Reshaping the Sensory Environment Sensory Accuracy Survival

Reshaping the Sensory Environment

Sensory Accuracy

Survival of all animals depends on the accuracy with which sensory information is processed by the nervous system. Integrating this information in an efficient and effective manner depends on dynamic strategies that the nervous system relies on to determine the reliability and accuracy of the sensory inputs (reviewed by Zaidel, Turner, and Angelaki, 2011). This essay examines contemporary theories that attempt to explain how these strategies function.

Reliability-Based Cue Combination (RBCC) Theory

The reliability of a sensory cue is believed to determine the weight an organism assigns to a given cue, such that a more reliable cue may have a greater influence on behavioral outcomes (reviewed by Zaidel, Turner, and Angelaki, 2011). Empirical support for this theory has come from studies examining the integration of multisensory information. This theory has been called reliability-based cue combination (RBCC).

Reliability, as defined by RBCC, is used interchangeably for sensory precision; however, these terms do not necessarily imply accuracy (reviewed by Zaidel, Turner, and Angelaki, 2011). Accuracy is defined as the degree of agreement between the sensory information and the environment. External feedback is theoretically required for determining accuracy, but in the absence of additional external information, the organism must make do with the degree of agreement between different sensory inputs. For example, the sound of an approaching car can be confirmed visually, but should someone experience hearing a loud bang outside their visual field, an internal validation process will likely occur because visual confirmation is impossible.

Reliability-Based Adaptation (RBA) Theory

In situations when multisensory inputs disagree, RBCC proposes that the sensory cue with the most reliability will have the greatest influence on behavior (Burges, Girshick, and Banks, 2010, p. 8). Accordingly, if visual information is providing the most reliable cues for a given environment, discrepant auditory or touch information will be discounted or undergo adaptation and/or recalibration relative to the visual information. This theory has been called reliability-based adaptation (RBA) (reviewed by Zaidel, Turner, and Angelaki, 2011). Although some researchers have suggested humans are visually dominant and therefore all other senses adapt to visual cues, recent research provides strong support for the theory that all senses are capable of adapting depending on the situation (Burges, Girshick, and Banks, 2010). For example, hearing a nearby train whistle at night may cause someone to hesitate before crossing a set of railroad tracks, even if they cannot see an approaching train. In this environment, the reliability of aural sensory information is increased relative to visual information because it is nighttime.

Despite the empirical evidence indicating the visual system can adapt to a more reliable sensory cue, other studies have provided evidence suggesting that visual information probably represents a default sensory reference in most situations where visual cues are provided. When barn owls had prisms placed in front of their eyes their prey-oriented flight paths suffered, despite the use of both hearing and vision to find and capture prey (reviewed in Burges, Girshick, and Banks, 2010, p. 10). With time however, the owls adapted to the prisms and the accuracy of their flight patterns increased. When their brains were compared to control animals the connections in the auditory cortex, rather than the visual cortex, were modified. Visual dominance, in the form of neurobiological correlates, therefore seems to exist in barn owls.

Bias

Bias is defined as the difference between what we sense, and the actual value of the sensed property in the environment (reviewed by Scarfe and Hibbard, 2011, p. 2). Sensory accuracy represents the magnitude of this bias. Bias can be introduced by a number of different mechanisms. For example, vibration in the environment can distort sensations of limb position, as can muscle fatigue and preconditioning (reviewed by Winter, Allen, and Proske, 2005, p. 1043-1044). If a person confronts a novel sensory experience, such as an astronaut conducting motor tasks for the first time in the absence of gravity, then bias increases.

In contrast, amputees, whether from trauma or chronic disease, frequently experience pain or movement sensations associated with a missing limb (Bestmann et al., 2006). There is little or no agreement between the sensory experience and environmental cues for such individuals and sensory accuracy is therefore minimal. When a phantom limb sensation was triggered in an amputee by transcranial magnetic stimulation and imaged with functional magnetic resonance imaging (fMRI), it was discovered that the motor cortex was primarily responsible for this experience. The role of the central nervous system therefore seems to extend beyond the tasks of integrating and calibrating sensory inputs, and includes the internal generation of sensory experience independent of environmental cues.

Discussion

Sensory information from multiple sources, such as vision, hearing, and touch, are managed by the nervous system to produce an accurate representation of environmental conditions. Because of the complexity inherent in such a system, there is considerable potential for error. When error does occur, the brain prioritizes sensory cues depending on how reliable this cue has been in the past in providing useful information. If external feedback is possible, sensory inputs can be recalibrated over time to increase the accuracy, and thus reliability, of the cues. Judging the accuracy of a sensory input therefore depends on how much agreement there is between multiple sensory inputs, how reliable these sensory inputs have been in the past in providing useful information for navigating the environment, and the novelty of the environmental situation.