Abstract
Area MT has traditionally been thought to operate in a retinotopic reference frame; however, there has been recent fMRI evidence that human MT has some spatiotopic properties (d'Avossa et al, 2007). Also, we have presented psychophysical evidence that area MT plays a spatiotopic role in the memory for motion process (Ong et al, 2009).
Here, we recorded from area MT in animals performing visually guided saccades during which a moving dot stimulus (100% coherence) or a circular stimulus was presented. The dot stimulus moved in the preferred direction of the recorded neuron in the pre-saccadic or post–saccadic receptive field for 500 ms; its onset occurring 80 ms before the saccade target appeared. The luminance-matched circle was flashed for 50 ms in the pre- or post-saccadic receptive field at random time intervals between 100ms before the saccade target appeared to 350 ms after. Mean saccadic latency was 192 ± 35 ms.
We recorded from 31 neurons and none of them showed pre-saccadic remapping with either stimulus. With the flashed circle, approximately 1/3 of the neurons showed late post-saccadic remapping, defined as when stimuli flashed shortly before the beginning of the saccade induced a neural response after the saccade in the post-saccadic receptive field.
We found that the post-saccadic response latencies of the moving dots were similar to onset latencies for most neurons, consistent with saccadic suppression. A subpopulation had shorter latencies, but none were pre-saccadic. These neurons were more likely to show late post-saccadic remapping of the flashed circle.
Although no neurons exhibited pre-saccadic remapping, the presence of the late post-saccadic response to a stimulus flashed entirely prior to the saccade indicates that a remapping mechanism may act on MT neurons and could explain results showing spatiotopic processing in area MT.
The National Eye Institute, the Kirchgessner Foundation, the Gerald Oppenheimer Family Foundation, the Klingenstein Fund, the McKnight Foundation, the Alfred P. Sloan Foundation.