Free Visual Exploration Research Articles

Interhemispheric balance of attention: A theta burst stimulation study

Interhemispheric imbalance is discussed as a pathophysiological mechanism in visuospatial neglect. It is suggested that after a lesion of the right hemisphere, the mutual transcallosal inhibition is impaired resulting in an increased activity of the left hemisphere. We investigated interhemispheric balance of attention in healthy subjects using a free visual exploration task and by interfering with the neural activity of the posterior parietal cortex (PPC) of either hemisphere by means of theta burst stimulation (TBS).

Exploring the visual world: The neural substrate of spatial orienting

Inspecting the visual environment, humans typically direct their attention across space by means of voluntary saccadic eye movements. Neuroimaging studies in healthy subjects have identified the superior parietal cortex and intraparietal sulcus as important structures involved in visual search. However, in apparent contrast, spatial disturbance of free exploration typically is observed after damage of brain structures located far more ventrally. Lesion studies in such patients disclosed the inferior parietal lobule (IPL) and temporo-parietal junction (TPJ), the superior temporal gyrus (STG) and insula, as well as the inferior frontal gyrus (IFG) of the right hemisphere. Here we used functional magnetic resonance imaging to investigate the involvement of these areas in active visual exploration in the intact brain. We conducted a region of interest analysis comparing free visual exploration of a dense stimulus array with the execution of stepwise horizontal and vertical saccades. The comparison of BOLD responses revealed significant signal increases during exploration in TPJ, STG, and IFG. This result calls for a reappraisal of the previous thinking on the function of these areas in visual search processes. In agreement with lesion studies, the data suggest that these areas are part of the network involved in human spatial orienting and exploration. The IPL dorsally of TPJ seem to be of minor importance for free visual exploration as these areas appear to be equally involved in the execution of spatially predetermined saccades.

Eye movements and visuoverbal descriptions exhibit heterogeneous and dissociated patterns before and after prismatic adaptation in unilateral spatial neglect

This prospective study examined the effects of prismatic adaptation on visual exploration strategies in patients with left unilateral spatial neglect (USN). Photo-oculographic gaze recordings were obtained, as the subjects (28 brain-damaged; 15 control) performed a free visual exploration task before and after a session of prismatic adaptation. (i) Before prismatic adaptation, the pattern of visual exploration described two subgroups of patients (symmetrical exploration of hemispaces - similar to the control subjects, deficient exploration of left hemispace). Twelve of 20 patients failed to describe significant elements in the left part of the displayed image. Several visuoverbal patterns were observed, some dissociating visual exploration and verbal description. (ii) Immediately after prismatic adaptation, patients with asymmetrical visual exploration presented a significant increase in the number of point fixations and saccades in the left hemispace. Patients with symmetrical exploration presented the opposite pattern. Improved pattern of visual exploration contrasted with an absence of improved verbal description. Eye movements and visuoverbal descriptions exhibit heterogeneous and dissociated patterns before and after prismatic adaptation. This results demonstrate that prismatic adaptation has no effect in certain patients, suggesting that therapeutic indications and evaluation of prismatic test results should take into consideration the heterogeneous nature of USN.

Visual receptive field modulation in the lateral intraparietal area during attentive fixation and free gaze.

The receptive field (RF) of neurons recorded from the lateral intraparietal area (LIP) was quantified using a rapid, computer-driven mapping procedure. For each neuron, the RF was mapped: (1) during attentive fixation and (2) during free visual exploration. RF location, size and internal structure were modulated by the mapping context in over two-thirds of the recorded neurons. The major trend was a proportionally larger amount of neuronal visual resources allocated to central space during fixation, and an attenuated center-to-periphery gradient in the visual field representation during free gaze. A population approach shows that these spatial modulations are accompanied by changes in the signal-to-noise ratio of the information carried in the RF substructure. We related these neurophysiological observations to behavior, by comparing the characteristics of saccades elicited during fixation and free gaze. Together, the results suggest that the dynamics of LIP visual RFs may characterize both the state of engagement of attention and the power of resolution of visual analysis: during fixation, the neural population is locked in a filter state concentrating the processing resources at the fovea, while during free gaze, the population shifts to a detector state spreading the resources more evenly across the visual field.