MEG & fMRI @ VSS
Lucia and Finn are just back from presenting a pair of posters at the Vision Science Society meeting in Naples, Fl this week. Check below for abstracts!
Deficit of temporal dynamics of detection of a moving object during egomotion in a stroke patient: a psychophysical and MEG study
Lucia-Maria Vaina, Kunjan Rana, Ferdinando Buonanno, Finnegan Calabro, Matti Hamalainen
To investigate the temporal dynamics underlying object motion detection during egomotion, we used psychophysics and MEG with a motion discrimination task. The display contained nine spheres moving for 1 second, eight moved consistent with forward observer translation, and one (the target) with independent motion within the scene (approaching or receding). Observers?s task was to detect the target. Seven healthy subjects (7HS) and patient PF with an infarct involving the left occipital-temporal cortex participated in both the psychophysical and MEG study. Psychophysical results showed that PF was severely impaired on this task. He was also impaired on the discrimination of radial motion (with even poorer performance on contraction) and 2D direction as well as on detecting motion discontinuity. We used anatomically constrained MEG and dynamic Granger causality to investigate the direction and dynamics of connectivity between the functional areas involved in the object-motion task and compared the results of 7HS and PF. The dynamics of the causal connections among the motion responsive cortical areas (MT, STS, IPS) during the first 200ms of the stimulus was similar in all subjects. However, in the later part of the stimulus (>200 ms) PF did not show significant causal connections among these areas. Also the 7HS had a strong, probably attention modulatory connection, between MPFC and MT, which was completely absent in PF. In PF and the 7HS, analysis of onset latencies revealed two stages of activations: early after motion onset (200-400 ms) bilateral activations in MT, IPS, and STS, followed (>500 ms) by activity in the postcentral sulcus and middle prefrontal cortex (MPFC). We suggest that the interaction of these early and late onset areas is critical to object motion detection during self-motion, and disrupted connections among late onset areas may have contributed to the perceptual deficits of patient PF.
Detection of object motion during self-motion: psychophysics and neuronal substrate
Finnegan Calabro, Lucia-Maria Vaina
The extraction of object motion from a visual scene is critical for planning direct interactions with one?s surroundings, and is of particular interest and difficulty when the observer is moving. To investigate the visual processes underlying object motion detection during self-motion, we presented observers (n=23) with a stimulus containing nine objects, eight of which moved consistent with forward observer translation, and one of which (the target) had independent motion within the scene. Results showed that observers? abilities to detect the target depended significantly on the speed of the object within the scene (Exp 1), but that performance was independent of observer speed, and therefore retinal velocity (Exp 2, n=7). Results were compared to predicted performances for target selection based on relative differences in speed and direction among the objects, and were not consistent with either strategy. Instead, these data suggest that observer performance used a flow parsing mechanism in which self-motion is estimated and subtracted from the flow field. In an event-related fMRI paradigm using the task from Exp 1, we found a distributed pattern of activations of occipito-temporal, posterior parietal and parieto-frontal areas. Granger causality analysis among these activated regions revealed two major highly connected networks. One network involved a set of interconnected early, bilateral, visually responsive areas (including KO, hMT+ and VIPS). We posit that these regions underlie the perception and formation of a visual representation of the stimulus. The second network was comprised of primarily higher-level, left hemisphere areas (including DIPSM, FEF, subcentral sulcus and postcentral gyrus) that have been reported to be involved in the use of sensory inputs for preparing motor commands. We suggest that these networks provide a link between the perceptual representation of the visual stimulus and its interpretation for action.