This paper surveys studies on the anisotropic perception of orientation in
the visual system. The vertical and horizontal orientations are known to be
perceived more accurately than, oblique orientations. This anisotropy; whi
ch called the oblique effect, was present in a large variety of tasks (dete
ction, discrimination, recognition, memorisation). The discussion about the
visual oblique effect (VOE) concerns the nature and level of the processes
that generate this effect. It is still unknown whether the VOE stems from
low levels of the visual processing system or/and from higher levels. The "
low level hypothesis" explains the VOE by the neural properties existing at
low levels of visual processing. This hypothesis is supported by three typ
es of arguments : 1 / the VOE is explained by the axis-dependent change in
the properties of orientation-selective neurons early in, the visual hierar
chy (the neurons that are tuned to horizontal and vertical orientations mig
ht be more sensitive, narrowly tuned or/and more numerous than those tuned
to oblique orientations); 2 / the VOE is not influenced by factors independ
ent of the visual stimulus (e.g.: environment, etc.);3 / the VOE follows a
retinal reference frame when the head or body of subjects is tilted with re
spect to gravity. The hypothesis of a "high level VOE" is supported by oppo
site arguments : I / the observations fail to follow the predictions of neu
ronal models; 2 / the magnitude of the VOE is influenced by some factors in
dependent of the visual stimulus; 3 / the VOE follows a gravitational rathe
r than a retinal reference frame in some experiments. The analyse of contra
dictory results shows that the origin of the visual oblique effects seems t
o be multi-component and to occur at different levels of processing accordi
ng to the nature of task.