Martin Jüttner

Peripheral vision and pattern recognition: a review.

We summarize the various strands of research on peripheral vision and relate them to theories of form perception. After a historical overview, we describe quantifications of the cortical magnification hypothesis, including an extension of Schwartzs cortical mapping function. The merits of this concept are considered across a wide range of psychophysical tasks, followed by a discussion of its limitations and the need for non-spatial scaling. We also review the eccentricity dependence of other low-level functions including reaction time, temporal resolution, and spatial summation, as well as perimetric methods. A central topic is then the recognition of characters in peripheral vision, both at low and high levels of contrast, and the impact of surrounding contours known as crowding. We demonstrate how Boumas law, specifying the critical distance for the onset of crowding, can be stated in terms of the retinocortical mapping. The recognition of more complex stimuli, like textures, faces, and scenes, reveals a substantial impact of mid-level vision and cognitive factors. We further consider eccentricity-dependent limitations of learning, both at the level of perceptual learning and pattern category learning. Generic limitations of extrafoveal vision are observed for the latter in categorization tasks involving multiple stimulus classes. Finally, models of peripheral form vision are discussed. We report that peripheral vision is limited with regard to pattern categorization by a distinctly lower representational complexity and processing speed. Taken together, the limitations of cognitive processing in peripheral vision appear to be as significant as those imposed on low-level functions and by way of crowding.

Occurrence of human express saccades depends on stimulus uncertainty and stimulus sequence

SummarySaccadic latencies measured in response to a step-wise displacement of the target may be substantially reduced if a gap separates the offset of the initial fixation point and the onset of the peripheral target. According to Fischer and Ramsperger (1984) this paradigm provokes a bimodal latency distribution which consists of a peak of very fast saccadic responses (express saccades) at about 110 ms and another peak arising from somewhat slower saccades (regular saccades). Using again the gap paradigm, we investigated the effect of an additional go/no-go (i.e. target trial/catch trial) decision on saccadic latencies. The experiments yielded the following results: (i) the distribution between the peaks of express and regular saccades strongly depends on the proportion of catch trials introduced into the trial sequence, which suggests the existence of different modes of operation of the decision processes for express and regular saccades. (ii) The catch trial effect on saccadic latency proved to be a local phenomenon in time: saccades which follow catch trials tend to be slower than those following target trials.

Reduced perceptual dimensionality in extrafoveal vision

The classification behaviour of human observers with respect to compound Gabor signals is tested at foveal and extrafoveal retinal positions. Classification performance is analysed in terms of a probabilistic classification model recently proposed by Rentschler, Jüttner and Caelli [(1994) Vision Research, 34, 669-687]. The analysis allows inferences about structure and dimensionality of the individual internal representations underlying the classification task and their temporal evolution during the learning process. Using this technique it is found that the internal representations of direct and eccentric viewing are intrinsically incommensurable, in the sense that extrafoveal pattern representations are characterized by a lower perceptual dimension in feature space relative to the corresponding physical input signals, whereas foveal representations are not. The observed deficits cannot be renormalized by size scaling (cortical magnification); however, they can be partially reduced by learning although the learning progress strongly depends on the observers practice. The structural incommensurability between foveal and extrafoveal representations poses constraints on possible forms of foveal-extrafoveal interaction, which might have implications on related perceptual phenomena such as visual stability across saccadic eye movements.

Lateral information transfer across saccadic eye movements

Our perception of the visual world remains stable and continuous despite the disruptions caused by retinal image displacements during saccadic eye movements. The problem of visual stability is closely related to the question of whether information is transferred across such eye movements-and if so, what sort of information is transferred. We report experiments carried out to investigate how presaccadic signals at the location of the saccade goal influence the visibility of postsaccadic test signals presented at the fovea. The signals were Landolt rings of differ-ent orientations. If the orientations of pre- and postsaccadic Landolt rings were different, the thresholds of the test signals were elevated by about 20%–25% relative to those at the static control condition. When the orientations were identical, no such elevation occurred. This selective threshold elevation effect proved to be a phenomenon different from ordinary saccadic suppression, although it was closely related to the execution of the saccadic eye movement. The consequences for visual stability are discussed.

Innate and learned components of human visual preference.

BACKGROUND Recent claims in neuroscience and evolutionary biology suggest that the aesthetic sense reflects preferences for image signals whose characteristics best fit innate brain mechanisms of visual recognition. RESULTS This hypothesis was tested by behaviourally measuring, for a set of initially unfamiliar images, the effects of category learning on preference judgements by humans, and by relating the observed data to computationally reconstructed internal representations of categorical concepts. Category learning induced complex shifts in preference behaviour. Two distinct factors - complexity and bilateral symmetry - could be identified from the data as determinants of preference judgements. The effect of the complexity factor varied with object knowledge acquired through category learning. In contrast, the impact of the symmetry factor proved to be unaffected by learning experience. Computer simulations suggested that the preference for pattern complexity relies on active (top-down) mechanisms of visual recognition, whereas the preference for pattern symmetry depends on automatic (bottom-up) mechanisms. CONCLUSIONS Human visual preferences are not fully determined by (objective) structural regularities of image stimuli but also depend on their learned (subjective) interpretation. These two aspects are reflected in distinct complementary factors underlying preference judgements, and may be related to complementary modes of visual processing in the brain.

Development of configural 3D object recognition

There is evidence for the late development in humans of configural face and animal recognition. We show that the recognition of artificial three-dimensional (3D) objects from part configurations develops similarly late. We also demonstrate that the cross-modal integration of object information reinforces the development of configural recognition more than the intra-modal integration does. Multimodal object representations in the brain may therefore play a role in configural object recognition.

Scale‐invariant superiority of foveal vision in perceptual categorization

The recognition of objects is exceedingly difficult in indirect view. This complication cannot be explained in terms of retino‐cortical magnification, as size scaling fails to establish position invariance both for character recognition [Strasburger, H. & Rentschler, I. (1996) Eur. J. Neurosci., 8 1787–1791] and pattern classification [Jüttner, M. & Rentschler, I. (1996) Vision Res., 36, 1007–1021]. Thus we compared, for two tasks of discrimination learning and category learning with respect to a common set of grey‐level patterns, how humans perform in foveal and extrafoveal vision. Observers learnt to discriminate (size‐scaled) images equally well in foveal and extrafoveal view, whereas they displayed profound deficiencies in extrafoveal category learning for the same patterns. From the behavioural learning data, internal representations of the learning signals were reconstructed by means of computer simulations. For foveal view, these representations were found to be veridical to their physical counterparts for both learning tasks. For extrafoveal view, they were severely distorted for category learning but not for discrimination learning. A variance reduction of the pattern classes by a factor of 100 reduced the dissociation between extrafoveal categorization and discrimination but did not remove it. These observations suggest a scale‐invariant superiority of foveal vision for learning object categories. This implies a high degree of space variance of visual cognition which is vastly underestimated by classical measures of visual performance, e.g. acuity, visual field and contrast sensitivity.

A developmental dissociation of view-dependent and view-invariant object recognition in adolescence.

Spatial generalization skills in school children aged 8-16 were studied with regard to unfamiliar objects that had been previously learned in a cross-modal priming and learning paradigm. We observed a developmental dissociation with younger children recognizing objects only from previously learnt perspectives whereas older children generalized acquired object knowledge to new viewpoints as well. Haptic and--to a lesser extent--visual priming improved spatial generalization in all but the youngest children. The data supports the idea of dissociable, view-dependent and view-invariant object representations with different developmental trajectories that are subject to modulatory effects of priming. Late-developing areas in the parietal or the prefrontal cortex may account for the retarded onset of view-invariant object recognition.

Similarity-based models of human visual recognition

Seven models of human visual recognition from cognitive psychology, visual psychophysics and connectionism were compared. They were used to predict psychophysical classification data obtained via supervised learning with parametrised grey-level patterns (compound Gabor signals). Four sets of learning patterns, as well as foveal and extrafoveal viewing conditions, were applied. Model performance was determined by comparing observed and predicted data with respect to root mean square deviation and to signal reconstruction via multidimensional scaling. Results show that a psychophysical theory of classification requires a similarity concept that is based both on physical signal description and on cognitive bias. The latter is less pronounced in foveal recognition, where all seven models performed almost equally well, but matters in extrafoveal recognition. Virtual prototype models (Rentschler et al. (1994), Vision Research 34, 669-687), which best accommodate stimulus- and observer-dependencies, are then of advantage. Concerning computational efficiency, a hyperBF model (Poggio and Girosi (1990), Science 247, 978) was much faster, and generalized signal detection models were much slower than the average.

Mirror-image relations in category learning

The discrimination of patterns that are mirror-symmetric counterparts of each other is difficult and requires substantial training. We explored whether mirror-image discrimination during expertise acquisition is based on associative learning strategies or involves a representational shift towards configural pattern descriptions that permit resolution of symmetry relations. Subjects were trained to discriminate between sets of unfamiliar grey-level patterns in two conditions, which either required the separation of mirror images or not. Both groups were subsequently tested in a 4-class category-learning task employing the same set of stimuli. The results show that subjects who had successfully learned to discriminate between mirror-symmetric counterparts were distinctly faster in the categorization task, indicating a transfer of conceptual knowledge between the two tasks. Additional computer simulations suggest that the development of such symmetry concepts involves the construction of configural, protoholistic descriptions, in which positions of pattern parts are encoded relative to a spatial frame of reference.