Table 2.

Comprehensive description of the function of the direct and indirect pathway, organized by type of experiment.

	dSPNs	iSPNs	Reference
Optogenetic activation	Increase in spontaneous locomotion and movement velocity; causes form of dyskinesia that altered overall performance in the lever press task (at least for DLS)	Increase in freezing and decrease movement velocity; ipsilateral turns	[96–98,118]
	Mild delay in initiation of movement	Abolish first half of a motor sequence	[103]
	Slow initiation as the animal switched to other behaviors (and slow performance)	Aborted initiation as the animal switched to other behaviors (and affected performance)	[101]
Optogenetic activation prior to reversal learning	Contralateral bias	Ipsilateral bias	[113]
Optogenetic activation during outcome presentation (in DMS)	Reduced switches following reward	Increase switches after unrewarded trials	[102]
Optogenetic activation during learning	Reinforces behavior and/or spatial preference paired with stimulation; reinforces velocity or other features of trained movements	Reduces performance of stimulation-paired behavior; increases aversion for a spatial location; reduces velocity of movement	[117,118]
Optogenetic inhibition	Affected movement initiation; increased probability that the animal disengages from the task	Affected movement initiation	[101]
	Slows action initiation	No changes in action initiation	[103]
Electrophysiology combined with optogenetic tagging (DLS only)	Sequence related sustained firing activity during action execution	Sequence related inhibited firing activity during action execution	[107]
	Portion of both (~ 40%) active during initiation and/or termination of movements
Electrophysiology combine with optogenetic tagging (DMS only)	Both involved in element-level action execution		[103]
		More active after change in strategy
Fluorescent calcium indicators	Both active during movement and less during immobility (particularly in contralateral inward movements); similarly active during training; both encode velocity; coactive specifically predicting movement initiation		[94,97,99,101–102,105,111]
Simultaneous photometry recording of dSPNs and iSPNs	Both encode the sequences of spontaneous motor behaviors (activation of both pathways necessary for action selection). Both similarly selective for similar actions		[112]
Single-cell recording	Similar activity clustering in regard to spatiotemporal features of the movement		[109]
Lesion of the DLS (and partially DMS)	More likely to signal sequence initiation and termination	Preferentially encodes the transition between sequences	[103]
DLS inhibition during reward-based lever press task	Critical for completion of responses in serial order (without affecting reversal learning)	Behavior not affected, except for a transient improvement on second step of the task	[176]
In DMS	No effect on rotarod performance; stimulates novel object recognition, more active during rewarded presentation; D1 availability correlates with instrumental learning bias from rewarded trials	Affects early training performance on rotarod; inhibits novel object recognition; important to inhibit competing actions; disengages in later stages of learning; modulates dSPNs inhibiting previously learned commands and allow new goal-directed learning; more active during unrewarded outcomes	[96,108,109,113,128]
In DLS	Active in late stages of learning; necessary for performance in early and late learning; necessary to develop novel strategy and habit learning	iMSNs active in early skill learning	[102]
D1R or D2R KO on rotarod	Decreased performance, later improved	Good performance at early training, without any improvement. Poorer performance if animals exposed to high speed first	[133]
D1R antagonist	Lower motor ability at high speed of treadmill, not related to changes in reward sensitivity	No changes