In two experiments, human subjects showed a reduced eyewink reflex to an airpuff (S2) if it was preceded by a light flash or a noise burst (S1). The first (N=6) showed that a light inhibited the blink maximally at a 100-msec lag. In the second (N=8), thresholds for noise pips were established with the method of limits. Then the pips were presented in multiples of threshold values as S1 100 msec before S2. Sujbects pressed a button whenever they heard the sound, on S1-S2 pairs, on S1 alone, or on S2 alone trials. Eyeblinks to S2 declined in amplitude and increased in latency as S1 intensity increased. The threshold S1 (approximately 2 dB over a 50-dB background) produced reliable inhibition in 7 of 8 subjects, and a weaker S1 which never evoked a button press was sufficient to produce reliable inhibition for the group as a whole. Inhibition in any particular trial was unrelated to button pressing. The sensitivity of the reflex technique and its relative invariance to task demands suggest its usefulness as a behavioral psychophysical procedure.

In Experiment 1. the amplitude of the acoustic startle reflex in rats was shown to increase linearly with increases in the intensity of the eliciting stimulus (Se). A preliminary stimulus (Si), a light flash, inhibited the reflex. The amplitude of the reflex was reduced by an equal amount regardless of Se intensity. In Experiment 2. the amplitude of the same reflex was shown to have a biphasic relation to the intensity of a white noise background (Sn) and reflex amplitudes peaked at the intermediate noise levels. Although the vigor of the reflex elicited in the presence of the most intense noise approximated that elicited by the weak Se in Experiment 1, in this condition the reflex was unaffected by the occurrence of the otherwise inhibitory light flash. It has been previously hypothesized that the decrement in reflex vigor obtained at high Sn results because sensory masking of Se by Sn reduces the effective intensity of Se. The different effect of Si in these two conditions suggests that this hypothesis is incorrect. A hypothetical neural model was devised to account for the difference between the two reflexes in their susceptibility to a preliminary stimulus. and the model was extended to other effects of stimulus strength and background noise on reflexive behavior.The strength of the acoustic startle reflex in the rat is substantially affected by the intensity of the eliciting stimulus (Se), by the level of the background noise present at the time of reflex elicitation (Sn), and by the occurrence of irrelevant stimuli just prior to that time (Si simple assumptions produce an invariant decrement in reflex amplitudes regardless of reflex strength, unless the amount of inhibition produced by Si is large relative to the strength of the reflex (i.e., a "floor effect" may deform the relationship). The present experiment assessed the generality of this invariance by examining the effectiveness of Si on the variously sized reflexes that occur in different noise backgrounds. The descending upper limb of the biphasic curve relating response amplitude to Sn intensity has been ascribed to a process akin to that responsible for the relationship between Se intensity and reflex strength. It has been proposed that, at noise levels higher than the optimum, the background noise increasingly masks Se and reduces its effective intensity (Davis, 1974; Ison & Hammond, 1971): an increment in Sn is formally identified with a decrement in Se. If this hypothesis is correct, then, in this range of relatively high noise levels past the optimal value, the preliminary stimulus should subtract a constant amount from reflex amplitude. METHOD SubjectsThe subjects were albino male rats of the Holtzman strain weighing 350-500 g at the time of testing. They were maintained on an ad-lib food and water schedule. ApparatusThe rats were restrained during testing in a small perforated Plexiglas stabilimeter cage (8 x 6 x 9 cm high) that was mounted on compression springs in an aluminum frame. An accelerator mounted on the back of the stabilimeter de...

Two experiments were conducted on normal hearing adults to assess the audiometric test potential of reflex modulation (RM) when administered according to the descending method of limits. RM describes the inhibitory effect of a tone upon a subsequently occurring eyeblink reflex. Although RM conducted according to the method of constant stimuli has already been shown to produce RM results which agree well with voluntary audiometry in normal and hearing-impaired adults and children, the present concern is with the potential clinical application of RM, and in this regard the descending method may offer certain distinct advantages. In study one, a 2-kHz tone was presented at several descending intensities whereas in study two, four different frequencies (0.5, 1, 2, and 4 kHz) were presented in concurrent fashion, but again, the intensity of the complete set was decreased in successive steps. Sensitivity in both cases was 10 to 15 dB SL. Implications for the clinical application of RM audiometry are discussed.

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A computerized pattern recognition system has been developed that is capable of identifying 40 separate spontaneously occurring behavioral acts of the primate Macaca fascicularis. The system, called PROBE (pattern recognition of behavioral events), is described in detail. In its present stage of development, PROBE classifies behavioral activity with a reliability comparable to trained human observers. The potential applications for and improvements to the PROBE system are discussed.Techniques used in computer graphics and pattern analysis have been applied to the tasks of observing, classifying, and recording spontaneous behavioral activities in the captive primate. The goal in designing this system was to provide a computer-based pattern recognition system capable of identifying normal activity patterns of the primate Macaca [ascicularis. If successful, this system can be used to detect alterations in these patterns under conditions of toxicologic and pharmacologic insult. In this article, the development of our system, which is called PROBE (pattern recognition of behavioral events), is discussed.Throughout this project, the purpose was to solve a unique application problem. Existing pattern recognition methods (Fu, 1968;Fukunaga, 1972;Pavlidis, 1977), consistent with the adequate solution of our problem, were applied. HARDWAREThe computer system consists of a Digital Equipment Corporation PDP-11/34 computer with a floatingpoint processor and 32K of core memory, a nine-track magnetic tape drive, card reader, line printer, console typewriter, Tektronics Model 4012 display scope, and a 44·million-word disk drive. A schematic view of the