| dc.description.abstract |
The Superior Colliculus (SC) is well established to be a structure that plays a major
role in orienting the eyes and head towards objects of interest in the surroundings.
Most of the studies in the SC thus far have been conducted on non-human primates.
Although the neural architecture is presumed to be similar, existence of differences
in anatomy, afferents and efferents between the SC of different mammals questions
functional extrapolation of such studies to humans. Also, there have been very few
electrophysiological studies that have investigated the role of the SC in reaching and
finger tapping movements. An fMRI study in humans conducted earlier in the lab
was the first to show that the SC is involved in reaching movements in humans. In
order to further elucidate the motor functions of the SC in humans, we conducted
three experiments and analysed one pre-acquired dataset. These studies used two
simple and classical paradigms – reaching, and finger tapping movements.
The SC is widely believed to be a visually driven structure, supported by numerous
studies in macaques. With our first study, we sought to decipher if the SC is also
active in response to reaching, guided by tactile stimulation. We designed a task where
subjects had to maintain fixation while performing reaching movements to peripheral
targets that were either visual or tactile in nature, separated by their respective
blocks. We found that the SC indeed shows activity in response to somatically or
visually guided reaching, but not to visual stimulation or somatic stimulation on their
own. In addition, we also observed a strong signal contribution from button presses
in tasks with control conditions, which were required to be performed by subjects in
response to oddball stimuli, to make sure that they were engaged in the tasks.
Our second study was divided into two parts. The first part consisted of an analysis
of the finger tapping task (motor task section) from the Human Connectome Project (HCP) database. The second part of the study investigated whether complex finger
tapping and simple finger tapping movements, paced by visual or auditory stimuli,
caused an activation in the SC. We found no activation in response to the finger
tapping task from the HCP data. We also found that the SC did not take part in
either the complex or simple finger tapping tasks that we conducted. This, together
with the results corresponding to finger tapping from the HCP data, and the finger
tapping (recorded as button presses) results from the first experiment, led us to
postulate that the SC might be involved only when finger tapping movements occur
in response to novel stimuli.
With our third and final study, we investigated the response of the SC to novel
stimuli. In the experiment, subjects responded with button presses or counting to
oddballs involving the same modalities as the first experiment: visual and somatic
stimulation. We found some activity in response to all oddball conditions combined
together. This activity was further weakened when oddballs were bifurcated into four
conditions by the sensory modalities involved - visual and somatic stimulation and the
corresponding responses - button presses and counting. Although these results hint
that the SC might respond to oddball stimulation, these effects were not consistent
across subjects warranting further investigation for more concrete conclusions.
Taken together, our results conclusively show that apart from visual information the
SC integrates sensory information such as touch towards the execution of reaching
movements. In addition, the SC does not play a role in the execution of movement
sequences that are repetitive, irrespective of whether they are complex or simple
movements. For a concrete conclusion regarding responses to novel stimuli, more
sensitive experiments are necessary. |
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