Mediaeval artists: masters in directing the observers' gaze.
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Fixation, Ocular↗
Biomedical subjects
Ute Leonards
Publications and source records attributed to Ute Leonards.
Luminosity--a perceptual "feature" of light-emitting objects?
Light-emitting objects are perceived as qualitatively different from light-reflecting objects, and the two categories elicit different cortical activity. However, it is unclear whether object luminosity is treated as an independent visual feature, comparable to orientation, motion or colour. Visual search tasks revealed that light-emitting targets led to efficient search when presented with light-reflecting distractors of similar luminance, but this efficiency was induced by the presence of luminance gradients producing the percept of luminosity rather than by luminosity itself. This implies that luminance gradients (not object luminosity) are encoded as features, questioning the existence of specific sensory mechanisms to detect light-emitting objects.
Adaptation, Ocular↗
Does contextual information influence positive and negative schizotypy scores in healthy individuals? The answer is maybe.
Defensive responding in schizotypy questionnaires might depend on context. Students completed a schizotypy questionnaire in a "psychiatric" context or a "creativity" context. Positive, but not negative, schizotypy scores were lower in the psychiatry than in the creativity group, but findings applied mainly to male participants. The implications of these findings are critically discussed.
Adolescent↗
Idiosyncratic initiation of saccadic face exploration in humans.
Visual processing and subsequent action are limited by the effectiveness of eye movement control: where the eyes fixate determines what part of the visual environment is seen in detail. Visual exploration consists of stereotypical sequences of saccadic eye movements which are known to depend upon both external factors, such as visual stimulus features, and internal cognition-related factors, such as attention and memory. However, how these two factors are balanced is unknown. One determinant might be the familiarity or ecological importance of the visual stimulus being explored. Recordings of saccades for human face stimuli revealed that their exploration was subject to strong individual biases for the initial saccade direction: subjects tended to look first to one particular side. We attribute this to internal factors. In contrast, exploration of landscapes, fractals or inverted faces showed no significant direction bias for initial saccades, suggesting more externally driven exploration patterns. Thus the balance between external and internal factors in scene exploration depends on stimulus type. An analysis of saccade latencies suggested that this individual preference for first saccade direction during face exploration leads to higher effectiveness through automation. The findings have implications for the understanding of both normal and abnormal eye movements.
Adolescent↗
Cortical distinction between the neural encoding of objects that appear to glow and those that do not.
Objects that appear to glow appear very different from those that do not. However, the neural representation of glow has not been investigated. We present data from an fMRI study which suggest that an extra-striate visual area is involved in the encoding of glowing stimuli, and that this activation does not arise from luminance or contrast factors. Possible functional reasons for the existence of such an area are discussed.
Adult↗
Comparison of early cortical networks in efficient and inefficient visual search: an event-related potential study.
Functional magnetic resonance imaging studies have indicated that efficient feature search (FS) and inefficient conjunction search (CS) activate partially distinct frontoparietal cortical networks. However, it remains a matter of debate whether the differences in these networks reflect differences in the early processing during FS and CS. In addition, the relationship between the differences in the networks and spatial shifts of attention also remains unknown. We examined these issues by applying a spatio-temporal analysis method to high-resolution visual event-related potentials (ERPs) and investigated how spatio-temporal activation patterns differ for FS and CS tasks. Within the first 450 msec after stimulus onset, scalp potential distributions (ERP maps) revealed 7 different electric field configurations for each search task. Configuration changes occurred simultaneously in the two tasks, suggesting that contributing processes were not significantly delayed in one task compared to the other. Despite this high spatial and temporal correlation, two ERP maps (120-190 and 250-300 msec) differed between the FS and CS. Lateralized distributions were observed only in the ERP map at 250-300 msec for the FS. This distribution corresponds to that previously described as the N2pc component (a negativity in the time range of the N2 complex over posterior electrodes of the hemisphere contralateral to the target hemifield), which has been associated with the focusing of attention onto potential target items in the search display. Thus, our results indicate that the cortical networks involved in feature and conjunction searching partially differ as early as 120 msec after stimulus onset and that the differences between the networks employed during the early stages of FS and CS are not necessarily caused by spatial attention shifts.
Adult↗
A new electrophysiological index for working memory load in humans.
Working memory, the ability to store and simultaneously manipulate information, is affected in several neuropsychiatric disorders which lead to severe cognitive and functional deficits. An electrophysiological marker for this process could help identify early cerebral function abnormalities. In subjects performing working memory-specific n-back tasks, event-related potential analysis revealed a positive-negative waveform (PNwm) component modulated in amplitude by working memory load. It occurs in the expected time range for this process, 140-280 ms after stimulus onset, superimposed on the classical P200 and N200 components. Independent Component Analysis extracted two functional components with latencies and topographical scalp distributions similar to the PNwm. Our results imply that the PNwm represents a new electrophysiological index for working memory load in humans.
Adult↗
Simulation of artificial vision: I. Eccentric reading of isolated words, and perceptual learning.
Simulations of artificial vision were performed to assess "minimum requirements for useful artificial vision". Retinal prostheses will be implanted at a fixed (and probably eccentric) location of the retina. To mimic this condition on normal observers, we projected stimuli of various sizes and content on a defined stabilised area of the visual field. In experiment 1, we asked subjects to read isolated 4-letter words presented at various degrees of pixelisation and at various eccentricities. Reading performance dropped abruptly when the number of pixels was reduced below a certain threshold. For central reading, a viewing area containing about 300 pixels was necessary for close to perfect reading (>90% correctly read words). At eccentricities beyond 10 degrees, close to perfect reading was never achieved even if more than 300 pixels were used. A control experiment using isolated letter recognition in the same conditions suggested that lower reading performance at high eccentricity was in part due to the "crowding effect". In experiment 2, we investigated whether the task of eccentric reading under such specific conditions could be improved by training. Two subjects, naive to this task, were trained to read pixelised 4-letter words presented at 15 degrees eccentricity. Reading performance of both subjects increased impressively throughout the experiment. Low initial reading scores (range 6%-23% correct) improved impressively (range 64%-85% correct) after about one month of training (about 1 h/day). Control tests demonstrated that the learning process consisted essentially in an adaptation to use an eccentric area of the retina for reading. These results indicate that functional retinal implants consisting of more than 300 stimulation contacts will be needed. They might successfully restore some reading abilities in blind patients, even if they have to be placed outside the foveal area. Reaching optimal performance may, however, require a significant adaptation process.
Adult↗
Perceptual learning of highly demanding visual search tasks.
Inefficient visual search can become efficient with practice [Vision Research 35 (1995) 2037; 40 (2000) 2925]. In this study, we wondered whether this improvement depends on unique visual features associated with the target, on differences in item-specific brightness distribution between target and distractors, or only on a change in the allocation of attention and thus global search strategy. We found that both, unique visual features and differences in brightness distribution lead to parallelisation with practice of originally inefficient search. Prolonged practice of inefficient search tasks lacking both unique visual features and differences in brightness distribution (conjunctions) does not lead to improved performance, thus indicating that perceptual learning in visual search does not solely reflect an unspecific global improvement in search strategy. Changing the brightness polarity of the stimuli leads to instantaneous, complete transfer to the new task. There is no transfer but rather trade-off between the learning based on unique visual features or on differences in brightness distribution between target and distractors.
Attention↗