Henderson and Smyth's study found that patients could at first evoke the phantom, temporarily, by merely thinking about it. This voluntary direction of thought was, like other factors, at first uniformly effective, but it did become gradually weaker and finally would disappear. It is one of the more enduring influences.

Johns Hopkins University

There are a number of factors that may impact the degree to which a patient detects the tingling, or phantom-like, sensation.
- Pain in the Stump
- Sedatives
- Climatic Conditions
- Thinking of Phantom
- Urinating and other Visceral Reflexes
- Loss of Balance
- Instinctive Actions
- Fright
- Emotion
- Attachment of Artificial Limb

Factors that Affect Phantom Perception

HENDERSON, W. R., & SMYTH, G. E. (1948). Phantom limbs. Journal of neurology, neurosurgery, and psychiatry, 11(2), 88–112. https://doi.org/10.1136/jnnp.11.2.88

HENDERSON, W. R., & SMYTH, G. E. (1948). Phantom limbs. 

Sedatives, shock, and toxæmia, by lowering the level of consciousness, delay the appearance of the phantom.

Impact of Sedatives on Phantom Perception

The weather has been noted to have a marked influence on the vast majority of phantoms, in that they are more noticeable in winter (especially during a cold spell). This is the most enduring influence and it persists long after other inducing and intensifying factors have ceased to be effective. In fact, after two or three years the phantom may be noticed only when there is a "change in the weather."

Impact of Climatic Conditions on Phantom

Thinking of the Phantom

There is a brief sensation in the stump/phantom during urination in at least 90% of thigh and leg amputations, usually mild or unnoticed. The sensation is one of two kinds, either a tingling in the phantom or a tingling on the end of the stump w/o any reference to the phantom. This rarely, if ever, occurs in upper limb phantoms.

The Effect of Urinating on the Phantom

When a patient stumbles or slips on crutches a strong sensation shoots down the limb, with the phantom being felt momentarily. Simultaneously, the stump moves out as if to save the patient from falling. This has been noted as one of the more enduring reflexes.

Effect of Loss of Balance on Phantom

When the stump begins to move to complete a task (like picking up a ball), the mistake in attempting to use the phantom limb is realized and simultaneously there is often, but not always, a monetary sensation of the phantom (however this ceases within a few months for the patients "quickly learn to avoid making mistakes"). One possible explanation for this is that the cortex is in a hypersensitive state, enabling the normally subconscious reflexes locally to reach a conscious level.

Effect of Instinctive Actions on Phantom

Sudden fright (ie. from a loud noise) is sometimes associated with momentary perception of the phantom.

Effect of Fright on Phantom Perception

Emotion such as anger or excitement makes the patient forget the phantom.

Effect of Emotion on Phantom Perception

Nearly 100 men were fitted with temporary limbs nine to eighteen months after amputation. Almost without exception the phantom was never felt while wearing the prosthetic but often reappeared when it was removed. 

Effect of Wearing an Artificial Limb on Phantom Perception

Following an amputation, severe pain in the stump can initially overshadow the perception of a phantom limb. Consequently, during the healing process, there may be a delay of several weeks before the phantom limb is perceived. One theory suggests that this delay occurs because the patient's attention is diverted from the phantom limb by the severe pain in the stump.

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