Task-specific neural mechanisms underlie biases in human orientation perception

Prior experience shapes both visual perception and its underlying neural circuits. This is exemplified by the oblique effect - a strong perceptual advantage for cardinal (horizontal/vertical) over oblique orientations - which reflects how the brain adapts to statistical regularities in the natural environment. It remains unclear whether such adaptations are generalised across visual cortex or are specific to circuits supporting different perceptual judgements. To investigate, we examined human performance in contrast detection and orientation discrimination, using identical stimuli for a range of spatial frequencies, paired with a biologically-inspired model of visual orientation processing. Behaviourally, a robust oblique effect emerged for orientation discrimination but was found only at higher frequencies for contrast detection. The model explained detection changes via an increased pooled response from cardinal-tuned neurons alongside spatial frequency-dependent narrowing of orientation bandwidths, consistent with known properties of cortical V1 neurons. However, the discrimination oblique effect required a different constraint, narrower orientation tuning for cardinal versus oblique neurons. No single model captured both effects simultaneously, suggesting that the oblique effect results from task-specific mechanisms. More broadly, these findings demonstrate how, rather than relying on a fixed strategy, the brain employs flexible computational strategies to optimise sensory encoding for specific tasks. Author SummaryEveryday scenes are dominated by horizontal and vertical contours, such as horizons, buildings, and other natural or man-made structures. The human visual system appears tuned to this regularity: people judge horizontal and vertical orientations more accurately than oblique ones. This bias is thought to arise from neural mechanisms that encode orientation, and we investigated whether this reflects a fixed property of orientation coding, or flexible adaptation to task demands. Participants performed a detection task, in which they reported the presence or absence of faint oriented patterns, and a discrimination task, in which they judged small changes in orientation. We built a computational model based on known response properties of orientation-selective neurons in visual cortex, to test which neural adaptations best-explained performance on each task. Participants exhibited advantages for horizontal and vertical orientations which differed between the two tasks. Critically, our model revealed that this bias could not be explained by a single, general-purpose neural adaptation applied uniformly across both tasks. Instead, our data suggest that the visual system contains distinct orientation-coding biases that are engaged in a task-dependent manner. Consequently, sensory processing is shaped not only by regularities of the environment, but also by the observers behavioural goals.