Give tactile sensation

The standard of care for both the LP and El is the blind technique, relying solely on the tactile sensation and the experience of the medical practitioner. Once the correct intervertebral level is determined, and the appropriate interspace is located by palpation, the spinal needle is advanced incrementally through the tissue until there is a give tactile sensation indicating that the needle tip has passed the ligamentum flavum. The needle is further advanced until there is a pop sensation, which indicates the dura mater has been punctured and the needle tip is now located in the subarachnoid space. The success of LP is indicated by the appearance of cerebrospinal fluid, which slowly drains through the hoUow needle due to the intracranial pressure. 

Chronic use of amphetamine or cocaine can give rise to visual, tactile, olfactory and auditory hallucinations (Miller, 1991 Sims, 1988). Formication is characteristic in which the user experiences the sensation of little animals or insects crawling over the body or just under the skin or sees his body covered in sores or vermin. These hallucinations may lead to delusions of infestation. 

The visual perception of depth (when viewing 3D) data is helped enormously if the viewer can move the data himself. There are many depth cues (stereo, depth from motion, depth from perspective), but depth from motion is the strongest. That is why maximum intensity projections (MIP) are preferably viewed dynamically. By self-tracking also the muscle s proprioceptors are giving feedback to the brain, adding to the visual sensation. The combination with human s superb eye-hand coordination has led to the concept of the Dextroscope (www.dex-troscope.com), where a (computer generated) view or object can be manipulated under a half-transparent mirror, through which the viewer sees the display. Displays can also be equipped with haptic (tactile) feedback systems, which are now commercially available (Fig. 9.13). 

---