The Role of Visual Features in Text-Based CAPTCHAs: An fNIRS Study for Usable Security

Table 1

Selected features to be analyzed with difficulty levels.

Features	# of difficulties	Security features defined for each level
Line	3 levels	No line	Line thin	Line thick
Waving	3 levels	No wave	Wave light	Wave high
Rotation	2 levels	No rotation	Rotation {−20, −15, 15, 20}	—