Neural fate of seen and unseen faces in visuospatial neglect: A combined event-related functional MRI and event-related potential study

Abstract

To compare neural activity produced by visual events that escape or reach conscious awareness, we used event-related MRI and evoked potentials in a patient who had neglect and extinction after focal right parietal damage, but intact visual fields. This neurological disorder entails a loss of awareness for stimuli in the field contralateral to a brain lesion when stimuli are simultaneously presented on the ipsilateral side, even though early visual areas may be intact, and single contralateral stimuli may still be perceived. Functional MRI and event-related potential study were performed during a task where faces or shapes appeared in the right, left, or both fields. Unilateral stimuli produced normal responses in V1 and extrastriate areas. In bilateral events, left faces that were not perceived still activated right V1 and inferior temporal cortex and evoked nonsignificantly reduced N1 potentials, with preserved face-specific negative potentials at 170 ms. When left faces were perceived, the same stimuli produced greater activity in a distributed network of areas including right V1 and cuneus, bilateral fusiform gyri, and left parietal cortex. Also, effective connectivity between visual, parietal, and frontal areas increased during perception of faces. These results suggest that activity can occur in V1 and ventral temporal cortex without awareness, whereas coupling with dorsal parietal and frontal areas may be critical for such activity to afford conscious perception. R ight parietal damage may cause a loss of awareness for contralateral (left) sensory inputs, such as hemispatial neglect and extinction (1-3). Visual extinction is the failure to perceive a stimulus in the contralesional field when presented together with an ipsilesional stimulus (bilateral simultaneous stimulation, BSS), even though occipital visual areas are intact and unilateral contralesional stimuli can be perceived when presented alone. It reflects a deficit of spatial attention toward the contralesional side, excluding left inputs from awareness in the presence of competing stimuli (2, 3). Spatial attention involves a complex neural network centered on the right parietal lobe (4, 5), but how parietal and related areas interact with sensory processing in distant cortices is largely unknown.Here we combined event-related functional MRI (fMRI) and event-related potentials (ERPs) to study the regional pattern and temporal course of brain activity produced by seen and unseen stimuli in a patient with chronic neglect and extinction caused by parietal damage. In keeping with intact early visual areas in such patients, behavioral studies suggest that some residual processing may still occur for contralesional stimuli without attention, or without awareness, including ''preattentive'' grouping (e.g., refs. 6 and 7) and semantic priming (e.g., ref. 8). It has been speculated (3, 9) that such effects might relate to separate cortical visual streams, with temporal areas extracting object features for identification, and parietal areas encoding spatial...

The electrophysiology of tactile extinction: ERP correlates of unconscious somatosensory processing

“…In this respect, our results seem reminiscent of recent visual studies which used fMRI to show that activation of striate and extrastriate cortex can analogously be insufficient to generate corresponding visual awareness in extinction patients (e.g. [34,42]). In the tactile domain, our results may also accord with recent evidence on unconscious somatosensory processing from a different tactile disorder.…”

“…Using event-related potential (ERP) and/or fMRI measures to study visual extinction, several recent studies have shown that, on the one hand, relatively early components of visual processing may be abnormal for contralesional stimuli in visual extinction (e.g. [25,26,42]); while, on the other hand, extinguished visual stimuli can evidently still undergo considerable unconscious residual processing that activates intact cortical visual areas, despite being undetected (e.g. [14,15,34,42]).…”

“…[25,26,42]); while, on the other hand, extinguished visual stimuli can evidently still undergo considerable unconscious residual processing that activates intact cortical visual areas, despite being undetected (e.g. [14,15,34,42]). Here, we address similar issues with ERP measures, but for the tactile modality instead.…”

Neural Basis of Individual Differences in Synesthetic Experiences

“…One possible explanation is that prefrontal and modality-specific brain areas interact in creating the synesthetic experiences. This can be through associative learning (Passingham et al, 2000) and memory retrieval (Fletcher et al, 1998), or in terms of supporting the (change in) conscious perception (Kleinschmidt et al, 1998;Lumer et al, 1998;Rees, 2001;Vuilleumier et al, 2001). An alternative explanation is that the prefrontal brain areas are not involved in creating the synesthetic experiences per se, but reflect increased dependence on control mechanisms (Ridderinkhof et al, 2004).…”

Multiple Mechanisms of Consciousness: The Neural Correlates of Emotional Awareness

“…Thus, awareness depends not only on activation in category-selective regions, but also on input from superordinate structures. Candidate regions for performing this "topdown" function include lateral prefrontal and inferior parietal cortices (Beck et al, 2001;Vuilleumier et al, 2001a). Indeed, a recent lesion study suggests a causal role for prefrontal cortex in allowing stimuli access to consciousness (Del Cul et al, 2009).…”