Garage Psych is an early, rawer, more sinister and intense form of psychedelic rock, with as much emphasis on the psychotic as the psychedelic... Roughly predominant 1965-68 before the styles branched into psychedelic rock proper, hard rock and heavy psych (see Yardbirds -> New Yardbirds -> Led Zeppelin for a rough illustration of the evolution and interconnectedness of these styles). Acid Punk such as the Stooges and more modern Icarus Line have their roots in this era of rock.
As far as influnces go, the usual garage culprits (notably Jeff Beck's incarnation of the Yardbirds), Hendrix, regional psychedelic scenes plus the bands' own unique experiences with various substances; there is a genuine feeling of artists making it up as they're going along, following their muses into whatever directions they saw fit...
While staying within the garage band format, tracks variously feature:
-soaring/ screeching lead guitar work, overdriven into fuzz tones and stranger sounds/noises created with novel effects pedals; pulsating bass,
-manic singing/ yelling/ spaced-out rambling
-sometimes experimentation with drone and repetition within a “pop” format
-studio trickery
-Yardbirds-style rave-ups taken away from the blues into something more genuinely shamanic
-eastern-infused melody lines / raga scales and pounding, sometimes tribal drums/ rhythms
-occasional inclusion of atypical instrumentation such as organ, percussion instruments, though the majority of novel sounds were created by pushing the boundaries of the guitar
-less-rigid song structure or rhythmic continuity
-usual garage swagger but with warped blues stylings bordering on the Beefheart... .
