Learned changes in the sensitivity of stimulus representations: Associative and nonassociative mechanisms

Central to associative learning theory is the proposal that the concurrent activation of a pair of event representations will establish or strengthen a link between them. Associative theorists have devoted much energy to establishing what representations are involved in any given learning paradigm and the rules that determine the degree to which the link is strengthened. They have paid less attention to the question of what determines that a representation will be activated, assuming, for the case of classical conditioning, that presentation of an appropriately intense stimulus from an appropriate modality will be enough. But this assumption is unjustified. Ipresent the results of experiments on the effects of stimulus exposure in rats that suggest that mere exposure to a stimulus can influence its perceptual effectiveness—that the ability of a stimulus to activate its representation can be changed by experience. This conclusion is of interest for two reasons. First, it supplies a direct explanation for the phenomenon of perceptual learning—the enhancement of stimulus discriminability produced by some forms of stimulus exposure. Second, it poses a theoretical challenge in that it seems to require the existence of a learning mechanism outside the scope of those envisaged by current formal theories of associative learning. I offer some speculations as to how this mechanism might be incorporated into such theories.     

Relative contributions of envelope maxima and minima to comodulation masking release

Comodulation masking release (CMR) is a phenomenon that demonstrates the sensitivity of the auditory system to across-frequency differences in the temporal modulation pattern of a complex waveform. In this paper, we review briefly some of the data on the physical parameters that affect CMR and describe models that have been proposed to account for CMR -- namely, models based upon envelope equalization/cancellation, across-frequency envelope correlation, and “dip listening”. The present literature is ambiguous with regard to the relative importance of energy in the peak and dip regions of the waveform envelope. We therefore performed a series of experiments to investigate this issue. In the first experiment, we examined CMR for signals that resulted either in a uniform increment or in uniform decrement in the masking noise centred on the signal frequency. This was accomplished by using a 20-Hz-wide noise band centred on 700 Hz as both the masker and as the signal, adjusting the phase angle between the signal and masker to either 0° (increment) or 180° (decrement). Conditions were examined where either zero, one, two, four, or six comodulated flanking bands were present. Results indicated positive CMRs for all conditions in which a comodulated flanking band was present. CMR increased as the number of flanking bands increased for intensity increments, but not for intensity decrements. The remaining experiments examined conditions where signals were present only in masker peaks, or only in masker dips. The results of these experiments indicated relatively large CMRs when the signal occurred in dip regions, but no CMR when the signal occurred in peak regions. Whereas some of the results of the above experiments would be difficult to account for in terms of the dip listening hypothesis of CMR, the present findings did indicate that the stimulus cues that give rise to CMR appear to be derived primarily from the dip regions of the masking noise.     

Editorial preface

International Journal for Philosophy of Religion -