Within mode quality and intensity changes of habituated stimuli

Decrement of the skin conductance response to repeated volitional inspiration

OBJECTIVE

To examine response decrement of the recently reported inspiratory skin conductance response (SCR) [Lim CL, Seto-Poon M, Clouston PD, Morris JG. Sudomotor nerve conduction velocity and central processing time of the skin conductance response. Clin Neurophysiol 2003;114:2172-80].

METHODS

Twelve healthy adult volunteers performed 3 tasks (A) a control task of maintaining tidal breathing and then two randomized tasks, (B) a deep inspiration to a target oral pressure and (C) tapping with a finger. Each task was performed 30 times on cue every 20s in 3 runs with 5 min of rest between runs. The SCR, oral pressure, airflow, inspired volume and cue signal were recorded continuously and analysed offline. SCR amplitude was logarithmically transformed and then statistically analysed, using a linear mixed effects model, as a function of run number, trial number and absolute error between target and actual oral pressures.

RESULTS

Inspiratory efforts elicited exponentially decreasing SCR amplitude with increasing trial number during each run (P < 0.0001). After adjusting for trial number, the mean SCR amplitude of the second and the third run were, respectively, 24.2 (95% CI (0.175, 0.336), P < 0.001) and 14.4% (95% CI (0.104, 0.200), P < 0.001) of the first run amplitude.

CONCLUSIONS

Volitional deep inspiration reliably activates an SCR that exhibits response decrement with repetition, which may be habituation.

SIGNIFICANCE

The volitional inspiratory SCR may assist in the assessment of sympathetic autonomic status in patients with peripheral afferent neuropathy.

Sudomotor nerve conduction velocity and central processing time of the skin conductance response

OBJECTIVE

To estimate the sudomotor nerve conduction velocity (CV), the central processing time (CPT) and habituation of the skin conductance response (SCR).

METHODS

SCRs in response to a single deep inspiratory breath, an electrical stimulus and a sound click were obtained from the fingers and toes of 30 healthy adults. Sudomotor nerve conduction velocities were determined after measuring extremity length and latency differences. CPT was estimated by subtracting the efferent time and the known afferent times and neuroeffector times from the onset latency.

RESULTS

The inspiratory SCR habituated slower than the auditory or electrical SCRs. CVs of the 3 modalities did not differ statistically and their mean was 1.07 m s(-1) (95% CI: 1.01-1.13). The inspiratory SCR arrived at the fingers 1.26+/-0.09 s after the onset of chest wall movement. Electrical and auditory SCR onset latencies at the fingers were 1.60+/-0.03 and 1.75+/-0.04 s, respectively. Their CPTs were 140 and 160 ms, estimated from the electrical and auditory SCR onset latencies to the fingers. The CPT for inspiratory SCR was estimated to occur during the inspiratory CPT after the inspiratory decision and before chest movement.

CONCLUSIONS

In contrast to the SCR following an electrical or auditory stimulus, initiation of deep inspiratory SCR occurs before the inspiratory act, precluding any possible input from respiratory afferent receptors and implicating a central generator.

SIGNIFICANCE

This study provides new insights into the origin of the SCR following inspiration.

The orienting response and stimulus significance: further comments

Siddle's (1979) assertion that the significance hypothesis is based on tautological argument regarding the effects of intensity decrease refer only to two papers which first suggested the need for research into determinants other than mismatch. Studies testing specific predictions followed in various laboratories, and it is their results that support the hypothesis. The hypothesis never depended on intensity decrease, and the charge of nonreplicability is incorrect. Siddle's argument concerning response stereotypy cannot account for the data showing response to significant 'novelty' coupled with nonresponse to nonsignificant 'novelty' in the same OR component, or for that showing consistent differences in various components between randomly selected groups. Arguments based on trait-lability are inadequate since tests of the lability hypothesis have confounded trait factors with the differences in stimulus assessment they hope to test. The significance hypothesis emphasizes stimulus information in the OR, significance serving to limit access to central processing channels to those signals whose information is relevant for the organism. OR components are seen as long-latency responses elicited after automatic early detection of uncertainty and significance, and reflect higher processing of significant information. A distinction between involuntary and voluntary phases of attention is acknowledged, but it is questioned whether OR components can be assigned to the former because of the frequent failure of 'automatic' ORs to innocuous stimuli, the long latency of most components, and failure to demonstrate structural differences between the classes of response.

The orienting response and stimulus significance: some comments

Bernstein (1969, 1979) has argued that Skolov's (1963) model is not sufficient to account for the data concerning orienting response (OR) elicitation to stimulus change. He has argued that following the registration of stimulus mismatch, an additional stage of stimulus evaluation occurs and that only stimuli which represent change and are judged as 'significant' elicit ORs. The present paper takes issue with this assertion. It is argued that the data on which the 'significance hypothesis' was originally based are not replicable and that the argument, as it was formulated originally, is tautological. It is also argued that individual differences in OR responsiveness are more parsimoniously explained in terms of individual response stereotypy and that differential responsiveness to different sorts of stimulus change poses problems for the significance hypothesis. Evidence is also reviewed which suggests that the relationship between stimulus change and stimulus significance is additive rather than multiplicative as suggested by Bernstein, i.e., stimulus change and stimulus significance are each sufficient for OR elicitation. Finally, it is concluded that the concept of 'significance', introduced operationally, may be of value in terms of examining the role of the OR in information processing.