Activity In Occipital Cortex Research Articles

Right Occipital Cortex Activation Correlates with Superior Odor Processing Performance in the Early Blind

Using functional magnetic resonance imaging (fMRI) in ten early blind humans, we found robust occipital activation during two odor-processing tasks (discrimination or categorization of fruit and flower odors), as well as during control auditory-verbal conditions (discrimination or categorization of fruit and flower names). We also found evidence for reorganization and specialization of the ventral part of the occipital cortex, with dissociation according to stimulus modality: the right fusiform gyrus was most activated during olfactory conditions while part of the left ventral lateral occipital complex showed a preference for auditory-verbal processing. Only little occipital activation was found in sighted subjects, but the same right-olfactory/left-auditory-verbal hemispheric lateralization was found overall in their brain. This difference between the groups was mirrored by superior performance of the blind in various odor-processing tasks. Moreover, the level of right fusiform gyrus activation during the olfactory conditions was highly correlated with individual scores in a variety of odor recognition tests, indicating that the additional occipital activation may play a functional role in odor processing.

Occipital cortex activation by long-term repetitive tactile stimulation is necessary for object recognition in blinds: A case report

Tactile vision has been approached from a variety of angles using different techniques. So far, a certain kind of object (and text) recognition has been shown, though seeing as such has not been achieved yet, and it remains unclear. Trough repetitive passive tactile stimulation perceptual processing is transferred from temporo-parietal to occipital areas, which affects object recognition. We report the results of passive tactile stimulation, as well as rTMS, applied to a 50 year old left handed blind male with over 97% loss of vision, who suffers from Peter’s anomaly and microphthalmia. After 15 weeks of passive tactile stimulation, the subject showed increased activity in occipital areas associated with the development of visual-like perception which remained unchanged after three months without passive tactile stimulation. Inhibitory rTMS over the visual cortex led to noticeable reduction of spatial recognition performance and visual sensations in this subject. Stable changes in occipital cortical activity can be associated with subjective sensations of seeing. Once occipital activation has been achieved, it is necessary for spatial object recognition. Both facts highlight the implication of occipital areas in tactile vision and the cortical plasticity of passive tactile long-term stimulation in people with blindness.

Cortical activation deficits during facial emotion processing in youth at high risk for the development of substance use disorders

BackgroundRecent longitudinal studies demonstrate that addiction risk may be influenced by a cognitive, affective and behavioral phenotype that emerges during childhood. Relatively little research has focused on the affective or emotional risk components of this high-risk phenotype, including the relevant neurobiology. MethodsNon-substance abusing youth (N=19; mean age=12.2) with externalizing psychopathology and paternal history of a substance use disorder and demographically matched healthy comparisons (N=18; mean age=11.9) were tested on a facial emotion matching task during functional MRI. This task involved matching faces by emotions (angry, anxious) or matching shape orientation. ResultsHigh-risk youth exhibited increased medial prefrontal, precuneus and occipital cortex activation compared to the healthy comparison group during the face matching condition, relative to the control shape condition. The occipital activation correlated positively with parent-rated emotion regulation impairments in the high-risk group. ConclusionsThese findings suggest a preexisting abnormality in cortical activation in response to facial emotion matching in youth at high risk for the development of problem drug or alcohol use. These cortical deficits may underlie impaired affective processing and regulation, which in turn may contribute to escalating drug use in adolescence.

Differential Contribution of Cortical and Subcortical Visual Pathways to the Implicit Processing of Emotional Faces: A tDCS Study

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.

Shape-specific activation of occipital cortex in an early blind echolocation expert

We have previously reported that an early-blind echolocating individual (EB) showed robust occipital activation when he identified distant, silent objects based on echoes from his tongue clicks (Thaler, Arnott, & Goodale, 2011). In the present study we investigated the extent to which echolocation activation in EB's occipital cortex reflected general echolocation processing per se versus feature-specific processing. In the first experiment, echolocation audio sessions were captured with in-ear microphones in an anechoic chamber or hallway alcove as EB produced tongue clicks in front of a concave or flat object covered in aluminum foil or a cotton towel. All eight echolocation sessions (2 shapes×2 surface materials×2 environments) were then randomly presented to him during a sparse-temporal scanning fMRI session. While fMRI contrasts of chamber versus alcove-recorded echolocation stimuli underscored the importance of auditory cortex for extracting echo information, main task comparisons demonstrated a prominent role of occipital cortex in shape-specific echo processing in a manner consistent with latent, multisensory cortical specialization. Specifically, relative to surface composition judgments, shape judgments elicited greater BOLD activity in ventrolateral occipital areas and bilateral occipital pole. A second echolocation experiment involving shape judgments of objects located 20° to the left or right of straight ahead activated more rostral areas of EB's calcarine cortex relative to location judgments of those same objects and, as we previously reported, such calcarine activity was largest when the object was located in contralateral hemispace. Interestingly, other echolocating experts (i.e., a congenitally blind individual in Experiment 1, and a late blind individual in Experiment 2) did not show the same pattern of feature-specific echo-processing calcarine activity as EB, suggesting the possible significance of early visual experience and early echolocation training. Together, our findings indicate that the echolocation activation in EB's occipital cortex is feature-specific, and that these object representations appear to be organized in a topographic manner.

Processing counterfactual and hypothetical conditionals: An fMRI investigation

Counterfactual thinking is ubiquitous in everyday life and an important aspect of cognition and emotion. Although counterfactual thought has been argued to differ from processing factual or hypothetical information, imaging data which elucidate these differences on a neural level are still scarce. We investigated the neural correlates of processing counterfactual sentences under visual and aural presentation. We compared conditionals in subjunctive mood which explicitly contradicted previously presented facts (i.e. counterfactuals) to conditionals framed in indicative mood which did not contradict factual world knowledge and thus conveyed a hypothetical supposition. Our results show activation in right occipital cortex (cuneus) and right basal ganglia (caudate nucleus) during counterfactual sentence processing. Importantly the occipital activation is not only present under visual presentation but also with purely auditory stimulus presentation, precluding a visual processing artifact. Thus our results can be interpreted as reflecting the fact that counterfactual conditionals pragmatically imply the relevance of keeping in mind both factual and supposed information whereas the hypothetical conditionals imply that real world information is irrelevant for processing the conditional and can be omitted. The need to sustain representations of factual and suppositional events during counterfactual sentence processing requires increased mental imagery and integration efforts. Our findings are compatible with predictions based on mental model theory.

Age-related vulnerability in the neural systems supporting semantic processing

Our ability to form abstract representations of objects in semantic memory is crucial to language and thought. The utility of this information relies both on the representations of sensory-motor feature knowledge stored in long-term memory and the executive processes required to retrieve, manipulate, and evaluate this semantic knowledge in a task-relevant manner. These complementary components of semantic memory can be differentially impacted by aging. We investigated semantic processing in normal aging using functional magnetic resonance imaging (fMRI). Young and older adults were asked to judge whether two printed object names match on a particular feature (for example, whether a tomato and strawberry have the same color). The task thus required both retrieval of relevant visual feature knowledge of object concepts and evaluating this information. Objects were drawn from either natural kinds or manufactured objects, and were queried on either color or shape in a factorial design. Behaviorally, all subjects performed well, but older adults could be divided into those whose performance matched that of young adults (better performers) and those whose performance was worse (poorer performers). All subjects activated several cortical regions while performing this task, including bilateral inferior and lateral temporal cortex and left frontal and prefrontal cortex. Better performing older adults showed increased overall activity in bilateral premotor cortex and left lateral occipital cortex compared to young adults, and increased activity in these brain regions relative to poorer performing older adults who also showed gray matter atrophy in premotor cortex. These findings highlight the contribution of domain-general executive processing brain regions to semantic memory, and illustrate differences in how these regions are recruited in healthy older adults.

FMRI investigation of visual change detection in adults with autism

People with autism spectrum disorders (ASD) may show unusual reactions to unexpected changes that appear in their environment. Although several studies have highlighted atypical auditory change processing in ASD, little is known in this disorder about the brain processes involved in visual automatic change detection. The present fMRI study was designed to localize brain activity elicited by unexpected visual changing stimuli in adults with ASD compared to controls. Twelve patients with ASD and 17 healthy adults participated in the experiment in which subjects were presented with a visual oddball sequence while performing a concurrent target detection task. Combined results across participants highlight the involvement of both occipital (BA 18/19) and frontal (BA 6/8) regions during visual change detection. However, adults with ASD display greater activity in the bilateral occipital cortex and in the anterior cingulate cortex (ACC) associated with smaller activation in the superior and middle frontal gyri than controls. A psychophysiological interaction (PPI) analysis was performed with ACC as the seed region and revealed greater functionally connectivity to sensory regions in ASD than in controls, but less connectivity to prefrontal and orbito-frontal cortices. Thus, compared to controls, larger sensory activation associated with reduced frontal activation was seen in ASD during automatic visual change detection. Atypical psychophysiological interactions between frontal and occipital regions were also found, congruent with the idea of atypical connectivity between these regions in ASD. The atypical involvement of the ACC in visual change detection can be related to abnormalities previously observed in the auditory modality, thus supporting the hypothesis of an altered general mechanism of change detection in patients with ASD that would underlie their unusual reaction to change.

Perceptual Criteria in the Human Brain

A critical component of decision making is the ability to adjust criteria for classifying stimuli. fMRI and drift diffusion models were used to explore the neural representations of perceptual criteria in decision making. The specific focus was on the relative engagement of perceptual- and decision-related neural systems in response to adjustments in perceptual criteria. Human participants classified visual stimuli as big or small based on criteria of different sizes, which effectively biased their choices toward one response over the other. A drift diffusion model was fit to the behavioral data to extract estimates of stimulus size, criterion size, and difficulty for each participant and condition. These parameter values were used as modulated regressors to create a highly constrained model for the fMRI analysis that accounted for several components of the decision process. The results show that perceptual criteria values were reflected by activity in left inferior temporal cortex, a region known to represent objects and their physical properties, whereas stimulus size was reflected by activation in occipital cortex. A frontoparietal network of regions, including dorsolateral prefrontal cortex and superior parietal lobule, corresponded to the decision variables resulting from the downstream stimulus-criterion comparison, independent of stimulus type. The results provide novel evidence that perceptual criteria are represented in stimulus space and serve as inputs to be compared with the presented stimulus, recruiting a common network of decision regions shown to be active in other simple decisions. This work advances our understanding of the neural correlates of decision flexibility and adjustments of behavioral bias.

Neural correlates of the encoding of multimodal contextual features

Functional magnetic resonance imaging (fMRI) was employed to identify neural regions engaged during the encoding of contextual features belonging to different modalities. Subjects studied objects that were presented to the left or right of fixation. Each object was paired with its name, spoken in either a male or a female voice. The test requirement was to discriminate studied from unstudied pictures and, for each picture judged old, to retrieve its study location and the gender of the voice that spoke its name. Study trials associated with accurate rather than inaccurate location memory demonstrated enhanced activity in the fusiform and parahippocampal cortex and the hippocampus and reduced activity (a negative subsequent memory effect) in the medial occipital cortex. Successful encoding of voice information was associated with enhanced study activity in the right middle superior temporal sulcus and activity reduction in the right superior frontal cortex. These findings support the proposal that encoding of a contextual feature is associated with enhanced activity in regions engaged during its online processing. In addition, they indicate that negative subsequent memory effects can also demonstrate feature-selectivity. Relative to other classes of study trials, trials for which both contextual features were later retrieved demonstrated enhanced activity in the lateral occipital complex and reduced activity in the temporo-parietal junction. These findings suggest that multifeatural encoding was facilitated when the study item was processed efficiently and study processing was not interrupted by redirection of attention toward extraneous events.

Successful memory encoding is associated with increased cross-frequency coupling between frontal theta and posterior gamma oscillations in human scalp-recorded EEG

Although previous studies have established that successful memory encoding is associated with increased synchronization of theta-band and gamma-band oscillations, it is unclear if there is a functional relationship between oscillations in these frequency bands. Using scalp-recorded EEG in healthy human participants, we demonstrate that cross-frequency coupling between frontal theta phase and posterior gamma power is enhanced during the encoding of visual stimuli which participants later on remember versus items which participants subsequently forget ("subsequent memory effect," SME). Conventional wavelet analyses and source localizations revealed SMEs in spectral power of theta-, alpha-, and gamma-band. Successful compared to unsuccessful encoding was reflected in increased theta-band activity in right frontal cortex as well as increased gamma-band activity in parietal-occipital regions. Moreover, decreased alpha-band activity in prefrontal and occipital cortex was also related to successful encoding. Overall, these findings support the idea that during the formation of new memories frontal cortex regions interact with cortical representations in posterior areas.

Attentional shifts induced by uninformative number symbols modulate neural activity in human occipital cortex

Number processing interacts with space encoding in a wide variety of experimental paradigms. Most intriguingly, the passive viewing of uninformative number symbols can shift visuo-spatial attention to different target locations according to the number magnitude, i.e., small/large numbers facilitate processing of left/right targets, respectively. The brain architecture dedicated to these attention shifts associated with numbers remains unknown. Evoked potential recordings indicate that both early and late stages are involved in this spatio-numerical interaction, but the neuro-functional anatomy needs to be specified. Here we use, for the first time, functional magnetic resonance imaging (fMRI) to investigate attentional orienting following uninformative Arabic digits. We show that BOLD response in occipital visual regions is modulated by the congruency between digit magnitude (small/large) and target side (left/right). Additionally, we report higher BOLD responses following large (8, 9) compared to small (1, 2) digits in two bilateral parietal regions, yielding a significant effect of digit magnitude. We propose and discuss the view that encoding of semantic representations related to number symbols in parietal cortex leads to shifts in visuo-spatial attention and enhances visual processing in the occipital cortex according to number-space congruency rules.

Losing the struggle to stay awake: divergent thalamic and cortical activity during microsleeps.

Maintaining alertness is critical for safe and successful performance of most human activities. Consequently, microsleeps during continuous visuomotor tasks, such as driving, can be very serious, not only disrupting performance but sometimes leading to injury or death due to accidents. We have investigated the neural activity underlying behavioral microsleeps--brief (0.5-15 s) episodes of complete failure to respond accompanied by slow eye-closures--and EEG theta activity during drowsiness in a continuous task. Twenty healthy normally-rested participants performed a 50-min continuous tracking task while fMRI, EEG, eye-video, and responses were simultaneously recorded. Visual rating of performance and eye-video revealed that 70% of the participants had frequent microsleeps. fMRI analysis revealed a transient decrease in thalamic, posterior cingulate, and occipital cortex activity and an increase in frontal, posterior parietal, and parahippocampal activity during microsleeps. The transient activity was modulated by the duration of the microsleep. In subjects with frequent microsleeps, power in the post-central EEG theta was positively correlated with the BOLD signal in the thalamus, basal forebrain, and visual, posterior parietal, and prefrontal cortices. These results provide evidence for distinct neural changes associated with microsleeps and with EEG theta activity during drowsiness in a continuous task. They also suggest that the occurrence of microsleeps during an active task is not a global deactivation process but involves localized activation of fronto-parietal cortex, which, despite a transient loss of arousal, may constitute a mechanism by which these regions try to restore responsiveness.

Migraine with Prolonged Aura Versus Migrainous Infarction: Multimodality Imaging in a Unique Case (P03.213)

Objective: To report a patient with very prolonged migrainous visual aura and to discuss the significance of multimodality imaging in this clinical setting. Background The International Headache Society9s (IHS) definition of migraine with prolonged aura indicates that at least one aura symptom should persist between 60 minutes and 7 days. Migrainous infarction is an attack of migraine during which at least one aura symptom is not fully reversible within 7 days or absence of neuroimaging evidence of corresponding ischemia. A rigid time frame separating these two entities may be artificial. There is limited experience and knowledge in utilizing magnetic resonance perfusion imaging (MRI-PI) and magnetoencephalography (MEG) in this clinical setting. Design/Methods: We describe a patient with unusually prolonged migrainous aura evaluated by the authors, on whom multimodality neuroimaging was completed. Results: A 54 year-old man presented with complaints of scintillations, photopsias, and visual loss in the left hemifield, along with right temporo-occipital headache, photophobia, nausea, and emesis. Neurologic examination confirmed a complete left homonymous hemianopsia. Brain MRI revealed no acute ischemic lesion and no large cerebral vessel occlusion on MRA. MRI-PI revealed slightly increased cerebral blood flow and blood volume in the medial right occipital lobe. EEG was unremarkable. During the resting state, MEG detected expected coherent activity in bilateral occipital cortices. Unexpectedly, maximal coherence was observed in the right medial temporal region. On left hemifield stimulation, there was no MEG activity in the right occipital cortex. Valproate resolved the headache within 4 days while Humphrey Visual Field Testing confirmed resolution of left homonymous hemianopsia after 7 weeks. Conclusions: Migrainous aura can last longer than 7 days. The distinction between prolonged migrainous aura and migrainous infarction should not rely only on a rigid time frame. Multimodality neuroimaging allows this distinction to be made with clarity. The IHS criteria may need to be revised. Disclosure: Dr. Ashraf has nothing to disclose. Dr. Bowyer has nothing to disclose. Dr. Mitsias has nothing to disclose.

Dynamic causal modeling of the network underlying the Müller-Lyer illusion

The Müller-Lyer illusion is an optical illusion that alters the perceived length of a line via arrows attached to its end. Recent imaging studies revealed that ventral and dorsal stream areas contribute to the processing of the Müller-Lyer illusion (Weidner et al., 2010; Weidner & Fink, 2007). Here we investigated [using dynamic causal modeling (DCM; Friston, Harrison, & Penny, 2003)] the effects of this illusion on the effective connectivity of the brain areas involved in coding illusion strength. Strength of the Müller-Lyer illusion was parametrically modulated while participants performed either a landmark task or a non-spatial luminance task. Relative to the luminance task, the landmark task increased neural activity in lateral occipital cortex and right superior parietal cortex dependent upon illusion strength. Based on these findings DCM was used to investigate the possible interactions between ventral and dorsal visual stream. Bayesian model selection (Stephan et al., 2009) indicated that a model involving bi-directional connections between dorsal and ventral stream areas most accurately accounted for the underlying network dynamics. In this model, illusion strength enhanced bi-directional couplings of bilateral lateral occipital cortex and the right superior parietal cortex. Thereby, both areas seem to fulfill differential roles: Whereas lateral occipital cortex seems to be directly related to size transformation processes, activation in right superior parietal cortex rather reflects subsequent levels of processing, including task-related supervisory functions. Furthermore, our data clearly demonstrate that the effect of illusion strength on these interactions between lateral occipital and superior parietal regions critically depends on an observer’s top-down settings.

Dissociable Neural Activations of Conscious Visibility and Attention

Recent neuroimaging evidence indicates that visual consciousness of objects is reflected by the activation in the lateral occipital cortex as well as in the frontal and parietal cortex. However, most previous studies used behavioral paradigms in which attention raised or enhanced visual consciousness (visibility or recognition performance). This co-occurrence made it difficult to reveal whether an observed cortical activation is related to visual consciousness or attention. The present fMRI study investigated the dissociability of neural activations underlying these two cognitive phenomena. Toward this aim, we used a visual backward masking paradigm in which directing attention could either enhance or reduce the object visibility. The participants' task was to report the level of subjective visibility for a briefly presented target object. The target was presented in the center with four flankers, which was followed by the same number of masks. Behavioral results showed that attention to the flankers enhanced the target visibility, whereas attention to the masks attenuated it. The fMRI results showed that the occipito-temporal sulcus increased activation in the attend flankers condition compared with the attend masks condition, and occipito-temporal sulcus activation levels positively correlated with the target visibility in both attentional conditions. On the other hand, the inferior frontal gyrus and the intraparietal sulcus increased activation in both the attend flankers and attend masks compared with an attend neither condition, and these activation levels were independent of target visibility. Taken together, present results showed a clear dissociation in neural activities between conscious visibility and attention.

Combining fiber tracking and functional brain imaging for revealing brain networks involved in auditory–visual integration in humans

Previous functional magnetic resonance imaging (MRI) found various brain areas in the temporal and occipital lobe involved in integrating auditory and visual object information. Fiber tracking based on diffusion-weighted MRI suggested neuroanatomical connections between auditory cortex and sub-regions of the temporal and occipital lobe. However, the relationship between functional activity and white-matter tracks remained unclear. Here, we combined probabilistic tracking and functional MRI in order to reveal the structural connections related to auditory–visual object perception. Ten healthy people were examined by diffusion-weighted and functional MRI. During functional examinations they viewed either movies of lip or body movements, listened to corresponding sounds (phonological sounds or body action sounds), or a combination of both. We found that phonological sounds elicited stronger activity in the lateral superior temporal gyrus (STG) than body action sounds. Body movements elicited stronger activity in the lateral occipital cortex than lip movements. Functional activity in the phonological STG region and the lateral occipital body area were mutually modulated (sub-additive) by combined auditory–visual stimulation. Moreover, bimodal stimuli engaged a region in the posterior superior temporal sulcus (STS). Probabilistic tracking revealed white-matter tracks between the auditory cortex and sub-regions of the STS (anterior and posterior) and occipital cortex. The posterior STS region was also found to be relevant for auditory–visual object perception. The anterior STS region showed connections to the phonological STG area and to the lateral occipital body area. Our findings suggest that multisensory networks in the temporal lobe are best revealed by combining functional and structural measures.

Activity in the lateral occipital cortex between 200 and 300 ms distinguishes between physically identical seen and unseen stimuli

There is converging evidence that electrophysiological responses over posterior cortical regions in the 200–300 ms range distinguish between physically identical stimuli that reach consciousness or remain unseen. Here, we attempt at determining the sources of this awareness-related activity using magneto-encephalographic (MEG). Fourteen subjects were presented with faint colored gratings at threshold for contrast and reported on each trial whether the grating was seen or unseen. Subjects were primed with a color cue that could be congruent or incongruent with the color of the grating, to probe to what extent two co-localized features (color and orientation) would be bound in consciousness. The contrast between neural responses to seen and unseen physically identical gratings revealed a sustained posterior difference between 190 and 350 ms, thereby replicating prior studies. We further show that the main sources of the awareness-related activity were localized bilaterally on the lateral convexity of the occipito-temporal region, in the Lateral Occipital (LO) complex, as well as in the right posterior infero-temporal region. No activity differentiating seen and unseen trials could be observed in frontal or parietal regions in this latency range, even at lower threshold. Color congruency did not improve grating's detection, and the awareness-related activity was independent from color congruency. However, at the neural level, color congruency was processed differently in grating-present and grating-absent trials. The pattern of results suggests the existence of a neural process of color congruency engaging left parietal regions that is affected by the mere presence of another feature, whether this feature reaches consciousness or not. Altogether, our results reveal an occipital source of visual awareness insensitive to color congruency, and a simultaneous parietal source not engaged in visual awareness, but sensitive to the manipulation of co-localized features.

Prism adaptation enhances activity of intact fronto-parietal areas in both hemispheres in neglect patients

Unilateral spatial neglect involves a failure to report or orient to stimuli in the contralesional (left) space due to right brain damage, with severe handicap in everyday activities and poor rehabilitation outcome. Because behavioral studies suggest that prism adaptation may reduce spatial neglect, we investigated the neural mechanisms underlying prism effects on visuo-spatial processing in neglect patients. We used functional magnetic resonance imaging (fMRI) to examine the effect of (right-deviating) prisms on seven patients with left neglect, by comparing brain activity while they performed three different spatial tasks on the same visual stimuli (bisection, search, and memory), before and after a single prism-adaptation session. Following prism adaptation, fMRI data showed increased activation in bilateral parietal, frontal, and occipital cortex during bisection and visual search, but not during the memory task. These increases were associated with significant behavioral improvement in the same two tasks. Changes in neural activity and behavior were seen only after prism adaptation, but not attributable to mere task repetition. These results show for the first time the neural substrates underlying the therapeutic benefits of prism adaptation, and demonstrate that visuo-motor adaptation induced by prism exposure can restore activation in bilateral brain networks controlling spatial attention and awareness. This bilateral recruitment of fronto-parietal networks may counteract the pathological biases produced by unilateral right hemisphere damage, consistent with recent proposals that neglect may reflect lateralized deficits induced by bilateral hemispheric dysfunction.

Neurophysiological investigation of idiopathic acquired auditory–visual synesthesia

We present a case of acquired auditory–visual synesthesia and its neurophysiological investigation in a healthy 42-year-old woman. She started experiencing persistent positive and intermittent negative visual phenomena at age 37 followed by auditory–visual synesthesia. Her neurophysiological investigation included video-EEG, fMRI, and MEG. Auditory stimuli (700 Hz, 50 ms duration, 0.5 s ISI) were presented binaurally at 60 db above the hearing threshold in a dark room. The patient had bilateral symmetrical auditory-evoked neuromagnetic responses followed by an occipital-evoked field 16.3 ms later. The activation of occipital cortex following auditory stimuli may represent recruitment of existing cross-modal sensory pathways.