Carlo Alberto Marzi

Neural Site of the Redundant Target Effect: Electrophysiological Evidence

The present study represents an attempt to find an electrophysiological correlate of the redundant targets effect, or RTE (i.e., the speeding up of reaction time, or RT, for redundant vs. single targets). Subjects made a speeded response either to one small checkerboard presented to the left or right of fixation or to a pair of identical checkerboards presented simultaneously to both hemifields. Both single and double targets could appear either in the upper or lower visual hemifield. The task required detection but not discrimination of the stimuli. During task performance, we recorded the event-related potentials (ERPs) elicited by the checkerboard targets. As in previous studies, we found that manual RTs to bilateral stimuli were faster than those to unilateral stimuli. This effect was more marked for lower-than for upper-field stimuli and could not be ascribed to probability summation. In addition, we found that the P1 and N1 components of the visual ERP had a shorter latency for bilateral than for summed unilateral stimuli presented to the two hemifields. In parallel with the behavioral findings, the latency values for the above components showed a larger RTE for lower-field stimuli. These findings indicate that the RTE occurs at the level of early visual processing, probably in the extrastriate visual cortex, rather than at late decisional or pre-motor stages.

Corpus callosum and simple visuomotor integration

Malcolm Jeeves was the first to demonstrate lengthened interhemispheric transmission times in subjects with agenesis of the corpus callosum by using a simple reaction time paradigm with lateralized unstructured light stimuli and crossed and uncrossed hand responses. Uncrossed responses can be integrated within one hemisphere, whereas crossed responses require a communication between the two hemispheres. In the normal brain this communication is effected rapidly by the corpus callosum, whereas in the acallosal brain it must occur much more slowly by way of less efficient alternative interhemispheric pathways. Using a similar experimental paradigm we have studied normal subjects, subjects with a complete callosal agenesis and epileptic patients with surgical callosal sections, either complete or partial. All subjects with complete callosal defects showed much lengthened interhemispheric times compared to normal controls. Virtually normal interhemispheric transmission times were found in subjects with partial callosal defects, whether anterior or posterior, suggesting a possible equipotentiality of different portions of the corpus callosum in the mediation of crossed manual responses. In both normals and acallosals there were no crossed-uncrossed differences in reaction time when responses were made unilaterally with lower limb effectors or para-axial upper limb effectors, as well as bilaterally with upper-limb proximal and para-axial effectors. Since these effectors can be controlled directly from either side of the brain via bilaterally distributed motor pathways, crossed responses using them, unlike crossed manual responses, do not require an interhemispheric integration.

Collicular vision guides nonconscious behavior

Following destruction or deafferentation of primary visual cortex (area V1, striate cortex), clinical blindness ensues, but residual visual functions may, nevertheless, persist without perceptual consciousness (a condition termed blindsight). The study of patients with such lesions thus offers a unique opportunity to investigate what visual capacities are mediated by the extrastriate pathways that bypass V1. Here we provide evidence for a crucial role of the collicular–extrastriate pathway in nonconscious visuomotor integration by showing that, in the absence of V1, the superior colliculus (SC) is essential to translate visual signals that cannot be consciously perceived into motor outputs. We found that a gray stimulus presented in the blind field of a patient with unilateral V1 loss, although not consciously seen, can influence his behavioral and pupillary responses to consciously perceived stimuli in the intact field (implicit bilateral summation). Notably, this effect was accompanied by selective activations in the SC and in occipito-temporal extrastriate areas. However, when instead of gray stimuli we presented purple stimuli, which predominantly draw on S-cones and are thus invisible to the SC, any evidence of implicit visuomotor integration disappeared and activations in the SC dropped significantly. The present findings show that the SC acts as an interface between sensory and motor processing in the human brain, thereby providing a contribution to visually guided behavior that may remain functionally and anatomically segregated from the geniculo-striate pathway and entirely outside conscious visual experience.

Implicit redundant-targets effect in visual extinction.

Patients with left visual extinction as a result of unilateral right hemisphere damage were tested on a redundant-targets effect paradigm (RTE). LED-generated brief flashes were lateralized either to the left or to the right visual hemifield or presented bilaterally. Subjects were asked to press a key as fast as possible following either unilateral or bilateral stimuli and immediately afterwards to report on the number of stimuli presented. As previously found in normal subjects, bilateral stimuli were responded to faster than unilateral ones, and this was evidence of a RTE. The main thrust of this study was that extinction patients showed a RTE not only for correctly perceived bilateral stimuli but also in trials in which they extinguished the stimulus on the field contralateral to the lesion. This result is compatible with a preserved processing of the extinguished input at least up to the stage at which it may interact with the input from the normal side to yield a speeded motor response. Interestingly, the implicit redundancy gain of extinction patients was found to fit a coactivation (i.e. neural) rather than a probabilistic model.

Dissociating Arbitrary Stimulus-Response Mapping from Movement Planning during Preparatory Period: Evidence from Event-Related Functional Magnetic Resonance Imaging

In the present study, we aimed to dissociate the neural correlates of two subprocesses involved in the preparatory period in the context of arbitrary, prelearned stimulus-response (S-R) associations, namely, S-R mapping and movement planning (MP). We teased apart these two subprocesses by comparing three tasks in which the complexity of both S-R mapping and MP were independently manipulated: simple reaction time (SRT) task, go/no-go reaction time (GNGRT) task, and choice reaction time (CRT) task. We found that a more complex S-R mapping, which is the common element differentiating CRT and GNGRT from SRT, was associated with higher brain activation in the left superior parietal lobe (SPL). Conversely, a greater number of planned finger movements, which is the common difference between CRT and both SRT and GNGRT, was associated with higher brain activation in a number of frontal areas, including the left supplementary motor area (SMA), left dorsal premotor cortex (dPM), and left anterior cingulate cortex (ACC). The left-hemisphere dominance for S-R mapping could be related to the fact that arbitrary S-R mapping is often verbally mediated in humans. Overall, these results suggest a clear dissociation in the preparatory-set period between the more abstract role of left SPL in activating the appropriate S-R associations and the more concrete role played by the SMA, dPM, and ACC in preparing the required motor programs.

Attention and Interhemispheric Transfer: A Behavioral and fMRI Study

When both detections and responses to visual stimuli are performed within one and the same hemisphere, manual reaction times (RTs) are faster than when the two operations are carried out in different hemispheres. A widely accepted explanation for this difference is that it reflects the time lost in callosal transmission. Interhemispheric transfer time can be estimated by subtracting RTs for uncrossed from RTs for crossed responses (crossed uncrossed difference, or CUD). In the present study, we wanted to ascertain the role of spatial attention in affecting the CUD and to chart the brain areas whose activity is related to these attentional effects on interhemispheric transfer. To accomplish this, we varied the proportion of crossed and uncrossed trials in different blocks. With this paradigm subjects are likely to focus attention either on the hemifield contralateral to the responding hand (blocks with 80 crossed trials) or on the ipsilateral hemifield (blocks with 80 uncrossed trials). We found an inverse correlation between the proportion of crossed trials in a block and the CUD and this effect can be attributed to spatial attention. As to the imaging results, we found that in the crossed minus uncrossed subtraction, an operation that highlights the neural processes underlying interhemispheric transfer, there was an activation of the genu of the corpus callosum as well as of a series of cortical areas. In a further commonality analysis, we assessed those areas which were activated specifically during focusing of attention onto one hemifield either contra- or ipsilateral to the responding hand. We found an activation of a number of cortical and subcortical areas, notably, parietal area BA 7 and the superior colliculi. We believe that the main thrust of the present study is to have teased apart areas important in interhemispheric transmission from those involved in spatial attention.

Electrophysiological Correlates of Conscious Vision: Evidence from Unilateral Extinction

To study the electrophysiological correlates of conscious vision, we recorded event-related potentials (ERPs) in a patient with partial unilateral visual extinction as a result of right-hemisphere damage. When, following bilateral presentations, contralesional stimuli were not perceived, there was an absence of the early attention-sensitive P1 (80-120 msec) and N1 (140-180 msec) components of the ERP response. In contrast, following unilateral presentations, or in those bilateral presentations in which contralesional stimuli were perceived (about 60), these ERP components were present. These results provide novel evidence that extinction involves the stage of early focusing of attention and that the P1 and N1 components of visual ERPs are reliable physiological correlates of conscious vision.

Ipsilateral inhibition and contralateral facilitation of simple reaction time to non-foveal visual targets from non-informative visual cues.

Orienting to an extrafoveal light cue without foveating it induces a temporary inhibition of responses to subsequent targets presented in the same visual hemifield, as evinced from the fact that reaction time (RT) to targets ipsilateral to the cue relative to fixation is longer than RT to targets contralateral to the cue. This study has tested the hypothesis that ipsilateral RT inhibition is associated with contralateral RT facilitation by attempting to divide the difference between ipsilateral and contralateral RTs into costs and benefits. A neutral condition suited to this purpose should involve a cue that does not require a lateral orientation. Such neutral condition was provided by measuring RT to lateralized light targets following a central overhead auditory cue (experiment 1) or a foveal visual cue (experiment 2). In both experiments RT in the neutral condition was intermediate between ipsilateral and contralateral RTs, and the differences reaches significance in the second experiment. Benefits over the neutral condition measured in the contralateral condition were thus associated with costs in the ipsilateral condition. These results suggest that a reciprocal antagonism between opposite turning tendencies underlies the organization of covert orienting. They also agree with general multi-channel theories of selective attention according to which the facilitation of given channels is an obligatory accompaniment of the inhibition of other competing channels and vice versa.

Attentional orienting induced by arrows and eye-gaze compared with an endogenous cue

Exogenous orienting has been widely studied by using peripheral cues whereas endogenous orienting has been studied with directional central cues. However, recent evidence has shown that centrally presented eye-gaze and arrows may produce an automatic rather than voluntary orienting of attention. Therefore, the aim of the present study was to investigate the behavioural and electrophysiological (event-related potentials-ERP) correlates of the attentional shift induced by arrows and eye-gaze. In order to have a control condition, we compared arrows and eye-gaze with a purely endogenous cue, i.e., a texture arbitrarily coding one direction. We analyzed the ERP components (P1, N1, P2a, P2p, P3) elicited by the cue stimuli and the early lateralised attentional effect (early directing attention negativity-EDAN). In addition, in order to investigate the topography of the neural mechanisms underlying the cortical activity in each cueing condition, we applied a temporal segmentation procedure. The results showed that the three cueing conditions induced a different strength of activation within the same cortical network. Occipito-parietal regions were involved in the early processing of visual information, followed by an involvement of frontal areas, likely implicated in learning associations. These data confirm the assumption that, in contrast to purely endogenous cues, arrows and eye-gaze induce a very fast attentional shift. However, the similarity of the ERP components and of the topographical cortical maps among conditions suggest that this early orienting of attention is more likely related to an overlearned association mechanism rather than to a real exogenous attentional process.

Transcranial magnetic stimulation selectively impairs interhemispheric transfer of visuo-motor information in humans

Abstract We investigated the cerebral cortical route by which visual information reaches motor cortex when visual signals are used for manual responses. Subjects responded unimanually to photic stimuli delivered to the hemifield ipsilateral or contralateral to the moving hand. On some trials, trans-cranial magnetic stimulation (TMS) was applied unilaterally over the occiput, with the aim of stimulating extrastriate visual areas and thereby modifying transmission of visual input. In association with the side of a visual stimulus and a motor response, TMS could change inter- or intra-hemispheric transmission needed to convey visual information to motor areas. Reaction time differences following TMS suggested that TMS exerted an inhibitory effect only when visuo-motor information had to be transferred interhemispherically. This result reinforces evidence for an extrastriate pathway of interhemispheric transfer of visuomotor information.