Four experiments were conducted to develop and test a method of determining the frequency-response characteristic of the observer when he listens for single-frequency signals presented against a continuous background of wide-band noise. After observers were trained to detect primary signals of a single frequency, probe signals of various other frequencies were presented infrequently, in lieu of the primary signal. Primary signals and all probe signals were presented with very similar amplitudes that would be expected to render them all equally detectable if presented alone in single-frequency experiments. Estimates of the detectability of the signals of the various frequencies were obtained concurrently in a two-alternative forced-choice procedure. The results from 14 observers were quite similar and show differences in detection as a function of signal frequency when the primary signal was of 1000 or of 1100 Hz. In general, the primary signal was correctly detected 75%–90% of the time while signals with frequencies at approximately 150 to 200 Hz on either side of the primary-signal frequency were detected at the chance level, 50% correct. In as few as three experimental sessions, the observer's frequency-response characteristic was obtained using the probe-signal method.
An experimental simulation technique is introduced to study memory constraints affecting comprehension of embedded sentences. Memory for embedded structure is measured in a pairing task that simulates requirements imposed by sentence comprehension, With increasing memory load (number of embeddings), the rate of performance decline is consistent across all (recall and comprehension) conditions, and reflects a loss of order (as opposed to item) information. Short-term memory overload, rather than linguistic complexity, appears sufficient to account for comprehension difficulty, Performance was imperfect even with singly embedded constructions, Recall showed no abrupt failure with multiple embeddings, indicating that memory constraints are not as discontinuous as English syntax might suggest.
Listeners rated the subjective loudness of tones presented in noise and made decisions as to the presence or absence of Signals. Loudness ratings and detection decisions were studied first in separate experiments and then in experiments which required both responses to be made on each trial. The loudness judgments were stable across sessions and Signal probability conditions, and the two responses could be made on the same trials with no discernible interference. The relation between the two responses was compared with predictions from psychophysical models. While the data support the view that detection responses are composed of sensory and decision stages, they are inconsistent with several traditional models. A variable criterion model is proposed which gives a good account of the detection data.
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