A. Jean Ayres (1964, 1972, 1979) first
identified characteristics of defensiveness related to aversive responses to tactile input, describing it as “the tendency
to react negatively and emotionally to touch sensations” (Ayres, 1979, p. 107). She noted a connection between hyperactivity
and tactile defensiveness (Ayres, 1972, 1979). Ayres believed that tactile defensiveness was associated with tactile discrimination
difficulties (Ayres, 1972).
Knickerbocker (1980) explored this in greater
detail, using the more general term sensory defensiveness. Current thinking of the time identified this disorganized response
to sensory input as resulting in an imbalance of central nervous system inhibition and excitation. This was then believed
to result in decreased inhibition which resulted in large amounts of input reaching higher CNS structures and subsequent defensive
behaviors (Knickerbocker, 1980; Larson, 1982; Fisher & Dunn, 1983). Knickerbocker, as did Jean Ayres before her, noted
defensiveness specifically in three system: the olfactory, tactile and auditory systems; and labeled this the OTA triad (Knickerbocker,
1980, Ayres, 1972). She further notes that defensive individuals demonstrate increased activity, distractibility, as well
as hyper-verbal and disorganized behavior. Knickerbocker also noted a category called sensory dormancy which appeared to demonstrate
increased central nervous system inhibition and decreased sensory arousal, with individuals appearing more quiet and compliant.
In 1983, Fisher & Dunn addressed the
relationship between tactile defensiveness and tactile discrimination suggesting that they two separate disorders and not
a continuum and while they may present together, they often present in isolation (Fisher & Dunn, 1983).
By the late 1980’s a continuum was
proposed with sensory defensiveness at one end of the continuum and sensory dormancy at the other (Cermack, 1988; Royeen,
1989; Royeen & Lane, 1991). It was noted that this continuum may present with a circular function for some individuals
where environmental sensory input can bring about changes between one extreme and the other, sensory defensiveness and sensory
dormancy. Lai, Parham & Johnson-Ecker (1999) looked at the relationship between these two disorders and suggested that,
while there appears to be a “significant association”, the evidence is insufficient to suggest that they are “expressions
of the same modulation disorder” (p. 4).
Winnie Dunn, author of the ‘Sensory
Profile’ (1999) developed a conceptual model of sensory processing linking neurological threshold to their manner of
responding behaviorally (Dunn, 1997; Dunn, 1999 Dunn, 2001). She described an under-responsive child as having a high threshold
and an over-responsive child as having a low threshold. Children who are over-responsive to sensory input, i.e. sensory defensiveness,
are described as ‘sensory sensitive’ or ‘sensory avoidant’ (Dunn, 1999). Those classified as sensory
sensitive are believed to give in to low thresholds and demonstrate behavior that appears hyperactive or distractible. Those
classified as sensory avoidant are believed to attempt to avoid activities that activate their threshold and may avoid participation.
Both groups of children may be identified
as being sensory defensiveness because they react negatively to sensations that are generally considered non-noxious through
fight-flight-fright or withdrawal and/ or avoidant behaviors.
While these developing theories have been
beneficial in increasing the growing knowledge in the area of sensory modulation disorders and sensory defensiveness in particular,
current understanding is that the processes that these authors are attempting to conceptualize are far more complex and multidimensional
than linear models suggest (Dunn, 1997; Parham & Mailloux, 1996; Wilbarger & Wilbarger, 1991; Wilbarger & Murnan-Stackhouse,
1998). The presentation of a continuum of over-responsiveness and under-responsiveness does not account for this complexity
and therefore is no longer regarded as an adequate explanation of modulation (Lane, 2002). Furthermore, previous suggestions
of a dichotomy with sensory dormancy at one end and sensory defensiveness at the other end are no longer appropriate explanations,
as evidence is beginning to demonstrate that these difficulties actually occur in different CNS mechanisms (Wilbarger, 1998).
The clinical experience of authors suggest
that sensory modulation disorder is multidimensional and not a continuum (Dunn, 1997; Parham & Mailloux, 1996; Wilbarger,
1993; Wilbarger & Wilbarger, 1991; Wilbarger & Stackhouse, 1998).
Wilbarger & Stackhouse, 1998, in the
conclusion to their review of literature related to sensory modulation, address this limitation:
While the linear continuum models that have been the dominant means of conceptualizing SMD served
an important purpose, it is apparent that the nervous system mechanisms that support modulation are many, varied and complex.
As such, a new model that captures the dynamic, interacting networks is required.