Abstract
Abstract: :
Purpose: We have recently shown that visual area hMT+/ V5 is involved in segmentation and/or integration of global motion signals. Indeed, plaid stimuli which can be perceived either as two surfaces, one moving on top of the other, or as a single surface moving in intermediate direction, strongly activate this area. Physically constant stimuli can elicit both percepts in alternation, and such fluctuations in perception are strongly correlated with changes in activity in hMT+/ V5. This phenomenon might be related to adaptation mechanisms. Here, we searched for evidence of such mechanisms and asked whether these mechanisms would simply reflect neuronal habituation or more specific cross-directional inhibition. We have further assessed the role of selective attention in the modulation of responses to real or illusory motion.Methods: Functional imaging was performed in 9 subjects at 1.5 T. Stimuli were overlapping gratings (plaids). Data analysis included preprocessing, co-registration and Tailarach transformation of scans, volume and surface rendering, cortex reconstruction, and morphing (using BRAIN-VOYAGER 2000). For multiple regression analysis, a general linear model (GLM) with predictors for each experimental condition was computed. Results: Activity in hMT+/V5 is dependent on the previous history of stimulation: responses evoked by two moving surfaces is higher if preceded by presentation of one surface moving in a intermediate direction, than if preceded by a static stimulus. The converse was also true. In general, pre-adaptation to a stationary stimulus induced less subsequent motion-evoked activity than pre-adaptation to stimuli moving in distinct directions (p < 0,001). Responses to static stimuli also increased upon motion adaptation, which correlates with an illusion of motion going in opposite direction. If the static stimulus had similar spatial configuration to the moving stimuli, the likelihood of illusory motion was much enhanced, as well as the hMT+ response (Anova, p < 0.0001), even when selective attention was manipulated. Responses to real motion were also modulated by attention and type of previous static stimuli.Conclusions: The results suggest that global motion adaptation renders direction selective cell populations less prone to inhibit populations responding to other directions of movement. We propose a model for cross-directional inhibitory interactions in motion selective area hMT+/V5.
Keywords: motion-2D • visual cortex • imaging methods (CT, FA, ICG, MRI, OCT, RTA, S