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The anatomy of egocentric and allocentric spatial representations in man: a combined behavioural-fMRI-MEG study on normal subjects and hemispatial neglect patients

Final Activity Report Summary - AEASRM (The Anatomy of Egocentric and Allocentric Spatial Representations in man: a combined Behavioural-fMRI-MEG Study ...)

The main objectives of this research were the investigation, by means of functional magnetic resonance imaging (fMRI), of:

1. the neural fate and signature along the visual pathways, from V1 to the fusiform face area (FFA) and the parahippocampal place area (PPA), of unattended visual information, such as pictures of faces and houses, presented in the periphery of the visual field of normal subjects and in the neglected visual field of patients with neglect for the left side of space following a lesion to the right hemisphere
2. investigation of the neural correlates throughout the entire visual pathway of the rehabilitation of neglect through prismatic adaptation.

As a neural marker for the processing of visual information we considered the ‘adaptation effect’, whereby repetition of the same stimulus elicited a smaller fMRI signal than the presentation of two different stimuli (habituation of the BOLD signal). This decrease in signal, presumably arising from adaptation of neurons, provided a method for probing the extent to which the same population of neurons represented a given stimulus. Neuronal adaptation to unattended visual information was traced along the striate and extrastriate visual processing areas, from V1 to FFA and PPA, in normal controls and in unilateral neglect subjects before and after treatment with prismatic glasses. This design allowed us to investigate:

1. the neuronal correlates of unconscious processing of visual information and compare its characteristics in normal subjects and in unilateral neglect patients
2. the neuronal marker of rehabilitation of neglect by prismatic treatment.

Our hypothesis was that effective prismatic rehabilitation of neglect, which was known to increase awareness for contralesional events through a transitory redirection of attention contralesionally, should result in an increased neuronal adaptation response to visual stimulation from contralesional events, because neurons should become ‘better’ discriminators of ‘same-different’ pairs of stimuli as an effect of prismatic treatment.

The results from data collected from six normal subjects showed that, in the majority of normal controls, neuronal adaptation to unattended stimuli, i.e. faces and houses, was evident bilaterally throughout the visual pathways, from V1 to PPA and FFA through V2, V3 and V4. This effect also appeared in the lateral occipital complex (LOC), although decreased in magnitude. The data demonstrated that striate and extrastriate visual areas could detect similarities and differences among visual stimuli of which subjects were, subjectively, unaware of. Even more interestingly, the data collected from two neglect patients showed that the pictures of faces and houses presented in the neglected field elicited neuronal adaptation in the same visual areas as control subjects. The neuronal response to pictures of two ‘different’ stimuli presented in the neglected visual field was significantly increased if compared to the neuronal response to two identical stimuli. This adaptation effect was comparable in magnitude to that of normal controls.

Finally, a single session of treatment with prismatic glasses that was delivered to our patients reduced neglect severity and increased the neuronal adaptation response in striate and extrastriate visual areas. These data provided for the first time evidence that one element of the neurophysiological mechanisms mediating the rehabilitative effect of prismatic treatment of neglect consisted in an increment in the level of activation of the striate and extrastriate ipsilesional neuronal populations in response to stimuli presented in the neglected visual field. This ‘boosting’ of neural activity was reflected by an increased neuronal adaptation effect, i.e. by an increment in the ‘ability’ of neurons to differentially respond to similarities and differences between stimuli after a single session of prismatic treatment. This result suggested the possibility that the amelioration of spatial awareness for the left side of space observed in neglect patients after prismatic treatment was associated to a treatment-related plastic modulation of the level of activation of those neurones spared by the lesion.