Psychological research is, whenever possible, guided by theory. However, stringent hypothesis testing or reasonable practical application of a theory is often reliant on good indicators of processes or latent variables mentioned in the theory. In search of such indicators, a great deal of work has been invested by the international research community. But even in a field of research as fundamental as the investigation of orienting, it has not yet been convincingly established empirically whether two well-known autonomic responses to novel stimuli—the skin conductance response (SCR) and the heart rate (HR) response—indicate the brain activity of the orienting mechanism in an unambiguous and comparable manner. This mechanism was introduced to psychology years ago by an influential theory of orienting (cf. Sokolov, 1960
) and it is still crucial to the understanding of human behaviour and learning. For this reason, we address the question of whether these two peripheral responses similarly reflect novelty detection
, the theoretical basis or key process for the triggering of the involuntary orienting response
(OR). In this vein, we are interested in whether both of them are valid indicators of novelty detection and thus indicate involuntary orienting straightforwardly and independent of voluntary orienting.
, i.e., detection of a contextually or actually novel event, is typically linked to avoiding dangers, adapting to environmental changes and improving one’s knowledge. A novel stimulus by definition evokes the involuntary
OR, an evolutionarily arisen response mechanism that ensures that processing resources are automatically focussed on unknown events to incorporate them into an enhanced representation of the world (Lynn, 1966
; Pavlov, 1927
). Novelty is also considered to trigger involuntary orienting because of an inherent and thus unconditioned signal value—that is, unfamiliar stimuli point to potential but vital consequences that must be attended to (Campbell et al., 1997
). As soon as the OR releasing stimulus occurs repeatedly without any consequences it becomes familiar and its relevance for improvement of perception and further processing decreases rapidly. This selective adaptation is one of the most elementary forms of learning and memory (Ranganath & Rainer, 2003
is thus driven by novelty detection and inevitable coupled with an automatic or passive shift of attention because attentional control is in a sense captured by novelty (Johnston et al., 1990
). Nevertheless, involuntary orienting may subsequently be replaced by voluntary orienting
as soon as attention is controlled by recognising clues for a significant event that is relevant to goals or actions. The conception of two different operating or control modes underlying different orienting activities originates in the classical distinction between two basic varieties of attention
: (i) passive, non-voluntary, effortless attention; and (ii) active and voluntary attention (James, 1890
). In other words, it is taken for granted that selective processing can be performed passively and actively (Johnston & Dark, 1986
, p. 63). It is, however, important to bear in mind that attention (selection) is not orienting, and that a different attentional control as well as its triggering is only one defining feature of the differentiation between involuntary and voluntary orienting. Other distinctive features rest upon the involved processing operations and, what still has to be proved (see below), their consequences in peripheral response systems.
Thus, orienting is thought to be a brain response to novelty, significance, as well as an amalgam of both (e.g., Bernstein, 1969
; Dawson et al., 1989
; Maltzman, 1979
; Pendery & Maltzman, 1977
; Sokolov, 1963
). Some of the significant stimuli already have an inherent biological signal value, others acquire their signal value by conditioning or simply by instruction.
With this in mind, our current OR paradigm—a special variant of the well-known repetition-change paradigm—was implemented to examine the effects of novelty and signal value on two presumable response manifestations of the OR, the skin conductance response and the heart rate response. To elicit an involuntary orienting response
, a pure novelty OR, an OR associated with the involuntary and passive shift of attention, a contextually salient and unexpectedly occurring novel—and consequently unfamiliar—sound (change condition) was presented after a series of six familiar auditory stimuli. These familiar stimuli had a crucial feature in common, they belonged to a conceptual category (conceptual repetition condition): all of them were one-digit numerals. Moreover, independent of familiarity, the first stimulus in this series, in particular, can be regarded as contextually novel within the context of the particular experimental setting. This special event is thus expected to serve as a stimulant to another kind of novelty OR, an OR to contextual novelty
. Beyond that, the present paradigm differs from classical variants especially by assigning a special signal value to some of the familiar stimuli that promotes voluntary orienting
. Signal value
was established by an instruction that prompted participants to pay close attention to three out of the six one-digit numerals (targets) and their immediate consequences. This instruction takes account of the hypothesis that the “significance contribution to the OR is achieved via frontal lobe activation of voluntary attention” (Sokolov, 1990
, p. 99), and it aims to emulate exploration behaviour, a striking element of orienting outside the laboratory world (Berlyne, 1966
; Daffner et al., 1998
In a typical OR paradigm an exemplary OR manifestation is expected to show the unique features of the involuntary OR, response habituation
and response recovery
; Sokolov, 1963
). Response habituation is the expected and usually exponential amplitude decline with repeated stimulation. Response recovery is the subsequent increase in amplitude after a distinguishable change in stimulation. If this recovery does occur, the preceding response decline can be interpreted as a sign of a selective
central nervous system (CNS) inhibition process (e.g., Sokolov, 1960
; Voronin & Sokolov, 1960
), because a generalised CNS process such as fatigue is incommensurate with response recovery. This selective inhibition process is usually called (inferred process of) habituation.
By definition, every unequivocal manifestation of Sokolov’s involuntary OR (e.g., 1963
) needs to show response habituation and
response recovery. Sokolov (e.g., 1963
) even emphasised that the effects of repetition and change should appear in all
OR manifestations in a comparable manner
To explain the selective nature of habituation, Sokolov (1963
) introduced the concept of a “neuronal model of the stimulus”. Each occurrence of a redundant event increases the precision of this model and thus the (selective) inhibition of the OR, while the occurrence of a discrepancy, i.e., of a mismatch between stimulus and model, will again trigger an OR. The greater the mismatch, the larger will be the response recovery (Voronin & Sokolov, 1960
Referring to the question of whether the HR response is a manifestation of the involuntary OR pending issues mainly stem from conflicting empirical findings. Although, at times the HR exemplarily shows response recovery indicating (renewed) novelty processing (e.g., Bohlin et al., 1981
; Simons et al., 1987
, experiment II; Turpin et al., 1999
; Vossel & Zimmer, 1989a
; Zimmer, 2002
), the proofs about response habituation under pure stimulus repetition have been inconsistent (e.g., Barry, 1986
; MacDonald et al., 2015
; Simons et al., 1987
; Turpin et al., 1999
; Vossel & Zimmer, 1989a
). For a certain HR response—a distinctive deceleration—Zimmer (2002
) even found response habituation as well as
response recovery. However, considering the empirical contradictions, a clear link between the OR and HR deceleration is highly questionable. Possible HR modulations, which could result from various processing operations in different phases of processing, also impede the assumption of a simple relation (see below).
Resting upon the observation that recovery of the cardiac response is occasionally (a) a rapidly developing, (b) very pronounced and (c) relatively long-lasting slowing of the heartbeat frequency (cf. Bohlin et al., 1981
; Vossel & Zimmer, 1989a
; Zimmer, 2002
), it may be concluded that it consists of two or three phases of HR deceleration with different starting times: (i) a brief, rapid-onset deceleration or primary bradycardia
(Lacey & Lacey, 1980
) representing stimulus registration (Graham, 1992
), (ii) a second deceleration
reflecting involuntary orienting (e.g., Graham & Clifton, 1966
; Turpin, 1983
), and possibly (iii) a third deceleration
reflecting voluntary orienting activity associated with supervisory attentional control of processes involved in anticipation and preparation (Bohlin & Kjellberg, 1979
; Damen & Brunia, 1987
; Lacey & Lacey, 1978
Referring to a more recent concept of preparation, findings suggest that preparation is a set of processes (Jennings & Van der Molen, 2005
) and that transient heartbeat slowing is indicative of inhibitory processes necessary for an appropriate task-based preparation (Jennings & Van der Molen, 2002
). A task-based HR deceleration may thus be a useful psychophysiological window into the operating principle of the supervisory attentional system (Jennings & Van der Molen, 2002
, pp. 337–340).
In the transitional phase (involuntary orienting) between stimulus registration and voluntary orienting, various processes are likely to be triggered by novel stimuli. These may either (a) support slowing of the heartbeat frequency. Or, (b) depending on stimulus features (especially rise time and intensity; cf., e.g., Cook & Turpin, 1997
), signal value (e.g., Öhman et al., 2000
; Walter & Porges, 1976
) and, most importantly, the central processing requirements associated with the particular signal value (Graham, 1992
; Jennings, 1986
; Lacey, 1967
; Simons et al., 1998
), they might drive the heart rate up even above baseline, so that the cardiac response finally has a three-phase structure.
In the case of a novel event without any task or goal relevance and of moderate intensity and rise time, one of these processes, an automatic and consequently passive call for processing
(Öhman et al., 2000
), is assumed to trigger the slowing of the heartbeat frequency. This deceleration of the HR is presumably a pure novelty-dependent deceleration
because novelty detection,
the theoretical CNS core component of involuntary orienting
(e.g., Graham & Hackley, 1991
; Öhman et al., 2000
), is the presumed cause of this call. Duration of this “novelty deceleration”
is assumed to be longer than the primary bradycardia and shorter than a chronologically third deceleration, which we associate with voluntary orienting. Just like the latter, a novelty deceleration
is directly connected with reduced somatic activity and improvement of perception (cf. Öhman et al., 2000
, pp. 552–556).
(A) In light of the above considerations regarding the component structure of the cardiac response and its susceptibility to various CNS processes, we hypothesize (I) that the reaction of the heart to our task-irrelevant and definitely unknown stimulus is a very pronounced, rapidly developing and relatively long-running HR deceleration, as conditions causing accelerative tendencies were non-existent, whereas at least two CNS causes of deceleration were given—(1) stimulus registration and, most importantly, (2) the call for processing.
In contrast to this clear-cut deceleration, we expect (II) the general cardiac response to our familiar stimuli to have a three-phase structure, consisting of a transient and purely stimulus-driven deceleration, followed by an acceleration and another deceleration. Regarding (a) the task-relevant familiar stimuli (targets), we assume that the acceleration and the ensuing deceleration are due to the particular signal value of the stimuli and thus to their demands on central processing (effortful memory processes and decision making) and active attentional processes (concerning perception and anticipation). In terms of (b) the remaining familiar stimuli (non-targets), the same (three-phase) response structure is expected to occur but to a lesser extent—due to lower demands on the central processing and the active attentional processes. The reason for these lower demands is that further effortful processing and supervisory attentional control are no longer necessary once a stimulus has been identified as a non-target.
However, if the cardiac response to the stimuli of our repetition-change paradigm were (merely) a pure indicator of novelty processing, it would have to be a strong and clear-cut novelty-dependent HR deceleration, behaving like an exemplary indicator of involuntary orienting.
(B) The skin is expected to respond to the novel change as well as to the preceding familiar stimuli in almost the same manner—an increase in conductance. This increase in skin conductance, the SCR, is expected to reflect contextual novelty (in the present case, particularly the OR to the first stimulus), actual novelty (in the present case, the recovery of the OR to the novel change), as well as the CNS process of habituation (as a result of conceptual repetition).
Specifically, conceptual repetition is expected to decrease the SCR and novelty as well as signal value (voluntary orienting) are expected to increase the SCR. In addition, response habituation, i.e., the decrease in the SCR with repeated conceptual stimulation, is expected to follow an exponential course.
No sound hypothesis can be formulated about the interactive effect of signal value and repetition on the amplitude of the SCR as well as on the component structure of the HR response, in particular because it is not clear whether voluntary orienting, as implemented in the present study, habituates.