Cathleen Grimsen

Dissociation of egocentric and allocentric coding of space in visual search after right middle cerebral artery stroke

Spatial representations rely on different frames of reference. Patients with unilateral neglect may behave as suffering from either egocentric or allocentric deficiency. The neural substrates representing these reference frames are still under discussion. Here we used a visual search paradigm to distinguish between egocentric and allocentric deficits in patients with right hemisphere cortical lesions. An attention demanding search task served to divide patients according to egocentric versus allocentric deficits. The results indicate that egocentric impairment was associated with damage in premotor cortex involving the frontal eye fields. Allocentric impairment on the other hand was linked to lesions in more ventral regions near the parahippocampal gyrus (PHG).

Ultra Rapid Object Categorization: Effects of Level, Animacy and Context

It is widely agreed that in object categorization bottom-up and top-down influences interact. How top-down processes affect categorization has been primarily investigated in isolation, with only one higher level process at a time being manipulated. Here, we investigate the combination of different top-down influences (by varying the level of category, the animacy and the background of the object) and their effect on rapid object categorization. Subjects participated in a two-alternative forced choice rapid categorization task, while we measured accuracy and reaction times. Subjects had to categorize objects on the superordinate, basic or subordinate level. Objects belonged to the category animal or vehicle and each object was presented on a gray, congruent (upright) or incongruent (inverted) background. The results show that each top-down manipulation impacts object categorization and that they interact strongly. The best categorization was achieved on the superordinate level, providing no advantage for basic level in rapid categorization. Categorization between vehicles was faster than between animals on the basic level and vice versa on the subordinate level. Objects in homogenous gray background (context) yielded better overall performance than objects embedded in complex scenes, an effect most prominent on the subordinate level. An inverted background had no negative effect on object categorization compared to upright scenes. These results show how different top-down manipulations, such as category level, category type and background information, are related. We discuss the implications of top-down interactions on the interpretation of categorization results.

Electrophysiological correlates of figure-ground segregation directly reflect perceptual saliency.

In a figure identification task, we investigated the influence of different visual cue configurations (spatial frequency, orientation or a combination of both) on the human EEG. Combining psychophysics with ERP and time-frequency analysis, we show that the neural response at about 200ms reflects perceptual saliency rather than physical cue contrast. Increasing saliency caused (i) a negative shift of the posterior P2 coinciding with a power decrease in the posterior theta-band and (ii) an amplitude and latency increase of the posterior P3. We demonstrate that visual cues interact for a percept that is non-linearly related to the physical figure-ground properties.

Patients with functional psychoses show similar visual backward masking deficits

Recent genetic, behavioral, and clinical studies suggest that functional psychoses (schizophrenia, bipolar disorder, schizoaffective disorder), previously thought to be distinct from each other, may belong to one continuum. The shine-through masking paradigm is a potential endophenotype of schizophrenia with high sensitivity and specificity for discriminating between patients, their clinically unaffected relatives, and healthy controls. Hence, if schizophrenia, bipolar disorder and schizoaffective disorder belong to one common disease, strong masking deficits are expected to occur in all three diseases whereas no masking deficits are expected for abstinent alcoholic or depressive patients. Indeed, we found masking to be much stronger in psychotic patients compared to controls and to depressive patients and abstinent alcoholics, who performed on similar levels.

Orientation specificity of learning vernier discriminations

Orientation selective neurons in the primary visual cortex typically respond to a range of orientations that covers 20 degrees or more, while in psychophysical experiments, orientation bandwidth is often clearly narrower. Here, we measure the orientation specificity of perceptual learning for vernier discriminations. More than 70 observers, in separate groups, practiced a vernier discrimination task with a constant stimulus orientation. After a 1h session of training, the vernier was rotated by 2 degrees, 4 degrees, 10 degrees, 20 degrees, 45 degrees or 90 degrees. Improvement through training in the first session transferred to the second session (tested on the next day) up to 10 degrees of stimulus rotation. We found no transfer for rotations of 20 degrees, 45 degrees and 90 degrees. Hence, the orientation half-bandwidth of perceptual learning is around 15 degrees, leading to a bandwidth of 30 degrees and corresponding to that of single neurons in early visual cortices, while being narrower than that in higher cortical areas.

Specificity of fast perceptual learning in shape localisation tasks based on detection versus form discrimination

Perceptual learning is defined as a long-lasting improvement of perception as a result of experience. Here we examined the role of task on fast perceptual learning for shape localisation either in simple detection or based on form discrimination in different visual submodalities, using identical stimulus position and stimulus types for both tasks. Thresholds for each submodality were identified by four-alternative-forced-choice tasks. Fast perceptual learning occurred for shape detection-based on luminance, motion and color differences but not for texture differences. In contradistinction, fast perceptual learning was not evident in shape localisation based on discrimination. Thresholds of all submodalities were stable across days. Fast perceptual learning seems to differ not only between different visual submodalities, but also across different tasks within the same visual submodality.

Human feature-based attention consists of two distinct spatiotemporal processes

In human and nonhuman primates, goal-directed behavior requires the selection of relevant pieces of information from the multitude of simultaneous sensory inputs. Feature-based attention (FBA) plays a crucial role in this selection by improving the neuronal representation of an attended stimulus feature. Of particular interest for understanding the neuronal mechanisms behind FBA is the processing fate of spatially unattended stimuli, either sharing the attended feature attribute or belonging to the attended or to a nonattended feature dimension. Using a wide range of cue/stimulus combinations, we investigated event-related potentials from the human brain, recorded under conditions of different feature attention but constant visual stimulation. We found that neural processing of visual stimuli sharing the dimension or the attribute of the attended target is associated with two distinct spatiotemporal processes, particularly prominent during the selection negativity period. Dimension-based modulation of neural signals first emerged over frontal electrode sites, and temporally preceded and accompanied attribute-specific FBA effects at occipital, parieto-occipital, and parietal electrodes. The findings suggest a process of FBA that not only increases responses of those neurons particularly tuned to the attended attribute but also modulates activity in the cortical module that is selective for the feature dimension to which the attended attribute belongs.

Functional modulation of contralateral bias in early and object-selective areas after stroke of the occipital ventral cortices

ABSTRACT Object agnosia is a rare symptom, occurring mainly after bilateral damage of the ventral visual cortex. Most patients suffering from unilateral ventral lesions are clinically non‐agnosic. Here, we studied the effect of unilateral occipito‐temporal lesions on object categorization and its underlying neural correlates in visual areas. Thirteen non‐agnosic stroke patients and twelve control subjects performed an event‐related rapid object categorization task in the fMRI scanner where images were presented either to the left or to the right of a fixed point. Eight patients had intact central visual fields within at least 10° eccentricity while five patients showed an incomplete hemianopia. Patients made more errors than controls for both contra‐ and ipsilesional presentation, meaning that object categorization was impaired bilaterally in both patient groups. The activity in cortical visual areas is usually higher when a stimulus is presented contralaterally compared to presented ipsilaterally (contralateral bias). A region of interest analysis of early visual (V1–V4) and object‐selective areas (lateral occipital complex, LOC; fusiform face area, FFA; and parahippocampal place area, PPA) revealed that the lesioned‐hemisphere of patients showed reduced contralateral bias in early visual areas and LOC. In contrast, literally no contralateral bias in FFA and PPA was found. These findings indicate disturbed processing in the lesioned hemisphere, which might be related to the processing of visually presented objects. Thus, unilateral occipito‐temporal damage leads to altered contralateral bias in the lesioned hemisphere, which might be the cause of impaired categorization performance in both visual hemifields in clinically non‐agnosic patients. We conclude that both hemispheres need to be functionally intact for unimpaired object processing. HIGHLIGHTSWe studied the effect of unilateral ventral lesions on object categorization and its neural correlates.Patients performed a rapid object categorization task in the fMRI scanner.Patients are impaired in categorizing objects both contra‐ and ipsilesionally.Functional changes in visual cortices only in patients lesioned hemisphere.The relationship between impaired categorization and neural alteration is discussed.

Contour Integration in Dynamic Scenes: Impaired Detection Performance in Extended Presentations

Since scenes in nature are highly dynamic, perception requires an on-going and robust integration of local information into global representations. In vision, contour integration (CI) is one of these tasks, and it is performed by our brain in a seemingly effortless manner. Following the rule of good continuation, oriented line segments are linked into contour percepts, thus supporting important visual computations such as the detection of object boundaries. This process has been studied almost exclusively using static stimuli, raising the question of whether the observed robustness and “pop-out” quality of CI carries over to dynamic scenes. We investigate contour detection in dynamic stimuli where targets appear at random times by Gabor elements aligning themselves to form contours. In briefly presented displays (230 ms), a situation comparable to classical paradigms in CI, performance is about 87%. Surprisingly, we find that detection performance decreases to 67% in extended presentations (about 1.9–3.8 s) for the same target stimuli. In order to observe the same reduction with briefly presented stimuli, presentation time has to be drastically decreased to intervals as short as 50 ms. Cueing a specific contour position or shape helps in partially compensating this deterioration, and only in extended presentations combining a location and a shape cue was more efficient than providing a single cue. Our findings challenge the notion of CI as a mainly stimulus-driven process leading to pop-out percepts, indicating that top-down processes play a much larger role in supporting fundamental integration processes in dynamic scenes than previously thought.

Attentional Effects in Contour Integration in Dynamic Scenes

Contour integration (CI) is an integral part of visual information processing requiring the combination of aligned edge configurations into coherent percepts. Subjects are efficient at detecting contours, reaching peak performances for stimulus presentation times of 100-200ms. CI research usually employs flashed static stimuli, making this an artificial situation compared to the continuous observation of everyday scenes. We addressed this discrepancy between commonly employed stimuli in CI and natural vision by employing novel dynamic stimuli comprising slowly rotating Gabor elements. Contours formed at predefined times and locations when 10 Gabor aligned. We expect these stimuli to better approximate natural vision as they require sustained observation and accounting of dynamic changes to generate a coherent picture of a visual scene in order to detect the appearance of a contour. Since CI is believed to be a pop-up effect we expected similar performances for brief and extended presentations. However, results show a statistically significant (t(8)=8.83, p< 0.001) drop in performance of 19.5% for extended presentations of varying lengths, between 1840ms and 3680ms, when compared to a peak performance of 87% for a presentation time of 230ms. This dramatic decrease in performance led us to hypothesize that contour perception in extended presentations is a more demanding task, hence, that it might draw higher benefits from attention than brief presentations. By employing distinct single cues, and their combinations, we evaluated the capabilities of the visual system to use and combine independent information about a target. Cues targeting feature-based attention and location-based attention lead to a 6.7% and a 5.5% improvement in performance, respectively, when compared to a no-cueing condition in extended presentations. Their combination lead to an apparent additive effect, significantly (t(8)=3.3, p< 0.05) improving performance by 12.2%, thus restoring most of the decrease in performance seen between the peak condition and the extended condition. Meeting abstract presented at VSS 2015.