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ABSTRACT: The visual processing of emotional faces is subserved by both a cortical and a subcortical route. To investigate the specific contribution of these two functional pathways, two groups of neurologically healthy humans were tested using transcranial direct current stimulation (tDCS). In Experiment 1, participants received sham and active cathodal-inhibitory tDCS over the left occipital cortex, while, in control Experiment 2, participants received sham and active cathodal-inhibitory tDCS over the vertex, to exclude any unspecific effect of tDCS. After tDCS, participants performed a go/no-go task responding to happy or fearful target faces presented in the left visual field, while backwardly masked faces (emotionally congruent, incongruent, or neutral) were concurrently displayed in the right visual field. After both suppressing activity in the vertex (Experiment 2) and sham stimulation (Experiment 1 and 2) a reduction of reaction times was found for pairs of emotionally congruent stimuli. However, after suppressing the activity in the left occipital cortex, the congruency-dependent response facilitation disappeared, while a specific facilitative affect was evident when masked fearful faces were coupled with happy target faces. These results parallel the performances of hemianopic patients and suggest that when the occipital cortex is damaged or inhibited, and the visual processing for emotional faces is mainly dependent on the activation of the "low road" subcortical route, fearful faces represent the only visually processed stimuli capable of facilitating a behavioral response. This effect might reflect an adaptive mechanism implemented by the brain to quickly react to potential threats before their conscious identification.
Journal of Neuroscience 04/2013; 33(15):6469-75. · 7.11 Impact Factor
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ABSTRACT: The ability to process unseen emotional signals might offer an evolutionary advantage in enabling threat-detection. In the present study, patients with visual field defects, without any subjective awareness of stimuli presented in the blind field and performing at the chance level in two alternative discrimination tasks (Experiment 1), were tested with go-no go tasks where they were asked to discriminate the emotional valence (Experiment 2) or the gender (Experiment 3) of faces displayed in the intact field, during the concurrent presentation of emotional faces in the blind field. The results showed a facilitative effect when fearful faces were presented in the blind field, both when the emotional content of the stimuli was relevant (Experiment 2) and irrelevant (Experiment 3) to the task. These findings are in contrast with performances of healthy subjects and patients tested in classical blindsight investigations, who showed response facilitation for congruent pairs of emotional stimuli. The observed implicit visual processing for unseen fearful stimuli might represent an adaptive mechanism for the implementation of efficient defensive responses, probably mediated by a spared sub-cortical and short-latency pathway.
Cortex 03/2012; · 6.08 Impact Factor
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ABSTRACT: The visual and auditory systems often concur to create a unified perceptual experience and to determine the localization of objects in the external world. Co-occurring auditory and visual stimuli in spatial coincidence are known to enhance performance of auditory localization due to the integration of stimuli from different sensory channels (i.e. multisensory integration). However, auditory localization of audiovisual stimuli presented at spatial disparity might also induce a mislocalization of the sound towards the visual stimulus (i.e. ventriloquism effect). Using repetitive transcranial magnetic stimulation we tested the role of right temporoparietal (rTPC), right occipital (rOC) and right posterior parietal (rPPC) cortex in an auditory localization task in which indices of ventriloquism and multisensory integration were computed. We found that suppression of rTPC excitability by means of continuous theta-burst stimulation (cTBS) reduced multisensory integration. No similar effect was found for cTBS over rOC. Moreover, inhibition of rOC, but not of rTPC, suppressed the visual bias in the contralateral hemifield. In contrast, cTBS over rPPC did not produce any modulation of ventriloquism or integrative effects. The double dissociation found in the present study suggests that ventriloquism and audiovisual multisensory integration are functionally independent phenomena and may be underpinned by partially different neural circuits.
European Journal of Neuroscience 05/2010; 31(10):1791-9. · 3.63 Impact Factor
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ABSTRACT: Patients with visual field disorders often exhibit impairments in visual exploration and a typical defective oculomotor scanning behaviour. Recent evidence [Bolognini, N., Rasi, F., Coccia, M., & Làdavas, E. (2005b). Visual search improvement in hemianopic patients after audio-visual stimulation. Brain, 128, 2830-2842] suggests that systematic audio-visual stimulation of the blind hemifield can improve accuracy and search times in visual exploration, probably due to the stimulation of Superior Colliculus (SC), an important multisensory structure involved in both the initiation and execution of saccades. The aim of the present study is to verify this hypothesis by studying the effects of multisensory training on oculomotor scanning behaviour. Oculomotor responses during a visual search task and a reading task were studied before and after visual (control) or audio-visual (experimental) training, in a group of 12 patients with chronic visual field defects and 12 controls subjects. Eye movements were recorded using an infra-red technique which measured a range of spatial and temporal variables. Prior to treatment, patients' performance was significantly different from that of controls in relation to fixations and saccade parameters; after Audio-Visual Training, all patients reported an improvement in ocular exploration characterized by fewer fixations and refixations, quicker and larger saccades, and reduced scanpath length. Overall, these improvements led to a reduction of total exploration time. Similarly, reading parameters were significantly affected by the training, with respect to specific impairments observed in both left- and right-hemianopia readers. Our findings provide evidence that Audio-Visual Training, by stimulating the SC, may induce a more organized pattern of visual exploration due to an implementation of efficient oculomotor strategies. Interestingly, the improvement was found to be stable at a 1 year follow-up control session, indicating a long-term persistence of treatment effects on the oculomotor system.
Neuropsychologia 11/2008; 47(2):546-55. · 3.64 Impact Factor
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ABSTRACT: Temporo-nasal asymmetry in visual responses has been observed in many behavioural studies. These observations have typically been attributed to the anatomical asymmetry of fibres projecting to the Superior Colliculus (SC), even though this attribution is debated. The present study investigates temporo-nasal asymmetry in multisensory integration, and, by exploiting the absence of S-cone input to the SC, measures a behavioural response dependent strictly on the activity of the SC itself. We used a redundant signal paradigm for simple reaction times, with visual stimuli (red or purple) presented in either the temporal or the nasal hemifield. Participants responded more quickly to concurrent audio-visual (AV) stimuli than to either an auditory or a visual stimulus alone, an established phenomenon known as the Redundant Target Effect (RTE). The nature of this effect was dependent on the colour of the visual stimuli, suggesting its modulation by collicular circuits. When spatially-coincident audio-visual stimuli were visible to the SC (i.e. red stimuli), the RTE depended on a neural coactivation mechanism, suggesting an integration of multisensory information. When using stimuli invisible to the SC (i.e. purple stimuli), the RTE depended only on a simple statistical facilitation effect, in which the two sensory stimuli were processed by independent channels. Finally, we demonstrate that the multisensory integration effect was stronger for stimuli presented to the temporal hemifield than to the nasal hemifield. Taken together, these findings suggested that multisensory stimulation can be differentially effective depending on specific stimulus parameters.
Brain research 05/2008; 1242:37-44. · 2.46 Impact Factor
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ABSTRACT: Animal studies have shown that the superior colliculus (SC) is important for synthesising information from multiple senses into a unified map of space. Here, we tested whether the SC is a critical neural substrate for multisensory spatial integration in humans. To do so, we took advantage of neurophysiological findings revealing that the SC does not receive direct projections from short-wavelength-sensitive S cones. In a simple reaction-time task, participants responded more quickly to concurrent peripheral (extra-foveal) audiovisual (AV) stimuli than to an auditory or visual stimulus alone, a phenomenon known as the redundant target effect (RTE). We show that the nature of this RTE was dependent on the colour of the visual stimulus. When using purple short-wavelength stimuli, to which the SC is blind, RTE was simply explained by probability summation, indicating that the redundant auditory and visual channels are independent. Conversely, with red long-wavelength stimuli, visible to the SC, the RTE was related to nonlinear neural summation, which constitutes evidence of integration of different sensory information. We also demonstrate that when AV stimuli were presented at fixation, so that the spatial orienting component of the task was reduced, neural summation was possible regardless of stimulus colour. Together, these findings provide support for a pivotal role of the SC in mediating multisensory spatial integration in humans, when behaviour involves spatial orienting responses.
Experimental Brain Research 04/2008; 186(1):67-77. · 2.39 Impact Factor
