|Default Health and Armor||
|Max Health and Armor||
60 (HD:TFE), 61 (HD:TSE, SS3)
|Enemy Health Multiplier||
|Enemy Damage Multiplier||
|Enemy Speed Multiplier||
x1.125 (SS3), x1.25 (SS1)
|Enemy "Think Frequency" Multiplier (the higher it is, the faster they act)||
|Enemy Initial Delay Multiplier (Delay before they detect player)||
|Auto-aim Factor (1 means no autoaim, above 1 means there is autoaim, below 1 means inaccuracy)||
A unique twist on the available difficulty settings, Mental Difficulty is exactly like Hard Difficulty with the only difference being that enemies will fade from being visible, to completely invisible. An enemy's vitality can still be identified by pointing the aiming reticle over an invisible enemy, which makes the feature somewhat more useful in Mental Difficulty. In Mental Difficulty, it is important to not just look around for enemies, but to utilize sound as a way to track enemies.
Since Mental Difficulty does not borrow the changes from Serious Difficulty, it can be considered easier to a degree. It does not contain the largest caliber of enemies like Serious Difficulty does, but it still takes the enemy's numbers, and strengths from Hard Difficulty. So while it may not be as overwhelming as Serious, it compensates this by the invisibility granted by enemies which may disorientate inexperienced players. In Serious Sam HD, and Serious Sam 3: BFE, players are also granted a score multiplier of 60/61; the highest of all difficulties.
Since enemies fade from visible to invisible indefinitely, it can make certain enemies such as the Beheaded Kamikaze, Marsh Hopper, and the already-transparent invisible variety of Gnaar much more challenging to target, and track. This will mean during a mass of enemies, the player will have to occasionally guess where an enemy is located when briefly invisible while still moving and dodging others. Mistakes where the player might fail to notice a Kleer Skeleton, or an invisible Kamikaze approaching from the rear may prove dangerous, and fatal if the player is not prepared for a possible mistake.
For a mass of Marsh Hoppers, it is incredibly easy to lose track of a group and leave oneself open for an attack. So memory, estimation, and sounds are important for survival. Since estimated attack shots are sometimes required of the player, ammo conservation is also important, and unlike Serious Difficulty, the player is not granted double the maximum ammunition capacity, just like on Hard. This means the player must guess as accurately as possible to where an enemy might be, but to not waste a large degree of ammo doing so.
Mental Difficulty is suggested for generally experienced players who can handle the amount of enemies on Hard, but are experienced enough to know where some enemies may spawn so as to not be unprepared. The only difference from going to Hard to Mental should technically be the invisibility addition on enemies. Players who have unlocked Mental through completion on Serious Difficulty may find Mental to be both easier and slightly more challenging due to the reduced number of enemies, maximum ammunition reduction (HD remakes only), and invisibility addition respectively.