1. neurophysiological study of the cat superior colliculus
Eye position and extracellular single unit activity were recorded while the animals were performing a visually-guided saccade task. Single spikes with their waveforms were extracted off-l ...
1. neurophysiological study of the cat superior colliculus
Eye position and extracellular single unit activity were recorded while the animals were performing a visually-guided saccade task. Single spikes with their waveforms were extracted off-line from the extracellular potentials above a certain threshold. Spike trains of the cells from 45 channels were wavelet transformed. Wavelets from one to eighty hertz with 1ms time index were convoluted with every spike train of every trial. Dominant frequency ranged between 11-20Hz (12 Hz on average) and the amplitude reached the peak at 87.68ms. The rhythm remained significant for 149.19ms. Alpha and beta frequency were dominant after the target onset, and alpha was dominant before saccade onset and offset. The rhythm appeared with the visual response, reached the peak approximately 90ms after the target onset, remained significant for sometime, and then faded away. In order to verify that the rhythm had been visually induced, the inter-trial coherence (ITC) was calculated from the wavelet transformed data. The peak magnitude of ITC ranged in alpha band (9.59 ± 2.31 Hz), and it was concluded that the dominant rhythm was visually induced. In order to investigate whether, and how much, the cells in distant population oscillates together, ICC (inter-channel coherence) was calculated. Overall, the ICCs at the low frequency band were significant. Consistent to previous results, ICC reached the peak magnitude 69.48±22.10ms after target onset. According to the power spectrum, wavelet spectrum, ITC, and ICC, the frequency of the rhythmic oscillation was dominant at alpha range. In addition, it could be concluded that the rhythm was induced by visual stimulus, not by saccadic eye movement or other events.
2. Dynamic modulation of spatial short-term memory
Spatial location of a visual target can be registered with respect to concurrent visual references, or with respect to the observer. When a visual reference is not available, e.g. in the dark, spatial localization depends on only egocentric cues, and its accuracy is often compromised. When a visual probe was used to report the remembered location of the target, the accuracy of the target localization depended on the initial position of the probe. When the probe initially appeared on the same side as the fixation with respect to the target (SS condition), the remembered target location was systematically biased beyond the target eccentricity. On the other hand, in the OS trials where the probe initially appeared on the side opposite to the fixation with respect to the target, localization was relatively accurate. The difference in localization bias between the SS and OS conditions persisted regardless of the gaze direction during the response period or the response device tested. Also, this difference persisted, although modulated, when the temporal gap between the fixation target offset and target onset was increased to 1s or when the temporal gap between the target offset and probe onset was increased to 1.2s. We also found this difference in the presence of a persistent external reference at the initial fixation. The asymmetric pattern of mislocalization between the OS and SS conditions is the closest to the prediction made by the hypothesis that the target memory is repulsed from both the fixation and the probe. Thus, repulsion from the fixation and probe either annihilate to result in near veridical localization in the OS condition, or accumulate to result in a larger localization error in the SS condition. The relatively accurate localization in the OS trials suggests that repulsions from the fixation and probe are comparable in magnitude. Under our experimental condition, the fixation and probe repulsed the target memory approximately by 0.15 and 0.18 deg, respectively, for each degree of target eccentricity.