Reflex inhibition and reflex strength

Abstract

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...

Enhancement and depression of tactile and acoustic startle reflexes with variation in background noise level

“…The intratympanic reflex cannot affect tactile input, nor can this stimulus be masked by the noise, since the two modalities excite different receptors and are processed through a different afferent system. It is possible that the motor overarousal hypothesis suggested by Ison and Reiter (1980) may play some additional role in determining reflex amplitudes at the very highest noise levels: It may be responsible, for example, for limiting the growth of the facilitative effect of noise beyond 90 dB for the electrotactile stimulus, but it cannot account for the early appearance of depression obtained with acoustic stimuli.…”

“…It may otherwise result as a refractory-like consequence of the motor activity observed with intense noise backgrounds (lson & Silverstein, 1978), because this activity may reduce the excitability of the motor neurons that also subserve reflex elicitation. This hypothesis was suggested by Ison and Reiter (1980) as part of a one-process arousal explanation of the biphasic inverted U. They proposed that initial facilitation may result because increased arousal prepares the motor system by a near-threshold excitation of motor units, which are then more sensitive to the startle stimulus.…”

Forward and backward classical conditioning of the flexion reflex in the spinal cat

“…This may be a result of teleologically derived processes designed to "protect" the nervous system from undergoing associatively produced changes by largely irrelevant, simultaneous sensory inputs that constantly occur in the environment. Such processes may function to supply brief inhibitory influences from one component of a sensory system onto other sensory and/or effector system components, and may be involved in other classes of observation, such as masking, blocking, overshadowing, and reflex inhibition (Estes, 1975;Ison and Reiter, 1980;Kling and Riggs, 197 1). As a result of such processes, the nervous system components responsible for associative learning changes are less likely to be influenced by sensory inputs with near-simultaneous onsets.…”

Reflex modification in the domain of startle: I. Some empirical findings and their implications for how the nervous system processes sensory input.

“…Ordinarily, startle is a motor reaction to sudden intense stimulation. For example, with 1 Under some conditions, particularly these involving preliminary stimuli of moderate strength, amplitude increases may also be seen (e.g., Ison, McAdam, & Hammond, 1973). 2 Under most conditions this change in amplitude is accompanied by an increase in latency.…”