact and concluded that (a)synchrony as such was 'not responsible for the perception of form in these or earlier displays'. Here, we present experiments that avoid contrast artefacts but still produce figure-ground segregation based on purely temporal cues. Our results show that subjects can segregate figure from ground even though being unable to use motion reversals as such. Subjects detect the figure when either (i) motion stops (leading to contrast artefacts), or (ii) motion directions differ between figure and ground. Segregation requires minimum delays of about 15 ms. We argue that whatever the underlying cues and mechanisms, a second stage beyond motion detection is required to globally compare the outputs of local motion detectors and to segregate figure from ground. Since analogous changes take place in both figure and ground in rapid succession, this second stage has to detect the asynchrony with high temporal precision.

Artifacts↗
PubMed2003

Differential contribution of early visual areas to the perceptual process of contour processing.

We investigated contour processing and figure-ground detection within human retinotopic areas using event-related functional magnetic resonance imaging (fMRI) in 6 healthy and naïve subjects. A figure (6 degrees side length) was created by a 2nd-order texture contour. An independent and demanding foveal letter-discrimination task prevented subjects from noticing this more peripheral contour stimulus. The contour subdivided our stimulus into a figure and a ground. Using localizers and retinotopic mapping stimuli we were able to subdivide each early visual area into 3 eccentricity regions corresponding to 1) the central figure, 2) the area along the contour, and 3) the background. In these subregions we investigated the hemodynamic responses to our stimuli and compared responses with or without the contour defining the figure. No contour-related blood oxygenation level-dependent modulation in early visual areas V1, V3, VP, and MT+ was found. Significant signal modulation in the contour subregions of V2v, V2d, V3a, and LO occurred. This activation pattern was different from comparable studies, which might be attributable to the letter-discrimination task reducing confounding attentional modulation. In V3a, but not in any other retinotopic area, signal modulation corresponding to the central figure could be detected. Such contextual modulation will be discussed in light of the recurrent processing hypothesis and the role of visual awareness.

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

Electrophysiological correlates of purely temporal figure-ground segregation.

Inhomogenous displays, in contrast to homogenous ones, evoke a specific potential in the VEP (tsVEP) which appears across different classical visual stimulus dimensions defining figure-ground segregation, such as luminance, orientation, (first-order) motion, and stereoscopic depth. This negative potential has a peak latency of about 200-300 ms and a peak amplitude of about -3 to -10 microV [Doc Ophthalmol. 95 (1998) 335]. Previously, we demonstrated that human subjects reliably segregate figure from ground, even in the absence of the classical cues, leaving time of change as the only cue for segregation. The results of the present study demonstrate that also purely temporally defined checkerboards evoke a tsVEP resembling the motion-defined tsVEP regarding polarity (negative), latency (two peaks at 180 and 270 ms, respectively), amplitude of the first negativity (-5.6 microV), and overall form of its components.

Adult↗
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

Mechanisms of time-based figure-ground segregation.

Figure-ground segregation can rely on purely temporal information, that is, on short temporal delays between positional changes of elements in figure and ground (Kandil, F.I. & Fahle, M. (2001) Eur. J. Neurosci., 13, 2004-2008). Here, we investigate the underlying mechanisms by measuring temporal segregation thresholds for various kinds of motion cues. Segregation can rely on monocular first-order motion (based on luminance modulation) and second-order motion cues (contrast modulation) with a high temporal resolution of approximately 20 ms. The mechanism can also use isoluminant motion with a reduced temporal resolution of 60 ms. Figure-ground segregation can be achieved even at presentation frequencies too high for human subjects to inspect successive frames individually. In contrast, when stimuli are presented dichoptically, i.e. separately to both eyes, subjects are unable to perceive any segregation, irrespective of temporal frequency. We propose that segregation in these displays is detected by a mechanism consisting of at least two stages. On the first level, standard motion or flicker detectors signal local positional changes (flips). On the second level, a segregation mechanism combines the local activities of the low-level detectors with high temporal precision. Our findings suggest that the segregation mechanism can rely on monocular detectors but not on binocular mechanisms. Moreover, the results oppose the idea that segregation in these displays is achieved by motion detectors of a higher order (motion-from-motion), but favour mechanisms sensitive to short temporal delays even without activation of higher-order motion detectors.

Adult↗
PubMed2002

Effects of grouping in contextual modulation.

Perception of a visual target and the responses of cortical neurons can be strongly influenced by a context surrounding the target. This observation relates to the fundamental issue of how cortical neurons code objects of the external world. In high-contrast regimes, embedding a target in an iso-oriented context reduces neural responses and deteriorates performance in psychophysical experiments. Performance from orthogonal surrounds is better than that from iso-oriented ones. This contextual interference is often postulated to be caused by long- or short-range interactions between neurons tuned to orientation. Here we show, using a new illusion called 'shine-through' as a sensitive psychophysical probe, that the orientation difference between target and context does not determine performance. Instead, contextual modulation depends on the overall spatial structure of the context. We propose that contextual suppression vanishes if the contextual elements are grouped to an independent and coherent object.

Humans↗
PubMed2002

Perceptual learning in amnesia.

Evidence from experiments on perceptual learning, accumulated during the last few years, increasingly indicates that the relative 'front end' parts of the visual system are more plastic even in adults than was previously expected. Hence, it might be possible that perceptual learning is similar in several respects to procedural learning and may be achieved even without (declarative) memory traces present. Results on six patients suffering from global amnesia due to damage to hippocampal-diencephalic systems demonstrate, for the first time, that at least some amnesic patients are able to significantly improve performance in a visual hyperacuity task as a result of training, showing improvement as good as the observers in the control group. This result corroborates the notion of a relatively 'front end' location of at least some forms of perceptual learning.

Adult↗
PubMed2002

Modulations of the processing of line discontinuities under selective attention conditions?

We examined whether the processing of discontinuities involved in figure-ground segmentation, like line ends, can be modulated under selective attention conditions. Subjects decided whether a gap in collinear or parallel lines was located to the right or left. Two stimuli were displayed in immediate succession. When the gaps were on the same side, reaction times (RTs) for the second stimulus increased when collinear lines followed parallel lines, or the reverse, but only when the two stimuli shared the same orientation and location. The effect did not depend on the global form of the stimuli or on the relative orientation of the gaps. A frame drawn around collinear elements affected the results, suggesting a crucial role of the "amodal" orthogonal lines produced when line ends are aligned. Including several gaps in the first stimulus also eliminated RT variations. By contrast, RT variations remained stable across several experimental blocks and were significant for interstimulus intervals from 50 to 600 msec between the two stimuli. These results are interpreted in terms of a modulation of the processing of line ends or the production of amodal lines, arising when attention is selectively drawn to a gap.

Attention↗