Abstract: Phosphenes represent a perceptual effect of transcranial magnetic stimulation (TMS) or electric stimulation of visual cortical areas. One likely neural basis for the generation of static phosphenes is the primary visual cortex (V1) although evidence is controversial. A peculiar feature of V1 is that it has sparse callosal connections with the exception of a central portion of visual field representation. In contrast, visually responsive cortical areas in the parietal lobe have widespread callosal connections. Thus, interhemispheric transfer (IT) time of off-centre phosphenes should be slower when generated by V1 than by visual parietal areas. To verify this possibility, in Exp. 1 we measured IT of phosphenes generated by TMS applied to V1 and in Exp. 2 we measured IT of phosphenes obtained by TMS applied to posterior parietal cortex. In both experiments, we obtained static bright circular phosphenes appearing in the contralateral hemifield. We measured IT time behaviorally by comparing unimanual simple reaction time to the onset of a phosphene under crossed or uncrossed hemifield-hand condition (Poffenberger paradigm). In keeping with our prediction, we found a substantially longer IT time for V1 than for parietal phosphenes. Additionally, an IT similar to that obtained with V1 stimulation was found when participants were asked to imagine the phosphenes previously experienced during TMS. In conclusion, the present results suggest that IT of phosphenes either generated by V1 TMS or imagined is subserved by slower callosal channels than those of real visual stimuli or parietal phosphenes.

Abstract: Task-irrelevant visual cues with near zero visibility proved apt to retard reaction time for the detection of supraliminal visual targets presented at the cued location. The time course of the effect was similar to that of the so-called inhibition-of return (IOR), which is assumed to be due to the withdrawal of attention from the inhibited location. However the present subliminal cues consistently failed to induce an RT facilitation prior to the RT inhibition, contrary to what would be expected if the cue were able to attract attention to the cued location. Since the RT inhibition from subliminal cues could not be attributed to the withdrawal of attention from the cued location, it can be argued that such cues acted both outside of consciousness and without the influence of attention. Therefore, the RT inhibitory effect seems best accounted for by an automatic, unconscious and attention-independent self-inhibition of response tendencies instructed by irrelevant information, akin to that postulated by (Eimer, M., & Schlaghecken, F. (1998). Effects of masked stimuli on motor activation: behavioural and electrophysiological evidence. Journal of Experimental Psychology Human Perception and Performance, 24, 1737-1747.) to explain the negative compatibility effect.

Abstract: To try and cast light on the processing locus of the redundant signal effect (RSE), i.e. the speeding of reaction time (RT) with two rather than one stimulus, we manipulated three features of redundant visual stimuli, i.e. exposure duration, intensity and interstimulus interval (ISI). We found an inverse relationship between stimulus duration or intensity and the maximum length of ISI at which an RSE occurred. These effects are broadly similar to those found in the measurement of visible persistence, i.e. the phenomenon that the sensation produced by a brief visual stimulus can outlast the duration of the physical stimulation. Therefore, we suggest that the RSE occurs at a visual processing stage. This conclusion does not rule out other subsequent stages when employing different redundant stimuli and task paradigms.

Abstract: We employed two reaction time paradigms to find out whether imagined visual stimuli can be integrated between the two cerebral hemispheres. In a first experiment we found that interhemispheric transfer time, as assessed with the Poffenberger paradigm, was much longer for imagined than visible stimuli and this suggests that the callosal site of transfer is different in the two conditions. In a second experiment we found that interhemispheric summation, as assessed with the redundant signal effect paradigm, was present for both visible and imagined stimuli and could be accounted for by a neural coactivation mechanism rather than by a probabilistic explanation. Taken together, these results support the view that that there is an equivalence between perceptual and imagery processes that goes beyond early processing stages and includes the interhemispheric exchange of information.

Abstract: The brain effectively integrates multisensory information to enhance perception. For example, audiovisual stimuli typically yield faster responses than isolated unimodal ones (redundant signal effect, RSE). Here, we show that the audiovisual RSE is likely subserved by a neural site of integration (neural coactivation), rather than by an independent-channels mechanism such as race models. This neural site is probably the superior colliculus (SC), because an RSE explainable by neural coactivation does not occur with purple or blue stimuli, which are invisible to the SC; such an RSE only occurs for spatially and temporally coincident audiovisual stimuli, in strict adherence with the multisensory responses in the SC of the cat. These data suggest that audiovisual integration in humans occurs very early during sensory processing, in the SC.

Abstract: An abnormal pattern of hemispheric asymmetry, possibly as a result of disturbed interhemispheric communication, is widely, albeit by no means unanimously, held as a major cause of schizophrenia. To behaviourally test interhemispheric communication in schizophrenia we used a task that has been shown to be a reliable indicator of callosal functioning, namely, the redundant signals effect (RSE). It consists of the speeding of simple reaction time when responding to double as opposed to single visual stimuli. When the stimuli in a pair are presented to different hemispheres patients who underwent total commissurotomy or suffer from callosal agenesis show a paradoxically enhanced RSE with respect to healthy controls. Therefore, if schizophrenia patients have a callosal abnormality they ought to show a similar effect. In three experiments we tested a total of 55 patients with a diagnosis of schizophrenia and 51 healthy controls. In Experiment 1 we presented unilateral single stimuli and bilateral simultaneous double stimuli. The RSE was reliably larger in schizophrenics than in controls. In Experiment 2 the temporal interval between the two stimuli in a pair was varied. We found that while in controls the RSE disappeared with interstimulus intervals longer than 17ms, in schizophrenia patients there was a RSE only for simultaneous double stimuli. Finally, in Experiment 3 we found that there was no enhanced redundancy gain in schizophrenics when the double stimuli were presented to one and the same hemisphere, and therefore, with no need for callosal transmission. All in all, the present results provide evidence of a callosal dysfunction in schizophrenia that impairs interhemispheric integration.

Abstract: Neglect can be ego-centered or object-centered depending on the reference frames for "left" and "right", of either the body or an object. It has been suggested that object-centered neglect is not a general phenomenon but is limited to words because only they have a true canonical representation. Here, we examined whether object-centered neglect could be observed for non-verbal material by creating, after repeated exposure, a canonical representation of a nonsense figure. Fourteen neglect patients repeatedly bisected a series of asymmetrical nonsense drawings containing two different shapes at their right and left end-points (canonical trials). In the critical trials, which were the last three in the series, the position of the two shapes was mirror-reversed. Afterwards, neglect patients were asked to draw the stimulus, which provided a further measure of whether a canonical representation of the object has been built by the patients. All the patients made rightward errors with the canonical stimuli. With mirror-reversed stimuli, the bisection errors were reversed to the contralesional side in one patient, returned to zero in one patient and significantly decreased in three patients. In addition, 10 patients reliably drew the canonical stimulus at the end of the series of trials, providing an indication that they built up a canonical representation of the stimulus. The present data provide evidence that object-centered neglect is a phenomenon that is not limited to words. The nature of a stimulus, verbal or non-verbal, is not critical for observing object-centered neglect. What is critical is the way in which material is represented by the patients.

Abstract: Inconsistent observations have been reported in the literature regarding the asymmetrical contribution of higher visual areas of the left and right hemispheres to visual motion processing. In the present experiment, we tested for hemispheric asymmetry of the middle-temporal complex (V5/MT), which is a key-component of the visual motion network, by using rTMS applied over left or right V5/MT during a visual trajectory perception task. The results showed that the effect of rTMS was to enhance individual hemispheric asymmetries present when the test was performed without rTMS. The more general meaning of these results is that there are robust individual hemispheric asymmetries in motion perception but no general pattern of hemispheric differences.

Abstract: Neglect patients, when asked to bisect a horizontal line, typically show large rightward errors with long lines and a decreased error with medium length lines. With very short lines the bisection bias reverses from the right to left side of the line physical centre (the so-called crossover effect). It is commonly pointed out that such a leftward bias is difficult to explain by traditional theories of neglect. Several accounts propose two distinct mechanisms, one that works for short lines and one that works for long. In the present study we demonstrated that the crossover effect can be explained by means of a unitary mechanism that derives from the space anisometry hypothesis. This hypothesis postulates that in neglect patients representational space is progressively 'relaxed' contralesionally and progressively 'compressed' ipsilesionally. In a series of five experiments, we investigated the crossover effect in 26 right-brain damaged patients: 17 with neglect without hemianopia, 4 with neglect and hemianopia and 6 without neglect or hemianopia. Patients were to bisect or extend lines of objectively and subjectively different lengths. The modulation of subjective length was created by an Oppel-Kundt illusion that is thought to resemble the distortion of representational space that occurs with neglect. All groups, except for the patients with neglect and hemianopia, were prone to the illusion. The rightward bias was reduced when the illusion induced a perceptual distortion opposite to that thought to underlie neglect. Importantly, the strength of the illusion decreased with reducing the physical line length and reversed with very short lines. These results argue for a simple and unitary explanation of the crossover effect in spatial neglect within the framework of the space anisometry hypothesis.

Abstract: It is now common knowledge that the total surgical section of the corpus callosum (CC) and of the other forebrain commissures prevents interhemispheric transfer (IT) of a host of mental functions. By contrast, IT of simple sensorimotor functions, although severely delayed, is not abolished, and an important question concerns the pathways subserving this residual IT. To answer this question we assessed visuomotor IT in split-brain patients using the Poffenberger paradigm (PP), that is, a behavioral paradigm in which simple reaction time (RT) to visual stimuli presented to the hemifield ipsilateral to the responding hand is compared to stimuli presented to the contralateral hemifield, a condition requiring an IT. We tested the possibility that the residual IT is mediated by the collicular commissure interconnecting the two sides of the superior colliculus (SC). To this purpose, we used short-wavelength visual stimuli, which in neurophysiological studies in non-human primates have been shown to be undetectable by collicular neurons. We found that, in both totally and partially callosotomised patients, IT was considerably longer with S-cone input than with L-cone input or with achromatic stimuli. This was not the case in healthy participants in whom IT was not affected by color. These data clearly show that the SC plays an important role in IT of sensorimotor information in the absence of the corpus callosum.

Abstract: To cast light on the possible neural substrate of visual imagery we tested normal participants and one hemianopic patient on simple reaction time (RT) to real and imagined visual stimuli. In one experiment participants were to detect as quickly as possible a luminous square presented at one out of two different retinal eccentricities. A well known effect with visual stimuli is that RT is slower for peripheral versus central stimuli. We found that imagined stimuli showed an eccentricity effect similar to that obtained with real stimuli. However, this was not the case in a patient with a hemianopic visual field loss (quadrantanopia) as a result of damage to the optic radiation. Even though the patient showed no difficulty in imaging stimuli in the affected hemifield she did not show an eccentricity effect as was the case in her intact side. In a second experiment, normal participants showed faster RT to stimuli of larger size with either real or imagined stimuli. Overall, these results show that visual perception and imagination share a similar visuotopic organisation that is disrupted following deafferentation of the visual cortex.

Abstract: 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.

Abstract: The aim of this chapter is to discuss evidence bearing on two related issues, namely, first, whether the neural pathways of subliminal perception are the same as those subserving suprathreshold perception. Second, whether the pathways for subliminal perception in normals are similar to those subserving blindsight in brain-damaged patients. As to the former question, the overall balance is in favor of the different-pathway hypothesis while a tentative answer to the second question might be that blindsight is basically similar to subliminal perception in normals. The differences undoubtedly existing between the two conditions depend mainly on the differences in the stimuli used to reveal them.

Abstract: In a first experiment, 11 neglect patients repeatedly bisected the elongated caricature of a basset hound with head on the right and tail on the left side with respect to the viewer. On the last (critical) trial, in which the figure was left-right reversed, the bisection error towards the ipsilesional side reversed its direction in three patients and significantly decreased in one patient. In a second experiment, 13 different neglect patients had to bisect the elongated caricature of the basset hound with head on the left and tail on the right side. On the last trial, the bisection error reversed its direction in three patients and significantly decreased in three patients.These results suggest that object-centred neglect (OCN) may affect newly established knowledge about the canonical orientation of a non-verbal visual stimulus.

Abstract: The redundant target effect (RTE) consists in the speeding of reaction time with single versus multiple targets and can be explained either by a neural coactivation or by a race model. To try to understand the role of the magnocellular and parvocellular systems in the determination of the RTE we carried out three experiments using onset or feature singletons. The former are likely to be mainly processed by the magnocellular system while the latter are mainly processed by the parvocellular system. In experiment 1 we found an RTE both when the target (red disk) was presented in isolation and when it was surrounded by equiluminant green distractors. Thus, the RTE occurred both with onset and feature singletons. However, with the former, the RTE could be accounted for by neural coactivation while with the latter it could be accounted for by a probabilistic explanation. In experiment 2 we tried to ascertain the role of distractors in yielding a probabilistic RTE: we used either targets in isolation or surrounded by distractors of lower luminance and found an RTE that could be explained by neural coactivation for both kinds of targets. This ruled out an effect of distractors per se in determining a probabilistic RTE. Finally, in experiment 3 we used targets of lower luminance than either the background or the distractors. We found that the RTE could be accounted for by neural coactivation with targets alone while it was probabilistic with distractors. Overall, these results show that stimuli presumably processed by the magnocellular system yield redundancy gains that result from a neural coactivation mechanism. In contrast, stimuli presumably processed by the parvocellular system are compatible with a probabilistic redundancy gain.

Abstract: Patients with unilateral neglect show disorders in horizontal space perception. It has been argued that these disorders may depend on a left-right relaxation of the representational medium that becomes progressively "relaxed" toward the contralesional space and progressively "compressed" toward the ipsilesional space. We tested this hypothesis in 31 right-brain-damaged patients, 17 with neglect and 14 without neglect in two different experiments. Patients were asked to read words in canonical and anisometric letter spacing. Only in neglect patients, the manipulation of letters spacing may ameliorate neglect dyslexia. These results support the idea that the abnormalities observed in typical neglect tests are due to a distorted internal representation of the outside world. In addition, the space distortion seems to depend on the degree of horizontal relaxation of the representational medium and it is unrelated to hemianopia.

Abstract: To verify the possibility that the superior colliculus (SC) subserves interhemispheric neural summation, we presented single or double white visual targets to one or both hemifields in normal participants and in patients lacking the corpus callosum (one with total callosotomy and one with callosal agenesis). Simple reaction time was typically faster with double than single stimuli, a phenomenon known as the redundant target effect (RTE); moreover, confirming previous results, we found a larger RTE in patients without callosum than in normals. In both groups, the redundancy gain was related to neural coactivation rather than to probability summation. The novel finding was that, when using monochromatic purple stimuli that are invisible to the SC, we found a similar redundancy gain in both groups; moreover, this redundancy gain was probabilistic rather than neural. Control experiments with monochromatic red stimuli yielded a RTE of the neural type similar to that with white stimuli and this confirmed that the probabilistic RTE found was specific for purple stimuli. In conclusion, visual input to the SC is necessary for interhemispheric neural summation in both normals and in individuals without the corpus callosum while probabilistic summation can occur without a collicular contribution.

Abstract: Neglect dyslexia is a reading disorder which affects the identification of portions of words or sentences and is typically due to right-brain damage. Caramazza and Hillis (1990a, b) proposed a model of representation of words whereby various patterns of neglect dyslexia are attributed to selective impairment to the different forms of representation. According to the model, patients with first and second level deficits will "neglect" those parts of horizontally presented words that fall on one side of the retina, or on one side of the string centre, respectively, but will read vertically presented words without errors. A patient with a third level deficit will neglect letters at the beginning of the word irrespective of whether they fall on the right or left half of the retina, or on the right or the left of the word centre. In the present paper I describe an atypical pattern of neglect errors that is not easily explicable by this model and is better interpreted as an object-based neglect within an ego-centred frame of reference.

Abstract: Array-centred and subarray-centred neglect were disambiguated in a group of 116 patients with left neglect by means of a modified version of the Albert test in which the central column of segments was deleted so as to create two separate sets of targets grouped by proximity.The results indicated that neglect was more frequent in array- than subarray-centred coordinates and that, in a minority of cases, neglect co-occurred in both coordinate-systems. The two types of neglect were functionally but not anatomically dissociated. Presence of visual field defects was not prevalent in one type of neglect with respect to the other.These data contribute further evidence to previous single-case and small-group studies by showing that neglect can occur in single or multiple reference frames simultaneously, in agreement with current neuropsychological, neurophysiological and computational concepts of space representation.

Abstract: Normal subjects react more quickly to a pair of visual stimuli than to a stimulus alone. This phenomenon is known as the redundant signal effect (RSE) and represents an example of divided visual attention in which signal processing is carried out in parallel to the advantage of response speed. A most interesting aspect of this phenomenon is that it can occur when one stimulus in a pair cannot be consciously detected because of hemianopia or unilateral extinction resulting from brain damage. Here, we report that a similar dissociation between visual awareness and visually guided behavior is present in normal subjects who show an RSE even when the luminance of one of a pair of stimuli is below detection threshold. The observed RSE cannot be attributed to probability summation because it violates Miller's race inequality and is likely to be related to neural summation between supra- and subthreshold stimuli. Given that a similar implicit RSE is present in hemispherectomy patients, we hypothesize that the site of this summation might be the superior colliculus (SC).