Neuroimaging of direction-selective mechanisms for first-order and second-order motion stimuli

Abstract

Psychophysical findings have indicated functional segregation of processing pathways for first-order motion (movement of luminance modulation, LM) and second-order motion (e.g., movement of contrast modulation, CM). However, neural correlates of those pathways are still controversial. To investigate whether there is corresponding anatomical segregation, we conducted a novel fMRI study with deliberate control of attention, to localize direction-selective cortical mechanisms for first- and second-order motion stimuli by measuring direction-contingent response changes induced by motion adaptation. We used Siemens 3T scanner along with a flattened occipital format. The stimulus was a rotating windmill defined by a 10% LM superimposed on a static random-dot noise, or by a 100% CM of a static noise. Every 32s, the rotating stimulus (LM or CM) gradually disappeared and reappeared with or without direction reversal between the 32-s periods (a reversal occurred every 64s). We asked subjects to maintain fixation at a central spot and to detect speed changes of the stimulus rotation, which occurred at unpredictable timings (independent of the direction reversals) and were as tiny as the detection threshold, in order to keep the subject's attention to the motion stimulus. We obtained time courses of MR signals from V1, V2, V3, VP, V3A, V4v and MT+, defined by other retinotopic stimuli. We compared cortical activations between the first 32s period (after direction reversal) and the second period (without reversal) to isolate the effects of direction-selective adaptation from those of motion onset. The results obtained so far suggest that the CM stimulus generated direction-selective adaptation in V3A and MT+, but not in V1 and V2. However, the LM stimuli produced clear direction-selective adaptation in V3A, MT+, and (if the LM depth was increased) also in V1 and V2. Therefore, the functional difference between the first- and second-order motions may originate in V1 and V2.