In the HAAT (human - activity - assistive technology) model presented in Module 1, the human was considered the "intrinsic enabler." In ergonomics, the human is often called the "human operator." In rehabilitation, the human is often called to "patient" or "client." In usability engineering, the human is called the "user." This module provides a perspective that is complementary to these but more of a "systems" perspective: the human as an "information processor."

From this perspective, the human system can be viewed as consisting of three components: sensors, central processing, and effectors:

Sensors: Input from sensors enables the system to obtain information from the environment. This is accomplished through sensory neurons, which have nerve endings embedded within tissue that function as sensory transducers for physical phenomena such as light, sound, pressure, temperature and length. Key considerations are sensitivity (minimum detectable level) and range (modes of variation in the phenomena that is measureable). Some assistive technologies are specifically designed to compensate for impaired sensory function (e.g., hearing aids, reading systems). Specific types of sensors will be covered in subsequent sections within this module.

Effectors: The observable output of the human system is through neuromusculoskeletal elements that provide movement (e.g., mobility of the human, postural adjustments, manipulation of objects, speech, head and eye movements). The "final common pathway" of the central nervous system (CNS) are the motor neurons, which provide excitation to muscles that serve as the actuators that produce (length-sensitive) forces that then act on skeletal structures to enable postural maintenance and movement (or motor output). Limitations can arise from impairments to neuromusculoskeletal elements (e.g., disease at neuromuscular junction, muscle injury, bone fracture). Effectors often provide motor outputs that can be used to control assistive technology systems (e.g., hand movements acting on joystick to control powered wheelchairs). In some cases such assistive technologies help the human compensate for an impairment to the effectors (e.g., muscle weakness).

Central Processing: In the "human as information processor" model, located between the sensors and effectors are central processing capabilities that are commonly broken into the functions of perception, cognition, motor planning and movement control, and memory. Interneurons serve as the physical units for this capability, and indeed constitute the vast majority of neurons.

• Perception: Interpretation and assignment of meaning to data received from sensors, through integration of information from sensors and memory.
• Cognition: The processes of problem-solving, decision-making, language, and other aspects of integrative performance based on cognitive function.
  • Problem-solving and decision-making represent a complex phenomena, and a useful sequence of steps bears resemblance to steps common in the engineering analysis and design process: problem recognition, problem definition, goal definition, strategy selection, alternative generation, alternative evaluation and alternative selection and execution.
  • Language is a form of communication that is commonly considered to consist of five basic elements (and associated rules): phonology (sounds used in a certain language and rules for their organization), morphology (rules for organizing the smallest meaningful units of language), syntax (rules for organizing words into measningful utterances), semantics (relationship between words and their meaning), and pragmatics (relationship between language and its users).
• Motor planning and control: the result of integration of sensory, perceptual and cognitive components into a motor pattern that is executed by the effectors: motor planning is a CNS process by which purposeful movements are executed to accomplish a purposeful task; motor learning occurs via practicing motor tasks and using sensory information to adjust central processing units such as neuronal synapses; motor control is the result of integration of sensory, perceptual and cognitive components into motor activity (motor pattern, motor sequence) that is executed by the effectors.
• Memory: Human memory is often considered as having three components:
  • Sensory memory (duration of 1/4 to 2 sec): a short-lasting memory that is used to help provide a temporarly sensory "feel."
  • Short-term memory (20-30 sec): a type of "working" memory that enables recall of items or tasks for up to 20-30 secs, with up to 7 items able to be stored in this active temporary memory (which may come from external or internal sources).
  • Long-term memory (duration is ~ permanent): memory with lasting value, encoded as information within neurocircuitry, that is potentially retrievable but can be "forgotten" (e.g., proactive interference where information learned previously interferes with storage and use of new information, and retroactive interferance where new information interferes with retrieval of old information).