Pro- and antisaccades in children elicited by visual and acoustic targets - does modality matter?

Peer reviewedPublisher PD

Bidirectional control of saccadic eye movements by the disconnected cerebral hemispheres

The present investigation demonstrates that callosotomy patient J.W. can generate either leftward or rightward saccades in response to color cues presented unilaterally. When asked to name the colors, performance was at chance for left visual field presentations, demonstrating a disability in interhemispheric transfer of chromatic information. The successful control of saccadic direction based on discriminative color cues that appear confined to a single hemisphere may suggest a capacity for bidirectional control of saccadic eye movements in the disconnected cerebral hemispheres.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46564/1/221_2004_Article_BF00231667.pd

Can Humans Initiate Saccadic Eye Movements to Targets They do not Consciously Notice?

Saccadic eye movements are fast ballistic gaze shifts, and the most frequent movement in the human body, even more than the heartbeat. They are divided into 2 main groups: as express saccades, initiated before 100ms, and as normal saccades, initiated after 100ms. The source of saccadic eye movements has been long debated as conscious or unconscious movements. We attempted to determine if saccades could be initiated towards objects not consciously seen, operating under the hypothesis that saccades could be initiated towards objects that the subject was not consciously aware of. they were unconsciously controlled. We did this by tracking eye movements to stimuli obscured by metacontrast masking, designing the experiment trials to maximize the chance to see express saccades in subjects with unimpaired vision. We had subjects attempt to move their eyes as fast as possible when they saw stimuli, and then had them rate how well they saw the stimulus on a perceptual awareness scale of 0-3, where 0 was “not seen at all,” and 3 was “definitely seen,” the most important distinction being between ratings of 0 and 1. Across 15 experimental subjects and 5 pilot subjects, only 2 participants made saccades unconsciously during the experiment. This lack of unconscious saccades limited our ability to analyze unconscious reaction times, and overall interpretation of the trends in reaction times. Instead, we compared reaction times between masked and unmasked saccades. Ultimately, the metacontrast mask interfered with the subjects’ abilities to consciously detect the stimuli. The average reaction time while masked (448.6ms) was decreased compared to the unmasked condition (539.8ms), although there was strong individual variance, with some subjects having a lower reaction time unmasked compared to their masked times. Unfortunately, statistical analysis of the results does not support our findings, so we can only tentatively assume that it is possible for express saccades to be initiated without conscious perception of stimuli. We would like to see this experiment repeated with a larger subject group, and with confirmation of neural activity during saccades, for example with electroencephalography or magnetoencephalography, to localize which regions of the brain and possible pathways are initiated when conscious awareness is absent. The next set of experiments should also control for the metacontrast mask as a possible cue for saccades, as some subjects were unable to produce saccades at all without masking, and the individual variance in reaction times between masked and unmasked conditions were strong

Strategic modulation of the fixation-offset effect: dissociable effects of target probability on prosaccades and antisaccades

In several previous experiments examining the effects of participants’ expectations on oculomotor performance, the manipulation of target probability has been confounded with factors such as target occurrence and saccade frequency. We report results from three experiments that manipulated target probability in isolation from systematic variations in such bottom-up factors. We present evidence for trial-by-trial, top-down modulation of the fixation-offset effect in prosaccade latency. Furthermore, fixation-stimulus offset and target-probability manipulations had additive effects on antisaccade latency, suggesting that these factors influence separable neural processes engaged for antisaccade performance. Based on these findings, we suggest that cognitive processes utilizing target-probability information influence task processes engaged for prosaccades that differ from those engaged for antisaccades.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46542/1/221_2005_Article_2242.pd

Neural Mechanism of Blindsight in a Macaque Model

Some patients with damage to the primary visual cortex (V1) exhibit visuomotor ability, despite loss of visual awareness, a phenomenon termed “blindsight”. We review a series of studies conducted mainly in our laboratory on macaque monkeys with unilateral V1 lesioning to reveal the neural pathways underlying visuomotor transformation and the cognitive capabilities retained in blindsight. After lesioning, it takes several weeks for the recovery of visually guided saccades toward the lesion-affected visual field. In addition to the lateral geniculate nucleus, the pathway from the superior colliculus to the pulvinar participates in visuomotor processing in blindsight. At the cortical level, bilateral lateral intraparietal regions become critically involved in the saccade control. These results suggest that the visual circuits experience drastic changes while the monkey acquires blindsight. In these animals, analysis based on signal detection theory adapted to behavior in the “Yes–No” task indicates reduced sensitivity to visual targets, suggesting that visual awareness is impaired. Saccades become less accurate, decisions become less deliberate, and some forms of bottom-up attention are impaired. However, a variety of cognitive functions are retained such as saliency detection during free viewing, top–down attention, short-term spatial memory, and associative learning. These observations indicate that blindsight is not a low-level sensory-motor response, but the residual visual inputs can access these cognitive capabilities. Based on these results we suggest that the macaque model of blindsight replicates type II blindsight patients who experience some “feeling” of objects, which guides cognitive capabilities that we naïvely think are not possible without phenomenal consciousness

Neural bases of visual processing of moving and stationary stimuli presented to the blind hemifield of hemianopic patients

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The tectum/superior colliculus as the vertebrate solution for spatial sensory integration and action

The superior colliculus, or tectum in the case of non-mammalian vertebrates, is a part of the brain that registers events in the surrounding space, often through vision and hearing, but also through electrosensation, infrared detection, and other sensory modalities in diverse vertebrate lineages. This information is used to form maps of the surrounding space and the positions of different salient stimuli in relation to the individual. The sensory maps are arranged in layers with visual input in the uppermost layer, other senses in deeper positions, and a spatially aligned motor map in the deepest layer. Here, we will review the organization and intrinsic function of the tectum/superior colliculus and the information that is processed within tectal circuits. We will also discuss tectal/superior colliculus outputs that are conveyed directly to downstream motor circuits or via the thalamus to cortical areas to control various aspects of behavior. The tectum/superior colliculus is evolutionarily conserved among all vertebrates, but tailored to the sensory specialties of each lineage, and its roles have shifted with the emergence of the cerebral cortex in mammals. We will illustrate both the conserved and divergent properties of the tectum/superior colliculus through vertebrate evolution by comparing tectal processing in lampreys belonging to the oldest group of extant vertebrates, larval zebrafish, rodents, and other vertebrates including primates

Linguistic Competence and Related Functions in the Right Cerebral Hemisphere of Man Following Commissurotomy and Hemispherectomy

A simple new contact lens technique has been developed to permit the presentation of continuously lateralized visual information to one visual half field at a time. Free unilateral scanning of the information and monitoring of performance in the subjects' lap makes it possible to administer a variety of standard perceptual and cognitive tasks to either hemisphere in order to assess hemispheric specialization under natural conditions. Two representative commissurotomy patients have been fitted with the new device and have undergone an extensive series of language and related studies focusing on the right hemisphere. All tests were administered unilaterally to each hemisphere and subsequently in free vision. The results were correlated with data from a group of three hemispherectomy patients in two of whom the right (non dominant) and in one of whom the left (dominant) cerebral hemisphere has been surgically removed for the treatment of post infantile tumor. The case of dominant hemispherectomy is particularly rare and permits the study of language competence and performance in a girl whose language lateralization for speech and hearing was well under way (perhaps completed) when the tumor set in. Results of extensive clinical aphasia tests reveal a distinct hierarchy of language functions from a relatively good auditory comprehension through a more severe speech deficit, to almost complete alexia, agraphia and acalculia. Theoretical aphasiological analysis of the pattern of impairment in language functions here shows that in spite of characteristic nonfluency and anomia in speech, the syndrome is unique and does not correspond to either a Broca's or anomic aphasia. The agraphia and especially the alexia in this patient are more severe than in the separated right hemisphere of the two commissurotomy patients which can read a wide range of individual words and even short sentences. This is in contrast to her superior expressive speech relative to the right hemisphere of the same two commissurotomy patients. In a series of studies comparing the psycholinguistic abilities of the two hemispheres in the two commissurotomy and three hemispherectomy patients it was shown that the right cerebral hemisphere had extensive ability to elicit meaning from pictures and to recognize semantic associations and form concepts. In particular it was able to ignore perceptual for semantic similarity. Lateralized tests of visual closure reveal the conditions under which right hemisphere visual feature extraction mechanisms fail. Previous results on superior right hemisphere competence in completing patterns from fragmented information must now be qualified by the provision that when the gestalt of the visual ground is strong and in competition with the figure, the right hemisphere is unable to complete partial patterns. Neither can it recognize complete embedded figures in the face of distracting gestalt in the ground. Right hemisphere competence in various aspects of auditory language comprehension has been investigated with the aid of an experimental paradigm involving matching an auditory message to one of unilaterally presented alternative line drawings. Lower limit age estimates for right hemisphere comprehension of vocabulary were obtained and it was shown that the right hemisphere can comprehend not only abstract words but also a variety of syntactic structures including verbs, and sentential transformations and to a lesser degree long nonredundant and semantically abstract references. Right hemisphere pattern of syntactic competence has weak correlation with order of acquisition in children and somewhat stronger with aphasics. In contrast, aural vocabulary in the right hemisphere, although consistently inferior to the left, follows the same function of frequency as the left just as do children and aphasics.</p

Cortical mechanisms of visual attention in typically developing infants and adults

This thesis used a combined methodology of on-line eye tracking and high density EEG to study neural mechanisms of attention development in infants and adults. The extensively studied Fixation Shift Paradigm (FSP) measures the ability to shift attention between two stimuli (competition) or towards one single visible stimulus (non-competition) which improves during infancy. The novel method here overcame a number of methodological challenges to measure event-rated potentials during overt shifts of attention in competition and non-competition conditions. An experiment used eye tracking to test infants between 1 and 8 months on the FSP, establishing that this automated measure is suitable for non-verbal populations and adds precision to the developmental trends previously reported. An experiment successfully combined eye tracking and EEG to record patterns of brain activity during covert and overt attention shifts in adults. It found that neural mechanisms previously studied in covert attention shifts are similar to those in overt shifts, but differ in a frontal positivity, possibly reflecting saccade inhibition. Combined eye tracking and EEG with the original FSP showed that similar cortical mechanisms are involved in attention shifts under competition and non-competition conditions but that occipital response latencies differ at an early stage, reflecting the behavioural pattern of shorter latencies in non-competition conditions. Parallel measurements during infancy showed that the lateralisation of frontal brain responses coincides with developmental improvements in the ability to shift attention. In conclusion, the work demonstrates that, with suitable precautions taken to avoid artefacts, eye tracking and EEG can be successfully combined to monitor group-level brain mechanisms during overt attention shifts. Neurodevelopmental changes have been identified that underpin the increasing efficiency of neural attention pathways during infancy, and increased automation of responses from reliance on frontal pathways in infancy to occipital pathways in adulthood