Table 1. Studies using MTL^a or privacy-preserving approaches for various applications.

Study	Application	Signal	MTL	FLb	DPc	Multimodality
Zhao et al [21]	Network anomaly detection	Network signals	✓	✓
Smith et al [22]	Activity recognition	Acceleration, gyroscope	✓	✓		✓
Somandepalli et al [16]	Affect recognition	Speech, video, text		✓		✓
Sang et al [7]	Affect recognition	Face images	✓
Shome and Kar [23]	Affect recognition	Face images		✓
Chhikara et al [19]	Affect recognition	Face and speech		✓		✓
Feng et al [24]	Affect recognition	Speech		✓	✓
Can and Ersoy [15]	Affect recognition	PPGd		✓
Nandi and Xhafa [20]	Affect recognition	EEGe, EDAf		✓		✓
Wang et al [18]	Biometric	Face speaker		✓	✓	✓
This study	Affect recognition	PPG, EDA, ACCg, STh	✓	✓	✓	✓

^aMTL: multitask learning.

^bFL: federated learning.

^cDP: differential privacy.

^dPPG: photoplethysmography.

^eEEG: electroencephalography.

^fEDA: electrodermal activity.

^gACC: acceleration.

^hST: skin temperature.