The effects of visual cortex lesions on the perception of rotated shapes

Brain functional network connectivity based on a visual task: visual information processing-related brain regions are significantly activated in the task state

It is not clear whether the method used in functional brain-network related research can be applied to explore the feature binding mechanism of visual perception. In this study, we investigated feature binding of color and shape in visual perception. Functional magnetic resonance imaging data were collected from 38 healthy volunteers at rest and while performing a visual perception task to construct brain networks active during resting and task states. Results showed that brain regions involved in visual information processing were obviously activated during the task. The components were partitioned using a greedy algorithm, indicating the visual network existed during the resting state. Z-values in the vision-related brain regions were calculated, confirming the dynamic balance of the brain network. Connectivity between brain regions was determined, and the result showed that occipital and lingual gyri were stable brain regions in the visual system network, the parietal lobe played a very important role in the binding process of color features and shape features, and the fusiform and inferior temporal gyri were crucial for processing color and shape information. Experimental findings indicate that understanding visual feature binding and cognitive processes will help establish computational models of vision, improve image recognition technology, and provide a new theoretical mechanism for feature binding in visual perception.

Reflections on the hand: the use of a mirror highlights the contributions of interpreted and retinotopic representations in the rubber-hand illusion

In the rubber-hand illusion, observing a rubber hand stroked in synchrony with one's own hand results in mislocalisation of the own hand, which is perceived as being located closer to the rubber hand. This illusion depends on having the rubber hand placed at a plausible egocentric orientation with respect to the observer. In the present study, we took advantage of this finding in order to compare the relative influence on the illusion of the rubber hand's perceived retinotopic image against its real-world position. The rubber hand was positioned egocentrically (fingers away from the participant) or allocentrically (fingers towards the participant), while participants viewed it either directly or via a mirror that was placed facing the participant. In the mirror conditions, the orientation of the retinotopic image of the hand (either egocentric or allocentric) was opposed to its real-world orientation. We found that the illusion was elicited in both mirror conditions, to roughly the same extent. Thus either of two representations can elicit the rubber-hand illusion: a world-centred understanding of the scene, resulting from the inferred position of the hand based on its mirror reflection, or a purely visual retinotopic representation of the viewed hand. In the mirror conditions, the illusion was somewhat weaker than in the typical directly viewed egocentric condition. We attribute this to competition between two incompatible representations introduced by the presence of the mirror. Finally, in two control experiments we ruled out that this reduction was due to two properties of mirror reflections: the increased perceived distance of items and the reversal of the apparent handedness of the rubber hand.

Including cognitive biases and distance-based rewards in a connectionist model of complex problem solving

We present a cognitive, connectionist-based model of complex problem solving that integrates cognitive biases and distance-based and environmental rewards under a temporal-difference learning mechanism. The model is tested against experimental data obtained in a well-defined and planning-intensive problem. We show that incorporating cognitive biases (symmetry and simplicity) in a temporal-difference learning rule (SARSA) increases model adequacy-the solution space explored by biased models better fits observed human solutions. While learning from explicit rewards alone is intrinsically slow, adding distance-based rewards, a measure of closeness to goal, to the learning rule significantly accelerates learning. Finally, the model correctly predicts that explicit rewards have little impact on problem solvers' ability to discover optimal solutions.

On coding non-contiguous letter combinations

Starting from the hypothesis that printed word identification initially involves the parallel mapping of visual features onto location-specific letter identities, we analyze the type of information that would be involved in optimally mapping this location-specific orthographic code onto a location-invariant lexical code. We assume that some intermediate level of coding exists between individual letters and whole words, and that this involves the representation of letter combinations. We then investigate the nature of this intermediate level of coding given the constraints of optimality. This intermediate level of coding is expected to compress data while retaining as much information as possible about word identity. Information conveyed by letters is a function of how much they constrain word identity and how visible they are. Optimization of this coding is a combination of minimizing resources (using the most compact representations) and maximizing information. We show that in a large proportion of cases, non-contiguous letter sequences contain more information than contiguous sequences, while at the same time requiring less precise coding. Moreover, we found that the best predictor of human performance in orthographic priming experiments was within-word ranking of conditional probabilities, rather than average conditional probabilities. We conclude that from an optimality perspective, readers learn to select certain contiguous and non-contiguous letter combinations as information that provides the best cue to word identity.

"What" and "where" in word reading: ventral coding of written words revealed by parietal atrophy

The visual system of literate adults develops a remarkable perceptual expertise for printed words. To delineate the aspects of this competence intrinsic to the occipitotemporal "what" pathway, we studied a patient with bilateral lesions of the occipitoparietal "where" pathway. Depending on critical geometric features of the display (rotation angle, letter spacing, mirror reversal, etc.), she switched from a good performance, when her intact ventral pathway was sufficient to encode words, to severely impaired reading, when her parietal lesions prevented the use of alternative reading strategies as a result of spatial and attentional impairments. In particular, reading was disrupted (a) by rotating word by more than 50 degrees , providing an approximation of the invariance range for words encoding in the ventral pathway; (b) by separating letters with double spaces, revealing the limits of letter grouping into perceptual wholes; (c) by mirror-reversing words, showing that words escape the default mirror-invariant representation of visual objects in the ventral pathway. Moreover, because of her parietal lesions, she was unable to discriminate mirror images of common objects, although she was excellent with reversible pseudowords, confirming that the breaking of mirror symmetry was intrinsic to the occipitotemporal cortex. Thus, charting the display conditions associated with preserved or impaired performance allowed us to infer properties of word coding in the normal ventral pathway and to delineate the roles of the parietal lobes in single-word recognition.

Differential Effects of Transcranial Magnetic Stimulation of Left and Right Posterior Parietal Cortex on Mental Rotation Tasks

A recently published study used the interference strategy of transcranial magnetic stimulation (TMS) to demonstrate the role of the right posterior parietal cortex (PPC) in the mental rotation of alphanumeric stimuli. We used similar stimulation parameters over the same left and right PPC regions, and examined the ability to rotate more complex 3D Shepard and Metzler (1971) images. There was reduced accuracy of performance with both right and left PPC stimulation for different angles of rotation of the visual stimuli. Right PPC stimulation led to reduced accuracy to rotate stimuli by 1200, whereas left PPC stimulation affected 180 degrees C rotation. We hypothesise that the two hemispheres make different contributions to the processing underlying visuospatial mental imagery: the right PPC is important for spatial rotations through smaller angles; the left hemisphere has a unique role when the stimuli to be compared are rotated through 180 degrees C, a task that engages verbal strategies due to the well-documented special nature of enantiomorphs.

The nature of foveal representation

A fundamental question in visual perception is whether the representation of the fovea is split at the midline between the two hemispheres, or bilaterally represented by overlapping projections of the fovea in each hemisphere. Here we examine psychophysical, anatomical, neuropsychological and brain stimulation experiments that have addressed this question, and argue for a shift from the current default view of bilateral representation to that of a split representation, to provide a greater understanding of higher visual processes.

Brain changes after learning to read and play music

Musically naive participants were scanned before and after a period of 15 weeks during which they were taught to read music and play the keyboard. When participants played melodies from musical notation after training, activation was seen in a cluster of voxels within the bilateral superior parietal cortex. A subset of these voxels were activated in a second experiment in which musical notation was present, but irrelevant for task performance. These activations suggest that music reading involves the automatic sensorimotor translation of a spatial code (written music) into a series of motor responses (keypresses).

Irrelevant digits affect feature-based attention depending on the overlap of neural circuits

Feature-based attention was investigated by examining the effect of irrelevant information on the processing of relevant information. In all experiments, irrelevant information consisted of digits whose semantic information is known to be processed in parietal areas. Between experiments we varied the degree of parietal involvement in the processing of the relevant feature. The influence of the irrelevant digit on the binary manual response task on the relevant feature was measured by the SNARC effect, a spatial numerical association of response codes demonstrating faster left than right hand responses for small numbers and faster right than left hand responses for large numbers. When processing of the relevant feature depended on parietal cortex, as is the case for orientation processing (exps. 1 and 4), there was an effect of the digit's semantic value on response times. Conversely, there was no effect of the irrelevant digit on the processing of color (exps. 2 and 3) or shape (exp. 5), which rely only minimally on parietal resources. After ruling out alternative explanations we conclude that the efficiency of feature-based attention is determined by the degree of neural overlap of structures dedicated to process relevant and irrelevant information.

Orientation discrimination of objects and gratings compared: an fMRI study

We used functional magnetic resonance imaging to compare the human brain regions involved in orientation discrimination of two-dimensional (2D) objects and gratings. The orientation discrimination tasks, identification and successive discrimination, were contrasted to a dimming detection control condition with identical retinal input. Regions involved in orientation discrimination were very similar for the two types of tasks and for the two types of stimuli and both belonged to the dorsal and ventral visual pathways. They included posterior occipital, lingual, posterior fusiform, inferior temporal, dorsal intraparietal and medial parietal regions. The main difference between the two types of stimuli was a larger activation of precuneus when 2D objects were used compared to gratings. The main difference between discrimination tasks was an enhanced activity, at the group level, in superior frontal sulcus in identification compared to successive discrimination, and at least at the single subject level, a larger activity in right fusiform cortex in successive discriminations compared to identification. Thus, in contradiction to generally accepted views, orientation discrimination of gratings and objects involve largely similar networks including both ventral and dorsal visual regions.

Failure at object identification improves mirror image matching

In a previous report ([5]: Davidoff J & Warrington EK. The bare bones of object recognition: implications from a case of object recognition impairment. Neuropsychologia 1999;37:279-92) the inability to differentiate between mirror images was recorded in a patient with excellent canonical view recognition. We now extend our investigation to a patient (JBA) with probable Alzheimer's disease in whom canonical view recognition was compromised. The reciprocal inhibition of two aspects of object processing are demonstrated in JBA. The patient's ability to detect mirror image rotations was dependent on her inability to identify the object. Paradoxically, her performance was more impaired for those stimuli that she was able to identify than those she was not.

Skill learning: bringing cognition to its senses

The brain areas involved in a task may change their contribution as one acquires expertise. An understanding of how this occurs relies on a good psychological theory of the processing requirements of a task and knowledge of which regions of the brain can perform the necessary component computations.

Functional imaging of mirror and inverse reading reveals separate coactivated networks for oculomotion and spatial transformations

Echoplanar functional magnetic resonance imaging (fMRI) was used to localize the cortical areas involved in the analysis of spatially transformed letter strings. Significant increases of the blood oxygen level-dependent (BOLD) signal for transformed vs normal reading were observed in the superior parietal lobule (SPL), along the intraparietal sulcus (IPS), in the frontal eye fields (FEF), and in the latero-occipital area LO. The respective contributions of oculomotor and spatial transformation areas to this activation pattern were separated by means of a control condition involving the execution and suppression of eye movements. Areas activated in association with the control of eye movements included the superior parietal lobule and the frontal eye fields. The cooperation of different brain areas was analysed by correlating the time course of task-dependent BOLD signal changes in these areas. This correlation analysis revealed coactivation of occipitotemporal object recognition areas and a spatial transformation area in the intraparietal sulcus during the reading of transformed letter strings. We suggest that cortical systems that are coactivated during complex cognitive tasks can be differentiated by the correlation analysis of BOLD signal time courses in spatially separate brain areas.

Perceptual learning in visual search: some evidence of specificities

To test a recent suggestion that perceptual learning in visual search is non-specific, two groups of subjects were trained on visual search tasks and tested for transfer of learning to new tasks. One group was trained on parallel ("pop-out") tasks and transferred to serial, conjunction tasks and the other group trained on conjunction and transferred to pop-out. Some (not all) tasks which are initially serial, rapidly became parallel. Some transfer occurred between the different types of tasks. Under some conditions transfer was either absent or even negative. The specificities observed may reflect the roles of the brain regions involved in learning.

The brain as a symbol-processing machine

The knowledge accumulated about the biochemistry of the synapsis in the last decades completely changes the notion of brain processing founded exclusively over an electrical mechanism, toward that supported by a complex chemical message exchange occurring both locally, at the synaptic site, as well as at other localities, depending on the solubility of the involved chemical substances in the extracellular compartment. These biochemical transactions support a rich symbolic processing of the information both encoded by the genes and provided by actual data collected from the surrounding environment, by means of either special molecular or cellular receptor systems. In this processing, molecules play the role of symbols and chemical affinity shared by them specifies the syntax for symbol manipulation in order to process and to produce chemical messages. In this context, neurons are conceived as message-exchanging agents. Chemical strings are produced and stored at defined places, and ionic currents are used to speed up message delivery. Synaptic transactions can no longer be assumed to correspond to a simple process of propagating numbers powered by a factor measuring the presynaptic capacity to influence the postsynaptic electrical activity, but they must be modeled by more powerful formal tools supporting both numerical and symbolic calculations. It is proposed here that formal language theory is the adequate mathematical tool to handle such symbolic processing. The purpose of the present review is therefore: (a) to discuss the relevant and recent literature about trophic factors, signal transduction mechanisms, neuromodulators and neurotransmitters in order (b) to point out the common features of these correlated processes; and (c) to show how they may be organized into a formal model supported by the theory of fuzzy formal languages (d) to model the brain as a distributed intelligent problem solver.

Agnosia for object orientation: implications for theories of object recognition

Instances in which objects are copied accurately, but are dramatically rotated relative to the original, have been interpreted as evidence for viewpoint-independent accounts of the object recognition process. In two case reports, we demonstrate that patients who show rotation in copying also show difficulties in informing the examiner of the canonical orientation of known objects. In copying rotated versions of familiar objects, one subject showed a tendency to copy them in their canonical upright orientation, and both subjects copied non-representational line drawings with their principal axis vertically aligned, and with the irregular end pointing 'upwards'.

Reflections on mirror images. Neuropsychology

About half of the readers of this article would not be able to tell whether a picture of the Mona Lisa is correct or mirror-reversed. Neuropsychological studies may help explain this striking dissociation between the ability to identify images and memory for their handedness.

Perception: the seeing ear

Some people perceive colours when they hear words--a phenomenon known as synaesthesia. Brain imaging techniques have revealed the areas of the cortex involved in this rare mingling of the senses.