Auditory edge detection: the dynamics of the construction of auditory perceptual representations

Maria Chait

This dissertation investigates aspects of auditory scene analysis such as the detection of a new object in the environment. Specifically I try to learn about these processes by studying the temporal dynamics of magnetic signals recorded from outside the scalp of human listeners, and comparing these dynamics with psychophysical measures. In total nine behavioral and Magneto-encephalography (MEG) brain-imaging experiments are reported. These studies relate to the extraction of tonal targets from background noise and the detection of change within ongoing sounds. The MEG deflections we observe between 50-200 ms post transition reflect the first stages of perceptual organization. I interpret the temporal dynamics of these responses in terms of activation of cortical systems that participate in the detection of acoustic events and the discrimination of targets from backgrounds. The data shed light on the statistical heuristics with which our brains sample, represent, and detect changes in the world, including changes that are not the immediate focus of attention. In particular, the asymmetry of responses to transitions between 'order' and 'disorder' within a stimulus can be interpreted in terms of different requirements for temporal integration. The similarity of these transition-responses with commonly observed onset M50 and M100 auditory-evoked fields allows us to suggest a hypothesis as to their underlying functional significance, which so far has remained unclear. The comparison of MEG and psychophysics demonstrates a striking dissociation between higher level mechanisms related to conscious detection and the lower-level, pre-attentive cortical mechanisms that sub-serve the early organization of auditory information. The implications of these data for the processes that underlie the creation of perceptual representations are discussed. A comparison of the behavior of normal and dyslexic subjects in a tone-in-noise detection task revealed a general difficulty in extracting tonal objects from background noise, manifested by a globally delayed detection speed, associated with dyslexia. This finding may enable us to tease apart the physiological and behavioral corollaries of these early, pre-attentive processes. In conclusion, the sum of these results suggests that the combination of behavioral and MEG investigative tools can provide new insights into the processes by which perceptual representations emerge from sensory input.