Mental shield

A mental shield (not to be confused with the mutation Mental Mirror) is a property that makes creatures immune to mental damage and most mental effects, and blocks Telepathy. It is possessed by creatures without psionically accessible minds, whether because they are mindless, have artificial minds, or are otherwise immune to psychic abilities.

Details

Creatures with mental shields cannot take mental damage, such as that caused by psionic weapons or Sunder Mind. They cannot be affected by Telepathy. The following effects cannot be applied to creatures with a mental shield:^[1]

Beguiled
Confused
Dominated
Shaken
Shamed (Berate)
Psionically cleaved
Stunned by gas
Terrified (Intimidate and Menacing Stare)

By default, robots, oozes, fungi, plants, and nests have mental shields. However, the mental shield is removed from sapient plants and fungi.

The only creatures with mental shields outside of those categories are girshworms, twinning lampreys, and trining lampreys.
Pax Klanq is the only fungus without a mental shield.
Euclid, and the members of the Consortium of Phyta (Yurl, Sixshrew, Tillifergaewicz, the oddly-hued glowpad, Asphodel, Earl of Omonporch, and asphodelytes) are sapient and thus do not have mental shields.

Esper Hunters

If the player has a high enough glimmer and summons a psychic assassin, there is a chance for the hunter to be a creature that normally has a mental shield. In this case, the hunter's mental shield is removed, and thus the hunter is fully affected by mental attacks and effects.^[2] Creatures with mental shields are unable to become psychic thralls.

Relation to Mental Armor (MA)

The MA value of creatures with a mental shield is not often used, because they are invulnerable to mental attack. However, certain scenarios exist where the creature's MA value can still be used, such as the Rebuke Robot skill, Proselytize, or the ganglionic teleprojector, which each have their success based in part on the target's MA value.