Poster No.: 880
fMRI response of human MT complex is selective to change of direction of optic flow fields
M Concetta Morrone, Michela Tosetti, Domenico Montanaro, David C Burr, Adriana Fiorentini, Giovanni Cioni

Istituto di Neurofisiologia del CNR - Pisa, Italy. Istituto Scientifico Stella Maris, Calambrone, Pisa, Italy. U.O. Neuroradiologia - Ospedale S. Chiara Pisa, Italy. Dipartimento di Psicologia, Università di Firenze, Italy. Divisione di Neuropsichiatria Infantile, Università di Pisa, Italy.

Purpose
Previous fMRI and PET studies have shown that moving stimuli can elicit a strong cortical response, particularly in the region referred to as “MT complex”. In this study we investigate the BOLD fMRI response to stimulation by complex global “flow-field” motion (translation, expansion, rotation etc.), typical of that produced during self motion. Previous psychophysical studies have suggested that there may exist specific neural units in humans (possibly corresponding to the units in monkey MSTd), specialised for this type of complex global motion.

Methods
Six healthy young subjects with normal vision served as subjects. The stimuli comprised 50 dots (diameter 0.5^o, half white half black, 90% contrast), randomly positioned over a 20 X 20^o visual field of 300 cd/m² average luminance. Each dot moved at a local speed of 7 deg/sec for a “limited lifetime” of 10 frames (0.32 sec), after which it was reborn at a new random position. In the active condition all dots moved coherently to define circular, radial or translational motion. The motion was either constant throughout the 30 second epoch, or reversed direction every 2 seconds (clockwise-counterclockwise, expansion-contraction, left-right etc.). In the control condition the dots were either stationary (but of limited lifetime) or dynamic, with each dot following a randomly oriented trajectory (linear for translational motion and spiral for radial and circular motion), either continuous or reversing trajectory every 2 seconds. BOLD responses were acquired by 1.5 T General Electric Signa Horizon System (GE, Milwaukee, USA), equipped with Echo-speed gradient coil and amplifier hardware, using a standard quadrature head-coil. Activation images were acquired using echoplanar imaging (EPI) gradient-recalled echo sequence (TR/TE/flip angle = 3s/50ms/90°, FOV =280x210, matrix = 128x96, Acq.Time: 3.13 min). Volumes consisting in contiguous 5 mm thick slices parallel to the calcarine scissure were acquired every 3 s. Time course series of 64 images for each volume were collected in 6 epochs alternating between control and active conditions. The original sampling volume matrix was resampled to 128x128x8-10, resulting in a final voxel size of 2.19x2.19x5 mm³. An additional set of anatomical high resolution 2D SPGR data set (TR/TE/flip angle = 150ms/2.3ms/120°; RBW=12.8 kHz; FOV =280x210, matrix = 256x192; NEX:3; Acq.Time:1.41 min) matched to the fMRI images were acquired in order to identify subsequent localisation of the activation areas. The correlation coefficient threshold used to create the functional maps was set at 0.2, corresponding to an effective probability value of p=0.007. We also required a cluster size at 8 pixels for significance.

Results
When coherent motion (of any type) was tested against the stationary control, there was a very strong BOLD response in an area positioned in the postero-medial portion of the Brodmann area 37, usually referred as MT complex (corresponding to Talairach atlas at average co-ordinates x,y,z: 53 ,-54, -4). There was no significant response elsewhere, suggesting that the controls were well matched in their spatial and temporal properties. When continuous coherent motion was tested against the dynamic control, there was no measurable response for any type of motion, in any brain area. However, when the coherent motion was caused to reverse direction every 2 seconds (with a dynamic random control, also alternating), the response of MT complex (and no other area) was very strong. In some subjects the alternating stimulus also elicited a response in Brodmann area 39-40, purported to be important for directing attention over time.

Conclusion
When erroneous responses are minimised by use of controls carefully matched in space and time, MT complex seems to show response selectivity to change of flow field motion, rather than to a continuous presentation.