What is Sensory Integration?
Sensory integration is our ability to take in and utilize information from our senses to respond to the demands of our environment. We receive this sensory information constantly, and our brains must be able organize it effectively to allow us to function successfully in our day-to-day lives, making sensory integration a vital part of our nervous system functioning.

When we talk about ‘sensory information’ we are referring to the information we receive from not only our five ‘main’ senses – touch, sight, hearing, taste, and smell – but also two other ‘hidden senses’ known as proprioception, our sense of body awareness, and vestibular, our sense of movement and balance. Our seven senses work together to make up the three major components of sensory processing: sensory modulation, sensory discrimination, and praxis.
About the Senses
• Our sense of touch, called the tactile system, gives us information such as the shape, size, and texture of objects, which helps us better understand our environment and enable our recognition of how to manipulate objects. It is also vital to the development of body schema and emotional regulation.

• Our hearing, or auditory system, allows us identify the quality and direction of sounds. It allows us, for example, to recognize without looking that a person is talking, who that person is, who they are talking to, and how far away they are.

• Our sense of sight, the visual system, is critical to identifying and interpreting shapes, colors, symbols, body language, and more. In turn, our interpretations of these visual cues guides our movements and social interactions.

• Taste and smell, the gustatory and olfactory systems, work together to allow us to enjoy good and safe food while making us react negatively to unpleasant or dangerous sensations.

• The proprioceptive system is our sense of body awareness. It takes the information we receive from our muscles and joints to help us identify our body’s position in relation to itself and other objects.

• And finally, located in our inner ear is the vestibular system, our sense of movement and balance. This system tells us about the position of our head in relation to gravity, and the speed and direction of movement.
About the Components of Sensory Processing
Sensory modulation is our ability to take in sensory information, decide what is relevant, and to make an appropriate behavioral response.

Sensory discrimination allows us to understand the specific qualities of the information derived from sensory modulation. For example, the texture of an object or the direction of a sound.

Praxis, in the context of sensory integration, is the ability to conceptualize, plan, and sequence the steps of a motor task to adaptively meet environmental demands.
These basic concepts form the foundation of the sensory integration theory, and from there we can begin to understand the concept of sensory integration dysfunction.
What is SPD?
Sensory processing is the term we use to describe how our central nervous system manages sensory information. Sensory integration, sensory discrimination, and sensory modulation are all a part of sensory processing. Sometimes the central nervous system is unable to manage sensory information effectively to the point of delaying foundational skills and interfering with an individual’s daily life; we call this condition Sensory Processing Disorder, or SPD. While ‘Sensory Processing Disorder’ is becoming the standard description for this condition in both medical and research circles, you may also see the terms Sensory Integration Dysfunction (SID), Dysfunction in Sensory Integration (DSI), and Sensory Processing Dysfunction (SPD); all these terms refer to the same condition.
What Does SPD Look Like?
Though there are common clusters of symptoms, there is no universal “SPD Experience.” Symptoms exist on a sort of spectrum, varying greatly in type and severity across people of all genders, ages, and races.

Like sensory processing itself, Sensory Processing Disorder can be broken down into three different components: discrimination problems, modulation problems, and praxis problems. Each of our seven sensory systems has both a modulation and a discrimination component, which in turn impact our praxis skills in different ways. Sensory Processing Disorder may affect only one sense, but just as often will affect two or more, or even all seven, possibly in different ways. One person, for example, may only have visual discrimination problems, while another person may have the same problem in addition to auditory modulation difficulties. Yet another person may have praxis difficulties as a result of visual processing problems, but may not otherwise have significant visual discrimination or modulation problems. SPD is often, but not always, comorbid with other diagnoses that present with sensory related problems, such as attention deficit/attention deficit hyperactivity disorder, autism, and various learning disabilities.

More information about Sensory Processing Disorder is available on the Spiral Foundation's SPD Toolkit pages.
What Causes SPD?
Pilot studies suggest a variety of factors that may lead to sensory integration problems later in life, including  genetics, prematurity, birth trauma, and environmental factors, however there is no conclusive research in this area at this time. We just don't know yet, but. The citations below lead to a few articles, some of which are available for free, on some of the currently available research:

Ben-Sasson, A., Carter, A. S., & Briggs-Gowan, M. J. (2010). The development of sensory over-responsivity from infancy to elementary school. Journal of Abnormal Child Psychology, 38 (8). doi: 10.1007/s10802-010-9435-9.

Ben-Sasson, A., Carter, A. S., & Briggs-Gowan, M. J. (2009). Sensory over-responsivity in elementary school: Prevalence and social-emotional correlates. Journal of Abnormal Child Psychology, 37 (5). doi: 10.1007/s10802-008-9295-8.

Davies, P. L. & Gavin, W. J. (2007). Validating the diagnosis of sensory processing disorders using EEG technology. American Journal of Occupational Therapy, 61. doi:10.5014/ajot.61.2.176.

Gill, S. V., May-Benson, T. A., Teasdale, A., & Munsell, E. G. (2013). Birth and developmental correlates of birth weight in a sample of children with potential sensory processing disorder. BMC Pediatrics, 13, 29. doi:10.1186/1471-2431-13-29.

Goldsmith, H. H., Van Hulle, C. A., Arneson, C. L., Schreiber, J. E., & Gernsbacher, M. A. (2006). A population-based twin study of parentally reported tactile and auditory defensiveness in young children. Journal of Abnormal Child Psychology, 34(3), 393–407. doi:10.1007/s10802-006-9024-0.

May-Benson, T. A., Koomar, J. A., & Teasdale, A. (2009). Incidence of pre-, peri-, and post-natal birth and developmental problems of children with sensory processing disorder and children with autism spectrum disorder. Frontiers of Integrative Neuroscience, 3 (31).

Owen, J. P., Marco, E. J., Desai, S., Fourie, E., Harris, J., Hill, S. S., & Mukherjee, P. (2013). Abnormal white matter microstructure in children with sensory processing disorders. NeuroImage : Clinical, 2, 844–853. doi:10.1016/j.nicl.2013.06.009.