Functional analysis of middle temporal visual area and its associated behavior

Abstract

Our lab's long-term goal is to elucidate the circuitry of the visual cortex, to develop quantitative computational models of neuronal function in the visual cortex, and to establish how these models may relate to visual perception and visually guided behavior. Central to this goal is the analysis of functional architecture, which is crucial to an understanding of how the brain works. In my thesis research, I applied behavioral and microstimulation techniques to demonstrate the causal connections between neural activity and behavior. Understanding these relationships is one of the fundamental issues needed to be addressed in Neurobiology. Specifically, I focused on the functional analysis of the middle temporal visual area (MT) and the behavior associated with it. MT is an extrastriate area that is primarily involved in visual motion processing. A very important function within MT is a segregation of center-surround interactions which plays a critical role in processing visual motion cues. There are two types of motion center-surround interactions in MT neurons: surrounds may reinforce (at wide-field sites) or suppress (at local-motion sites) the centers' directional responses. They are important in representing the initial stages of a functional segregation between wide-field and local-contrast motion processing. To further study the computational model used by the brain to readout sensory information, I conducted microstimulation experiments in MT by changing stimulation amplitudes (from 10/LA to 160tA) and frequencies (from 25Hz to 500Hz). Microstimulation can introduce an additional velocity signal into MT and the pursuit and saccadic systems usually compute a vector average of the visually evoked and microstimulation-induced velocity signals.

(cont.) Increasing either amplitude or frequency generally increases the relative weight of the electrical velocity signal,' with the effects of amplitude being slightly more prominent. In addition, applying higher current fre-quencies appears to preserve the directionality of microstimulation better than does applying higher current amplitudes. With increasing frequencies, the magnitude of the electrical velocity signal either increased or remained constant, while its direction remained consistent. In contrast, increasing current amplitude tended to decrease the magnitude of the signal and increased its variability in direction. This finding is consistent with the idea that large current amplitudes, which presumably activate many MT columns signaling different directions, introduce noise into the behavior. My preliminary results have demonstrated that microstimulation in MT can also introduce an additional positional signal into the saccade system and that this electrical signal is combined with the visually evoked signal through a vector summation mechanism. The direction of this electrical signal is highly correlated with the position of the receptive field relative to the fixation point. To test the notion that the center-surround properties of MT neurons may be important for signaling the relative motion between object and background, we conducted behavioral experiments by using real background motion to simulate the microstimulation experiments at wide-field sites ...