Zaira Cattaneo

Imagery and spatial processes in blindness and visual impairment.

The objective of this review is to examine and evaluate recent findings on cognitive functioning (in particular imagery processes) in individuals with congenital visual impairments, including total blindness, low-vision and monocular vision. As one might expect, the performance of blind individuals in many behaviours and tasks requiring imagery can be inferior to that of sighted subjects; however, surprisingly often this is not the case. Interestingly, there is evidence that the blind often employ different cognitive mechanisms than sighted subjects, suggesting that compensatory mechanisms can overcome the limitations of sight loss. Taken together, these studies suggest that the nature of perceptual input on which we commonly rely strongly affects the organization of our mental processes. We also review recent neuroimaging studies on the neural correlates of sensory perception and mental imagery in visually impaired individuals that have cast light on the plastic functional reorganization mechanisms associated with visual deprivation.

Transcranial direct current stimulation over Broca's region improves phonemic and semantic fluency in healthy individuals.

Previous studies have demonstrated that transcranial direct current stimulation (tDCS) can be proficiently used to modulate attentional and cognitive functions. For instance, in the language domain there is evidence that tDCS can fasten picture naming in both healthy individuals and aphasic patients, or improve grammar learning. In this study, we investigated whether tDCS can be used to increase healthy subjects performance in phonemic and semantic fluency tasks, that are typically used in clinical assessment of language. Ten healthy individuals performed a semantic and a phonemic fluency task following anodal tDCS applied over Brocas region. Each participant underwent a real and a sham tDCS session. Participants were found to produce more words following real anodal tDCS both in the phonemic and in the semantic fluency. Control experiments ascertained that this finding did not depend upon unspecific effects of tDCS over levels of general arousal or attention or upon participants expectations. These data confirm the efficacy of tDCS in transiently improving language functions by showing that anodal stimulation of Brocas region can enhance verbal fluency. Implications of these results for the treatment of language functions in aphasia are considered.

Baseline cortical excitability determines whether TMS disrupts or facilitates behavior.

Transcranial magnetic stimulation (TMS) is increasingly used to modify brain activity noninvasively and to study brain-behavior relations. However, results can be variable and the conditions that affect the functional efficacy of TMS remain unclear. Here we show that on-line TMS can either facilitate or suppress perceptual functions depending on the baseline level of activity of the targeted brain region. When TMS was applied over the motion selective region V5/MT during a simple motion-detection task, subjects motion-detection ability was impaired. Similarly, suppression of V5/MT activity using off-line 1 Hz repetitive TMS (rTMS) disrupted performance in a subsequent motion-detection task. However, paradoxically, on-line V5/MT TMS had a facilitatory effect on motion detection if V5/MT had been suppressed by off-line 1-Hz rTMS prior to the motion-detection task. These results demonstrate that TMS can have an unexpected facilitatory effect on behavior when the targeted neural population is in a suppressed state. Our findings provide further evidence for the view that the effects of TMS are modulated by the initial activation state of the targeted neural population.

The causal role of category-specific neuronal representations in the left ventral premotor cortex (PMv) in semantic processing

The left ventral premotor cortex (PMv) is preferentially activated by exemplars of tools, suggestive of category specificity in this region. Here we used state-dependent transcranial magnetic stimulation (TMS) to investigate the causal role of such category-specific neuronal representations in the encoding of tool words. Priming to a category name (either Tool or Animal) was used with the objective of modulating the initial activation state of this region prior to application of TMS and the presentation of the target stimulus. When the target word was an exemplar of the Tool category, the effects of TMS applied over PMv (but not PMd) interacted with priming history by facilitating reaction times on incongruent trials while not affecting congruent trials. This congruency/TMS interaction implies that the Tool and Animal primes had a differential effect on the initial activation state of the left PMv and implies that this region is one neural locus of category-specific behavioral priming for the Tool category. TMS applied over PMv had no behavioral effect when the target stimulus was an exemplar of the Animal category, regardless of whether the target word was congruent or incongruent with the prime. That TMS applied over the left PMv interacted with a priming effect that extended from the category name (Tool) to exemplars of that category suggests that this region contains neuronal representation associated with a specific semantic category. Our results also demonstrate that the state-dependent effects obtained in the combination of visual priming and TMS are useful in the study of higher-level cognitive functions.

Contrasting early visual cortical activation states causally involved in visual imagery and short‐term memory

Whether visual imagery and visual short‐term memory (STM) share the same neural resources, and the extent to which the early visual cortex (V1/V2) is involved in these processes, has been the subject of much debate. Here, we used transcranial magnetic stimulation (TMS) in two separate experiments to contrast the neural states associated with visual imagery and visual STM in the early visual cortex. In Experiment 1, we investigated V1/V2 activation states at the end of the retention phase in a visual imagery and a visual STM task. V1/V2 TMS facilitated performance in both tasks; the finding that imagery and STM interacted with TMS in the same way suggests that the two processes have similar effects on early visual cortical excitability. In Experiment 2, we investigated V1/V2 activation states at the beginning of the retention phase. V1/V2 TMS impaired performance in the visual STM task, whereas it had no effect on the imagery task. Taken together, our findings show that the late phases of the early visual cortical activation state associated with visual imagery and visual STM are similar; differences between the two processes are apparent in the early phases of the tasks. Our results also suggest that the causal role of the early visual cortex in visual STM includes both the initial translation of the visual input into working memory and the subsequent maintenance of the mental representation. Finally, our findings indicate that visual STM sensory recruitment in working memory might act via excitability modulation of V1/V2 neurons.

Using state‐dependency of transcranial magnetic stimulation (TMS) to investigate letter selectivity in the left posterior parietal cortex: a comparison of TMS‐priming and TMS‐adaptation paradigms

The state‐dependency of transcranial magnetic stimulation (TMS) can be used to investigate the neural properties of subregions of the stimulated region. The objective of the present study was to determine whether state‐dependency can reveal letter selectivity in the left posterior parietal cortex (PPC), a region known to contain letter‐selective neurons. In two experiments, we used visual priming and adaptation to modulate the initial activation state of the left PPC prior to application of TMS. In the priming experiment, TMS was applied over the left PPC during the delay between the prime and the target stimulus on each experimental trial. Left PPC TMS reversed the effects of priming by facilitating the detection of non‐primed letters, whereas detection of primed letters was unaffected. As neurons tuned to non‐primed letters were less active at the time of TMS application than neurons tuned to the primed letters, this finding demonstrates that TMS preferentially facilitates the detection of attributes encoded by the less active neural populations. A similar facilitation of the less active neural populations was observed when adaptation was used to suppress letter‐selective neurons prior to application of TMS. Our study demonstrates that TMS‐priming and TMS‐adaptation paradigms can reveal letter selectivity in the left PPC and thus be useful in the study of language processes. Our results also show that the state‐dependent TMS effects obtained with visual priming are similar to those found with TMS adaptation: in both cases, attributes encoded by the less active neural populations are preferentially facilitated.

Transcranial magnetic stimulation reveals the content of visual short-term memory in the visual cortex

Cortical areas involved in sensory analysis are also believed to be involved in short-term storage of that sensory information. Here we investigated whether transcranial magnetic stimulation (TMS) can reveal the content of visual short-term memory (VSTM) by bringing this information to visual awareness. Subjects were presented with two random-dot displays (moving either to the left or to the right) and they were required to maintain one of these in VSTM. In Experiment 1, TMS was applied over the motion-selective area V5/MT+ above phosphene threshold during the maintenance phase. The reported phosphene contained motion features of the memory item, when the phosphene spatially overlapped with memory item. Specifically, phosphene motion was enhanced when the memory item moved in the same direction as the subjects V5/MT+ baseline phosphene, whereas it was reduced when the motion direction of the memory item was incongruent with that of the baseline V5/MT+ phosphene. There was no effect on phosphene reports when there was no spatial overlap between the phosphene and the memory item. In Experiment 2, VSTM maintenance did not influence the appearance of phosphenes induced from the lateral occipital region. These interactions between VSTM maintenance and phosphene appearance demonstrate that activity in V5/MT+ reflects the motion qualities of items maintained in VSTM. Furthermore, these results also demonstrate that information in VSTM can modulate the pattern of visual activation reaching awareness, providing evidence for the view that overlapping neuronal populations are involved in conscious visual perception and VSTM.

Processing of featural and configural aspects of faces is lateralized in dorsolateral prefrontal cortex: A TMS study

Facial recognition relies on distinct and parallel types of processing: featural processing focuses on the individual components of a face (e.g., the shape or the size of the eyes), whereas configural (or relational) processing considers the spatial interrelationships among the single facial components (e.g., distance of the mouth from the nose). Previous neuroimaging evidence has suggested that featural and configural processes may rely on different brain circuits. By using rTMS, here we show for the first time a double dissociation in dorsolateral prefrontal cortex for different aspects of face processing: in particular, TMS over the left middle frontal gyrus (BA8) selectively disrupted featural processing, whereas TMS over the right inferior frontal gyrus (BA44) selectively interfered with configural processing of faces. By establishing a causal link between activation in left and right prefrontal areas and different modes of face processing, our data extend previous neuroimaging evidence and may have important implications in the study of face-processing deficits, such as those manifested in prosopagnosia and autistic spectrum disorders.

Neural correlates of the semantic interference effect: New evidence from transcranial direct current stimulation

In two experiments, we combined a semantic blocked naming paradigm with anodal transcranial direct current stimulation (tDCS) to shed light on the neural correlates of the semantic interference (SI) effect. In particular, prior to the naming task, anodal tDCS was applied over the left superior temporal gyrus (STG, Experiment 1) or the left inferior frontal gyrus (IFG, Experiment 2) to enhance cortical excitability in these regions. In both experiments, participants were tested in two sessions in which either real or sham tDCS was delivered. We found that anodal tDCS over the left STG significantly increased the SI effect, whereas anodal tDCS over the left IFG led to a reduction of the SI effect. Overall, our data confirm the existence of a distributed cortical network involved in lexical retrieval and show that both the left IFG and the left STG play a causal role in this process. In particular, the left IFG is likely to be critical in resolving the conflict between competitor lexical representations, while the left STG seems to be the neural locus of the lexical representational system, where competition among different lexical representations occurs.

The Role of the Human Extrastriate Visual Cortex in Mirror Symmetry Discrimination: A TMS-Adaptation Study.

The human visual system is able to efficiently extract symmetry information from the visual environment. Prior neuroimaging evidence has revealed symmetry-preferring neuronal representations in the dorsolateral extrastriate visual cortex; the objective of the present study was to investigate the necessity of these representations in symmetry discrimination. This was accomplished by the use of state-dependent transcranial magnetic stimulation, which combines the fine resolution of adaptation paradigms with the assessment of causality. Subjects were presented with adapters and targets consisting of dot configurations that could be symmetric along either the vertical or horizontal axis (or they could be non-symmetric), and they were asked to perform a symmetry discrimination task on the targets while fixating the center of the screen. TMS was applied during the delay between the adapter and the test stimulus over one of four different sites: Left or Right V1/V2, or left or right dorsolateral extrastriate cortex (DLO). TMS over both Left and Right DLO reduced the adaptation effect in detecting vertical and horizontal symmetry, although the Left DLO effect on horizontal symmetry and the Right DLO effect on both vertical and horizontal symmetry were present only when considering subjects who showed a behavioral adaptation effect in the baseline No-TMS condition. Application of TMS over the Left or Right V1/V2 did not modulate the adaptation effect. Overall, these data suggest that both the Left and Right DLO contain neuronal representations tuned to mirror symmetry which play a causal role in symmetry discrimination.