Karoline Spang

Feature-specific electrophysiological correlates of texture segregation

Discrimination between a figure and its surround is an important first step of pattern recognition. This discrimination usually relies, as a first step, on the detection of borders between a figure and its surround, for example based on spatial gradients in luminance, colour, or texture. There is evidence that neurones in the visual cortex are specifically activated by segregation between textures, but the relation between segregation based on different types of features such as colour, luminance, and motion is unclear. Evoked EEG potentials specific to texture segregation were investigated in 17 observers in two separate experiments and by means of functional magnetic resonance imaging in a separate study (Fahle et al., in preparation). Differences in either luminance, colour, line orientation, motion, or stereoscopic depth defined a checkerboard pattern. Patterns defined by each of these features elicited segregation-specific potentials. In contrast to earlier reports (Vision Research 37 (1997) 1409), however, we find pronounced differences between the segregation-specific potentials evoked through different features, especially regarding their peak latencies. The topographical distribution of the activity evoked reveals different polarities and partly specific locations for different stimulus features, indicating the existence of different processors for texture segregation based on different features.

How Much of the “Unconscious” is Just Pre – Threshold?

Visual awareness is a specific form of consciousness. Binocular rivalry, the alternation of visual consciousness resulting when the two eyes view differing stimuli, allows one to experimentally investigate visual awareness. Observers usually indicate the gradual changes of conscious perception in binocular rivalry by a binary measure: pressing a button. However, in our experiments we used gradual measures such as pupil and joystick movements and found reactions to start around 590 ms before observers press a button, apparently accessing even pre-conscious processes. Our gradual measures permit monitoring the somewhat gradual built-up of decision processes. Therefore these decision processes should not be considered as abrupt events. This is best illustrated by the fact that the process to take a decision may start but then stop before an action has been taken – which we will call an abandoned decision process here. Changes in analog measures occurring before button presses by which observers have to communicate that a decision process has taken place do not prove that these decisions are taken by a force other than the observer – hence eliminating “free will” – but just that they are prepared “pre-thresholdly,” before the observer considers the decision as taken.

Visuospatial hemi-inattention following cerebellar/brain stem bleeding.

Neglect is a unilateral lack of responsiveness to stimuli caused by visuospatial hemi-inattention, a unilateral representation deficit and/or a unilateral hypokinesia. It results most frequently from right-hemisphere brain damage, particularly of the parietal lobe but also of the frontal cortex, the basal ganglia, the thalamus, and recently it has also been described after a cerebellar lesion. We report a patient with right-sided bleeding of the posterior inferior cerebellar artery, who developed a left-sided visual hemi-inattention. She had no visual field defects, yet she had problems detecting left-sided targets in visual extinction. Furthermore, she was impaired in detecting complex motion on the left side and targets in a fixation offset paradigm. Reactions to left-sided targets in covert shifts of attention were slowed in the in valid condition. Her text reading was impaired as she could not always find the initial word of the next line. However, she was aware of her deficit. Her visuoconstructive ability was normal and she gave no indication of tactile or acoustic extinction. As the cerebellar lesion was located in the right hemisphere and the inattention involved the left side of space, we suggest that the damage to the right brain stem led to a transient imbalance of the noradrenergic ascending activation system which may explain her hemi-inattention.

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.

Cortical correlates of stereoscopic depth produced by temporal delay

Stereoscopic depth processing for static objects depends on retinal disparities between the two eyes and has been shown in previous functional imaging (fMRI) work to involve widely distributed activity in the human visual cortex, including both dorsal and ventral streams. Stereoscopic depth processing of moving objects, on the other hand, can be produced by purely temporal lags between the eyes, and the cortical basis for this kind of stereopsis has received less attention. Using fMRI in human subjects, we measured the activations produced by dynamic visual noise both when it was in phase between the eyes and appeared two-dimensional (2D) and when an interocular delay made it appear like a 3D rotating cylinder. When observers attended to the depth, the stimulus with the interocular delay produced more activity than the 2D stimulus in a large variety of cortical areas, including V1, V3A, caudal intraparietal sulcus (cIPS), and MT. When, on the other hand, observers attended to a digit counting task in the fovea, the stimulus with the interocular delay tended to decrease the BOLD response in V1 while still increasing it significantly in area cIPS. The areas that are activated by interocular delay even when not attending to depth (MT, cIPS) are similar to those previously described for traditional stereoscopic stimuli, and we conclude that the dorsal-stream mechanisms for processing interocular delay are not different at the level of spatial resolution of this study.

Differential impact of parvocellular and magnocellular pathways on visual impairment in apperceptive agnosia

The term “visual form agnosia” describes a disorder characterized by problems recognizing objects, poor copying, and distinguishing between simple geometric shapes despite normal intellectual abilities. Visual agnosia has been interpreted as a disorder of the magnocellular visual system, caused by an inability to separate figure from ground by sampling information from extended regions of space and to integrate it with fine-grain local information. However, this interpretation has hardly been tested with neuropsychological or functional brain imaging methods, mainly because the magnocellular and parvocellular structures are highly interconnected in the visual system. We studied a patient (AM) who had suffered a sudden heart arrest, causing hypoxic brain damage. He was/is severely agnosic, as apparent in both the Birmingham Object Recognition Battery and the Visual Object and Space Battery. First- and especially second-order motion perception was also impaired, but AM experienced no problems in grasping and navigating through space. The patient revealed a normal P100 in visual evoked potentials both with colored and fine-grained achromatic checkerboards. But the amplitude of the P100 was clearly decreased if a coarse achromatic checkerboard was presented. The physiological and neuropsychological findings indicate that AM experienced problems integrating information over extended regions of space and in detecting second-order motion. This may be interpreted as a disorder of the magnocellular system, with intact parvocellular system and therefore preserved ability to detect both local features and colors.

Electrophysiological correlates of binocular stereo depth without binocular disparities.

A small region of background presented to only one eye in an otherwise binocular display may, under certain conditions, be resolved in the visual system by interpreting the region as a small gap between two similar objects placed at different depths, with the gap hidden in one eye by parallax. This has been called monocular gap stereopsis. We investigated the electrophysiological correlate of this type of stereopsis by means of sum potential recordings in 12 observers, comparing VEPs for this stimulus (“Gillam Stereo”, Author BG has strong reservations about this term) with those for similar stimuli containing disparity based depth and with no depth (flat). In addition we included several control stimuli. The results show a pronounced early negative potential at a latency of around 170 ms (N170) for all stimuli containing non- identical elements, be they a difference caused by binocular disparity or by completely unmatched monocular contours. A second negative potential with latency around 270 ms (N270), on the other hand, is present only with stimuli leading to fusion and the perception of depth. This second component is similar for disparity-based stereopsis and monocular gap, or “Gillam Stereo” although slightly more pronounced for the former. We conjecture that the first component is related to the detection of differences between the images of the two eyes that may then either be fused, leading to stereopsis and the corresponding second potential, or else to inhibition and rivalry without a later trace in the VEP. The finding that that “Gillam Stereo” leads to cortical responses at the same short latencies as disparity based stereopsis indicates that it may partly rely on quite early cortical mechanisms.

On the Mechanics of Immediate Corrections and Aftereffects in Prism Adaptation

Prisms laterally shifting the perceived visual world cause arm movements to deviate from intended targets. The resulting error—the direct effect—both for pointing and throwing movements, usually corresponds to only around half of the prism’s optical power due to an “immediate correction effect”. We investigated the mechanisms of this immediate correction effect. In three experiments with 73 healthy subjects we find that the immediate correction effect is associated with a head and/or eye rotation. Since these rotations are subconscious they are not taken into account by the participants. These subconscious rotations compensate for a large portion of the prism’s optical effect and change the subjective straight ahead. These movements seem to be induced only in a rich visual environment and hence do not take place in the dark. They correspond to the difference between the direct effect and the optical power of the prisms and seem to cause the immediate correction effect. Hence, eye-hand adaptation only adapts to the prism’s optical power minus unconscious head rotation and hence is much smaller than the optical power of the prisms.

Limited Plasticity of Prismatic Visuomotor Adaptation

Movements toward an object displaced optically through prisms adapt quickly, a striking example for the plasticity of neuronal visuomotor programs. We investigated the degree and time course of this system’s plasticity. Participants performed goal-directed throwing or pointing movements with terminal feedback before, during, and after wearing prism goggles shifting the visual world laterally either to the right or to the left. Prism adaptation was incomplete even after 240 throwing movements, still deviating significantly laterally by on average of 0.8° (CI = 0.20°) at the end of the adaptation period. The remaining lateral deviation was significant for pointing movements only with left shifting prisms. In both tasks, removal of the prisms led to an aftereffect which disappeared in the course of further training. This incomplete prism adaptation may be caused by movement variability combined with an adaptive neuronal control system exhibiting a finite capacity for evaluating movement errors.

What happens early in ultra rapid object recognition

0% 20% 40% Introduction Selecting gaze is in every day living nearly effortless and most of the time without conscious thought. So it is not surprising that humans and other primates are able to shift gaze quickly (around 120 ms) when confronted with two natural scenes to that one, which contains an object of interest (like a car, a face or an animal). It has been argued, that this is possible due to temporal coding instead of firing rate coding, which is inherently more time consuming and depends on temporal integration. Here we test, if temporal integration is crucial for ultra rapid object recognition for three different kind of tasks: manual responses (step wise), saccadic responses (step wise) and saccadic responses (gap).