Oculomotor plasticity and visuo-spatial attention
À propos
Attention and Saccadic Adaptation (SA) are critical components of visual perception, the former enhancing sensory processing of selected objects, the latter maintaining the eye movements accuracy towards them. Also, a similar dichotomy could be applied to both: voluntary saccades and endogenous attentional shifts follow internal goals while reactive saccades and exogenous shifts are elicited by sudden changes in the environment. Further, their neural substrates partially overlap and they impact each other behaviorally. My PhD work investigated the hypothesis of a functional coupling linking attention and SA in healthy humans.
Our experimental contributions all reled on the measurement of attentional performances before and after an exposure to SA (or control). In the first study, we recorded brain magnetic fields to investigate neurophysiological bases of the reactive/exogenous coupling. In the second study, we compared exogenous orienting measured in a Posner-like paradigm before and after reactive SA. Finally, using the same design, the third study investigated the voluntary/endogenous modality. We found that SA boosted the orienting of spatial attention and increased gamma band activity in the reactive/exogenous modality.
We thus proposed that the functional coupling between attention and SA relies on neuronal populations co-activated by both oculomotor plasticity and attention in the Posterior Parietal Cortex (PPC). The initial activation would emerge from a dual effect of the cerebellum inhibiting the left PPC and activating the right PPC. This effect would increase the right hemispheric dominance and the leftward attentional bias. This work opens new perspectives for the rehabilitation of visuoattentional deficits.
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PhD Thesis
Nicolas, J. (2019). On the link between saccadic adaptation and visuospatial attention.
Results
01.
Saccadic Adaptation Boosts Ongoing Gamma Activity in a Subsequent Visuoattentional Task
The main finding of this study is that saccadic adaptation increases the gamma band activity in a widespread brain network. During the saccadic task immediately after the exposure to backward adaptation of leftward reactive saccades, this increase was found in the right hemisphere centered on the TPJ and including area MT/V5. As the analysis contrasted between SA and control exposures, this increase could be specifically linked to saccadic adaptation. This result echoes Gerardin et al.'s (2012) findings, namely that the adaptation of leftward reactive saccades recruited the TPJ, area MT/V5 (and also the inferior precentral sulcus not highlighted in our study) in the right hemisphere.
During the subsequent discrimination task, the gamma band activity increase was more prominent in the left hemisphere and the medial part of the right hemisphere. The fact that we only found a small overlap between the networks highlighted in the saccadic and the discrimination task can be explained by the difference in the tasks. Indeed, during the discrimination task, no eye movements were allowed, whereas in the saccadic task, well, of course, subjects made saccades.
02.
Reactive saccade adaptation boosts orienting of visuospatial attention
The major finding of this study is that the backward (but not forward) adaptation of leftward saccades increases the cue benefit in the left hemifield. This boost is observed for the four target eccentricities. Moreover, the adaptation rate and the cue benefit boost are not significantly correlated, suggesting an all-or-none boosting effect. However this boost started in the second block and reached significance in the third block of the attentional task. This latter finding suggests that the boosting effect developed on a slow time-course. The finding of the hemifield selectivity was in our prediction. Indeed, we discussed in the State of the art, the dominance of the right cerebral hemisphere for attentional processes (left hemifield), especially concerning the exogenous orienting system.
03.
Inducing oculomotor plasticity to disclose the functional link between voluntary saccades and endogenous attention deployed perifoveally
The main result of this study is that the backward adaptation of leftward, but not rightward, voluntary saccades boosted the cue benefit for targets presented at an eccentricity of 3° but not of 7.5°. Moreover, the adaptation rate and the cue benefit boost were not significantly correlated, suggesting again an all-or-none boosting effect. This effect was however not specific to the adapted hemifield, being revealed both by -3° and +3° targets. Neuroimaging studies have shown that the endogenous orienting of attention activated the dorsal network in both hemispheres (although this activation was stronger for the contralateral one) probably accounting for this effect.
Interestingly, we found that only leftward adaptation produces a boosting effect on orienting of attention. In the absence of neuroimaging investigation of the adaptation of rightward saccades, we hypothesized that the regions activated by rightward VS adaptation would be the same as those of leftward adaptation but in the left hemisphere counterparts. Our results do not confirm this prediction and rather open the possibility that SA and/or attention substrates are not symmetrical and, in any case, do not overlap with the attentional network of the left hemisphere.
