Physiology & Behavior, Vol. 22, pp. 57-62. Pergamon Press and Brain Research Publ., 1979. Printed in the U.S.A.

Habituation and Sensitization in the Modulation of Reflex Amplitude I P A U L W E D E K I N G A N D P E T E R L. C A R L T O N "

Department o f Psychiatry, CMDNJ-Rutgers Medical School, Piscataway, N J 08854 ( R e c e i v e d 26 June 1978) WEDEKING, PAUL AND PETER L. CARLTON. Habituation and sensitization in the modulation of reflex amplitude. PHYSIOL. BEHAV. 22(1) 57-62, 1979.-- The interaction of decrement due to repeated presentation of an auditory eliciting stimulus with an ancillary visual stimulus that itself increased reflex amplitude was studied in 4 separate experiments. This interaction produced a multiplicity of complex reflex changes that cannot readily be accounted for by current conceptualizations of the processes of habituation (underlying reflex decrement) and sensitization (underlying reflex increment) that presumably modulate reflex change. Habituation

Sensitization

Startle reflex

Reflex

A STIMULUS that initially elicits a reflex can, with repeated presentation, come to lose its efficacy in eliciting that reflex. This waning of stimulus effectiveness, reflected in reflex decrement, has been termed habituation and has generally been viewed as an elemental form of nervous system plasticity. The hypothetical process that underlies this reflex decrement has also been referred to by the same term, habituation. Thus, there is potential confusion about the referrent of the term, observed phenomenon or inferred process. For this reason we have chosen to denote the phenomenon itself as reflex decrement (RD) and the hypothetical process as habituation. Stimulus presentation may also increase the amplitude of the elicited reflex. In one of the most important theories developed to account for this phenomenon of reflex increment (RI), as well as for RD itself, RI has been attributed to a process called sensitization, whereas RD has been attributed to the process of habituation [3]. These two processes have been postulated to be independent of one another. In addition, habituation (H) has been supposed to be a monotonic decreasing process, whereas sensitization (S) has been proposed to be a process that initially increases and subsequently decrements. Finally, H and S have evidently been assumed to summate in determining reflex amplitude. The empirical underpinnings of these two independent processes come from a series of experiments involving the recording of single unit activity within the spinal cord of the cat; some units show a waning of activity with repeated stimulation, whereas others show an initial increment followed by a decline. These units have been termed H and S units, respectively.

Although a linkage of the activity of these units to elicited, reflexive, muscular contraction has not been directly demonstrated, it is not entirely unreasonable to suppose that the activities of these units control the RI and RD observed with repeated elicitation of the reflex in question. What is less clear is whether the action of H and S can be directly applied to other instances of reflex elicitation in the intact animal as well as to behaviors that are emitted rather than elicited, as has been postulated by Thompson et al. [7]. One problem that inheres in such application is the fact that two hypothetical processes have been invoked to account for a single behavioral phenomenon. In the absence of independent measurement of at least one of these processes, virtually any set of data can be accounted for by the dualprocess theory. If, for example, we make the simplest assumption that a response (R) is an additive function of S and H, then the simplest form of interaction of these two becomes: R = S + H. But, if both S and H can be inferred solely from R, then a decrease in R with repeated stimulus presentations can be attributed to a greater role of H, an increase in R can be attributed to a greater role of S, and no change in R can he attributed to a complementary relation of the two; essentially all empirical outcomes can be "accounted for" in such a purely tautological manner. Thus, useful inferences about two processes can only be made on the basis of, not a single measure, but at least two. For example, because S is independent of H and can therefore be varied independently, it should be possible to estimate the role of this process alone in determining reflex amplitude. One procedure for independently varying S is based on the report by Groves and Thompson to the effect that intense

~This research was supported, in part, by General Research Support funds granted by the College of Medicine and Dentistry of New Jersey--Rutgers Medical School to Dr. Peter L. Carlton. "Requests for reprints should be sent to Dr. Peter L. Carlton, Department of Psychiatry, CMDNJ--Rotgers Medical School, Piscataway, New Jersey 08854.

C o p y r i g h t ,c 1979 Brain R e s e a r c h Publications Inc.m0031-9384/79/010057-06502.00/0

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WEDEKING AND CARLTON

visual stimulation can increase the startle response of the intact rat. Because this increase was interpreted by them as being due to S, any difference between the reflex magnitudes of animals receiving such stimulation and animals not receiving it can presumably be attributed to that process. This prospect was examined in the experiments reported here.

EXPERIMENT 1 Experiment 1 was concerned with the effects of a continuously presented visual stimulus on reflexes elicited by repetitive auditory stimulation. METHOD

GENERAL METHOD Anhnals

Experimentally naive, male albino Charles River rats (Charles River Distributors, Wilmington, MA), weighing between 200-350 g, were used. The rats were housed individually and were under continuous illumination; food and water were available ad lib except during the experimental session. Apparatus

During an experimental session a rat was placed in a clear, perforated Plexil~as chamber (21 x 8.5 x 8.5 cm) having a grid floor. The chamber was bolted onto a platform, which was mounted on the floor of a ventilated, light- and soundattenuated Industrial Acoustics Chamber (IAC), via 4, partially compressed, automotive-valve springs. An 8 ohm speaker, mounted on the ceiling of the IAC chamber, was used to deliver a constant background white-noise (82 dB). A 60 cm Altec midrange driver and horn (Model 27A), through which the stimulus for eliciting the startle reflex was delivered, was mounted behind the Plexiglas chamber. The stimulus consisted of a 100 msec presentation of an 8 K Hz tone of 110 dB intensity (re. 0.0002 dynes/cm") with a risedecay time of

Habituation and sensitization in the modulation of reflex amplitude.

Physiology & Behavior, Vol. 22, pp. 57-62. Pergamon Press and Brain Research Publ., 1979. Printed in the U.S.A. Habituation and Sensitization in the...
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