Author: Isabel Bleimeister

Temporal-nasal asymmetry: See it out of the corner of your eye

I have now been working on my thesis for two weeks. During that short amount of time, my project has already changed drastically. While I was warned by previous fellows that I need to be flexible because aspects of my thesis will change, I had not expected it to happen so early on. Then again, perhaps I am lucky that it did.

With my thesis, I am attempting to determine whether the superior colliculus plays a role in the subcortical visual processing of numerical stimuli. While subcortical visual processing has been well documented over the last few years through the presence of monocular advantage (i.e., an effect where people are faster at making same-different comparisons between sequentially presented stimuli when the stimuli are presented to one eye as opposed to both), the actual mechanism by which it works with respect to numerical stimuli is still not clear.

Although I only recently began the research for my thesis, I have already had to re-evaluate my methodology. Initially, I decided to target the superior colliculus using purple stimuli. Because of the short wavelength of purple light, purple images activate S-cones (short wavelength cones). Up until recently, S-cones were thought to not activate the superior colliculus. Thus, a failure to induce monocular advantage using purple stimuli would be indicative of the superior colliculus’ involvement in subcortical visual processing. As a technique used for decades to target the superior colliculus, this seemed like the best way to test its involvement. However, Hall & Colby (2014) found by recording the activation of individual neurons in the superior colliculus that the superior colliculus can be activated by S-cone-specific visual stimuli. This realization forced me to rethink my approach to the experiment.

A less commonly used technique to target the superior colliculus is temporal-nasal asymmetry. This technique takes advantage of the fact that the superior colliculus is part of the tectopulvinar pathway (a visual pathway that relies more on subcortical structures than the more cortically reliant geniculostriate visual pathway). In temporal-nasal asymmetry, stimuli are presented in either the temporal or nasal hemifields. Because of the asymmetry of the tectopulvinar pathway, temporal stimuli activate the superior colliculus more than nasal stimuli, in what is called a temporal hemifield advantage. Consequently, if stimuli that have previously uncovered monocular advantage result in a temporal hemifield advantage when presented in the temporal and nasal hemifields, this would suggest that the monocular advantage previously found may have been due to the superior colliculus.

My thesis has only just begun but my methods have already been altered. While such drastic changes so early on in the process are somewhat daunting, I look forward to pursuing this new direction. In the long run, I expect that it will be beneficial that I caught this problem now; it is easier to rectify a problem when it has only just begun to manifest rather than after everything has been completed.