The Neuronal Basis of Signal Detection in the Presence of Spatially Separated Noise: A Study in the Frog Inferior Colliculus
Psychophysical results from studies in humans have shown that when an auditory signal and noise are presented simultaneously, the threshold for signal detection decreases when the angular separation between the two sound sources is increased. Signal detection threshold is highest when the two sound sources are colocalized in space. Although this phenomenon has been known for many years, and has recently been confirmed in other vertebrates, the response of neurons in the auditory system to spatial separation of sound sources is not known. To study the neural basis of this phenomenon, the leopard frog Rana pipiens was chosen as the experimental subject. It is known that females of this species use only auditory cues to select and move towards mates calling in a dense heterospecific chorus. Extracellular recordings were made from single auditory units in the inferior colliculus (IC), while the frog was subject to free-field stimulation with sinusoidally amplitude modulated tone bursts (signal) in the presence of spatially separated broad-band noise (masker). Results of the study showed that while many IC neurons (called A-type) demonstrated the psychophysically observed behavior, many other neurons (called U-type) showed the opposite effect. That is, U-type neurons demonstrated an increase in signal detection threshold with an increase in angular separation of sound sources. Together, both these types of neurons accounted for nearly half the population sampled while the remainder of the population was a mix of the two types. When the type of the unit was correlated with the temporal discharge pattern of the unit it was found that most A-type units had phasic discharge patterns whereas most U-type units had tonic discharge patterns. It was also found that the response to signal plus noise was highly nonlinear and that detection thresholds for a unit could not be estimated by examining the neuron's response to signal alone and noise alone. The study concludes with a discussion of the nonlinear interactions in the system and discusses the possible role of A- and U-type units in signal detection.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign