Retroactive enhancement of a skin sensation by a delayed cortical stimulus in man: Evidence for delay of a conscious sensory experience

Sensation elicited by a skin stimulus (S) was subjectively reported to feel stronger when followed by a stimulus to somatosensory cerebral cortex (C), even when C was delayed by up to 400 ms or more. This expands the potentiality for retroactive effects beyond that previously known as backward masking. It also demonstrates that the content of a sensory experience can be altered by another cerebral input introduced after the sensory signal arrives at the cortex. The long effective S-C intervals support the thesis that a duration of cortical activity of up to 0.5 s is required before awareness of a sensory stimulus is developed.     

Apparent fluctuations of a sensory threshold

An account is given of a method for the nearly continuous recording of sensory thresholds and other psycho-physical variables. Its application to the difference-threshold for sound-intensity is described. It is found that this threshold, so recorded, shows irregular fluctuations in time, excursions of up to 100 per cent, being commonly present. There is no prima facie indication of any regular periodicity in the fluctuations. No evidence was found of any general trend in amount of fluctuation throughout a half-hour period of observation. Nor, except in one subject, was there any tendency for the amount of fluctuation to increase or decrease over a series of six half-hour periods of observation. There are marked, and fairly consistent, differences between individual subjects as regards the amount and pattern of fluctuation. The possible locus and causes of these effects are briefly considered.     

Changes in threshold level produced by a signal preceding or following the threshold stimulus

In a previous experiment (Howarth and Treisman, 1958) it was shown that when a warning preceded a stimulus by a fixed interval, the threshold level for the stimulus was higher the longer the fixed interval. A model of this effect was proposed, assuming that the threshold criterion adopted by the subject at any moment was modified by his estimate of the probability of a stimulus at that moment.

Three predictions are derived and tested. It is found that: the standard deviation of the response is constant despite the shifts in threshold induced; the threshold continues to fall as the interval from warning to stimulus is decreased to zero, with the lowest threshold when the warning signal and the stimulus are simultaneous; and that a warning signal following the stimulus by an interval of less than 0 5 to 1 sec. also lowers the threshold.     

The influence of stimulus scale-interval upon the assessment of pain threshold

An experiment concerning the influence of the scale of stimulus values upon the perception of heat-pain is reported in which it is found that the value of the threshold stimulus is dependent upon the size of the steps between successive stimuli.

The results are analysed in the light of a paper by Brown and Cane (1959) in which they point out that the value of a sensory threshold yielded by the Limiting Method is mathematically dependent upon the size of the steps between successive values of the variable stimulus. The threshold values reported here are found to be dependent on the step-size between stimuli to a greater extent than that which would be predicted by Brown and Cane.

In view of these results, an attempt is made to explain the wide variety of pain threshold values reported in the literature.     

Retroactive semantic priming in a lexical decision task

The present study reports two experiments that required subjects to name target items preceded by a masked prime. Additionally, and subsequent to the naming task, subjects were required to indicate whether or not the prime was a word, along with a confidence rating of their lexical decision. Experiment 1 demonstrates that the processing of masked primes is facilitated by related targets when such targets are presented either 100 or 200 msec after the onset of the prime. Experiment 2 extends the finding of “retroactive” priming to a 1000=msec separation in prime-target presentation (SOA). The extent of retroactive priming is not dependent on SOA between prime and target, nor is it affected by the prime-mask SOA, which varied from 10 to 180 msec. Priming of targets was also independent of prime-target and prime-mask SOA, providing that primes had been classified as words. For word primes classified as non-words there was no semantic priming on target naming reaction time. Implications of these findings with respect to the nature of retroactive priming and the current controversy concerning subliminal priming effects were discussed.     

Electrocutaneous stimulation L The effects of stimulus parameters on absolute threshold

Thresholds for detecting electrical stimulation were measured as a function of (1) body locus, (2) electrode configuration, (3) stimulus waveform, and (4) pulse duration. The results were: (1) the forehead gave slightly, but not reliably, lower thresholds than the abdomen; (2) concentric electrodes gave slightly, but not reliably, lower thresholds than unifocal electrodes; (3) cathodal monophasic (?) pulses and biphasic pulses (+/? and ?/+) gave identical thresholds, while anodal monophasic (+) pulses gave higher thresholds; and (4) thresholds decreased as pulse duration increased up to .5 msec, but changed less with longer pulses.     

Retroactive adjustment of perceived time

Accurately timing acoustic events in dynamic scenes is fundamental to scene analysis. To detect events in busy scenes, listeners must often identify a change in the pattern of ongoing fluctuation, resulting in many ubiquitous events being detected later than when they occurred. This raises the question of how delayed detection time affects the manner in which such events are perceived relative to other events in the environment.To model these situations, we use sequences of tone-pips with a time–frequency pattern that changes from regular to random (‘REG–RAND’) or vice versa (‘RAND–REG’). REG–RAND transitions are detected rapidly, but the emergence of regularity cannot be established immediately, and thus RAND–REG transitions take significantly longer to detect. Using a temporal order judgment task, and a light-flash as a temporal marker, we demonstrate that listeners do not perceive the onset of RAND–REG transitions at the point of detection (∼530 ms post transition), but automatically re-adjust their estimate ∼300 ms closer to the nominal transition.These results demonstrate that the auditory system possesses mechanisms that survey the proximal history of an ongoing stimulus and automatically adjust perception to compensate for prolonged detection time, allowing listeners to build meaningful representations of the environment.     

Effect of contralateral shock stimulation on laterality differences in the two shock threshold

Contralaterally presented shock was found to bring about marked changes in two shock threshold determined for the right finger but not for the left finger. The results are discussed in terms of a suggestion that the level of excitability of the left hemisphere may be more easily disturbed than that of the right hemisphere.     

Lowering of an auditory threshold by a near threshold warning signal

Following previous experiments on the effect of suprathreshold “warning” signals in lowering the threshold for stimuli in another modality, we have used visual “warning” signals which are themselves within the threshold range. These near threshold “warnings” appear to act in exactly the same way as suprathreshold “warnings” provided they are seen. Very weak “warnings” have a reduced effect, probably because they are not always seen. The experiment is of interest in that it supports the theory that the effect is due to the “warning” signal reducing the subject's uncertainty about when the threshold stimulus will occur, and secondly because it indicates that under these conditions at least, the subject can attend effectively to two simultaneous near-threshold stimuli.     

Control of avoidance by a response-produced stimulus

Three pigeons were trained to press a foot pedal to postpone electric shock. The safety signal was a tone of 1000 Hz, which sounded for 5 sec following each press; silence therefore served as a warning signal. After the performance approached asymptote, test sessions were interspersed among the training sessions. Following a warm up, the shock was omitted (extinction), and no tone or a tone of 250, 500, 1000, 2000, or 4000 Hz was presented following each press for the remainder of the session. Gradients of generalization plotted in terms of the number of times the tone was produced on each test peaked at 1000 Hz, indicating that the performance was controlled by the frequency. Since this dimension is orthogonal to absence of tone, the gradient cannot be attributed to differences in the magnitude of change in the warning signal. It was concluded that response-produced stimuli reinforce avoidance behavior.     

Difference threshold for stimulus length under simultaneous and nonsimultaneous viewing conditions

The present study was concerned with Weber’s Law as it is related to the discriminability of the lengths of lines. Experiments were conducted to investigate three questions: (I) Is Weber’s Law equally applicable to simultaneous and nonsimultaneous viewing conditions? (2) Is the relationship between the stimulus sizes and the values ofDLs described more adequately by the function proposed by Weber or a generalized Weber’s Law stated by Miller? and (3) Is Weber’s Law better approximated by proximal or distal size? It was demonstrated that the discriminability of the lengths of lines follows Weber’s Law under the nonsimultaneous viewing condition, but not under the simultaneous viewing condition. Under the nonsimultaneous viewing condition, it was noted that the generalized Weber’s Law as stated by Miller described the relationship between the DL and stimulus size significantly better than the function proposed by Weber. From the results pertaining to the third question, it was not possible to determine whether the proximal or the distal size follows Weber’s Law more closely.     

The effect of stimulus amplitude on the threshold of fusion of paired flashes

Four experiments are described in which the threshold of fusion of paired flashes was measured. The intensity of the single flashes making up the pairs was varied independently. The results showed that when the first flash of the pair was the more intense, the threshold of fusion increased linearly as the difference in the log intensity of the two flashes. A proposal put forward to explain the results suggested the “noise” created by the first flash of the pair has to dissipate to a critical extent to allow the “signal” arising from the second flash to be detected.     

Retroactive inhibition as a function of the locus of categorized response sets

The effects which list-level taxonomic attributes should have upon retroactive inhibition (RI) producing mechanisms in both response-set interference theory (Postman & Stark, 1969) and Petrich’s (1975) retrieval model are discussed. A test of the theories was performed by manipulating the presence (1) or absence (0) of categorized response sets in both original learning (OL) and interpolated learning IIL), and comparing their predictions concerning the relative degree of RI expected in the four conditions (0-0, 0-1, 1-0, 1-1). In line with Petrich’s predictions, OL categorization significantly reduced RI, IL categorization significantly increased RI, and 0-1 produced significantly greater RI than 1-0. Moreover, the obtained ranking of the means for the experimental groups in degree of RI (0–1>0-0>1-0 ? 1-1) was in close agreement with the ranking predicted by Petrich’s (1975) model. The results provide support for Petrich’s (1975) retrieval model and pose explanatory problems for response-set interference theory (Postman & Stark, 1969).     

Retroactive inhibition as a function of similarity of meaning in free-recall learning

Summary Retroactive inhibition in free-recall learning was measured under six degrees of meaningful similarity: zero, low, low-medium, medium, medium-high, and high. For each degree of similarity, an experimental and a control group of 20 Ss each learned a list of 20 unrelated adjectives through three cycles of alternate study and unpaced free recall. Immediately thereafter, the experimental group learned a second list of 20 adjectives, while the control group rested. Finally, a single recall trial was administered to both groups to measure first-list retention. The results have revealed a relationship which is at variance with the relationships observed in both serial and paired-associate learning but conforms to the curvilinear function earlier formulated by Skaggs and Robinson. Fresh arguments are offered in defence of the Skaggs-Robinson hypothesis.