|M.Sc Student||Wasmuht Dante|
|Subject||Adaptation Paradigms for the Study of Auditory and Visual|
Motion in the Optic Tectum of the Barn Owl
|Department||Department of Medicine||Supervisor||Professor Yoram Gutfreund|
|Full Thesis text|
The representation and processing of visual motion has been studied extensively but much less is known about the representation of auditory motion in the brain. Two main hypotheses have been postulated: 1) That, as in vision, separated and dedicated pathways process sound motion. 2) That in the auditory pathways only location and time is represented and thus motion representation is inseparable from spatial representation (the snapshot hypothesis). In this study we addressed this question by recording, in barn owls (Tyto alba), responses of neurons from the optic tectum (OT; homolog to the superior colliculus in mammals) to moving auditory cues.
Neurons in the OT tend to respond stronger to rarely presented sounds (novel sounds or deviants) than to the same sounds when presented frequently (common). This phenomenon, known as stimulus-specific adaptation (SSA) was previously shown using various sound features such as frequency, intensity, interaural time difference (ITD) and interaural level difference (ILD). Here we presented sweeps of ITD, mimicking a sound moving from right to left or from left to right. Stimuli were presented every two seconds, 15 times sweeping in one direction, 15 times in the other direction and then again, continuing for about 10 minutes. At the end of the paradigm the adaptation pattern was analyzed. If SSA exists, we expect to observe an increase in the neural response every time the direction is switched. We found that, in most cases, a change in the sweep direction, alone, did not evoke SSA. However when the two directions were also separated by the interaural level (ILD), i.e., mimicking a separation in elevation, SSA was commonly induced.
We repeated the same experiments under free-field conditions, using a 144 speaker array to produce apparent motion. Consistent with the results from ear-phone stimulation, in free-field condition no SSA to the direction of motion was observed.
For comparison, we repeated the experiments with visual stimuli, recording from the same population of neurons. For visual moving objects SSA occurred in both conditions: motion on the same horizontal level as well as motion along vertically separated levels. Thus, we conclude that there is a fundamental difference between the way auditory and visual motion are represented in the brain. We hypothesize that in the visual pathways a dedicated circuit for processing motion information exists, while in the auditory system such a system is lack?ing, consistent with the snapshot hypothesis.